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In the present work, titanium alloys doped with different amounts of impurity solutes were studied as regards their strain rate sensitivity, work-hardening and fracture manner. The experimental results revealed a transition of both the strain hardening behavior and fracture manner with increasing the impurity content. Namely, the low impurity content Ti exhibited features of intergranular crack, whereas the high impurity content Ti showed transgranular cracking instead. In addition, the physical activation volumes were found to reduce with the impurity content, which is attributed to both dislocation pinning and solute drag effects. A constitutive model was proposed to understand the mechanical behavior of the materials. The highlights of the model are of separating deforming grains into plastic and elastic groups and of giving them different properties. To describe the volume of grains changing from elastic to plastic deformation with strain, a controlling function was applied based on an empirical analysis. It turns out that the proposed model predicted stress-strain curves in well agreement with the experimental results.

Given that commercial purity titanium (CP-Ti) doped with a high oxygen concentration has recently been reported with high mechanical performance, the present work investigated the failure behavior and its mechanisms of the material under uniaxial impact loading. Besides the strong hardening effect of oxygen, it was found that increasing oxygen content led to increased propensity for adiabatic shear failure in CP-Ti. The texture or grain orientation was also found to have profound influence on the formation of adiabatic shear band (ASB). Microstructural examinations on the postmortem high oxygen CP-Ti suggested that uniform and equi-axed nano-grains were produced within the ASB. The orientations of these nano-grains were analyzed using the precession electron diffraction (PED) technique. It provides direct evidence of phase transformation occurring in ASB. Then we demonstrate that the nano-grains in ASB formed from parent grains by phase transformation, indicating that the α→β→α phase transformation process takes part in ASB evolution and is an underlying mechanism of grain refinement. As such, this result is a supplement for traditional well-accepted dynamic recrystallization mechanism of ASB evolution.

Oxygen and nitrogen have been both known to have a strong hardening effect on Ti and its alloys, while also imposing a serious embrittlement effect. In the present work, Ti samples with non-uniformly dispersed oxygen and nitrogen solid solutions were produced using high purity Ti via powder metallurgy (PM) methods. The experimental results suggested that, when the solutes are non-uniformly distributed, the high solute content region can serve as strengthening particles as that in metal matrix composites. In light of this, we propose a design of novel structure in high oxygen/nitrogen Ti materials to achieve both improved strengths and ductility. The highlight of the structure is a design of strain gradient at the matrix-particle interface, which can mitigate the strain compatibility that commonly reported in general MMCs as the crack initiator. Instead of using ceramics as reinforcements, here we propose to use the matrix phase itself that hardened by high oxygen/nitrogen solutes as reinforcing particles.

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.

Nano SiC layer is innovatively introduced into carbon nanotubes/Al (CNTs/Al) interface to improve the interface bonding and control the interface reaction between CNTs and Al matrix. SiC-cladded CNTs (CNTs@SiC) reinforced aluminum matrix composites (AMCs) were prepared by variable speed ball milling process-powder metallurgy-hot extrusion route. The results show that the introduction of SiC layer hinders the occurrence of interfacial reaction between CNTs/Al and inhibits the formation of Al4C3 brittle phase which should improve the structural stability of AMCs. The dispersive CNTs@SiC reinforcements play an effective pinning role in the sintering process of AMCs at high temperature, which inhibits the growth and mergence of Al matrix grains. Nano SiC transition layer transforms the reactive interface of CNTs/Al4C3/Al into the diffusion and mechanical interface of CNTs/SiC/Al, which improves the wettability and interface bonding between Al matrix and CNTs@SiC reinforcements. It also acts as a firm bridge to transfer physical information such as stress and phonon, which effectively promotes the load transfer efficiency between Al and CNTs. Accordingly, the tensile strength and elongation of 0.5CNTs@SiC-Al reach 190 MPa and 22.9 %, which shows further improvement in strength-ductility matching for CNTs reinforced AMCs while maintaining the good electrical conductivity and structural stability of Al matrix.

Dynamic recrystallization and strengthening-toughening effects were investigated in the processed near- Ti-xSi alloy by hot extrusion. The yielding and ultimate tensile strengths were enhanced by 138.5 MPa and 118.7 MPa in the hot-extruded Ti-Si alloy compared with pure Ti. The double GSS effects (gradient silicon solutes - gradient shearing strain) led to the bimodal microstructure (21.8 µm deformed strips and 5.2 µm recrystallized grains) and grain refinement by dragging the recrystallization nucleation and grains growth during extrusion. The yielding strength enhancement was resulted from multi-factor Si solid solution, grain refinement and oxygen solid solution. Multi-factor Si solid solution was found to be the dominant mechanism, which was a summation of size misfit, modulus mismatch, strain hardening, narrow slip band and texture strengthening. Obvious strain hardening and late necking occurrence would be positive in obtaining the excellent ultimate tensile strength. Ductility was maintained at 22.9% from strain compatibility and cracks suppression. This study may provide a new understanding on the relationship among the mechanical properties, microstructure and processing of Ti-xSi alloy.

Twinning mechanism of powder metallurgy Ti-Si alloy was investigated by interrupted in-situ tensile tests. Extension twins {10−12} in the fine-grained Ti-Si alloy were found in the uniform deformation period, but no twinning in the coarse pure Ti. Three deformation twinning nucleation mechanisms were proposed: i) local stress concentration by neighboured slip incompatibility, ii) slip/twin oriented relationship in the parent grain and iii) slip/twin transfer by high Luster-Morris oriented relationship. The interior back-stress state, grains rotation and dislocations pile-up drove the occurrence of detwinning phenomenon. Silicon-facilitation twinning verified the hypothesis that the substitutional Si solutes affected the core structures and thus the mobility of screw dislocations. Enhanced driving force and decreased energy barrier of nucleation in the micro/atomic scale were further proposed in the twinning activation. It was expected to deepen the understanding of twinning/detwinning behaviors and supply direct evidences building immature twinning mechanism. In-depth understanding about the relationship among the processing, mechanical properties and microstructure of Ti alloy was facilitated in the present work.

In this study, pure magnesium (Mg) and Mg-0.6 wt% yttrium (Y) binary alloy were fabricated via casting followed by room temperature equal channel angular pressing (ECAP) using an applied back pressure (BP). Microstructural examination after ECAP-BP revealed a fine-grained Mg-Y alloy with a high residual stress level, whereas, the pure Mg exhibited a well-recrystallized microstructure with uniform and equiaxed grains, but retaining very little residual stress. The Y atoms were present in the Mg matrix as solid solutes and acted as dislocation and grain boundary blockers, thus suppressing dynamic recovery and/or recrystallization during the ECAP process. The Mg-Y alloy had an average grain size of ~400 nm, approximately one order smaller than that of pure Mg. The combination of high residual stress and ultrafine grains of the Mg-Y alloy gave rise to a significant difference in its mechanical behavior from that of the pure Mg, under both quasi-static and dynamic compressive loading.

Aluminum composites reinforced with carbon-based nano-particles or fibers have been widely studied. Yet, the rate dependence of their properties has been barely reported. In the present study, CNTs-reinforced Al composites with CNTs of two aspect ratios were produced by different powder metallurgy methods, followed by spark plasma sintering and hot extrusion. The mechanical properties and the underlying mechanisms of CNTs-reinforced Al composites at various loading rates were studied, with the aim of exploring the role of CNTs on strengthening, esp. on rate-dependent properties. The mechanical experiments revealed that the addition of CNTs not only increased the strength but also the strain rate sensitivity in comparison with pure Al. It was found that, under dynamic loading, the materials showed an increased strength and elongation-to-failure simultaneously, due to improved strain hardening rates. A careful analysis suggested that the long-range back stress produced at the CNTs-Al interfaces and the geometrically necessary dislocations accumulated due to strain gradient along the interface, mainly contributed to the strain hardening of CNTs/Al composites. A novel microstructure-based model using microscopically non-uniform dispersion of CNTs was proposed to account for the mechanical properties with better prediction than the traditional models. The results might shed some light on understanding metal matrix composites reinforced with nano-particles or fibers.

A new strategy for the fabrication of γ-Al2O3/Al composites by controlled oxidation method was proposed to regulate Al2O3 content. The content of formed nano-sized γ-Al2O3 increased with prolonging the stepwise oxidation time. Accordingly, the tensile strength of 7.86 vol% γ-Al2O3/Al composite gradually raised from 118 MPa (pure Al) to 385 MPa.

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.

TiB nano-whiskers reinforced titanium matrix composites (Ti-TiBw) with a unique nano-reticulated microstructure was prepared via composite powder by selective laser melting (SLM). The composite powder was designed and produced by electrode induction melting gas atomization as starting materials for SLM other than the conventional premixed Ti–TiB2 powders. The new composite powder had a micron-scale reticulated structure with micro-nano TiBw along the grain boundaries, which was further refined to nano-scale during SLM attributing to the rapid solidification rate. The SLMed Ti-TiBw composites exhibited exceptional strength (σb = 851 MPa) while maintaining good ductility (δ = 10.2%), which was tightly related to our novel strategy of microstructural configuration.

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.

Ti-based materials with high oxygen solute contents were fabricated from mixtures of Ti powder and TiO2 particles by selective laser melting (SLM). Uniformly dissolved oxygen (O) from the TiO2 particles caused c lattice expansion in α-Ti crystals, which effectively increased the strengths of as-built SLM Ti–O materials. The as-built SLM Ti–O material using 1.5 wt% TiO2 showed a yield stress of 962 MPa and 15.3% elongation. The yield stress increases calculated by the Hall–Petch equation and Labusch model were equivalent to those observed experimentally, and O solid-solution strengthening was dominant in increasing yield stress.

In this study, the origin of the high tensile ductility (>10% strain to fracture) of a new class of high-strength (yield strength >500 MPa) aluminium (Al) metal matrix composites (Al MMCs) co-reinforced by ex-situ carbon nanotubes (CNTs) and in-situ γ−Al2O3 nanoparticles was investigated. The microstructures of these unique Al MMCs prior to and post tensile testing were characterized in detail using high resolution transmission electron microscopy, in order to understand their response to tensile deformation. The pathways to the in-situ formation of γ−Al2O3 nanoparticles were identified and attributed to the high energy ball milling process, and the subsequent spark plasma sintering, which converted the amorphous Al-oxide into γ−Al2O3 nanoparticles. The in-situ γ−Al2O3 nanoparticles and Al matrix exhibited a semi-coherent interface, which led to a markedly increased dislocation density in the matrix around the nanoparticles during tensile deformation. These dislocations were precursors to microvoid formation and consequent dimples on further deformation. The co-operation of CNTs and γ−Al2O3 nanoparticles resulted in a long strain-softening stage and therefore excellent tensile ductility. The results suggested that the exploitation of hybrid reinforcement by ex-situ CNTs and in-situ nanoparticles can enable the fabrication of high-performance metal matrix composites.

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.

In view of the research of carbon nanotubes (CNTs) reinforced aluminum matrix composites (AMCs), weak CNTs/Al interface bonding and adverse interface reaction are the key scientific problems which restrict the strengthening effect of CNTs in AMCs. This study proposed a new idea of introducing SiC transition layer between CNTs/Al interface based on the composite configuration and interface structure design of AMCs. The results showed that CNTs-SiC composite powders with controllable structure were successfully synthesized by carefully controlling the ratio between Si and CNTs. The adverse interface reaction was regulated and the interface coalescence between CNTs and Al matrix was enhanced effectively. It was also found that the thicker SiC layer consumed more energy in peeling and fracture during loading due to the carbide-linked inner walls provided stronger interlocked bonding, corresponding stronger ability to transfer the load from matrix to reinforcements and strengthen the load bearing capacity of CNTs in AMCs. Therefore, the mechanical performance of CNTs reinforced AMCs were significantly improved by further achieving the compounding effect of CNTs, which provide an effectual method and basis for the development and application of CNTs reinforced AMCs.

This investigation presents a selective laser melting strategy for fabricating high nitrogen-content Ti (Ti-N) materials. Unlike conventional cast Ti materials, Ti-N materials produced in this study exhibited a good balance of mechanical properties with an ultimate tensile strength of 976 MPa and 21.7% elongation due to solid-solution strengthening by N atoms originating in the Ti2N layers covering pure Ti spherical particles.

In the present work, the effect of strain rate on the mechanical properties, particularly the plastic deformation behavior of a magnesium alloy, AMX602 (Mg–6%Al–0.5%Mn–2%Ca; all wt%), fabricated by powder metallurgy, has been investigated under both quasi-static (strain rate 1×10−3 s−1) and dynamic (strain rate 4×103 s−1) compressive loading. The alloyed powder was extruded at three different temperatures. The microstructure of the alloy was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that AMX602 exhibits an impressive mechanical behavior but with a slight anisotropy along different directions in both strength and compressive ductility (or malleability). The strength was found to be nearly independent of the extrusion temperature, particularly, under dynamic loading. Nanoindentation strain rate jump test reveals a strain rate sensitivity of ~0.018 to ~0.015, depending on the extrusion temperature. Sub-micrometer-scale particles of the intermetallic compound Al2Ca were found with sizes ranging from ~100 nm to ~1.0 μm. These intermetallic particles are believed to have precipitated out during the extrusion process. They contribute to the formation of the ultrafine equiaxed grains which, in turn, help to improve the strength of the alloy by acting as barriers to dislocation motion. Adiabatic shear bands (ASBs) were observed in the dynamically loaded samples, the propagation of which eventually leads to final fracture of the specimens.

Titanium matrix composites (TMCs) were prepared via reactive Ti-B4C system by powder metallurgy and hot extrusion route. The effect of the volume fraction of in-situ synthesized TiC particles and TiB whiskers on microstructure and mechanical behavior of TMCs were investigated. TiB whiskers showed favorable alignment oriented parallel to the deformation axis after hot extrusion; TiC particles and TiB whiskers distributed homogenously throughout the as-extruded composites. σUTS of the synthesized TMCs increased gradually from 654 MPa to 1138 MPa when the volume fractions of hybrid TiC and TiB were raised from zero to theoretical value of 13.6 vol%; the elongation decreased from 32.4% to 2.6% correspondingly. The strengthening effect of in-situ formed TiC–TiB hybrid reinforcements at room and elevated temperature was studied based on the load-transfer mechanism between the titanium matrix and TiC–TiB reinforcements. It was found that synergistic of the TiC particle and TiB whisker showed beneficial strengthening effect on mechanical properties of TMCs.

The mechanical behavior of a lanthanum doped Mg alloy, AZXE7111, (Mg–7Al–1Zn–1Ca–1La, all in wt%) extruded at different temperatures has been investigated under both quasi-static (strain rate ~1×10 −3 s −1 ) and dynamic (strain rate ~4×10 3 s −1 ) compressive loading. Comparison has been made against the experimental results of two conventional Mg alloys, AZ91E and WE43. It was observed via transmission electron microscopy (TEM) that the nanoscale intermetallic compounds of Al 2 Ca and Al 11 La 3 , have presumably formed during the hot extrusion process. These compounds are believed to contribute significantly to the strength by reducing the grain size and acting as dislocation barriers. Additionally, twinning has been considered as the main mechanism for the higher strain hardening rate at high strain rates than that at low strain rates. It has been found that the ultimate strength of the alloy is only ~10% higher at dynamic loading rate than at quasi-static loading rate. Localized micro-shear fracture was observed and adiabatic shear mode was suggested by further examination of dynamically loaded specimens. The shear localization is further discussed in detail and it is suggested that reduced strain hardening rate is responsible for shear localization and subsequent fracture at both low and high strain rates.

Titanium powder metallurgy materials with solid solution nitrogen elements were developed by using pure Ti powder with high nitrogen contents (Ti-(N)), which were prepared via heat treatment from 640 °C to 800 °C in a N2 gas atmosphere. Ti2N compound layers were formed at the Ti-(N) powder surface as a shell, and solid solution nitrogen elements were also detected in the matrix of this powder. Ti-(N) powder was consolidated by spark plasma sintering (SPS), where Ti2N compounds were completely decomposed, and hot extrusion was then used to fabricate fully dense titanium with solid solution nitrogen elements. X-ray diffraction analysis showed that the lattice constant of α-Ti along the c-axis clearly increased in proportion to the nitrogen content due to the nitrogen solid solution behavior. Tensile test results revealed that the powder metallurgy titanium with 0.52 mass% nitrogen exhibited a 0.2% yield stress of 813 MPa and 31% elongation, which were remarkably superior to the strength-ductility balance of pure titanium with a 0.2% yield stress of 305 MPa and 25% elongation. In this study, the Hall-Petch equation and Labusch model were used to quantitatively evaluate the mechanism for the α-Ti grain refinement and solid solution strengthening effects of Ti-(N) powder metallurgy materials.

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.

Ti matrix composites (TMCs) were prepared by using spark plasma sintering (SPS) and hot extrusion from Ti–B4C powder system with different sizes of B4C addition. Tensile properties, O/N contents, grain sizes, microstructures, phase compositions and fractured surfaces were investigated systematically. Then, the effect of B4C particle size on the strength and ductility of TMCs were discussed based on the experimental results and strengthening theory. Results showed that coarse B4C powders led to incomplete reaction and decrease of the amount of both TiCp and TiBw in the Ti matrix, thus resulting in the significant decrease of strength but increase of elongation. The strength improvement of TMCs could be mainly attributed to the strengthening effect of in-situ formed TiCp and TiBw, especially the load-bearing transformation of TiBw. However, the limitation of the TiBw on the deformation of Ti matrix decreased the ductility of the TMCs dramatically.

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.

Fully dense amorphous Al86Ni6Y4.5Co2La1.5 alloy with a 10mm diameter has been fabricated by spark plasma sintering (SPS). It showed record fracture strength of 1250MPa for aluminium alloys. Transmission electron microscopy (TEM) confirmed good interfacial bonding. The activation energy for the SPS process depends on the applied pressure and is substantially higher than that for the self-diffusion of aluminium.

The applications of Ti and its alloys are limited to high-performance products because of the expensive material cost and poor plastic formability. In order to develop a cost-effective processing route for pure Ti and its alloys, pure Ti powder was used as raw material and consolidated by different powder metallurgy routes in this study. Warm compaction and cold compaction were employed to consolidate Ti powder and spark plasma sintering (SPS) was used as a reference method. The obtained compacts were hot extruded subsequently. The microstructures and mechanical properties of the hot-extruded pure Ti were evaluated. It was found that the samples prepared by warm compaction showed a higher ultimate tensile strength of 973.6 MPa, a better elongation of 26% and a higher Vickers hardness of 389.8 Hv compared with the other two methods. Effects of grain orientation, grain refinement and solid solution strengthening on mechanical properties were discussed. It was found that the main strengthening mechanism for the sample prepared by warm compaction was oxygen solid solution strengthening resulting from in-process in this study. The strengthening effect of oxygen solid solution was calculated as 769.8 MPa/mass% [O].

Effect of extrusion temperature on the microstructure and mechanical properties of the powder metallurgy Cu40Zn–CrFeTiSn alloy were investigated, through analysis on the solid solution of alloying elements, phase transformation and precipitation behavior. The extrusion was carried out between 500 °C and 650 °C using an extrusion ratio of 37:1. After extrusion, the microstructure was found consisting of the dual α + β phase structure and precipitates dispersing uniformly in the matrix. The tensile strength gradually decreased when the extrusion preheating temperature was increased. Solid solubility of Ti and Cr decreased steadily and grain coarsening occurred with increase in the extrusion temperature. Consequently, the lower extrusion preheating temperature was necessary to obtain excellent final mechanical properties, thus the extrusion temperature of 500 °C was enough to produce good ductility while maintaining the promising values of mechanical properties after extrusion.

The novel fine-grained and coarse-grained non-combustive AMX602 (Mg–6% Al–0.5% Mn–2% Ca) magnesium plates with average grain sizes of 1.06 and 6.14 μm respectively were subjected to the friction stir welding. The effect of the initial grain size on the microstructure and mechanical properties of the joints were studied. The results showed that the grains growth occurred in the stir zone of the fine-grained specimens after welding. However, grain refinement occurred in the coarse-grained counterpart. The texture evolution of the fine and coarse grained materials joints had similar trends but with different texture intensities. It is suggested that the initial grain size affects the texture intensity through the slip inhomogeneity caused by different amount of grain boundaries. The microhardness of the fine-grained material joints was lower than that of the base material, whereas it became higher in the coarse-grained specimens. The results of the tensile test indicated that the grain size and grain orientation of the joints had an effect on the mechanical properties and fracture locations of the joints. Although both the fine-grained and coarse-grained material joints had similar yield strengths, the fine-grained specimen had a higher ultimate tensile strength of 261.6 MPa and the coarse-grained specimen had a relatively lower one of 203.6 MPa.

The effects of Ti and Sn alloying elements on the microstructural and mechanical properties of 60/40 brass were studied by powder metallurgy processing. The super-saturated solid solution of Ti creates a high precipitation reaction chemical potential in water-atomized BS40-1.0Ti brass powder. Consequently, BS40–1.0Ti brass was remarkably strengthened by the addition of Ti. However, Ti readily segregated in the primary particle boundaries at elevated temperatures, which detrimentally affected the mechanical properties of BS40–1.0Ti brass. Accordingly, Sn was proposed as an additive to BS40–0.6Sn1.0Ti to inhibit the segregation of Ti. Consequently, the Ti precipitate was retained in the form of CuSn 3 Ti 5 in the interior of grains and grain boundaries rather than in the primary particle boundaries. This result demonstrates that the addition of Sn can effectively hinder Ti segregation in the primary particle boundaries. Sn addition produced significant grain refinement and mechanical strengthening effects in BS40–0.6Sn1.0Ti brass. As a result, outstanding strengthening effects were observed for BS40–0.6Sn1.0Ti sintered at 600 °C, which exhibited a yield strength of 315 MPa, an ultimate tensile strength of 598 MPa, and a Vickers micro-hardness of 216 Hv. These values represent increases of 27.5%, 20.1% and 45.6%, over those of extruded BS40–1.0Ti brass.

High-strength, lead-free machinable α–β duplex phase brass (Cu–40Zn) with 0.3 mass% chromium, 0.2 mass% iron, 0.6 mass% tin, and 1–3 mass% bismuth (Cu–40Zn–Cr–Fe–Sn–Bi) were prepared using a casting process, and their microstructures, mechanical properties, and machinability were investigated. Cast Cu–40Zn–Cr–Fe–Sn–Bi exhibited α–β duplex phase structures dispersed with Cr–Fe intermetallic compounds (IMCs) and spherical Bi particles that existed in the β-phase. The Bi particle size in this phase was smaller than that of irregularly shaped Bi particles in or around the α-phase; thus, cast specimens with large area fractions of the β-phase had more finely dispersed Bi particles. Furthermore, the additional Bi did not react with the added solid solution strengthening elements of Cr, Fe, and Sn. Conversely, the extruded Cu–40Zn–Cr–Fe–Sn–Bi consisted of fine, uniform α–β duplex phases dispersed with fine, discrete Cr–Fe IMCs, and the Bi particles were also slightly elongated along the extrusion direction. Analysis of back-scattered SEM images determined that the number of Bi particles in the wrought alloy matrix was 1500–3000/mm 2 in the transverse cross-section of the extrusion direction. The average yield strength (YS) and average ultimate tensile strength (UTS) of the extruded Cu–40Zn–Cr–Fe–Sn–Bi alloy were 288 MPa and 601 MPa, respectively. Based on the similar tensile properties of this alloy to those of Cu–40Zn–Cr–Fe–Sn, the main strengthening mechanism in the former alloy was due to solid solution strengthening with elemental additives and the increased area present as the hard β-phase. Furthermore, this extruded alloy exhibited an increase of 29% YS and 40% UTS compared to traditional machinable brass Cu-40Zn with 3.2 mass% lead (Cu–40Zn–Pb). The machinability of the extruded Cu–40Zn–Cr–Fe–Sn–Bi was also 25% lower than that of the Cu–40Zn–Pb alloy.

Laser welding characteristics of a hot extruded AZ31B magnesium alloy produced by rapid solidification powder metallurgy (P/M) were investigated. Extremely porous weld metals were formed under all the employed welding conditions although the base material possessed a fully dense microstructure without pores. Features of porosity morphology in the weld metals as well as X-ray transmission real-time observation results of the molten pool during welding implied that pores originated from the base material. Investigation of the chemical compositions of the base material revealed the presence of a small amount of oxygen and nitrogen. However, the high pressure applied in the initial production processes of the base material caused these gases to expand to large volumes once released when the material was melted during welding, resulting in the formation of large pores. The use of insert-layer of conventionally extruded AZ31B alloy did not improve the situation because a large amount of porosity was formed even when melting of the P/M alloy was minimized.

Carbon nanotube (CNT) is an effective reinforcement used to improve the mechanical and thermal responses of metal matrix composites. It is, however, obvious that segregation of CNTs due to their strong van der Waals forces will produce material defects, decreasing the material properties. An advanced powder metallurgy process that disperses un-bundled nanotubes has been developed, and it is applied to fabricate a Mg matrix composite reinforced with CNTs in the present study. When approximately 1 vol.% CNTs were added, the extruded pure Mg and AZ31B alloy composites displayed an extremely large increase of the tensile yield stress of 25–40%, compared to Mg materials containing no CNT. HR-TEM verified the presence of MgO thin layers of 2–4 nm thickness, originating in the oxide surface films of the raw Mg and its alloy powders. These layers exist at the interface between α-Mg and the un-bundled nanotubes. The oxide layer showed a homogeneous mixture containing both α-Mg and CNT. This mixture resulted in an effective tensile loading transfer at the interface, which significantly improved the tensile strength (TS) and yield stress (YS) of the Mg composites. However, the elongation was less than 5%, and the composites exhibited a very poor ductility. This was because the MgO layers at the interface between α-Mg and CNT were very ductile.

In order to improve and evaluate the wear behavior of the sintered magnesium materials under dry sliding conditions, the effect of carbon nanotubes (CNTs) and Mg2Si/MgO compounds, which were reinforcements of the sintered material, on the friction coefficient and wear loss under dry conditions was discussed. One of the raw materials was amorphous and porous silica particles originated from rice husks, which were coated with CNTs and contained nanotubes in the pores. The in situ synthesis of Mg2Si and MgO via deoxidization and oxidation reaction occurred from the elemental mixture of pure magnesium and CNT-SiO2 composite particles via the spark plasma sintering process. The friction coefficient of the composite material was low and stable because of no adhesion and stick-slip phenomenon in contacting with the SUS304 stainless steel ball as a counter material. The wear rate of sintered magnesium materials decreases in increasing the content of CNTs and Mg2Si. The friction coefficient was proportional to the total wear loss of magnesium materials. These results were due to both of the defensive by hard Mg2Si dispersoids and self-lubricant effect by network nanotubes on the sliding surface.

The cavitation erosion resistance of P/M aluminum alloy sintered composite with AlN dispersoids, prepared via the in situ synthesis and the conventional premixing process, was evaluated by using a magnetostrictive-vibration type equipment. In situ synthesized AlN particles were effective for the improvement of the erosion resistance of the composite because of their good bonding with the aluminum matrix. The additive AlN by the premixing process were easily detached from the specimen surface due to the insufficient coherence with the matrix, and caused the poor resistance. The cavitation resistance also depended on the porosity of the sintered composite. The continuously opened pores accelerated the wear phenomena by the cavitation due to the high pressure attack on the primary particle boundaries of sintered materials in the collapse of the bubbles.