Microstructure Examination and Sliding Wear Behavior of Al-15%Mg2Si-xGd In Situ Composites before and after Hot Extrusion

In current study; the effect of various Gadolinium (Gd) additions on the microstructure and sliding wear behaviour of Al-15%Mg2Si composite before and after the hot extrusion process was examined. Optical microscopy (OM), scanning electron microscopy (SEM) equipped with EDX facility and X-ray diffraction (XRD) were used to characterize the microstructure. The results showed that with addition of 1.0 wt.% Gd to Al-15%Mg2Si composite, the primary Mg2Si particles size reduced from 44 µm to 23 µm and its morphology altered from dendritic to polygonal shape. Further refinement of primary Mg2Si particles was achieved after conducting hot extrusion which resulted in a decrease in its size to 19 µm with a transfer to near-spherical morphology. The Vickers hardness value increased from 55.6 HV in the as-cast and unmodified composite to 72.9 HV in the extruded 1.0% Gd modified composite. The wear test results revealed that composites treated with Gd possess higher wear resistance in comparison with those of without Gd. The highest wear resistance obtained with the lowest wear rates of 0.19 mm3/km and 0.14 mm3/km in the Al-15%Mg2Si-1.0% Gd before and after the hot extrusion, respectively. The high wear resistance of extruded Gd-modified Al-15%Mg2Si composite is due to the refinement of primary Mg2Si particles with uniform distribution in the composite matrix along with fragmentation of Gd intermetallic compounds.


Introduction
Al-Mg-Si alloys are extensively utilized in the aerospace and automotive applications owing to their high strength to weight ratio, good formability and favorable cast ability [1]. It is claimed that the excellent features of the Al-Mg-Si alloy comes from Mg 2 Si particles due to its attractive features which made the Al-Mg 2 Si system a replacement for Al-Si alloys [2]. Nevertheless, coarse dendritic primary Mg 2 Si particles with sharp corners are formed under low solidification rates of Al-Mg 2 Si composites, which could affect the properties of the fabricated composites and constrict their usage in industrial applications [3]. Therefore, modification of primary Mg 2 Si particles in Al-Mg 2 Si composites is essential to induce adequate mechanical and tribological properties. Palta et al., [4] reported that with the addition of Cu to Al-12Si-20Mg alloy, the size and volume of the Mg 2 Si particle, formed within the matrix, decreased which can be due to the increasing the number of nuclei or by poisoning the surface of the in-situ Mg 2 Si nuclei owing to the segregation of Cu at the liquid-solid interface. Furthermore, the addition of Cu results in formation of CuAl 2 and CuMg 5 -Si 4 Al 4 intermetallics within the matrix which caused improvement of the wear 2 of 13 resistance. In another study, Wu el al., [5] found that by increasing the Bi content to 4 wt.%, the morphology of primary Mg 2 Si changes from irregular or dendritic to polyhedral shape with decreasing in its average size significantly which results in enhancement of wear and friction properties of the composite. The reason for the refinement of Mg 2 Si crystals by the addition of Bi may be related to the composition supercooling caused by the segregation of Bi at the front of the solid/liquid interface of Mg 2 Si during solidification. Farahany et al., [6] claimed that addition of 0.01 wt.% Sr to Al-20%Mg 2 Si-2%Cu composite results in the refinement of primary Mg 2 Si particles as well as improvement of mechanical and wear behaviour of the Sr-treated composite compared to the untreated one. It is suggested that the refinement of primary Mg 2 Si particles as a result of Sr addition can be attributed to the formation of heterogeneous nucleation substrates for primary Mg 2 Si particles. Other reasons for enhancement of wear resistance of the composite are good distribution of the fine Mg 2 Si particles in the matrix as well as presence of Al 2 Si 2 Sr intermetallic compound. It has been stated that addition of rare earth elements to Al-Mg 2 Si bring about significant influence of refinement of primary Mg 2 Si particles and as a consequence the mechanical and tribological properties are enhanced. Wu et al., [7] found that with the addition of 0.5 wt.% Nd addition to Al-18%Mg 2 Si composite, the dendritic morphology of primary Mg 2 Si changed to polyhedral shape with a decrease in its average size as well as enhancement in wear properties. In fact, Nd segregates at the front of the solid/liquid interface of Mg 2 Si during solidification and caused composition supercooling as a responsible mechanism for refinement of Mg 2 Si particles. In another study Jafari Nodooshan et al., [8] claimed that addition of 0.5 wt.% Y element as well as conducting a heat treatment process resulted in improvement in hardness and wear resistance of the Al-15%Mg 2 Si composite. The formation of Al 2 Y intermetallics in the composite matrix hindered the growth of primary Mg 2 Si particles through restriction of their growth. In addition, Soltani et al., [9] reported that increasing the extrusion ratio leads to considerable enhancement in hardness and wear resistance of Al-15%Mg 2 Si composite. The improvement was related to the role of hot extrusion on refinement and better distribution of primary Mg 2 Si particles as compared to the as-cast composite. Although there are some studies in the literature about the influence of rare earth additions and hot extrusion on microstructure and wear behaviour of Al-Mg 2 Si composites, there is limitation on combined effect of rare earth element and hot extrusion on microstructure alteration and tribological behaviour of the Al-Mg 2 Si composites. Hence, in the present study the effect of various contents of Gd additions and the hot extrusion process on microstructural and sliding wear behaviour of Al-15%Mg 2 Si composite is investigated.

Materials Fabrication
Industrially pure Al (99.8%), Mg (99.9%), and Si (99.5%) were utilized to fabricate an ingot of Al-15%Mg 2 Si composite. The chemical composition of the fabricated ingot is shown in Table 1. The procedure of Al-Mg 2 Si composite preparation is reported elsewhere [10]. An induction furnace was used to melt the composite ingot which was placed inside a graphite crucible with 1 kg capacity. When the temperature of the melt reached 750 • C, 0.3 wt. % Gd was introduced into the melt and then the melt was degassed with tablets containing C 2 Cl 6 for 3 min. Once the dross was skimmed the composite melt was poured into a preheated cast iron mold (110 • C). A similar approach was repeated for addition of 0.5 wt. %, 1.0 wt. % and 2.0 wt.% Gd.

Hot Extrusion
The composite samples with different content of Gd were machined to achieve the dimension of 30 and 29 in length and diameter respectively. The homogenisation treatment was then applied on the samples using an electrical furnace at 520 • C for 4 h followed by furnace cooling prior to hot extrusion. The hot extrusion process was carried out at constant parameters of ram speed (1 mm/s) and extrusion ratio (12:1) at 450 • C using graphite-based oil as lubrication.

Microstructural Examination
For microstructural observation, the grinding and polishing procedures were conducted on the composite samples using SiC papers and colloidal silica suspension, respectively. To detect the microstructure of Al-15%Mg 2 Si composites and perform quantitative analysis on primary Mg 2 Si particles size, an optical microscope (Olympus BX60) equipped with an i-Solution image analyzer was utilized, in which the quantitative metallographic characteristic of the microstructure such as mean size was calculated using Equation (1) as follows: where Li is the size, n is the number of Mg 2 Si particles measured in a single field (0.60126 mm 2 ), and m is the number of the evaluated fields. In addition, the composite samples were inspected using scanning electron microscopy (SEM) (Philips XL40) coupled with energy-dispersive spectroscopy (EDS). To identify the phase formation, X-ray diffraction (XRD) (Siemens-D500) with Cu Ka line produced at 40 kV and 35mA was utilized.

Hardness and Dry Sliding Wear Test
The hardness of the Al-15%Mg 2 Si composite samples comprising different content of Gd before and after hot extrusion was measured by Vickers's hardness machine (Matsuzawa DVK-2). Before conducting the experiment, a standard sample was utilized for machine calibration. A square diamond pyramid as an indenter was used for the Vickers hardness test. In each hardness test, and the applied parameters were a 5 kg loading force and 10 s test time. For conducting the dry sliding wear test of the fabricated composite, a pin on disk wear apparatus was used (Ducom, TR20-LE). The as-cast and extruded samples were machined to produce the wear pins with dimensions of 6 mm in diameter and 25 mm in length. The pins with two different applied loads of 20 N and 40 N were loaded against a steel disk (EN31, hardness = 60 HRC). The dry sliding wear test was conducted at room temperature under a fixed rotating speed of 200 rpm, sliding distance of 2000 m, wear track diameter of 60 mm and sliding time of 1 h. The counter disk open surfaces as well as pin surface were cleaned with acetone before every test. The cumulative wear in µm and frictional force in Newtons were calculated and measured during the sliding wear tests. Figure 1 depicts the optical images of Al-15%Mg 2 Si composites with various Gd concentrations before and after hot extrusion, in which the primary Mg 2 Si particles size is depicted in Figure 2. As seen in Figure 1(1a), in the unmodified composite, the primary Mg 2 Si particles possess dendritic and irregular morphology which are surrounded with eutectic Mg 2 Si cells. In addition, as can be seen there is a hole at the centre of some of the primary Mg 2 Si particles which is filled with α-Al as a result of nucleation of α-Al on Mg 2 Si particles [2]. When the composite was treated with 0.3 wt.% Gd, the shape of primary Mg 2 Si altered to polygon, and reduced in size as illustrated in Figure 1(1b) and Figure 2 compared to unmodified composite. With the addition of 0.5 wt.% Gd the particles changed into a regular polygonal morphology with a further reduction in size, as revealed in Figure 1(1c) and Figure 2. Addition of 1.0 wt.% of Gd displays the most refined primary Mg 2 Si particles, as depicted in Figure 1(1d) and Figure 2. Nonetheless, with an increase of Gd content to 2.0 wt.% the particles started to coarsen as demonstrated in Figure 1(1e) and Figure 2 which can be due to the over-modification phenomenon [11]. A similar behaviour can be detected in the size of primary Mg 2 Si articles in unmodified and Gd modified composites after conducting a hot extrusion process as seen in Figure 1(2a-2e) and Figure 2. The decrease in the size of primary Mg 2 Si particles after hot extrusion can be attributed to the mechanical fragmentation of Mg 2 Si particles caused by hot extrusion [12].

Microstructural Characterization
Mg2Si altered to polygon, and reduced in size as illustrated in Figures 1(1b) and 2 com pared to unmodified composite. With the addition of 0.5 wt.% Gd the particles changed into a regular polygonal morphology with a further reduction in size, as revealed in Fig  ures 1(1c) and 2. Addition of 1.0 wt.% of Gd displays the most refined primary Mg2S particles, as depicted in Figures 1(1d) and 2. Nonetheless, with an increase of Gd conten to 2.0 wt.% the particles started to coarsen as demonstrated in Figures 1(1e) and 2 which can be due to the over-modification phenomenon [11]. A similar behaviour can be de tected in the size of primary Mg2Si articles in unmodified and Gd modified composite after conducting a hot extrusion process as seen in Figures 1(2a-2e) and 2. The decreas in the size of primary Mg2Si particles after hot extrusion can be attributed to the mechan ical fragmentation of Mg2Si particles caused by hot extrusion [12].           There are three different Gd phases in the Al-15%Mg2Si-2%Gd composite which possess bulky, needle-like and Chinese-script-like morphologies which are shown in Figure  5a-c. The corresponding EDS results are tabulated in Table 2. The EDS analysis on bulky and needle-like phases in Figure 5a,b reveals the various atomic % of Al, Si and Gd correspond to Al2Si2Gd and AlSiGd IMCs, respectively. Nevertheless, the EDS result on the Gd phase in Figure 5c which has a Chinese-script structure depicts the composition of MgGd. It is apparent that with addition of Gd, new phase with various morphologies form in the composites microstructure and distributed at the interdendritic regions or grain boundaries. It is expected that the morphologies of Gd IMCs influence the mechanical and tribological properties of the Al-15%Mg2Si composite; hence, assessment of the Gd IMCs morphologies in Figure 3a-d should be taken into account. As observed in Figure 3a,b, addition of 0.3 wt. % and 0.5 wt.% Gd results in the formation of bulky (Al2Si2Gd) and needlelike (AlSiGd) morphology of Gd IMCs. Further addition of Gd to 1.0 wt.% Gd causes formation of script-type structure (Al2Si2Gd) as illustrated in Figure 3c. However, when Al-15%Mg2Si composite is modified with 2.0 wt.% Gd, the size of Gd IMCs has further increased leading to the formation of twin needle-like IMCs in addition to a bulky shape and a Chinese script type (MgGd) as shown in Figure 3d. There are three different Gd phases in the Al-15%Mg 2 Si-2%Gd composite which possess bulky, needle-like and Chinese-script-like morphologies which are shown in Figure 5a-c. The corresponding EDS results are tabulated in Table 2. The EDS analysis on bulky and needle-like phases in Figure 5a,b reveals the various atomic % of Al, Si and Gd correspond to Al 2 Si 2 Gd and AlSiGd IMCs, respectively. Nevertheless, the EDS result on the Gd phase in Figure 5c which has a Chinese-script structure depicts the composition of MgGd. It is apparent that with addition of Gd, new phase with various morphologies form in the composites microstructure and distributed at the interdendritic regions or grain boundaries. It is expected that the morphologies of Gd IMCs influence the mechanical and tribological properties of the Al-15%Mg 2 Si composite; hence, assessment of the Gd IMCs morphologies in Figure 3a-d should be taken into account. As observed in Figure 3a,b, addition of 0.3 wt. % and 0.5 wt.% Gd results in the formation of bulky (Al 2 Si 2 Gd) and needle-like (AlSiGd) morphology of Gd IMCs. Further addition of Gd to 1.0 wt.% Gd causes formation of script-type structure (Al 2 Si 2 Gd) as illustrated in Figure 3c. However, when Al-15%Mg 2 Si composite is modified with 2.0 wt.% Gd, the size of Gd IMCs has further increased leading to the formation of twin needle-like IMCs in addition to a bulky shape and a Chinese script type (MgGd) as shown in Figure 3d.   AlSiGd MgGd Figure 6a,b illustrates the BSE micrograph of Al-15%Mg2Si-1.0% Gd in as-cast and after extrusion, respectively. As observed, the Gd IMCs are fragmented throughout the composite matrix after extrusion. The higher magnification of Gd IMCs and corresponding elemental mapping is depicted in Figure 6c which clearly illustrates the role of hot extrusion in significantly altering the size and morphology of Gd IMCs in comparison with the as-cast composite.  Figure 6a,b illustrates the BSE micrograph of Al-15%Mg 2 Si-1.0% Gd in as-cast and after extrusion, respectively. As observed, the Gd IMCs are fragmented throughout the composite matrix after extrusion. The higher magnification of Gd IMCs and corresponding elemental mapping is depicted in Figure 6c which clearly illustrates the role of hot extrusion in significantly altering the size and morphology of Gd IMCs in comparison with the ascast composite. Figure 7 illustrates the Vickers hardness property of the Al-15%Mg 2 Si composite modified with various content of Gd. As seen in Figure 7, the hardness value of the composites increased as a result of Gd addition as compared to unmodified composite. The improvement can be attributed to the modification of Mg 2 Si phase and better distribution of particles that could withstand stress positively. The hardness value of the as-cast materials increased from 55.6 HV in unmodified composite to 56.00 HV when 0.3 wt.% Gd was added. The hardness value further increased to 67.80 HV and 68.6 HV when the composite is added with 0.5 wt.% Gd and 1.0 wt.% Gd, respectively. The similar trend is observed for extruded composites. In addition, conducting the hot extrusion process caused increasing hardness value compared to as-cast composites, in which the hardness values for extruded unmodified composite, 0.3 wt.% Gd, 0.5 wt.% Gd and 1.0 wt.% Gd were 60.4 HV, 61.8 HV, 69.38 and 72.9 HV, respectively, showing that 1.0 wt.% Gd possesses the highest value due to further refinement of Mg 2 Si particles, better particle distribution in the composite, reduction of porosity content, as well as fragmentation of Gd IMCs after a hot extrusion process (Figures 1 and 6). In fact, the presence of hard primary Mg 2 Si particles and Gd IMC particles act as a barrier against the dislocation motion which caused higher resistance to indentation [13]. However, the hardness value was found to decrease when the Gd concentrations exceeded the 1.0 wt.%. In fact, with the addition of 2.0 wt.% Gd into the composite, hardness value reduced to 65.9 HV and 67.8 HV for as-cast and extruded composite, respectively. This phenomenon happened due to the over-modification of Gd addition whereas the formation of primary Mg 2 Si particles per unit area (density) decreased; hence, the number of barriers against dislocation motion and consequent resistance to indentation is decreased which results in hardness value lessening. A similar behaviour was observed by [14] whereas the Na addition exceeded 1.0 wt.% in Al-Mg 2 Si composite.  Figure 7 illustrates the Vickers hardness property of the Al-15%Mg2Si composite modified with various content of Gd. As seen in Figure 7, the hardness value of the composites increased as a result of Gd addition as compared to unmodified composite. The improvement can be attributed to the modification of Mg2Si phase and better distribution of particles that could withstand stress positively. The hardness value of the as-cast materials increased from 55.6 HV in unmodified composite to 56.00 HV when 0.3 wt.% Gd was added. The hardness value further increased to 67.80 HV and 68.6 HV when the composite is added with 0.5 wt.% Gd and 1.0 wt.% Gd, respectively. The similar trend is observed for extruded composites. In addition, conducting the hot extrusion process caused in- extruded composite, respectively. This phenomenon happened due to the over-modification of Gd addition whereas the formation of primary Mg2Si particles per unit area (density) decreased; hence, the number of barriers against dislocation motion and consequent resistance to indentation is decreased which results in hardness value lessening. A similar behaviour was observed by [14] whereas the Na addition exceeded 1.0 wt.% in Al-Mg2Si composite.   (Figures 1 and 2) as well as formation of Gd-IMCs ( Figure  3). In fact, refinement of Mg2Si particles and formation of Gd IMCs lead to increasing the hardness of the composites (Figure 7) which in turn caused the reduction in the composites wear rate. Indeed, with increasing the hardness of the composite the depth of penetration caused by hardened steel disk asperities govern the results in higher wear resistance [15]. Further improvement of the wear resistance of Al-15%Mg2Si composites with lower wear rate was obtained after performing a hot extrusion process compared to the as-cast composites as observed in Figure 8a,b irrespective of the effect of Gd addition. This was because the extrusion process has a substantial effect on the reduction of Mg2Si particles size (Figure 2), porosity content as well as breakage of Gd IMCs in the matrix ( Figure 6) which in turn results in higher hardness. It was reported that hot extrusion not only promoted a decrease of average size but also caused a better distribution of the fine   (Figures 1 and 2) as well as formation of Gd-IMCs (Figure 3). In fact, refinement of Mg 2 Si particles and formation of Gd IMCs lead to increasing the hardness of the composites (Figure 7) which in turn caused the reduction in the composites wear rate. Indeed, with increasing the hardness of the composite the depth of penetration caused by hardened steel disk asperities govern the results in higher wear resistance [15]. Further improvement of the wear resistance of Al-15%Mg 2 Si composites with lower wear rate was obtained after performing a hot extrusion process compared to the as-cast composites as observed in Figure 8a,b irrespective of the effect of Gd addition. This was because the extrusion process has a substantial effect on the reduction of Mg 2 Si particles size (Figure 2), porosity content as well as breakage of Gd IMCs in the matrix ( Figure 6) which in turn results in higher hardness. It was reported that hot extrusion not only promoted a decrease of average size but also caused a better distribution of the fine Mg 2 Si particles [9]. Indeed, the fragmented primary Mg 2 Si particles and Gd IMCs had a considerable influence in reducing the extent of materials removal by supporting the contact stresses through lessening high plastic deformation and abrasion between the pin and disc [16]. However, as observed in Figure 8 with further addition of Gd to 2.0 wt.%, the wear rate increased for both as-cast and extruded composites, which was due to the coarsening of primary Mg 2 Si particles (Figures 1 and 2) and decreasing of the hardness value (Figure 7) due to over-modification phenomenon. Another reason for increasing the wear rate of as-cast and extruded composite modified with 2.0 wt.% Gd was the formation of a high fraction of brittle Gd IMCs in the composite matrix (Figure 3d) which own low load-supporting ability during the wear test. The similar result is reported by [7] in the Al-18%Mg 2 Si treated with Nd addition. Thus, combination of 1.0 wt.% Gd addition and the hot extrusion process promotes the lowest wear rate, 0.14 mm 3 ·km −1 in the Al-15%Mg 2 Si composite under applied load of 20 N. Another indicative feature in Figure 8b was that with increasing the loads from 20 N to 40 N the wear rate was increased for all samples. In fact in the higher applied load the generated stress on the wear surface was greater and caused an increase in the number of contact asperities, which resulted in faster wearing of the material surface. (Figure 7) due to over-modification phenomenon. Another reason for increasing the wear rate of as-cast and extruded composite modified with 2.0 wt.% Gd was the formation of a high fraction of brittle Gd IMCs in the composite matrix (Figure 3d) which own low loadsupporting ability during the wear test. The similar result is reported by [7] in the Al-18%Mg2Si treated with Nd addition. Thus, combination of 1.0 wt.% Gd addition and the hot extrusion process promotes the lowest wear rate, 0.14 mm 3 ·km −1 in the Al-15%Mg2Si composite under applied load of 20 N. Another indicative feature in Figure 8b was that with increasing the loads from 20 N to 40 N the wear rate was increased for all samples. In fact in the higher applied load the generated stress on the wear surface was greater and caused an increase in the number of contact asperities, which resulted in faster wearing of the material surface.   (Figure 9a). The coefficient friction further decreased to 0.38 after conducting hot extrusion. The decrease in the friction coefficient of the Al-Mg2Si-1.0% Gd composite compared to the unmodified one was due to the presence of fine primary Mg2Si particles and fragmented Gd IMCs with large ability of loadsupporting during the sliding wear test to decrease the touching area between the pin and disc. Nevertheless, when the Gd content exceeded 1.0 wt.%, the friction coefficient of the composites increased which was due to the existence of large primary Mg2Si particles and brittle Gd-IMCs in the composite matrix. Similar behaviour is reported in mischmetal (MM) treated Al-7.0 wt.% Si-0.3 wt.% Mg alloy [17]. In addition, Figure 9b illustrates that increasing the load from 20 N to 40 N did not induce considerable alteration in the friction coefficient value. However, under the applied load of 40 N the friction coefficient was slightly higher.   (Figure 9a). The coefficient friction further decreased to 0.38 after conducting hot extrusion. The decrease in the friction coefficient of the Al-Mg 2 Si-1.0% Gd composite compared to the unmodified one was due to the presence of fine primary Mg 2 Si particles and fragmented Gd IMCs with large ability of load-supporting during the sliding wear test to decrease the touching area between the pin and disc. Nevertheless, when the Gd content exceeded 1.0 wt.%, the friction coefficient of the composites increased which was due to the existence of large primary Mg 2 Si particles and brittle Gd-IMCs in the composite matrix. Similar behaviour is reported in mischmetal (MM) treated Al-7.0 wt.% Si-0.3 wt.% Mg alloy [17]. In addition, Figure 9b illustrates that increasing the load from 20 N to 40 N did not induce considerable alteration in the friction coefficient value. However, under the applied load of 40 N the friction coefficient was slightly higher.

Wear Properties
The wear behaviour and its corresponding mechanism (s) can be revealed by examination of the worn surfaces of the fabricated composites under SEM. Figure 10a-f shows the SEM images of worn surfaces of the Al-15%Mg 2 Si-x Gd (x = 0, 1.0 and 2.0 wt.%) composites before and after hot extrusion process tested at a load of 20 N. As seen in Figure 10a, the worn surface of unmodified Al-15%Mg 2 Si composite shows deep and wide plowing grooves parallel to the motion direction. This demonstrates that a combination of abrasion wear and delamination wear appear to be the main wear mechanism of the composite. When conducting the hot extrusion process, the groove become narrow and the pits shallower than those as-cast composites which results in alteration of the wear mechanism to abrasion and adhesive wear (Figure 10b). When the Al-15%Mg 2 Si composite is modified with 1.0 wt.% Gd, the worn surface of the composite possesses a rather smooth appearance with grooves which is indicative of a single abrasion wear mechanism (Figure 10c). The grooves become narrower when the composite undergone a hot extrusion process, in which mild abrasion wear is the governing wear mechanism as can be observed on the worn surface as shown in Figure 10d. However, abrasion and pits appear again when increasing the content of Gd to 2.0 wt.% as illustrated in Figure 10e. Performing the hot extrusion on Al-15%Mg 2 Si-2% Gd composite caused enhancement as it lessened the grooves and pits from the worn surface of the composite (Figure 10f). In fact, the presence of coarse primary Mg 2 Si particles and brittle Gd IMCs in Al-15%Mg 2 Si-2% Gd composite caused worsening of the worn surface compared to Al-15%Mg 2 Si-1% Gd composite. During the wear test these hard particles fractured and were entrapped between the composite and counterface, acting as third body abraders which in turn damage the worn surface. As a result, the plowing grooves formed on the worn surface of the composite (Figure 10e) and results in reduction of wear resistance of the composite which is consistent with a higher wear rate and friction coefficient (Figures 8 and 9). The wear behaviour and its corresponding mechanism (s) can be revealed by examination of the worn surfaces of the fabricated composites under SEM. Figures 10a-f shows the SEM images of worn surfaces of the Al-15%Mg2Si-x Gd (x = 0, 1.0 and 2.0 wt.%) composites before and after hot extrusion process tested at a load of 20 N. As seen in Figure  10a, the worn surface of unmodified Al-15%Mg2Si composite shows deep and wide plowing grooves parallel to the motion direction. This demonstrates that a combination of abrasion wear and delamination wear appear to be the main wear mechanism of the composite. When conducting the hot extrusion process, the groove become narrow and the pits shallower than those as-cast composites which results in alteration of the wear mechanism to abrasion and adhesive wear (Figure 10b). When the Al-15%Mg2Si composite is modified with 1.0 wt.% Gd, the worn surface of the composite possesses a rather smooth appearance with grooves which is indicative of a single abrasion wear mechanism ( Figure  10c). The grooves become narrower when the composite undergone a hot extrusion process, in which mild abrasion wear is the governing wear mechanism as can be observed on the worn surface as shown in Figure 10d. However, abrasion and pits appear again when increasing the content of Gd to 2.0 wt.% as illustrated in Figure 10e. Performing the hot extrusion on Al-15%Mg2Si-2% Gd composite caused enhancement as it lessened the grooves and pits from the worn surface of the composite (Figure 10f). In fact, the presence of coarse primary Mg2Si particles and brittle Gd IMCs in Al-15%Mg2Si-2% Gd composite caused worsening of the worn surface compared to Al-15%Mg2Si-1% Gd composite. During the wear test these hard particles fractured and were entrapped between the composite and counterface, acting as third body abraders which in turn damage the worn surface. As a result, the plowing grooves formed on the worn surface of the composite ( Figure  10e) and results in reduction of wear resistance of the composite which is consistent with a higher wear rate and friction coefficient (Figures 8 and 9). As discussed above, performing a hot extrusion process on the composite samples results in improvement of wear resistance by reducing the wear rate and friction coefficient compared to as-cast composites. Another reason for such an enhancement is the creation of a mechanically mixed layer (MML) on the worn surfaces which is clearly observed on the worn surfaces of the extruded composites with and without Gd addition (Figure 10b,d,f). This layer shields the underlying material from severe plastic deformation or brittle fracture but simultaneously is delaminated to from the wear particles and again restored by intermixing and transfer of components from both pin and disc. In this study, hard fragmented Gd intermetallic compounds plays an important role as initial structural components together with other elements from both pin and disc which are deteriorated and transformed into the MML. Therefore, formation of this stable and compacted layer controls the wear mechanism of the fabricated composites. Formation of the MML protective layer with a mixture of materials (Al, Si, Fe and Cr) (face and counterface) has been detected on the worn surface of A356-10%B 4 C ex-situ composite [18]. Lubricants 2022, 10, x FOR PEER REVIEW 12 of 14 As discussed above, performing a hot extrusion process on the composite samples results in improvement of wear resistance by reducing the wear rate and friction coefficient compared to as-cast composites. Another reason for such an enhancement is the creation of a mechanically mixed layer (MML) on the worn surfaces which is clearly observed on the worn surfaces of the extruded composites with and without Gd addition ( Figure  10b,d,f). This layer shields the underlying material from severe plastic deformation or brittle fracture but simultaneously is delaminated to from the wear particles and again restored by intermixing and transfer of components from both pin and disc. In this study, hard fragmented Gd intermetallic compounds plays an important role as initial structural components together with other elements from both pin and disc which are deteriorated and transformed into the MML. Therefore, formation of this stable and compacted layer controls the wear mechanism of the fabricated composites. Formation of the MML protective layer with a mixture of materials (Al, Si, Fe and Cr) (face and counterface) has been detected on the worn surface of A356-10%B4C ex-situ composite [18].

Conclusions
The following conclusions can be drawn based on the experimental work conducted: (1) For both as-cast and extruded composites, 1.0 wt.% Gd was found to be the best content to induce refinement/modification of primary Mg 2 Si particles in Al-15%Mg 2 Si composite. However, exceeding this amount has an adverse influence on the modification of primary Mg 2 Si particles. (2) Addition of Gd element up to 1.0 wt.% to Al-15%Mg 2 Si composite as well as conducting hot extrusion results in improvement of the hardness value by 31% as compared to the as-cast unmodified composite. (3) The wear properties including wear rate and coefficient of friction of as-cast Al-15%Mg 2 Si composite modified with 1.0 wt.% Gd was found to be considerably higher than the unmodified composite due to refinement/modification of primary Mg 2 Si particles and formation of Gd IMCs. Further improvement of the wear properties was achieved after conducting hot extrusion due to fragmentation of primary Mg 2 Si particles and Gd IMCs.
(4) When the as-cast Al-15%Mg 2 Si composite was modified with 1.0 wt.% Gd, the wear mechanism changed to abrasion and adhesion compared to abrasion and delamination in the as-cast unmodified composite. The wear mechanism changed to mild abrasion after performing the hot extrusion on Al-15%Mg 2 Si-1.0% Gd composite.

Conflicts of Interest:
The authors declare that they have no known competing financial interest or personal relationship that could have appeared to influence the work reported in this paper.