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Inconel 718 alloy is difficult to machine using conventional methods due to its physical properties. Thereby, additive manufacturing (AM) of IN718 components with near-net shapes has been excessively studied. Even though AM processes provide shape and size accuracy, there is still the need for the machining of the AM-processed components. Laser powder bed fusion (LPBF) has been successfully utilized to fabricate near-net shape IN718 components, moreover, the microstructure of LPBF-IN718 was unique owing to the AM processing, resulting in differences in grain size, grain boundary characteristics, and grain orientations. Furthermore, these microstructural characteristics are expected to alter the machining performance of IN718. Therefore, this study investigated the wire electro-discharge machining (WEDM) performance of LPBF-718 samples compared to wrought IN718 while focusing on the unique microstructure characteristics of LPBF-IN718 samples (lamella, single-crystal, ploy-crystal). Three different cutting strategies (rough, semi-finish, and finish) were implemented to understand the performance of the multi-pass cutting phenomenon and its effect on the surface of IN718. For all samples, rough (single pass) cutting displayed high roughness while finish (three passes) cutting exhibited good surface quality. Compositional analyses on the machined surface showed debris formation including Zn and Cu-containing recast material, indicating wire erosion. The surface of single-crystal LPBF-IN718 after the WEDM process was smooth owing to its big grain size and less amount of grain boundary, resulting in slow cutting speed but a good surface finish. Thus, this study, for the first time, investigated the effect of unique microstructural characteristics of LPBF-fabricated IN718 on WEDM performance and machined surface quality.

Metallic biomaterials improve the life quality of the people. In particular, efforts to develop porous biomaterials are getting more attention recently. Among the metallic materials, titanium alloys are commonly preferred in biomedical application due to their excellent biocompatibility, high corrosion resistance and mechanical strength. Researches into porous titanium-based implant materials have resulted in several fabrication methods, with many others under development. In this study, powder metallurgical different processing techniques such as partial sintering of powders, replication, additive manufacturing, plasma spray coating, microwave sintering, space holder method, spark plasma sintering, metal injection molding were investigated. What are the requirements, problems and developments in this field? After considering these issues, we will discuss different conventional and technological processing methods, along with their effects on the structure and properties of implant materials.

Although magnesium is one of the materials with the lowest density, its application is limited amongst other commercial materials due to insufficient strength. Generally, the strength of magnesium can be increased by introducing the Mg17Al12 phase into the structure with the addition of aluminum. In this study, as a different approach, Mg17Al12 particles were added to pure magnesium to form a composite structure. For this purpose, Mg composites with different Mg17Al12 contents were fabricated by powder metallurgy methods. Pure Mg and Mg17Al12 particles were mixed in a vacuum ball milling equipment for 1 h using zirconia balls to obtain the Mg metal matrix composite with homogeneously distributed intermetallic reinforcements. While for pure Mg powders, the sintering temperature was selected as 600°C, for Mg + Mg17Al12 mixtures, it was 420°C due to the eutectic transformation between Mg and Al. The sintering time and pressure of the Mg-based mixtures were held constant for 30 min and 50 MPa, respectively. All sintering processes were carried out under a 10–2 Pa vacuum atmosphere. Microstructural characterization was performed using optical microscopy and scanning electron microscopy. The hardness and wear properties of the composites produced have also been studied. The hardness and wear resistance of composites increased significantly as the Mg17Al12 particle content increased. Although the particle-enhanced composite density increased from 1.72 to 1.92 g/cm3, a 2.4-fold increase in hardness and a two-fold increase in wear resistance was observed. This magnesium composite produced with a new design could be used in many different areas.

In the current study, the effect of Mo content (1-10 wt.%) on the microstructure, hardness, bending and wear properties of Ti6Al4V-xMo alloys was investigated. The pre-alloyed and Plasma Rotating Electrode Process (PREP) atomized Ti6Al4V alloy powders and elemental Mo particles were mechanically mixed for 45 min in a zirconia jar. A uniaxial vacuum hot pressing was applied at 950°C for 30 min under 50 MPa pressure. The Ti6Al4V-xMo alloys were prepared metallographically and characterized by optical and scanning electron microscopy. The chemical composition of the different zones in the structure was determined using energy-dispersive x-ray (EDX) analysis. Mo appeared among the TiAl64V alloy particles and caused the formation of different diffusion zones. The formation of grain boundary α was effectively prevented, and instead, fine α’ and β zones were formed. Various phases formed along the particle boundaries of the Ti6Al4V alloy, and effective improvements in hardness, bending and wear resistance were obtained. However, the highest Mo content caused a decrease in mechanical properties. Ti6Al4V-5Mo alloy showed superior hardness and wear resistance.

Ti-based alloys namely, Ti-Ni, has been used as biomedical material due to its superior properties. However, it has been reported recently that nickel is toxic to the human body and thus alternative nickel-free alloys has been rigorously studied. The best candidate materials are Ti-Nb and Ti-Ta which are known to be biocompatible. These alloys maybe produced either via casting or through powder metallurgy route. Both production and fabrication methods produced dense and porous materials respectively which have different applications in biomedical field. This paper describes the influence of production and fabrication methods on the microstructures and hardness of nickel-free Ti-alloys, namely, Ti-Nb and Ti-Ta alloys with Ti-Ni as a reference alloy. The alloys were produced by three different methods: casting in vacuum induction furnace, compaction of powders followed by microwave sintering and compaction of powders followed by spark plasma sintering. The presence of porosity was analysed on the as cast and sintered samples by Archimedes drainage method while microstructures analysis was done using standard materials characterisation techniques such as optical and electron microscopy as well as X-ray diffractometry. Hardness test was also conducted by Vickers hardness test. It was found that the alloys with different production and fabrication methods give slight variation in the microstructures in terms of grain size and structures, porosity intensities and pore size. As-cast Ti alloys have the lowest amount of porosity and highest hardness values while Ti alloys fabricated by microwave sintering have the highest porosity content with lowest hardness values.

The production of metals by additive manufacturing is developing rapidly. After production, support structures emerge as waste materials. In this study, Ti6Al4V support structures recycled from the selective laser melting process were consolidated by vacuum hot pressing. Spherical and irregularly shaped Ti6Al4V alloy particles were also used for the comparison as raw materials. All raw materials have been subjected to the same sintering process by hot pressing. The microstructures of the samples were carried out, and their dry sliding wear performance was studied. The samples produced from support structures showed the highest wear performance compared to the powder forms of raw materials. This study showed that the support structures from additive manufacturing could be recycled and transformed into full dense structures by pressure-assisted sintering techniques, and enhanced wear performance can be obtained.

Despite the growing interest in using powder metallurgy shape memory alloys, there is limited research on their joining and welding aiming to expand their applications. In this research, we utilized a high-power, high-brightness disk laser welding process to join spark-plasma-sintered Ti-51 at.% Ni shape memory alloy and studied the primary factors that affect the strength and functionality of the created sound welds such as intermetallics and martensite (B19′) phase formation. The introduction of high-power laser could impede the formation of B19′ and undesirable intermetallics such as TiNi3 and Ti2Ni; however, the desired Ti3Ni4 would also be suppressed and post-weld heat treatment was necessary to induce them again. The microstructure of fusion zones was significantly altered, producing coarse columnar grains with centerline and equiaxed weld centers. Additionally, high-power laser welding of powder metallurgy Ti-Ni alloy triggered amorphous phase formation in the welds due to rapid cooling. It was found that a laser power of 3 kW and welding speed of 6 m/min are optimal welding parameters, since the produced weld solidified along favorable [001] direction of strain recovery in Ti-Ni shape memory alloys.

A powder metallurgy Ti–35 at.% Ta alloy was produced using a spark plasma sintering process. The sintered alloy exhibited Ta-rich regions at a sintering temperature of 1100 °C. Complete diffusion of elemental powders was achieved via homogenization heat treatment at 1500 °C for 12 h, but the ductility declined drastically. The brittle behavior of the homogenized sample was due to the high-oxygen content, continuous grain boundary α-phase and high-angle grain boundaries. Excellent mechanical properties were achieved when hot forging was used along with preheating the homogenized specimen at 1100 °C. The mechanisms underlying this phenomenon were suppression of the solid grain boundary α-phase, high fraction of low-angle grain boundaries and dynamic recrystallization. Tensile strength improved to the maximum value of 868 MPa. The strain recovery ratio increased to 90% in the first cycle during a cyclic loading–unloading test. Incomplete strain recovery of the first cycle was due to the presence of the martensite phase (α″).

This study aims to improve the mechanical properties of a Ti–2Fe base alloy by adding W solute and performing hot extrusion at a high temperature (1000 °C). W was added at 0, 1, 2, and 3 wt% using the powder metallurgy route and homogenization heat treatment. The as-extruded materials predominantly consisted of α phase with different microstructure morphologies; Ti–2Fe and Ti–2Fe–1W contained equiaxed α grains, while Ti–2Fe–2W and Ti–2Fe–3W showed equiaxed + acicular and acicular shape, respectively. Effective grain refinement was obtained in Ti–2Fe–2W (average grain size: ~1.64 μm), which greatly contributed to the strengthening. The solid solution of W was studied with X-ray powder diffraction, where a proportional increment of β lattice constant occurred as the W solute increased in the matrix (Ti–Fe). Additionally, electron backscatter diffraction analysis revealed that the W solution reduced the intensity of the prismatic texture along the extrusion direction. Based on the experimental evaluations, extruded Ti–2Fe–2W alloy exhibited a maximum yield strength of 925 MPa with excellent elongation 30% at room temperature, indicating a remarkable trade-off in strength and ductility.

Porous Ti-23%Nb (mole fraction) shape memory alloys (SMAs) were prepared successfully by microwave sintering with excellent outer finishing (without space holder). The effects of microwave-sintering on the microstructure, phase composition, phase-transformation temperature, mechanical properties and shape-memory effect were investigated. The results show that the density and size of porosity vary based on the sintering time and temperature, in which the smallest size and the most uniform pore shape are exhibited with Ti-23%Nb SMA after being sintered at 900 °C for 30 min. The microstructure of porous Ti-Nb SMA consists of predominant α″, α, and β phases in needle-like and plate-like morphologies, and their volume fractions vary based on the sintering time and temperature. The β phase represents the largest phase due to the higher content of β stabilizer element with little intensities of α and α″ phases. The highest ultimate strength and its strain are indicated for the sample sintered at 900 °C for 30 min, while the best superelasticity is for the sample sintered at 1200 °C for 30 min. The low-elastic modulus enables these alloys to avoid the problem of “stress shielding”. Therefore, microwave heating can be employed to sinter Ti-alloys for biomedical applications and improve the mechanical properties of these alloys.

Elemental titanium (Ti) and nickel (Ni) powders were consolidated by spark plasma sintering (SPS) to fabricate Ti–51%Ni (mole fraction) shape-memory alloys (SMAs). The objective of this study is to enhance the superelasticity of SPS produced Ti–Ni alloy using free forging as a secondary process. Products from two processes (with and without free forging) were compared in terms of microstructure, transformation temperature and superelasticity. The results showed that, free forging effectively improved the tensile and shape-memory properties. Ductility increased from 6.8% to 9.2% after forging. The maximum strain during superelasticity increased from 5% to 7.5% and the strain recovery rate increased from 72% to 92%. The microstructure of produced Ti–51%Ni SMA consists of the cubic austenite (B2) matrix, monoclinic martensite (B19′), secondary phases (Ti3Ni4, Ti2Ni and TiNi3) and oxides (Ti4Ni2O and Ti3O5). There was a shift towards higher temperatures in the martensitic transformation of free forged specimen (aged at 500 °C) due to the decrease in Ni content of B2 matrix. This is related to the presence of Ti3Ni4 precipitates, which were observed using transmission electron microscope (TEM). In conclusion, free forging could improve superelasticity and mechanical properties of Ti–51%Ni SMA.

Light elements such as oxygen (O) and nitrogen (N) significantly impact the microstructure and mechanical properties of Ti-based materials through solid solution strengthening. The microstructures of Ti-based materials processed via selective laser melting (SLM) have also been observed to contain a martensitic phase that improves the tensile strength. However, this improvement is achieved at the cost of reduced ductility. This study considered the use of post-heat treatment N dissolution to enhance the ductility of SLM-processed α-Ti materials. Tensile testing of the as-fabricated SLM Ti-(N) revealed a significantly increased strength of ~1200 MPa and a low ductility of 5% for N content of 0.5 wt%. However, the quenched samples exhibited increased ductility by up to 20%, with the microstructure, including primary α (αp) and transformed β structures. Further examination via electron probe micro-analysis (EPMA), transmission electron microscopy (TEM), in-situ high-temperature SEM observation and in-situ EBSD observation during tensile testing revealed that the enhancement in ductility of the quenched SLM-processed Ti-(N) samples was significantly due to alteration of the grain morphology, dislocations and N distribution. The findings of this study further clarify the microstructural evolution and deformation response of SLM-processed Ti-(N) materials under water quenching.

Tensile test showed that the strength of the welded joints of powder metallurgy Ti–51%atNi shape-memory alloys (SMAs) decreases after disk-laser welding due to the presence of pores and columnar structure in the fusion zone. There was a reduction in the superelastic behavior of the welded joints compared to the base metal. Superelastic properties of specimen LW1 (with welding speed of 6 m/min and laser power input of 3 kW) were close to the base metal after loading and unloading at room temperature. Welded specimens, which were subjected to solution treatment at 1000 °C followed by aging at 500 °C showed an increment in superelasticity properties. Superelasticity improved significantly after heat treatment in all the welded joints except LW2 (with welding speed of 7.5 m/min and laser power input of 3 kW), because it contained high amounts of porosity in the fusion zone. LW1 exhibited the best properties after heat treatment with superelasticity strain increasing up to 7%, which was higher than the base metal. Differential scanning calorimetry (DSC) investigations showed that heat-treated weld seams had similar phase transformation peaks of martensite, austenite, and R phases to the base metal, which may be due to the recovering of Ti 3 Ni 4 precipitates.

Ti–Ta and Ti–Ta–xSn shape memory alloys (SMAs) were produced successfully by microwave sintering. Tin element was added to Ti–Ta SMA with three different atomic percentages (0.37, 0.745 and 2.26 at.%). The influences of Sn addition on microstructure, transformation temperatures, mechanical properties, shape memory behaviours, corrosion resistance and bioactivity were investigated. It was found that the morphologies of the modified and unmodified Ti–Ta alloys contain a plate-like and needle-like structure, where the former structure is related to the β phase and the latter related to the α phase. The transformation temperatures of M s to M f with and without Sn additions were observed to be in the range of 389.65 to 355 K, while the transformation temperatures of A s to A f were found to be in the range of 250.42 to 365.8 K. By increasing the proportion of Sn, the compressive fracture strength and shape memory behaviour were enhanced; however, the corrosion resistance tended to be decreased. Based on the bioactivity results, antibacterial activity was improved with the addition of Sn. In conclusion, the existent results indicate that Ti–30 at.% Ta SMAs may be a convenient alternative to Ni–Ti for certain biomedical applications.

Thermomechanical processing and solid-solution strengthening are effective ways to improve the mechanical properties of Ti alloys with elements such as W and Fe. In this study, we present a novel extruded Ti-Fe-W alloys with improved mechanical properties. To analyze the effect of W on the mechanical properties and microstructure of the alloys, Ti-4Fe-xW (x = 0–3 wt%) alloys were prepared by spark plasma sintering followed by homogenization heat treatment and hot extrusion. The microstructure of the as-sintered specimens included undissolved W particles distributed in the acicular α + β matrix. However, heat treatment at 1300 °C for 1 h led to the complete dissolution of W. After hot extrusion at 850 °C, an ultrafine equiaxed (globular) microstructure was observed, in which Fe and W preferentially diffused into the β phase. The main effect of W on the microstructure was a remarkable grain refinement by activation of dynamic recrystallization and impediment of the grain boundary mobility in Ti-4Fe-(1–3)W (~1 µm) when compared to Ti-4Fe (~3 µm). Additionally, W increment resulted in a gradual decrease of the average size of α grains. The tensile yield strength increased as W content increased so that the Ti-4Fe-3W alloy exhibited a remarkably high tensile strength, yielding at ~1123 MPa with an elongation of ~26%. Finally, theoretical and experimental analyses suggested that grain refinement and solid-solution strengthening were the main mechanisms contributing to the strengthening phenomenon in W-containing alloys.

Ti−51at%Ni shape memory alloys (SMAs) were successfully produced via a powder metallurgy and microwave sintering technique. The influence of sintering parameters on porosity reduction, microstructure, phase transformation temperatures, and mechanical properties were investigated by optical microscopy, field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), compression tests, and microhardness tests. Varying the microwave temperature and holding time was found to strongly affect the density of porosity, presence of precipitates, transformation temperatures, and mechanical properties. The lowest density and smallest pore size were observed in the Ti−51at%Ni samples sintered at 900°C for 5 min or at 900°C for 30 min. The predominant martensite phases of β2 and β19′ were observed in the microstructure of Ti−51at%Ni, and their existence varied in accordance with the sintering temperature and the holding time. In the DSC thermograms, multi-transformation peaks were observed during heating, whereas a single peak was observed during cooling; these peaks correspond to the presence of the β2, R, and β19′ phases. The maximum strength and strain among the Ti−51at%Ni SMAs were 1376 MPa and 29%, respectively, for the sample sintered at 900°C for 30 min because of this sample’s minimal porosity.

Extensive research on the effect of spark-plasma sintering and free forging on the microstructure, transformation temperature, and superelasticity of Ti-23 at% Nb shape-memory alloys (SMAs) was undertaken. Based on the obtained results, it was found that the specimen showed significant results after deformation than the homogenization step, from which it can be concluded that the deformation has considerable effects on the mechanical properties and shape-memory characteristics of Ti-based SMAs. The microstructure of homogenized and deformed Ti-23 at% Nb SMAs consists of β, α, α′ and ω phases, while the sintered sample shows only β and α phases. Endothermic and exothermic peaks of the austenite ↔ martensite transformation are seen in the homogenized sample, then after being deformed, the exothermic peak disappeared. The highest austenite start and finish temperatures were observed with the deformed sample due to disappearance of TiNb4 intermetallic compounds and a high percentage of β phase. The ductility of the deformed sample shows the highest value compared with the homogenized sample due the elimination of coarse α precipitates and brittle intermetallic compounds of TiNb4, along with the suppression of the brittle ω phase and retaining β matrix. Keywords: Free forging, Spark-plasma sintering, Superelasticity and powder metallurgy

The influence of defocusing distance of the fiber laser welding on Ti-51at%Ni, Ti-28%atNb and Ti-30at%Ta shape memory alloys (SMAs) was investigated. These alloys were produced by powder metallurgy (P/M) and sintered by furnace and microwave processes. The experimental results showed that the furnace-sintered Ti-Ni welds have the maximum porosity density and the lowest weld quality. While, the microwave sintered alloys of Ti-Nb and Ti-Ta SMAs presented better weldability, even though porosities were still observed. Ti-Ta SMA showed the best welding quality compared with the other welded joints. It can be concluded that porosities play important role in the welding of powder metallurgy alloys and in order to achieve high-quality welds, the porosities should be minimized.

A Nd:YAG laser welding technique with varying pulse widths was employed to weld Ti-27 at.% Nb shape memory alloy plates, and subsequent phase transformation characteristics and microhardness of welds were systematically investigated. From microstructure observation, it is apparent that weld seams contain a columnar growth with the low pulse and transferred into a combined growth of columnar and planar with the high pulse width. Moreover, grains in the weld zone exhibited a finer size when compared to the unwelded zone. A small amount of NbTi4 was also found in the welds. In the forward transformation, transformation temperatures did not exhibit peaks, indicating the same behaviour as the original TiNb alloy. Microhardness of the TiNb welds increased as the pulse width increased when compared to the original TiNb alloy. However, little reduction in hardness occurred after a pulse width of 5 ms.

Research related to the substitution of ubiquitous elements for high-cost rare elements during the fabrication of high-performance Ti alloys has attracted significant attention. The microstructure evolution is, however, challenging when several elements with different properties are used. This research aims to study the effect of Si addition on the microstructure formation of the dual-phase Ti alloys in the presence of solid solution-forming elements such as Fe and Cu, and a high oxygen concentration. Ti–4Fe–0.5O–3Cu (TFOC) alloys with different Si contents (0.2, 0.4, and 0.6 wt%) were fabricated from the elemental powders using spark plasma sintering, followed by hot extrusion. The TFOC sintered alloys with 0.2 and 0.4% Si showed very similar microstructures in the lamellae α+β dual-phase. The coarsening of α colonies and primary β grains was remarkable when Si content increased to 0.6%. The obtained primary β grains were, however, significantly smaller than those in previous studies. In contrast, the refined globular α grains (respective grain sizes = 2.6 and 2.2 μm) embedded in the β matrix were observed in the hot extruded TFOC–0.2Si and TFOC–0.4Si alloys due to dynamic globularization. Additionally, EBSD analysis clarified that the hot extruded alloys had the strong fiber textures of (10 1 ¯ 0) and (101) for α and β phases respectively, which were consistent with the XRD results. High-resolution TEM examination demonstrated that Ti2Cu and Ti3Cu intermetallics of a length of 100–400 nm could form preferentially at α/β interfaces. The experimental results showed that 0.4 wt% Si added to TFOC alloys formed fine globular microstructures in the presence of hot plastic deformation.

Fully dense powder metallurgy (PM) Ti–4Fe–0.2O–3Cu–0.4Si alloy was produced employing spark plasma sintering (SPS) and hot extrusion aiming to develop an inexpensive high strength dual-phase (α+β) Ti alloy. The microstructure consisted of globularized fine grain α phase dispersed in the β matrix having the sharp texture of 1 ‾ 0>//extrusion direction. The tensile strength of alloy (~1060 MPa) is comparatively higher than the conventional Ti–6Al–4V alloy produced by a similar method. Based on theoretical calculations the grain refinement and solid solution phenomena were the principal strengthening mechanisms. Additionally, it was found that Si solute remarkably enhanced the microstructure stability (using in situ EBSD observation) and yield strength of alloy at high-temperature.

Single β phase titanium alloys are used for fabricating biomedical devices, and it is desirable to improve the strength of these alloys. In this study, the tensile strength of Ti-35 at.% Ta alloys with a single β phase was improved using oxygen solid solution and the strengthening mechanism was also investigated. The lattice constants of these alloys calculated from X-ray diffraction data increased with an increase in the oxygen content. This result suggests that oxygen atoms formed solutes in the alloys, which was supported by results obtained from first-principles calculations. Microstructural analyses of the alloys indicate that Ti, Ta, and O were homogeneously distributed. Tensile tests were conducted for the alloys, and the 0.2%YS increased with the increase in the O content of the alloy. The increments of 0.2%YS which was determined experimentally correspond well with the theoretically determined trend, and this observation is a result of the effect of the change in grain size and O solid solution. The contribution of the grain refinement was negligibly small compared with that of the O solid solution, and the latter is the major reason for the observed increase in the 0.2%YS of the Ti-35 at.% Ta alloys.

The demand for using high-performance Cu matrix composites in the electronic industry has recently increased rapidly owing to the low strength and tribological properties of pure copper. Accordingly, research and development of composite processing and related properties have increased. In this study, the Cu matrix was reinforced with different amounts of TiB2 particles by mechanical alloying of elemental powders followed by consolidation employing spark plasma sintering (SPS) whereby high relative density (∼99%) and electric conductivity (∼83–88% IACS) were obtained. Tensile properties and deformation mechanisms of the composites were studied utilizing conventional and in-situ tensile tests with simultaneous FESEM and EBSD observations. In comparison with the reference pure copper, it was found that adding 0.5 wt%TiB2 results in significant ductility increment, which was consistent with the longer strain-softening behavior observed after necking. In addition, using in-situ microstructure characterization, twinning with slip bands were confirmed as deformation mechanisms. By increasing the amount of reinforcement to 5 wt%TiB2, strengthening was remarkable while sustaining a considerable elongation. The Hall-Petch and dislocation strengthening mechanisms were the key models of strengthening based on the calculations and microstructure characterizations since by increasing the TiB2 particles, significant grain refinement and dislocation density were shown.

In this study, interstitial solid solution behavior of nitrogen atoms dissolved in commercially pure Ti fabricated by spark plasma sintering (SPS) followed by the extrusion process was investigated. Microstructural observations of the extruded Ti-(N) samples before homogenization, ExTi-(N), revealed the nitrogen element's segregated regions as a result of a short time of the SPS process. Because of the inadequate sintering time, nitrogen atoms were unable to dissolve in the α-Ti matrix homogeneously, which caused a material degradation through brittle failure initiating at high nitrogen absorbed regions. Applying homogenization heat treatment at 1123 K for 180 min followed by hot extrusion, improved microstructural homogeneity without the nitrogen segregation. Furthermore, grain morphology was transformed from coarse elongated grains with substructures into refined equiaxed α-Ti grains. The yield strength and ultimate tensile strength of the Ti-(N) sample with 0.69 wt%N reached maximum 957 MPa and 1117 MPa, respectively, which represented 148% and 214% increments over as-extruded pure Ti sample. In addition, a 50% enhancement of elongation of homogenized Ti-(N) alloys compared to the non-homogenizing condition was achieved. The improvement of strength and elongation was primarily attributed to grain refinement and solid solution strengthening. The quantitative evaluation of strengthening and mechanisms was discussed in detail by the Hall-Petch formula and Labusch model.

The prevalence of low fracture strain in metal matrix composites (MMCs), in comparison to the unreinforced matrix material, has always been a challenge, especially in CNT reinforced composites. The study investigates the improvement in the ductility of carbon nanotubes (CNT) reinforced aluminium matrix composite, through the introduction of 2.5 μm titanium carbide (TiC) particles as a second reinforcement phase. The bimodal hybrid composite is fabricated by successive application of flake powder metallurgy of Al powder, solution coating of CNT on Al powder, spark plasma sintering, and hot extrusion of the resulting billet. Field-emission scanning electron microscopy (FE-SEM) analysis reveals that the solution coating process produced Al powder perfectly coated with individual CNTs. The pure Al so fabricated had a tensile strength of 125 MPa and elongation of 40%, while the tensile strength and elongation of the Al-0.5CNT composite of 232 MPa and 5.2%, respectively, is considered typical. However, introducing 2.5 wt% TiC microparticles to form Al-2.5TiC-0.5CNT hybrid composite, reduced the UTS to 186 MPa while the elongation increased to 33%. The role of dislocation generation and annihilation by the reinforcement phase is explored to explain the novel behaviour of the bimodal hybrid composite. The adverse effect of microparticles on the strength of the hybrid composite is contextualized in terms of strengthening mechanisms and a theoretical estimation.

Ti-Ni and Ti-Ni-Ag shape memory alloys (SMAs) were prepared by microwave sintering. In Ti (49 -%Ag)-Ni51-Ag (atomic percentage), the silver was added with three percentages of (0.246, 0.5 and 1.51) at. %, respectively. The influence of Ag addition on the microstructure, phase composition, transformation temperatures and mechanical properties were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC) and compression test. The microstructure shows needles and plates inside Ti-rich region. The R phase appears at the plane (-112) and the plane (300). This phase has appeared during cooling and heating of the baseline of DSC test. The compression strain at maximum strength was improved, while the compression strength was reduced. The highest compressive strain was for the sample with 0.246 at. % Ag. The elastic modulus decreases with the increasing of Ag content. The elastic modulus of these alloys was low that make it proper for biomedical applications such as natural human bone due to the sintering method and also improve by adding silver.

The present investigation aims to enhance the mechanical properties and shape memory characteristics of Cu–Ni–Al shape memory alloys (SMAs) by alloying additional elements. These additions were found to control the phase morphology and grain size, along with the formation of different volume fractions, sizes, and distributions of precipitates. The features of the precipitates were mainly dependent on the type of alloying element. It was found that a Co (1.14 wt%) alloy gave the best overall improvement in terms of the transformation temperatures, ductility, and shape memory recovery. These improvements were mainly due to the exceptionally high presence of the γ2 phase in the microstructures of the modified alloy. The results of the current investigation were analyzed and compared to those of previous studies related to Cu–Al–Ni SMAs.

In this paper, dissimilar joining of 316L stainless steel to low carbon steel was carried out using gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). Samples were welded using AWS: ER309L welding electrode for GMAW and AWS: ER316L welding electrode for GTAW process. Determination of mechanical properties and material characterization on the welded joints were carried out using the Instron tensile test machine and an optical microscope respectively. The cross section area of the welded joint consists of three main areas namely the base metal (BM), heat affected zone (HAZ), and weld metal (WM). It was found that, the yield and tensile strengths of welded samples using ER316L filler metal were slightly higher than the welded sample using ER309L welding electrode. All welded samples fractured at low carbon steel base metal indicating that the regions of ER316L stainless steel base metal, ER316L filler metal and heat affected zone (HAZ) have a higher strength than low carbon steel base metal. It was also found that ER316L welding electrode was the best filler to be used for welding two dissimilar metals between carbon and stainless steel.

The trade-off between strength and ductility has been a dilemma in copper matrix composites. In this work, we introduce a way of strengthening copper matrix composites containing TiO2 nanoparticles with remarkable ductility. The Cu–8wt%TiO2 composites were fabricated using the powder metallurgy route with spark plasma sintering (SPS) for different holding times incorporating hot extrusion. The sintered composites for longer holding times showed an inhomogeneous distribution of large dispersed particles located mostly along the grain boundaries. However, tensile strength was improved compared with pure copper while ductility was reduced. In contrast, after hot extrusion, a homogeneous distribution of reinforcement particles and grain refinement concurrently triggered strong strengthening and enhanced ductility. The contribution of synergistic strengthening mechanisms, i.e. Hall–Petch strengthening, Orowan pinning, dislocation density and load transfer effect, was studied and the key strengthening mechanisms were elucidated. It was found that composite yielding was strongly affected by sintering time, particulate size and interparticle spacing so that HExt-SPS@30min composite presented the excellent yield strength of 290 MPa, about 72% above pure copper. © 2019 Elsevier B.V.

Thus far, the joining of Nitinol alloys has been studied extensively using laser welding. Here, we studied the weldability of powder metallurgy (PM) Ti–51 at %Ni alloy by friction stir welding (FSW) via microstructural, superelasticity and mechanical property evaluations. The FSW welds were produced by a constant welding speed and different rotational speeds of 250 rpm, 300 rpm, and 350 rpm utilizing the cemented carbide tool. No Ti3Ni4 precipitate was observed after FSW welding, which agreed with XRD analysis. Additionally, it was found that FSW triggered the dynamically re-crystallized fine grains in which tangles of dislocations existed. The crystallographic orientations in the weld zone revealed locally high texture evolution along with diverse Schmid factors measured for active slip systems of Nitinol in different locations of the welds. These heterogeneous crystallographic orientations, dislocation density and Schmid factor variations caused a gradient in local mechanical properties across the welds during the uniaxial tensile test performed by digital image correlation (DIC). The 350 rpm weld exhibited the excellent mechanical properties, yielding at ~765 MPa (~37% higher than the base metal and all previously reported Nitinol welds) and reaching an ultimate strength of ~870 MPa and 8.1% elongation. However, the superelasticity of the FSW-welded joints curtailed upon cyclic loading-unloading; hence it is suggested that post-heat treatment is essential to recovering their functionality.

Ti-Ni and Ti-Ni-Ce shape memory alloys (SMAs) were successfully fabricated by microwave sintering. The improvement of the mechanical properties especially the elastic modulus is the most important criterion in this research. The high elastic modulus problems are the most critical issues frequently encountered in hard tissue replacement applications. The effect of Ce addition with four atomic percentages (0 %, 0.19 %, 0.385 % and, 1.165 %) on the microstructure, phase composition, transformation temperatures and mechanical properties was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Differential Scanning Calorimeter (DSC), and compression test. The microstructure shows plates-like with needles-like inside the titanium-rich region. The compression strain was improved, but reduces the compression strength. The addition of cerium improved the properties by reducing the elastic modulus to be very close to the natural human bone, also the microwave sintering gives TiNi SMAs with low elastic modulus comparing with other methods. Based on the results, the 0.385 at. % Ce exhibited a remarkable highest compressive strain and lower elastic modulus compared with the other percentages. In conclusion, the present results indicate that Ti-Ni-Ce SMAs could be a potential alternative to improve Ti-51 at %Ni SMAs for certain biomedical applications.

Objective: The influence of microwave sintering time and Homogenization Treatment (HT) on the microstructure, density, phase composition, mechanical properties and phase transformation temperatures of biocompatible Ti-51%Ni Shape Memory Alloys (SMAs) are determined. Methods/Statistical Analysis: These alloys are fabricated at 900˚C for two different sintering times such as 5 min. and 30 min. Findings: The Field Emission Scanning Electron Microscopy (FESEM) micrographs show microstructure of needle-like morphology except for the sample which sintered at 900˚C for 30 min. without HT. Applications/Improvements: Sample synthesized at 900˚C for 30 min. without HT revealed the highest performance in terms of maximum compressive strength (1376 MPa) at 29% strain, Austenite finish temperature (Af) of 27˚C and Martensite finish temperature (Mf) of 47˚C at 18% porosity. They showed the Af temperature very close to the human body temperature, thus prospective for biomedical applications. During heating, the Differential Scanning Calorimeter (DSC) baseline shows multi-endothermic peaks, while during the cooling process there is only one exothermic peak.

This paper describes an investigation on the effect of using three different filler metals to weld two dissimilar metals namely, stainless steel 316L and low alloy carbon steel A516 gr 70. Manual Gas Tungsten Arc welding (GTAW) with three filler metals including ER 80S-Ni1, ER309L, ER NiCrMo-3 were selected to weld the two metals. Radiography and penetrant tests were performed on the welded metals to ensure the surface and internal soundness of the welds based on the tensile tests results, all the specimens failed at the carbon steel A516 gr 70 base metals with fully ductile fracture mode (cup and cone). Welded samples using Inconel 615 filler metal has the highest strength of 512 MPa while other samples show almost similar strength of 481 and 487 MPa. The tensile strength of all the welded samples is found to be in between the tensile strength of the base metals. Micro-hardness test showed that ER80S-Ni1weld has the highest hardness, meanwhile hardness profile of ER309L presented a sharp drop in the stainless steel side and ER NiCrMo-3 weld metal illustrated hardness above the two base metals with fewer variations across the weld metal.