State of the Art in Metal Matrix Composites Research: A Bibliometric Analysis

: Metal matrix composites (MMC) are the materials of tomorrow. This paper presents an in-depth analysis of the MMC research articles published in Web of Science (WoS) during 2001–2020. The study ﬁrstly included year on year publications, publication types, sources, research directions as well as the most productive researchers, organizations and nations. Secondly, a detailed analysis of collaborations among various MMC researchers, organizations and countries has been presented. Thirdly, citations based linkages among the published articles, sources, researchers, institutions and places have been discussed relative to their respective collaborative link strengths. A co-occurrence analysis of MMC keywords was also conducted to highlight the most important keywords trending in this area. Finally, burst detection analyses of keywords and references were carried out to unearth sudden citation spikes of keywords and documents. Primary results indicate that research articles formed 80.54% of all MMC publications in WoS. The journal ‘Materials Science and Engineering A: Structural Materials, Properties, Microstructure and Processing’ published maximum MMC articles. Collaboration analysis results indicate that Zhang D, the Chinese Academy of Science and People’s Republic of China, attained topmost collaboration based total link strengths (TLS). Citations based analysis showed that Zhang D, the Shanghai Jiao Tong University (China), People’s Republic of China and the journal ‘Materials Science and Engineering A: Structural Materials, Properties, Microstructure and Processing’ received highest citation TLS values. Keyword ‘Graphene’ scored the highest citation burst strength (2018–2020). The future of MMC research lies in processing and characterization of novel nanocomposites with reinforcements such as graphene and boron carbide for various applications.


Introduction
Advancement in science and technology is directly correlated with the discovery, processing and applied investigations of novel materials. Modern materials are more complex, functionally efficient and commercially viable as compared to their simpler and functionally limited predecessors. Composites form a large group of new age materials that combine the positive attributes of two or more different constituents resulting in a superior combination. These materials help fulfill the conflicting requirements of maximising performance and minimising energy consumption simultaneously [1,2]. Composites typically consist of reinforcements added to base materials to make them stronger, tougher and more resilient [3][4][5]. The base materials can vary from metals to polymers, whereas the reinforcements may vary from ceramics to fibres and fly ash [6]. The current study focuses on the research trends pertaining to metal based composites reinforced with ceramic particles. The following subsection gives a brief overview of the properties, compositions, force/tool wear/surface generation mechanisms, machining challenges as well as the simulation/modeling techniques explored by researchers to characterize these complex materials.

Metal Matrix Composites
Metal matrix composites refer to a class of new age materials consisting of base metal alloys primarily reinforced with ceramic particles for enhanced functional properties. Figure 1 shows a pictorial representation of metal matrix composite structure reinforced with (a) single and (b) multiple types of particles. The composites with multiple types of reinforcement particles are called hybrid metal matrix composites. Composites with nano sized inclusions are termed nano metal matrix composites, or simply, nano-composites. Hybrid metal matrix composites can also consist combinations of metal matrices in the form of alloys such as aluminum-magnesium, aluminum-copper and titanium based compositions [7]. These materials offer superior mechanical characteristics as compared to monolithic alloys. These characteristics include strength to weight ratio, stiffness to weight ratio, wear resistance, corrosion resistance, strength, hardness and more [8]. However, these materials are difficult to prepare and to machine. One of the challenges involving these materials is to obtain uniform dispersion of the reinforcement particles in the base alloy matrix. Hence, researchers have explored various processing techniques such as stir casting and powder metallurgy to ensure the same [9]. Investigators have also studied various machining methods such as turning, milling, drilling and grinding to process these materials efficiently [10]. Others have tried different simulation and control methods towards accurate predictive modeling [11,12] and automation of the MMC machining process outcomes such as surface roughness [13][14][15][16], tool chip interface friction [17] and more. Aluminium is a commonly preferred base metal alloy for most MMC compositions because of its good strength, light weight machinability and aerospace applications [18][19][20][21]. Common reinforcement materials include silicon carbide, boron carbide, alumina and others. Recently, nano scale reinforcement particles such as carbon nanotubes and graphene are also becoming popular among MMC researchers [13].
Metal matrix composites exhibit very unique strengthening and fracture mechanisms that make them difficult to machine. Researchers have conducted detailed experiments to understand the behavior of MMCs under quasi static as well as high strain rate processing conditions [22][23][24]. For instance, particle reinforcements tend to enhance the sensitivity of strain hardening to strain rate effects of machining [25], resulting in higher strain hardening rates at smaller increments in machining speeds. The hard ceramic reinforcements tend to generate higher tool wear during machining of composites [26]. Higher volume fractions of reinforcements lead to greater tool wear and damage [27]. These enhanced tool wear modes include edge chipping, grooving and notch wear [28][29][30]. The tool wear modes are generated during composite machining include two body and three body abrasion mechanisms caused by the mutual rubbing of the hard ceramic particles with the work surface and the tool flank face [31][32][33][34]. The abrasions between the ceramic particles and the work/tool surfaces is enhanced because of completely/partially pulled out, debonded and fractured particles leading to greater tool wear [35][36][37][38]. Additionally, the particle reinforcements improve the yield strength of the base metal alloy leading to high cutting force generation during machining. Alloy yield strength gets enhanced due to the numerous dislocations created by the particle inclusions in the matrix microstructure [26,[39][40][41][42][43][44][45][46]. During machining, the dislodged and broken ceramic particles plough through the machined surface to generate higher machined surface roughness as compared to monolithic alloys [47][48][49][50]. In order to account for the various complex mechanisms active during MMC machining, the researchers categorised MMC machining forces into shearing, ploughing and particle fracture/debonding for improved force modeling and prediction [51][52][53][54]. These modeling theories are primarily based on the seminal works of Merchant (orthogonal cutting model) [55], Oxley (slip line field model) [56] and Colwell (equivalent cutting edge concept) [57]. The Johnson-Cook's constitutive equation [58] is also widely used by researchers to model shear stress generation during MMC machining. This model incorporates strain hardening, strain rate as well as thermal softening phenomena to predict material flow stress.
The foregoing discussion gives a brief insight into the complex nature of some aspects of metal matrix composites. Similarly, researchers have extensively worked upon other MMC aspects such as preparation methods, characterisation techniques, varied applications and more. In the present study, a detailed bibliometric analysis has been carried out to understand the important contributing authors, institutions, countries as well as the current and upcoming trends of this rapidly emerging field.

Bibliometric Studies
Bibliometric analysis complements the traditional literature reviews by providing an objective evaluation of the trends and future potential of a research area [59][60][61][62]. This analysis is useful to highlight the evolution of a particular research area over a period of years or decades [61]. There are many methods to review the scientific research developments in a particular field [63]. These methods can be categorised under two broad classificationsevaluative and relational reviews [64,65]. Evaluative reviews generally deal with the quantitative analysis of the absolute research impacts of research articles, authors, organizations and nations. This analysis includes various productivity metrics such as the number of publications, publications per year, number of citations and more [66][67][68][69]. Evaluative reviews can also be conducted qualitatively based on expert opinions of research impact metrics in a particular area [70]. On the other hand, the relational review methodology focuses on inter-relationships among the above-mentioned evaluative impact metrics. These inter-relationships are measured and expressed in terms of the number of collaborative outputs, collaborative linkages and respective link strengths. Other relational metrics include citations based linkages, co-occurrences and co-citations based analyses [71]. Co-authorship linkages are important indicators of knowledge sharing among different research groups [72]. Citation linkages are vital for understanding research relationships among productive authors and organizations [73]. Determination of linkage strengths reveals the extent of collaborations/co-citations across geographies [74]. The current study considers both evaluative as well as relational tools for a balanced bibliometric analysis of metal matrix composites research. Bibliometric surveys provide insights into the development of a research area over a time period of one or more decades [59,75]. These analyses typically yield vital information about research outputs of individual authors and respective affiliating organizations [68,69,76]; collaboration and citation networks among documents, authors, organizations and places [76][77][78]; trending topics and keywords [70,79]; top publishing journals, inter-journal linkages [66,80,81] and more.
The above mentioned bibiliometric results can be effectively analysed and displayed by employing visual data representation tools such as VoS viewer [82], CiteSpace [83,84], ATLAS.ti [85], BiPlot [86], BibExcel [87], Network Workbench [88,89], VantagePoint [90] and many more. Data visualization tools are vital for effectively communicating the analysis results as well as for quickly arriving at useful conclusions [91,92]. In the present work, VoSviewer and CiteSpace were utilized for the quantitative data analysis and visualization of MMC research published in WoS during 2001-2020. VoSviewer generates author and journal network maps based on bibliometric analysis of the collaboration/citation data. It can also create keyword maps showing inter-linkages among various co-occurring terms related to a research area [82,93]. CiteSpace is a Java application designed to analyse and visualize citation based networks [83], co-authorship links, time series analysis of important topics and citation/reference bursts [94].
Recently, few researchers have conducted bibliometric analysis on some aspects of metal matrix composites. Laad and Ghule [95] analysed MMCs reinforced with environment friendly silica. Vencl et al. [96] studied the tribological properties and applications of composite materials with special emphasis on eastern European nations. Saxena et al. [97] used bibliometric analysis to explore the impact of friction stir processing to produce MMCs for industry 4.0 applications. Ghule and Laad [98] conducted another bibliometric survey on energy storage applications of polymer matrix composites. Koo and Jeong [99] applied bibliometric analysis to survey research conducted only on carbon nanotube reinforced composites. Costa and Moura [100] analysed the performance of and strategy for the develpment of polymer matrix nanocomposites with a special emphasis on Brazil. Pecas et al. [101] reviewed various natural fibre composites including their applications using the bibliometric methodology. Bibliometric studies of general areas include an analysis of knowledge maps in metallurgy [102], applications of artificial neural networks in engineering [103] and an analysis of materials research in India with a focus on international linkages [104]. The above section shows that there is a need to report an in-depth, comprehensive bibliometric analysis on the subject of metal matrix composites. The present work aims to bridge this research gap by providing multi-dimensional insights to the researchers for identifying current and future trends in this exciting field.

Data Selection and Extraction
The web of science (WoS) repository was selected as the primary database for the bibliometric analysis carried out in the current study. This database is a premium collection of high quality and high impact research articles published in the most prestigious journals across the globe. It catalogues manuscripts published in indexed journals since the year 1990. The WoS core collection includes eight citation indices viz. Emerging Science Citation Index (ESCI), Science Citation Index Expanded (SCIE), Social Sciences Citation Index (SSCI), Arts and Humanities Citation Index (AHCI), Conference Proceedings Citation Index-Social Sciences and Humanities (CPCI-SSH), Book Citation Index-Social Sciences and Humanities (BCI-SSH), Conference Proceedings Citation Index-Science (CPCI-S) and the Book Citation Index-Science (BCI-S). This entire WoS core collection was searched for the keywords: ("nano" OR "aluminium" OR "machining" OR "fabrication" OR "characterization" OR (("mechanical' OR "electrical" OR "tribological") In addition, "properties") OR "mechanisms" OR "modeling" OR "preparation" OR "novel" OR "in situ" OR "scanning electron microscopy" OR "advanced" OR "carbon nanotube reinforced" OR "particle reinforced" OR "safety aspects of nano" OR "applications" OR "aerospace") AND "metal matrix composites" for two decades spanning from 2001 to 2020. The search options were set to 'full record and cited references' with regard to the aforementioned keywords. The search yielded a total of 10,018 research articles which were exported from the database in plain text forms on 3 June 2021. The most exact records matching the specified MMC keywords were obtained from the ESCI, SCIE, CPCI-S, CPCI-SSH and BCI-S indices. The following sections present and discuss the different MMC research insights revealed by the bibiliometric analysis.

Publication Distribution along Various Verticals
This section details upon the distribution of the searched articles on the basis of publication years, types, source titles, MMC research areas and their trends. Moreover, the top MMC research contributing nations, organizations, authors and their respective nationalities are also presented along with their respective publication counts.

Year on Year Vertical
The year on year progression of WoS research publications in the field of metal matrix composites is depicted in Figure 2.  The primary reason behind this phenomenon is the sheer variety of metal matrix compositions that can be prepared using various combinations of base alloys, particle inclusions, reinforcement fractions and processing techniques. There is also a rising interest in the synthesis and analysis of metal matrix composites infused with various nano particles. In addition, there is an almost limitless scope for MMC modeling, application specific deployments, conventional and non conventional MMC machining and much more. Hence, the interest of researchers in this ever green topic is likely to be sustained in the future as well. The following subsection discusses the diversity of the publication types in the area of metal matrix composites.

Publication Types
The complete corpus of 10,018 MMC publications obtained from the WoS database was categorized into research articles, proceedings, review papers, early access documents and more. All these publication types and their respective counts are pictorially depicted in Figure 3. Research articles (8068) make up a major chunk (80.54%) of the total publications. Proceedings' articles (2198) form the second largest group of MMC publications (21.94%), whereas review articles (309) comprise 3.08% of the entire corpus. Book chapters (111) and early access manuscripts (44) on metal matrix composites contributed 1.11% and 0.44% to the total, respectively. Remaining publication types viz. letters, corrections, editorials, data papers, reprints and others constituted less than 0.25% of the exported database.

Publication Sources
This subsection provides insights into the top sources publishing metal matrix composite research indexed in WoS.

Research Directions
After the previous subsection on the distribution of the publication sources, the present sub section throws light on the various research directions focused by investigators in the field of MMCs. Figure 5 provides these research directions with their specific article counts filtered by the WoS corpus search. The analysis results indicate that the maximum publications (7580, 75.66%) were categorized under the general topic of 'materials science', indicating the materials specific focus of most articles. The topic of 'metallurgy/metallurgical engineering' emerged as the next most investigated area (3242, 32.36%). This indicates the number of articles dedicated to the various processing methods of the metal matrix composites. The next most significant chunk of the publications (2703, 26.98%) belonged to the general area of 'engineering', indicating a more general engineering based approach to the problems associated with MMC fabrication, machining and testing. Similarly, 'science technology and other topics' included 1082 (10.80%) articles, showing the percentage of investigations into novel scientific and technological aspects of metal matrix composites. The articles dealing with the physical and chemical properties of the various particulate materials and corresponding reinforced metal matrices numbered 839 (8.38%) and 738 (7.37%), respectively. About 436 (4.35%) articles reported investigations upon the various mechanics and mechanisms involved in processing and machining these advanced materials. Researchers also published 151 (1.51%) papers aimed at the integration of MMC machining aspects in industrial control systems for automated manufacturing. In addition, 83 articles focused on the mining, extraction and treatment of micro and nano particles for imparting desirable properties to the reinforced base metal alloys. Investigators also worked on applications of metal matrix composites in the field of and fibre optics and optical instruments (72 articles). Figure 6 depicts the publication trends of each of the above-mentioned research directions during 2001-2020. These results clearly indicate the prevalence of 'materials science' as the highest trending general research area in the field of metal matrix composites. It is readily evident that the majority of all published MMC articles have 'materials science' as their central theme. The next most important trending topics are 'metallurgy metallurgical engineering' and 'engineering', implying the ever increasing general interest in processing, machining and engineering applications of various MMCs. The related topics of 'science technology', 'chemistry' and 'physics' witnessed moderately incremental interest over the past two decades. Research areas such as 'automation control systems', 'mining mineral processing' and 'optics' do not indicate any upward publication trend. Hence, these research directions offer maximum opportunities for carrying out novel MMC investigations.

Popular Places Vertical
This subsection focuses on the top ten publishing nations that have contributed MMC articles in WoS-Peoples Republic of China, India, USA, Germany, Iran, Turkey, South Korea, England, Japan and France. Figure 7 depicts the publishing trends of these countries across twenty years starting 2001. It is evident from this figure that, during the initial years

Productive Organizations and Researchers Vertical
This subsection discusses an analysis of the top ten institutes publishing MMC related articles in WoS during 2001-2020.     Table 1 lists the top ten researchers worldwide with maximum MMC publications in WoS. Although the topmost contributing author hails from Singapore (Gupta M, 140 articles, 1.40% of total corpus), all of the next nine top authors are affiliated with Chinese institutions. This analysis highlights the presence of high output MMC research groups in China. The next section showcases an analysis of the various research collaborations in the field of MMCs.

Collaboration Analysis
Collaboration is very important for useful sharing of knowledge, ideas and resources among the national/international research community. Collaborative teams tend to be more productive, efficient and innovative. The following subsections present analyses of MMC research collaborations at various levels, viz. researchers, organizations and countries. In the present work, VOSviewer software was utilised for obtaining the MMC collaboration insights.

Collaboration among Researchers
For researcher-collaboration analysis, the VoSviewer threshold was set at a minimum of five documents and zero citations per author. This criteria was met by 1616 authors out of a total of 16,735. These 1616 authors were examined for mutual co-authorship links and the researchers with the greatest total link strengths (TLS) were ranked. The VOSviewer revealed that these authors produced 1429 WoS articles through their mutual collaborations! The top ten collaborative researchers in the field of metal matrix composites are listed in Table 2. The column 'P' refers to the number of MMC articles published by the respective authors in WoS during 2001-2020. 'Links' column refers to the total number of investigators in collaboration with each of these authors (appearing as co-authors in 'P' publications). Thus, each link represents a collaboration between two researchers. The strength of this link is determined on the basis of the number of published articles featuring these two researchers. The total link strength of a researcher is computed as the sum of the strengths of all his/her links.  Tables 3 and 4 clearly depict the difference between the scale of co-author linkages of Fan G and Wang X. The top six collaborators of Fan G have many more joint publications with Fan G (minimum 22 articles) as compared to the number of joint publications of the top most collaborator of Wang X (17 papers). Therefore, higher research outputs may not always correspond with higher collaboration strengths. In fact, some authors, like Gupta, M with high research outputs (Table 1) are not even mentioned in the top ten TLS rankings because of their lower collaboration linkages (60) and corresponding link strengths (TLS 223). Figure 10 shows the collaboration network map of the 1616 researchers with their respective inter-linkages. This map highlights different network groups in different colors and shows link thicknesses based on the respective link strengths. Moreover, Figure 11 depicts the collaboration network of the author with the highest TLS as per Table 2, i.e., Zhang D. This figure provides information of the collaborative linkages and the relative link strengths of this author at a glance. Herein, the respective author 'nodes' are depicted with varying sizes depending on their individual TLS values. It is evident from the link 'thicknesses' that Zhang D. has more publications with co-authors such as Li Z, Su Y, Tan Z, Fan G and Guo Q relative to the number of articles produced with Ji G, Zhang J, Li S, Guo X and others. On the other hand, Figure 12 shows the collaboration network of Gupta M, the author with the most MMC publications in WoS. It may be noted that this author does not figure in the list of researchers with the top ten TLS values. This occurrence is readily understood on mutual comparison of the two author's (Gupta M and Zhang D) collaboration networks. Figure 12 shows that both the collaboration authors' nodal weights (TLS of co-authors) as well as the collaboration link strengths (number of articles published in every collaboration) of Gupta M are lesser in comparison to those of Zhang D. Ji, G 11 8 Xu, R 10 9 Zhang, J 8 10 Guo, C 7 Wang, J 7 7 Che, Z 6 8 Wu, K 5 9 Zhang, X 5 10 Hu, X 5   As is evident from Table 2, all MMC researchers with the top ten TLS hail from China. Together, these researchers were able to publish a comparatively larger number of WoS articles. It may be observed that some researchers with lesser number of publications/links have higher TLS values over some of their colleagues with more WoS articles. This is due to the higher number of co-authors collaborated by these researchers in a comparatively lesser number of articles. For instance, Fan G has 46 articles and 48 links in comparison to the 80 papers and 133 links of Wang X. Still, Fan G has higher TLS of 288, whereas the TLS of Wang X is 286. This implies that Fan G included many more co-authors per paper as compared to Wang X.

Collaboration among Organizations
This subsection discusses the analysis results pertaining to the collaboration networks among various organizations involved in the metal matrix composites research. For this analysis, the VoS viewer thresholds were set at a minimum of five documents and zero citations per organization. About 801 institutes met these criteria out of a total of 4462 organizations. At the outset, the analysis revealed a total 765 articles penned through inter-institute collaboration networks. This number is far lesser than the total number of articles (1429) published through researcher networks, highlighting the scope to improve cross organizational research networking. Table 5 depicts the top thirteen institutes with the highest TLS values. Five of these institutions hail from China, three from India and one each from Singapore, Afghanistan, USA, Germany and Japan. These numbers indicate higher inter-institute collaboration environment present in Chinese organizations over others. It is interesting to note how Anna University, India scores slightly better TLS of 95 in comparison to the National Institutes of Technology, India (TLS 91, links 45, papers 155) despite having less WoS publications (112). This observation highlights the higher number of co-authors from different institutes collaborating per paper published by the Anna University researchers over those of the National Institutes of Technology. Tables 6 and 7 list the top collaborators of these two organizations for a deeper analysis of their respective link strengths. The Anna University researchers have collaborated with those from Sri Sivasubramaniya Nadar College of Engineering, University College of Engineering Kancheepuram and the University of Witwatersrand (South Africa), among others. On the other hand, the MMC investigators affiliated with the National Institutes of Technology have scored joint publications with their collaborators at the Indian Institutes of Technology, Jadvapur University (India), Indian Institute of Science and the National University Singapore among others. A similar observation may be made (Table 5) Figure 13 shows a pictorial representation mapping the inter-institute research networks in the field of metal matrix composites. This figure shows collaborative networks of the top TLS institutes (

Collaboration among Places
For a macro perspective of MMC research collaborations amongst various nations, the VoS viewer thresholds were set at five documents and zero citations per country. These conditions were met by 67 countries out of a total of 93. Table 8 (Table 1) nor in the list of the top ten TLS researchers (Table 2). This unique observation provides insights into a large number of Indian researchers publishing less MMC articles per author with relatively lesser collaborations. Another remarkable observation is with respect to the comparison of MMC research outputs of USA and India. The researchers affiliated with the USA scored higher international co-author linkages (47) and greater publications via these linkages (TLS 537) despite publishing much fewer WoS articles (1104) as compared to India (links 45, TLS 350, papers 2509). This observation also highlights the greater international collaboration oriented research environment prevalent in the USA as compared to India. Tables 9 and 10     People Republic of China 13 10 Oman 9 Figure 14 shows a colorful representation of the various international MMC research collaboration networks. This figure depicts international networks of the top TLS nations like the People's Republic of China, USA, India, England, Canada and Spain using distinct colors. Figure 15 shows the international collaboration network of the People's Republic of China, the country with the maximum TLS. This figure reveals strong collaborations of Chinese researchers with their colleagues in Japan, Australia and South Korea. There is also a strong linkage among the networks of Chinese and USA investigators. Furthermore, this figure indicates relatively minor inter linkages of Chinese authors with international collaborators from India, Egypt, England, Canada, Iran, Spain, Denmark and North Ireland. Similarly, Figure 16 depicts international linkages from the perspective of MMC researchers located in India. The Indian authors' led network includes participants from Singapore, Egypt, Saudi Arabia, Qatar, Oman, United Arab Emirates, Norway and others. Indian researchers have also collaborated with the investigators of other international networks such as Germany, Austria, Poland, Sweden, Brazil, Spain, Chile, Mexico, South Africa and Japan. Lastly, Figure 17 showcases the diverse network span of USA led MMC research. The USA researchers have international collaborations with MMC investigators from Germany, Austria, Hungary, Turkey, Slovenia, Taiwan, Ethiopia and South Africa. Their cross network linkages include India, England, Spain, Canada, Poland and Japan. With regard to the relatively smaller networks, researchers of England published MMC articles in collaboration with their contemporaries located in Wales, Portugal, Romania, Brazil, Malaysia, Iraq and others ( Figure 14). Lastly, the researchers of Canada and Spain worked together on MMCs with partners from Chile and others ( Figure 14).

Citation Landscapes
Citations are an important indicator of the actual research impact of a published article. This section presents an analysis of the MMC articles' citations and citations based linkages during 2001-2020. The MMC article citation analysis has been categorized as per the cited papers, associated/linked researchers, organizations, countries, publishing sources and keywords to understand the relative impacts of MMC research outputs under these classifications.

Citation Landscape of Published Articles
The first subsection of citation landscapes is concerned with the MMC publications having topmost citation based linkages with other WoS documents in the same field. The VoS viewer threshold was set at a minimum of five citations per document. Out of a total of 10,018, 5796 documents fulfilled this criterion. Of these, 5381 number of documents were found to be linked together by mutual citations. Table 11 [111,112] deal with the microstructural analysis of processed metal matrix composites that includes the study of particle distribution and particle-matrix interface characteristics. The remaining two papers (Miracle, 2005 and Mortensen, 2010) [113,114] address general review of metal matrix composites and associated science, technology and application aspects. Figure 18 shows the mutual citation network graphs of the MMC articles published in WoS during 2001-2020.

Citation Landscape of Researchers
To determine the citations based links among various MMC researchers, the VoS viewer threshold was set at a minimum of five documents and zero citations per author. Of the total of 16,735 authors, only 1616 met this criterion. Together, these 1616 authors cited each other in 1612 WoS articles. Table 12 Figure 19 shows the network map of mutually cited authors in the field of metal matrix composites.  Tan, Z 184 7 Ji, G 97 8 Zhang, J 93 9 Zhao, L 87 10 Li, J 70 Perez, P 31 10 Adeva, P 31 Figure 19. The citation network graph of MMC authors.

Citation Landscape of Organizations
This subsection analyses the citation based links among the various organizations of the MMC researchers. For this analysis, the VoS viewer thresholds were set at a minimum of five documents and zero citations per organization. About 801 organizations qualified this criterion from a total of 446. These organizations cited each other's WoS articles 800 times. Table 15 lists the top thirteen organizations having maximum mutually cited TLS values in their respective networks. It may be observed that the Nanjing University of Aeronautics and Astronautics (NUAA) is missing from this list. This university was listed at number 12 in the top co-authored TLS list (Table 5). On the other hand, the University of Wisconsin-Madison, featuring at number thirteen in Table 15, did not appear at all in the top co-authored TLS list (Table 5). This observation delineates the difference between the co-authored and mutually cited linkages. For instance, the Chinese Academy Science is the top listed co-authored TLS institution (Table 5), whereas it only has the fourth highest TLS of mutual citations (Table 15). Conversely, the Shanghai Jiao Tong University is only the fourth highest co-authored TLS scoring institution in Table 5, whereas it has the highest mutual citations among its links making it the topmost TLS university (Table 15). Similarly, in Section 3.2, it was pointed out that the Anna University attained higher coauthored TLS over the National Institutes of Technology (India) despite having relatively less publications in WoS. This scenario is reversed in the case of citations based TLS rankings, wherein the National Institutes of Technology has much higher TLS value (2187) as compared to that of the Anna University (1602). These observations indicate higher mutual citation linkages for lower co-authored linkages or vice versa in the case of the above-mentioned institutions. Other organizations like the Indian Institutes of Technology, National University of Singapore and Northwestern Polytechnic University maintained their respective positions viz. third, fifth and ninth spots in both of the top co-authored as well as citations based TLS listings. These organizations exhibited balanced levels of collaborative as well as citation linkages amongst their network institutions. Taking a macro view of the co-authored and citation TLS lists, it is evident that mostly the same organizations feature in both of these top rankings. Hence, it may be inferred that interinstitute research collaborations lead to inter-institute citation links as well. In addition, the citation TLS values tend to be much higher than the co-authored TLS numbers. For example, the highest mutually cited TLS value is 4463 (of Shanghai Jiao Tong University), whereas the greatest co-authored TLS values is just 212 (of Chinese Academy Science). Tables 16 and 17 Figure 20 shows the citation based inter-institutional networks in different colors. University of Tehran Iran 26 10 Annamalai University India 23

Citation Landscape of Places
To determine the citation based linkages for MMC research among various countries, the VoS viewer threshold was set at a minimum of five WoS documents and zero citations per nation. Only 63 countries met this criterion out of a total of 97. Table 18 shows the top thirteen countries having the best citations based TLS values. Compared with the collaboration TLS rankings given in Table 8, it may be observed that the People's Republic of China has topped both the lists. The authors of this country have maximum co-authored links (681) in MMC papers as well as the highest number of mutual citations (20,878) in these WoS articles. India, listed as the nation with the third highest co-authored linkages (350), bagged the second spot in international citation linkage rankings (17,501). Conversely, the USA attained the third highest citation linkages (13,112), although it has the second highest co-authored links (537) in its MMC articles. Similarly, nations like Germany, England, France, Japan, Canada and Spain also attained relatively lower citation based TLS ranks as compared to their respective co-authored works based TLS rankings. However, some countries improved their citation rankings over collaboration ranks. These countries included India, Iran and South Korea. Australia maintained the same rank in both listings. The biggest rankings difference is observed in the case of Iran. Iran has the fourth highest mutually cited TLS (5938), whereas it is ranked only ninth in the co-authored MMC research linkages based TLS (145). This analysis shows that Iranian researchers have relatively higher mutual citations with their international colleagues as compared to other top TLS nations; and lesser internationally co-authored publications against the rest of the top linked countries. Table 19 lists the top ten countries having MMC articles' citation based linkages with the Iranian authors. The top three linked nations are India, People's Republic of China and the USA having their mutual MMC citation link strengths with Iran at 1407, 988 and 568, respectively. Figure 21 depicts the citation network graph among the various countries engaged in MMC research, whereas Figure 22 shows the international citation linkages of Iranian authors.   Republic of China  2475  43,374  66  20,878  2  India  2509  34,715  66  17,501  3  USA  1104  33,038  66  13,112  4  Iran  463  10,830  63  5938  5  Germany  525  11,654  64  5042  6  Turkey  392  7784  62  4678  7  England  325  10,849  64  3799  8  South Korea  372  6264  62  3598  9  Japan  312  6452  64  3489  10  France  282  7011  61  3397  11  Australia  172  5240  64  3077  12  Canada  259  6846  59  2922  13 Spain 245 5763 61 2797

Citation Landscape of Sources
The citation landscape of publication sources was determined using the VoS viewer threshold of a minimum of five WoS documents and zero citations per source. Out of a total of 1471 sources, only 270 made it to the threshold limit. Table 20 Figure 23 shows the citation based linkages between the various sources publishing MMC papers in the WoS. Figure 24 shows citation linkages of the top ranked TLS source-Materials Science and Engineering: A-Structural Materials: Properties, Microstructure and Processing. Composites Science and Technology 44 9 Advances in Manufacturing 41 10 International Journal of Mechanical Sciences 38 Composites Part a-applied Science and Manufacturing 80 5 Acta Materialia 67 6 Composites Part b-engineering 52 7 Composites Science and Technology 46 8 Advanced Engineering Materials 37 9 Carbon 34 10 Journal of Materials Science 33

Co-Occurrence Landscape of Keywords
This subsection describes the co-occurrence based analysis of the technical keywords most quoted by the MMC researchers in their WoS articles. For this analysis, the WoS viewer threshold was set at a minimum of five occurrences per keyword. From a total of 11,569 keywords, only 1068 could qualify for this condition. Table 23 lists thirteen keywords with top TLS values based on co-occurrence links with other keywords appearing in MMC publications. It is evident that the keyword 'Metal Matrix Composites' is the top TLS keyword having 2135 occurrences in WoS documents. It has co-occurred with 845 other keywords, with a total strength of such linkages at 5566. It is interesting to note that the fifth, eighth and the tenth ranked keywords, viz. 'Metal-Matrix Composites (mmcs)', 'Metal Matrix Composite' and 'Metal-Matrix Composites' are instances of the top TLS keyword itself. Similarly, the sixth and the thirteenth ranked keywords-'Composites' and 'Composite' are also generalized versions the keyword with the highest TLS. Thus, six out of the thirteen top co-occurred keywords refer to the same term. The second ranked keyword 'Mechanical Properties' indicates the focus of most WoS articles on investigating the physical properties of the MMCs for varied applications. The fourth and ninth ranked keywords-'Wear' and 'Hardness' reveal the most important mechanical properties considered by researchers during MMC performance tests. Similarly, the keywords ranked seventh and eleventh-'Powder Metallurgy' and 'Stir Casting' indicate the most important techniques explored by various researchers for preparing MMCs of various compositions. After mechanical properties, the next most widely studied aspect of MMCs is 'Microstructure', listed as the third highest TLS keyword in WoS articles. Lastly, the twelfth ranked keyword-'Aluminium' indicates the favorite base metal alloy material used most widely by the researchers for processing MMCs with different micro/nano particulate inclusions. Figure 25 shows the network map of these and other keywords co-appearing in WoS articles during 2001-2020. These figures show inter-linkages among keywords belonging to various MMC materials, methods of preparation, characterization, testing, analyses, processing/machining and applications. Figures 26-28 showcase the co-occurrence based linkages of the top three MMC keywords for deeper insights into their individual networks. For instance, the top TLS keyword 'Metal Matrix Composites' is connected to keywords related to MMC materials like nano structured materials and processing techniques such as the spark plasma sintering, selective laser melting, mechanical alloying, laser cladding, powder processing and friction stir processing among others. The term 'Metal Matrix Composite' is also connected to keywords pertaining to the properties such as tensile strength, plastic deformation, fracture, fatigue, residual stress, wear resistance and damage mechanics. The analyses based links of 'Metal Matrix Composites' include electron microscopy, thermal analysis, liquid-solid reactions, neutron diffraction, finite element analysis and modeling. Similarly, the keyword 'Mechanical Properties' is linked to important terms such as damping capacity, electrical properties, strengthening mechanism and tribological properties. Lastly, the keyword 'Microstructure' is connected to related terms such as transmission electron microscopy and SEM (scanning electron microscopy) among others.

Burst Detection Analysis
Burst detection analysis reveals the specific time periods wherein particular keywords and/or sources witnessed sudden increase in research interest (indicated by the number of citations) [63]. In the current study, burst detection analyses of keywords and references related to MMCs were carried out based on the research published during 2001-2020.

Keyword Burst Detection
Burst detection for the MMC keywords used by WoS articles during 2001-2020 was analysed using the CiteSpace software tool. Table 24 shows the top MMC research related keywords ranked as per their relative burst strengths, i.e., the degrees of sudden increase in their respective citation frequencies. As per this list, the keyword 'graphene' witnessed the highest spike in citations as compared to all other keywords related to MMC research. CiteSpace determined the strength of 'graphene' keyword burst at 33.6453. Moreover, this burst assumes additional significance because of its latest burst duration, i.e., 2018-2020. This particular citation spike indicates a sudden increase in nano-scale particle reinforced metal matrix composites research in the last two years. During the first decade of this century, keywords like 'fracture', 'damage', 'deformation', 'creep', 'flow', 'residual stresses' and 'particulate composites' witnessed citation bursts with respective strengths of 24 16.9666 (2016-2018), and 13.6994 (2018-2020), respectively. These citation bursts indicate a renewed emphasis on mechanisms and methods of preparing different metal matrix composites. In addition, the recent keyword bursts also indicate a lot of researchers exploring various kinds of nanocomposites reinforced with particulate materials like graphene and boron carbide for various applications.

Reference Burst Detection
References burst detection analysis was also carried out using the CiteSpace software to gain insights into the top MMC articles that received the highest citation bursts during 2001-2020. Table 25 lists fifteen publications having the highest burst strengths based on citations in WoS. Three of these articles (ranked 1, 2 and 15) are review papers dealing with the different particle reinforced metal matrix composites [115,116], associated preparation techniques and characteristics [117]. The remaining papers (ranked 3 to 14) reported investigations into specific mechanical properties of MMCs such as deformation [118], tensile strength [119,120], creep [121], fracture [122,123], dislocation generation [41,124], ductility [125], elastic energy [126] and others [127]. It may be observed that all of these articles were published before 2001. Seven of them were published during 1991-2000 and six articles were published during [1981][1982][1983][1984][1985][1986][1987][1988][1989][1990]. Two papers were published as far back as 1973 and 1957! This analysis shows that researchers have been working on metal matrix composites for the last five decades. All of the fifteen strongest citation bursts have been attained by the articles published before 2001, with zero instances of the MMC articles published during 2001-2020. In fact, all of these citation bursts have been recorded during the initial period of 2001-2005. Referring to Figure 2, it may be observed that relatively fewer MMC papers were published in WoS in that period. Hence, the documents with top burst strengths were cited at high frequencies by the MMC papers published during 2001-2005. In addition, none of the top cited articles listed in Table 11 have made it to the top citation burst list. This observation implies that the top cited MMC articles did not receive their respective citations in the form of short-term high frequency bursts.

Conclusions
This paper presents a comprehensive bibliometric analysis of metal matrix composites research articles published in the web of science database during the last two decades. The MMC related keywords generated a corpus of 10,018 articles from indices such as SCIE, ESCI, CPCI-S, CPCI-SSH and BCI-S. The bibilometric analysis was carried out under four major categories-publications distribution along different verticals, collaboration analysis, citation landscapes and burst detection. The first category of publication distribution was comprised of verticals such as year on year number of articles, publication types, sources, research directions, productive places, organizations and researchers. The second category included analysis of collaboration among various MMC researchers, affiliating organizations and countries. The third category dealt with mutual citation based linkages among MMC articles, researchers, organizations, places and sources. This section also included a co-occurrence based analysis of MMC related keywords quoted by researchers in their WoS articles. The fourth category provided insights into the top keywords and documents that witnessed strongest citation bursts during the last two decades. Following are the primary findings of the entire bibliometric analysis of metal matrix composites research presented in the current study.

1.
The MMC related keywords' search yielded a total of 10,018 articles from the WoS database.