Titanium Alloys and Titanium-Based Matrix Composites

Titanium alloys have been considered unique materials for many years [...]


Introduction and Scope
Titanium alloys have been considered unique materials for many years. Their development has led to the design of several groups of structural alloys, including single-phase α or β alloys, two-phase α + β alloys-the most popular ones-and TiAl intermetallic alloys. The main application areas of titanium alloys include transportation (mainly aerospace structures), machine building, the fuel-energy industry and medicine. Titanium alloys are also good materials for metal matrix composites (MMCs). Their main attractions are high strength and stiffness-which depend on the type of reinforcement. An important feature of titanium is its remarkable biocompatibility. The low Young's modulus of titanium alloys-especially β alloys-makes them valuable biomaterials used for bone implants.
The constant development of titanium-based materials is also related to new technologies introduced-e.g., 3D printing or friction welding. It has restored the need for further research on titanium components produced using modern manufacturing and processing methods. This Special Issue covers various aspects of research on titanium alloys and Ti-based metal matrix composites-related to the development of their microstructure and operational properties by chemical composition modification, heat treatment and plastic deformation, but also by modern manufacturing and processing methods such as powder metallurgy, additive manufacturing or friction stir welding.

Contributions
Twenty articles-18 research ones, 1 review and 1 technical note-have been published in the present Special Issue of Metals. Their subject matter is quite extensive, encompassing the fields of deformation behavior, microstructure and properties development, as well as special applications (biomedical and functional ones) and technologies. For the ease of the reader, papers have been divided into three groups, although among them thematic links can be established.

Deformation Behavior
Four papers in this section concern the hot deformation behavior of metallic materials, including the evaluation of microstructure evolution and processing maps elaboration. Liu et al. [1] investigated the hot deformation process of metastable β Ti-6Cr-5Mo-5V-4Al alloy (Ti-6554), which is considered as a new type of high-strength and high-toughness titanium alloy for manufacturing large-scale aircraft structural parts. It is of great significance to improve the properties of the alloy. The results of hot compression tests at a temperature range of 715-840 • C and strain rate range of 0.001-1 s −1 were the basis of a processing maps calculation, enabling the determination of the instability regions in the process and improvement of forging effectiveness. Zherebtsov et al. [2] analyzed the deformation behavior of Ti-15Mo/TiB composite produced by spark plasma sintering. They found its gradual softening with an increase in deformation temperature in the range of 500-1000 • C to be associated with the dynamic recovery and recrystallization processes, in addition to the reorientation and shortening of TiB whiskers. The effect of pre-deformation on the hot plasticity of Ti-6Al-4V alloy was researched by Wu et al. [3]. It was found that pre-strain of 0.05 enhanced flow stress and elongation in tensile tests at a temperature of 850 • C. Moreover, pre-strain promoted dynamic recrystallization by increasing the deformation substructure. Zhang et al. [4] analyzed the asymmetry of tension-compression behavior of an extruded Ti-6.5Al-2Zr-1Mo-1V alloy (TA15) at a temperature range of 650-750 • C. The corresponding yield stress and asymmetric strain hardening behavior were studied.

Development of Microstructure and Operational Properties
The largest section contains ten papers related to the development of the microstructure and operational properties of titanium-based materials-alloys and composites-by heat treatment, plastic deformation methods and modification of chemical composition.
The first six papers concern titanium alloys manufactured by conventional metallurgical methods, though with a reference to 3D printing techniques. Guo et al. [5] investigated the precipitation behavior of the ω phase and ω→α transformation in Ti-5Al-5Mo-5V-1Cr-1Fe near-β alloy (Ti-55511) during isothermal ageing at 450 • C. It was observed that the size of α precipitates increase with increasing ageing time, resulting from the β→α and ω→α transformations. Phase transformations upon ageing in the other β alloy were the subject of research conducted by Bartha et al. [6]. Ti-15Mo alloy was subjected to two techniques of intensive plastic deformation-high-pressure torsion and rotary swaging at room temperature-leading to the formation of an ultrafine-grained microstructure. It was found that, after isothermal ageing in both deformed conditions, precipitations of the α phase remain small and equiaxed even after ageing at 500 • C for 16 h. The other β alloy-Ti-Nb-Zr-Fe-O-was investigated by Cojocaru et al. [7]. The influence of complex thermo-mechanical processing on its mechanical properties was analyzed. The advantages and disadvantages of such treatment were analyzed for expanding the database of possible β-Ti bio-alloys that could be used, depending on the specific requirements of different biomedical implant applications. Ding et al. [8] evaluated the solidification microstructure of Ti-6Al-4V-xFe (x = 0.1, 0.3, 0.5, 0.7 and 0.9) alloys fabricated by levitation melting. The growth of grains in the function of Fe content, as well as the composition distribution mechanisms during the solidification process of the alloy, were discussed. Qiao et al. [9] researched the microstructure, corrosion behavior and tensile properties of a newly developed low-cost titanium alloy, Ti-4Al-2V-1Mo-1Fe, and compared it to the widespread Ti-6Al-4V alloy. It was revealed that the corrosion resistance and ductility of the Ti-4Al-2V-1Mo-1Fe alloy were higher than that of the Ti-6Al-4V alloy, with a slight reduction in strength. Motyka [10], in his review paper, pointed to another aspect. He noticed that the introduction of new manufacturing methods-e.g., 3D printing-requires further research on titanium alloys already considered as well-known and that have been described in detail in the case of conventional technologies. In his article, he focused on martensite formation and decomposition processes in two-phase titanium alloys and emphasized their important role in microstructure development during conventional and additive manufacturing processes.
The next four papers are devoted to titanium-based alloys and composites produced by powder metallurgy. The influence of the hot isostatic pressing (HIP) post-processing step on the microstructure, porosity and mechanical properties of Ti-35Nb-2Sn alloy was studied by Lario et al. [11]. They confirmed that field-assisted consolidation processes, such as HIP, can be employed to reduce residual porosity and to increase the chemical and phase homogeneity of sintered β titanium alloys intended for biomedical applications. Markovsky et al. [12] investigated the influence of a strain rate on the mechanical response and microstructure evolution of the selected titanium-based materials, i.e., Ti-6Al-4V alloy produced via a conventional cast and wrought technology, as well as fabricated using blended elemental powder metallurgy. It was found out, among others, that Ti-6Al-4V with a globular microstructure is characterized by high strength and high plasticity in comparison to this alloy with a lamellar microstructure, whereas Ti-6Al-4V obtained via the powder metallurgy method reveals the highest plastic flow stress, with good plasticity at the same time. Hou et al. [13] successfully manufactured TiB-whiskers-reinforced Ti-15Mo-3Al-2.7Nb-0.2Si alloys by adding TiB 2 powder and pre-sintering followed by canned hot extrusion. It was observed that the extrusion led to grain refinement and the strengthening of the composite, especially at the process temperature of 1000 • C. Powder-metallurgy-titaniumbased composites were also researched by Montealegre-Meléndez et al. [14]. Their study was aimed at the analysis of the reaction layer between the titanium matrix and reinforcement: B 4 C particles and/or intermetallic Ti x Al y . The authors revealed that composites with ceramic reinforcement showed excellent hardness and good wear resistance.

Special Applications and Technologies
The last section comprises six papers and concerns special applications of titaniumbased materials-such as the biomedical one-as well as modern technologies-such as additive manufacturing, plastic consolidation or friction stir processing. Regarding biomedicine, titanium alloys belong to the few biomaterials that naturally match the requirement of bone implants or bone tissue replacement in the human body. The mechanical and bio-functional behavior of a Ti-30Nb-13Ta alloy in the form of foam, fabricated by mechanical alloying and subsequent spark plasma sintering, was studied by Giner et al. [15]. The material was evaluated as a potential prosthetic biomaterial used for cortical bone replacement and compared with commercial, pure Ti used for bone replacement implants. Moiduddin et al. [16] proposed an integrated system methodology for the reconstruction of complex zygomatic bone defects, using Ti-6Al-4V ELI alloy, comprising several steps, right from the patient scan to implant fabrication, while maintaining proper aesthetic and facial symmetry. Finally, it was concluded that a mirror-designed titanium implant (fabricated by the electron beam melting method) satisfies the aesthetic, functional, and mechanical properties for efficient zygomatic bone reconstruction. Moreover, the authors stated that the proposed design methodology could also be applied for other bone reconstruction surgeries.
The next three papers describe the effects of unconventional manufacturing and processing methods. Topolski et al. [17] consolidated titanium chips into solid, bulk material by extrusion of the briquettes into the form of solid rods using the KOBO method. Structural aspects of the solid-state processing of various titanium chips were analyzed. It was found that the manufactured rods were consolidated and near fully dense. Kim et al. [18] applied the step rolling process for the production of ultrafine-grained Ti-6Al-4V sheets. This study clarified the effect of subsequent annealing on the tensile properties of step-rolled Ti-6Al-4V at room and elevated temperature. Mironov et al. [19] used friction stir processing for the development of a globular α microstructure in Ti-6Al-4V alloy. Characterization of the crystallographic aspects of such a microstructure was accomplished by the electron backscatter diffraction technique.
The last paper in this section covers issues related to micro-actuators based on shape memory alloys. The most popular among them is nitinol (NiTi), which in fact is a nickelbased alloy, but is quite often described together with functional titanium alloys. Shimoga et al., in their technical report [20], reflected on the characteristics of the NiTi coil spring structure with its phase transformations and thermal transformation properties. It was postulated that the micro-actuators based on NiTi could be used for advanced high-tech applications.

Conclusions and Outlook
A variety of interrelated topics have been raised in the present Special Issue of Metals, providing a wide overview of recent research developments on different aspects of titaniumbased materials. The number of articles and the wide range of topics prove the continued interest in this group of materials. It is worth mentioning that the authors represent scientific institutions from 13 countries: Austria, China, the Czech Republic, Japan, Korea, Pakistan, Poland, Romania, Russia, Saudi Arabia, Spain, Ukraine and the USA.
As a Guest Editor of this Special Issue, I am very happy with the final result, and hope that the published papers will be useful to researchers working on titanium alloys and titanium-based composites. I would like to warmly thank all the authors for their contributions, and all of the reviewers for their efforts in ensuring a high-quality publication. Thanks are also due to the Metals Editorial Office, especially to Toliver Guo, Assistant Editor, who managed and facilitated the publication process.