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Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic (FCC) structure, whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature (Trx). Microscopically, this hardening effect has been ascribed to several mechanisms, i.e. solute segregation to defects (dislocation and stacking fault) and short-range chemical ordering, etc. However, none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys (HEAs). Here we report the observations, using high-resolution electron channeling contrast imaging and transmission electron microscopy, of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below Trx. The dislocation substructures are observed to be thermally stable up to Trx, which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs. The microstructure feature is markedly different from that of conventional dilute solid solution alloys, in which dislocation substructures gradually vanish by recovery during annealing, leading to a strength drop. Furthermore, dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed (≤500 °C) HEA. This Al induced softening effect, could be associated with the anisotropic formation of dislocation substructure, resulting from enhanced dislocation planar slip due to glide plane softening effect. These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.

A Cu-10 Ni-2Ti wt.% alloy has been fabricated by induction melting and casting. The microstructure evolutions of the studied alloy treated with thermal-mechanical treatment were characterized. After homogenized at 950 °C for four hours, hot rolling by 80% at 900 °C solid solution treated at 970 °C for four hours, cold-rolled by 50%, aged at 450 °C for one hour, the hardness, electrical conductivity, tensile strength, yield strength, and the elongation of the studied alloy approached 181 HV, 22.1% IACS, 576 MPa, 486 MPa, and 14.6%, respectively. The studied alloy shows large products of strength and elongation (15.4 GPa·%, 9.3 GPa·%, 8.41 GPa·%, and 10.1 GPa·% for the samples treated with a solid solution, under aged, peak aged, and over-aged states), which are much higher than those of other high strength conductive copper alloys. These findings are helpful in designing new high damage tolerance conductive materials. Keywords: Cu alloy, Damage tolerance, Precipitation, Strength, Microstructure

Three Ni-base powder metallurgy superalloys have been developed recently, and tensile tests at temperatures ranging from room temperature (RT) to 815 °C were conducted on them. The results conformed their excellent tensile properties, in comparison with several other existed polycrystalline superalloys. In this work, by means of microstructure characterization, thermal dynamic calculations, and theoretical modeling, different strengthening mechanisms including precipitation strengthening, grain boundary strengthening, solid solution strengthening, and Orowan strengthening, were found to contribute to the yield strength in different degrees, which would help to further enhance the tensile properties of these alloys through composition design and processing optimization thereafter.

Corrosion behaviours in hydrofluoric acid solution and mechanical performances of Ni-16Cr-11Mo-6Fe-2Cu and Ni-30Co-16Cr-11Mo-6Fe-2Cu alloys were investigated by using a variety of methods. No precipitate is observed in the present low Mo alloys, suggesting the excellent structural stability. Co leads to grain refinement and localized microstructure after hot rolling, resulting in significant enhancement in both hardness and tensile strength. Owing to the addition of Cu, all alloys with and without hot rolling show near weight loss rates after immersion. This is ascribed to the dominant passive film of Cu, which demonstrates the superior stability among the constitute elements of alloys.

To investigate the microstructure evolution of powder metallurgy (P/M) superalloys during high-strain-rate deformation, a P/M nickel-base superalloy is subjected to thermal compression at temperatures ranging from 1000 °C to 1100 °C and strain rates ranging from 1 s−1 to 10 s−1. The analysis regarding the flow behaviors and microstructure observation indicates that discontinuous softening is significant at high strain rates. Moreover, based on experimental measurement and theoretic estimation, the adiabatic heating under high-strain-rate deformation is hardly neglectable, which enhances the dynamic softening via dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. In order to obtain refined and homogeneous grains, deformation at temperature range of 1025–1050 °C with strain rate rang of 1–10 s−1 or at 1000 °C/1 s−1 is suggested for high-strain-rate compression on the P/M superalloy, where DRX plays more dominating roles.

In this study, single and multi-step aging treatments (SAT and MAT) following a super-solvus solution treatment were employed to tailor the mechanical property of a prototypical powder metallurgy (PM) nickel-based superalloy. Vickers hardness and tensile test at room temperature were used to evaluate the mechanical properties after different heat treatments. Microstructural evolution after heat treatments and tensile tests were revealed through scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) techniques. Our results show that the MAT with a stabilization at 650 °C for 24 h between the solution and aging assists to achieve the highest strength corresponding to microstructure of bimodal distribution of γ’ precipitates. The associated mechanism for variation in microstructures and mechanical properties are discussed based on thermodynamic calculations and precipitation strengthening theory. In addition, an empirical relationship between Vickers hardness and tensile strength of nickel-based superalloys is examined which is believed to be assistance for rapid evaluating mechanical response of materials with different microstructures.

In the present work, the effects of Mo/Cr addition on the microstructure and the resultant property of cemented carbides were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) and Vickers hardness test. The results show that addition of Mo/Cr elements contributed to a significant improvement of WC-Co cemented carbides hardness. The enhanced hardness is ascribed to the decreased grain size of WC with increasing Cr content. In addition, Cr element addition would promote the segregation of M7C3 phase surrounding the η-phase, which inhibits the growth of η-phase, and results in finer and more disperse η-phase. The evolution mechanism of η-phase and the segregation of M7C3 phase during sintering process were discussed systematically.

Optimizing hot workability and processing parameters of the difficult-to-work powder metallurgy (P/M) superalloy is critical to obtain a desired component through thermo-mechanic processing (TMP). In this study, a P/M superalloy with tailored grain structure and γ′ particle was prepared through atomization and hot isostatic pressing, then thermal compression tests were performed to evaluate its hot workability based on processing maps and microstructure evolution. Considering the friction and temperature variations during deformation, the flow data were corrected via an integrated method, to construct dependable processing maps. The results illustrated that the P/M superalloy prepared by the modified processing method presents superior hot workability, comparing with the P/M superalloys produced by different routes.

To understand how to tailor microstructure of high-Co polycrystalline superalloys during hot processing, the flow curves of a nickel-based superalloy with high Co content was quantitatively analyzed, and the microstructure evolution was studied by a high-throughput method. The results suggested that hot working conditions, especially the temperature, strongly influenced the grain structure at annealing. In specific, deforming under low strain rate and high temperature conditions facilitated the recovery and grain growth to consume the stored strain energy, furthermore, the weakened pinning effects of γ′ precipitates accelerated these procedures, which made the high-Co superalloy more vulnerable to the formation of abnormally large grains during subsequent supersolvus annealing.

The tradeoff between the strength and the fracture elongation in the high-strength Cu–Ni–Si alloy became a hot research topic recently. Cu–Ni–Si–(Ti) alloys were fabricated in a vacuum induction melting furnace to study the effects of titanium on the microstructure and mechanical properties of Cu–Ni–Si alloys with different thermo-mechanical treatments. After homogenization at 900 °C for 4 h, hot-rolled by 80%, solution treatment at 970 °C for 2 h, cold-rolled by 50%, and finally aged at 450 °C for 180 min, the studied Cu–10Ni–Si–2Ti alloy achieved the hardness of HV 252.4, electrical conductivity of 23.6% IACS, tensile strength of 764.4 MPa, yield strength of 622.26 MPa, fracture elongation of 10.4%, and strength-elongation product of 7.95 GPa%, which are less than those of the studied Cu–10Ni–2Si alloy. The addition of Ti contributed to refining the microstructure, suppressing the decreasing trend in mechanical properties after peak hardening, and arousing a primary substructure strengthening mechanism rather than the precipitation strengthening in Cu–Ni–Si alloys. These findings provide essential understandings of the effects of the Ti on Cu–Ni–Si system alloys, and the designed Cu–Ni–Si alloys with high-strength and fracture elongation could fulfill some requirements of the electronic and electrical industry.

As an advanced material forming method, additive manufacturing (AM) has been rapidly developed in many industry fields. Due to the non-weldability of nickel-based single crystal (SX) superalloys and complex solidification and stress conditions of AM process, hot cracks can easily form in depositions. Previous research focused on the additive manufacturing of single-track thin-walled SX samples, while effective solutions to fabricate crack-free multi-track SX block samples are still limited. Directed energy deposition (DED) technique was used to fabricate multi-track block samples in this work. The effects of the overlapping ratio, scanning velocity, and laser power on the microstructure and cracking susceptibility were analyzed. The experimental results demonstrated that improper process parameters play significant roles in the formation of high-angle grain boundaries and large internal stress, both of which will increase the cracking susceptibility. Crack-free SX multi-track block was successfully fabricated after adjusting the process parameters. These findings provide a guide to reduce the cracking susceptibility by controlling the process parameters, which aids in fabrication of large-scale crack-free SX blocks through DED technique.

Owing to its high specific strength and low density, Al&ndash, Cu alloys have been extensively used in aerospace for lightweight components. Additive manufacturing techniques such as selective laser melting, which offers geometric freedom, is suitable for topology-optimized designs. In this study, the effect of processing parameters on the densification, microstructure, and mechanical properties of additively manufactured Al&ndash, Cu alloy 2124 by selective laser melting was investigated. Parameters such as laser power, scanning speed, hatch spacing, and use of a support were studied. The results revealed that a grille support with a hollow structure played a resistant role in the transfer of heat to the base plate, thus reducing the temperature gradient and lessening cracks in the building part. Smaller hatch spacing was beneficial for the achievement of a higher relative density and strength due to track re-melting and liquid phase backflow, which could fill cracks and pores during the building process. An ultimate tensile strength as high as 300 MPa of the vertically built sample was obtained at optimized processing parameters, while the elongation was relatively limited. Moreover, columnar grains were found to be responsible for the anisotropy of the mechanical properties of the as-printed 2124 alloy.

Mo, as one of the most important elements in superalloy, dramatically affected the microstructure and mechanical properties via solution strengthening, forming Mo-enriched phase, and atomic segregation (Suzuki effect). To verify the influence of Mo on the microstructure and mechanical properties of Ni–Co-based superalloy, two types of alloys containing 4 wt% (A4) and 8 wt% (A8) Mo were prepared. The microstructure was examined using X-ray diffraction, scanning electron microscope, and transmission electron microscope, while the mechanical properties were evaluated using tensile tests at both room temperature and elevated temperature (700 °C). The segregation of Mo at stacking faults was observed in both alloys after aging with pre-cold-rolling. At the same time, in alloy A8, direct precipitation of μ phase both along grain boundary and in the interior of matrix was observed after aging. In alloy A4, segregation of Mo along stacking fault ribbons resulted in formation of quasi-μ phase. The quasi-μ phase developed into μ phase during prolonged aging. There was no precipitate along grain boundary in A4 after aging resulting in poor ductility at both room and elevated temperature. The strength was enhanced by net effect of Suzuki segregation, γ′ phase strengthening, and fine μ phase.

Inconel 718 alloy rods were fabricated by electron–beam melting (EBM), where the cylindrical axes (CAs) deviated from the build directions (BD) by 0°, 45°, 55°, and 90°. The microstructures and high-temperature tensile properties of the rods were investigated by taking into account the effect of the BD. Columnar grain structures or mixtures of columnar and equiaxed grains were obtained in the rods. The direction of the grains tended to be oriented not only in the BD but also in the two horizontal EB scanning directions (SDs), which were perpendicular to each other. As a result, the crystal orientation of the rods could be controlled by appropriate choice of the CA. For instance, the rods oriented along the , , directions could be fabricated by choosing their CAs parallel to the BD, face diagonal, and space diagonal of the cubic space defined by the BD and the two SDs, respectively. The highest strength was obtained for the oriented rod. The dependence of strength on the rod orientation could be explained in terms of the anisotropies in the crystal orientation, columnar grain structure, and arrangement of the precipitate particles.

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.

Explosive dynamic recrystallization (DRX) of Co-29Cr-6Mo-0.16 N (CCMN) alloy with extremely low stacking fault energy (SFE) was observed in rapid hot-forging processes; the DRX was considered to be closely related to the prevailing planar-defects formation in parent grains such as mechanical twin and stacking fault bands as well as their interactions, that finally break the parent grains into fine pieces accompanied by instantaneous reorientation and recovery in the refined grains.

Powder metallurgy superalloys are widely used to produce critical rotating parts in turbine engines. However, the issues like what is the driving force of precipitation on prior particle boundary (PPB) during consolidation and how the PPB structure evolves during plastic deformation still remain to be resolved. In this study, with the characterization of the microstructure evolution from powder, consolidation, to plastic deformation, the segregation of alloying elements on the particle boundaries at elevated temperature was analyzed by equilibrium thermodynamic calculations, and the distortion of PPB structure under large plastic deformation was quantitively simulated by finite element (FE) methods. The results indicated that surface energy serves as the main driving force of PPB precipitation and hot extrusion could distort the PPB structure and yield more uniformly refined grains.

We combine pre-compressive test, reverse tensile test, re-compressive test, in-situ electron back-scattered diffraction, and high-resolution transmission electron microscopy to systematically investigate the effect of annealing on the reciprocal motion of twin boundary (TB) in pure Mg and Mg alloys AZ31 and AZ91. We find that the twin boundary mobility (TBM) can be enhanced by decreasing the dislocation density and increasing the number of coherent TBs after annealing for a short time. On the other hand, after prolonged annealing in Mg alloys, TBM decreases since TBs are stabilized by segregated solute atoms and precipitates. As a result, the TBM significantly depends on both the alloying element content and the annealing time. We demonstrate, for the first time, that friction stress and back stress can be applied to clarify the variation of TBM during annealing in Mg alloys. Our findings show that the TBM can be regulated by annealing, opening up a novel avenue for developing Mg alloys with high damping capacity or enhanced mechanical properties.

A novel Co-Ni-based superalloy was manufactured by powder metallurgy (PM) process. The microstructure and mechanical properties of PM alloy were analyzed in detail and compared with the corresponding casting alloy. After cold swaging to 66% and subsequent aging for 800 °C for 3 h on solution treated (ST) PM alloy, the yield strengths of PM superalloy can be doubled and tripled to be 1500 and 2265 MPa respectively, as compared with 700 MPa of ST PM alloy. In contrast, for the same casting alloy after subjected to similar treatments, their corresponding yield strengths were 1100 and 1650 MPa, respectively, which are much lower than these of PM alloy. Strengthening mechanism of both PM and casting alloys were quantitatively calculated, unmasking that the higher mechanical property of PM alloy can be mainly ascribed to the higher contribution from grain boundary and precipitation strengthening.

In this work, the high temperature creep properties of three newly developed P/M superalloys with fine grains were investigated under conditions of 650 °C/1100 MPa, 750 °C/690 MPa, and 815 °C/550 MPa. The creep behavior associated with microstructure were analyzed, considering the interactions of dislocation with grain boundaries and γ′ phase. The results indicated that the cracks were more likely to initiate at grain boundaries, resulting in intergranular cracking. Moreover, the dislocation movement in grains was strongly related with testing temperature and applied stress, which is explained by theoretical calculation on the composition-dependent critical stresses (CSs) to trigger various mechanisms, including dislocation shearing the γ′ phase by stacking faults (SFs) or APB coupled dislocation pairs, and dislocation climbing.

In this study, microstructures, mechanical properties, and electrical conductivities of powder metallurgy (PM) Cu-0.28Cr-0.19 Mg alloy after cold-rolling and aging were investigated in detail. PM Cu-Cr-Mg alloy exhibits a combination of both higher mechanical properties and superior electrical conductivity compared to the corresponding casting Cu-Cr-X alloys. Such excellent performances are closely related to the more uniform and finer microstructures generated in PM process. Strengthening from second phase precipitation, grain boundary, and dislocation in the present PM Cu-Cr-Mg alloy were calculated to be approximately 43.65, 24.50, and 23.12 % of yield strength, respectively. The grain boundary strengthening is the main reason for the higher strength of the present PM Cu-Cr-Mg alloy and is approximately 325 % higher than that of the corresponding casting Cu-Cr-Mg alloy. The gradually decreased activation energy of PM Cu-Cr-Mg alloy with increasing reduction rate suggests that deformation significantly accelerates the precipitation behavior during the subsequent aging process.

In the present research, the microstructure and phase composition of high-Nb TiAl powder, produced by plasma rotating electrode process (PREP) have been investigated in details. PREPed powder with a mean particle size larger than 150 μm demonstrates a dendrite structure, and that smaller than 75 μm indicates a martensitic structure. PREPed powder is mainly composed of a hexagonal a2 phase, while in fine powder and coarse powders minor β phase and γ phase are detected, respectively, which is related to solidification path and cooling rate of molten droplets in PREP process. Phase composition and microstructure of the high-Nb TiAl powder have intrinsic effects on the resultant microstructure of hot isostatic pressed (HIPed) billets. After HIP processing, most martensitic and dendritic particles transformed into near γ structure, while few coarse dendritic powder was prone to introduce defects, such as residual primary particle boundaries (PPBs) or coarse lamellae structure, which were harmful to the mechanical properties of HIPed billets. The residual primary particle boundaries (PPBs) or coarse lamellae structure obviously decreased the ultimate tensile strength at room and elevated temperature. Keywords: Microstructure, High–Nb TiAl, Powder, Densification, Mechanical properties

Electrochemical measurements in 1 M HCl solution were performed to investigate the corrosion behaviour of Ti–6Al–4V alloy fabricated by electron beam melting (EBM) with cylindrical axes deviating from the building direction by 0°, 45°, 55°, and 90°. Microstructure characterization before and after the tests was carried out by using a variety of methods The results suggested that the corrosion resistance of EBM alloy in 1 M HCl solution increased slightly in the order of 45°, 90°, 55°, and 0°. Such difference can be ascribed to the grain boundary density and β phase constitution, which varied with the building direction.

In this work, Cu-based microcomposites were fabricated by powder metallurgy of gas-atomized Cu-15wt% Fe alloy powder, followed by drawing to various strains. The microstructure evolution and the resultant mechanical and electrical properties were investigated by scanning electron microscopy, electron backscatter diffraction, and tensile testing, among other techniques. The results indicated that during drawing, the Fe phase evolved into nanoscale filaments along the longitudinal direction. With increasing drawing strain, both the Cu grains and the Fe filaments were refined gradually, giving rise to a mean thickness of approximately 25nm for the Fe filaments at a total strain of 4.0. The iron oxides particles formed due to contamination of oxygen maintained unchanged during drawing process. In addition, it was found that an intermediate aging during the drawing can significantly enhance the electrical conductivity of the microcomposites without sacrificing their strength. The present process involving an initial powder with a homogenous distribution of Cu and fine Fe phases makes it possible to obtain microcomposites with an ideal combination of electrical and mechanical properties at lower drawing strain, compared to those produced by conventional casting methods. Keywords: Cu-Fe microcomposite, In situ technique, Hot consolidation, Electrical conductivity, Strength, Microstructure evolution

In the present research, the influence of Co addition on the microstructure and hot workability of IN713C alloy was investigated in detail by scanning electron microscope (SEM), transition electron microscope (TEM) and electron backscattering diffraction (EBSD) etc. Substitution of Ni by Co leads to lower driving force for formation of γ′ phase. In addition, with the increase of Co fraction, the γ′ phase precipitates turned finer and more cuboidal shape. As a result, the hot workability of IN713C alloy was improved significantly by Co addition. This can be ascribed to the decrease in activation energy, misfit between γ/γ′ phase lattice, the stacking fault energy as well as the fraction of γ′ phase at elevated temperature after adding Co. Keywords: Cobalt addition, Hot workability, DRX, Superalloy, SFE, Misfit

By combining finite element analysis, compressive tests, reverse tensile tests, in-situ electron-backscattering diffraction, and high-resolution transmission electron microscopy, we investigate systematically twinning and detwinning behaviors of {10 1 ¯ 2} twins in pre-compressed pure Mg and AZ31 and AZ91 alloys. We find that the solute elements Al and Zn and the resultant precipitates disrupt the synchronous motion of atoms in twinning boundaries (TBs) during twin formation, yielding a large amount of incoherent TBs and low-angle boundaries from the pre-existing TBs after detwinning. The detwinning in pure Mg and its alloys is found to be linked to both twin boundary (TB) mobility and the interaction of their boundaries with dislocations. At a low strain level (0–1%), comparable detwinning activities in both pure Mg and its alloys are observed due to the selective detwinning in grains with the highest SF, while at high strain level (1–4%), the contribution of detwinning in Mg alloy is higher. This is because a small twin size in Mg alloys leads to strong interactions between basal slip and TBs, and also because in pure Mg, twin size is much larger and its tip is pinned by grain boundaries. In addition, the secondary twins are also found to be formed remarkably in the interior of the primary twins in Mg alloys with a high solute-element concentration owing to the low mobility of incoherent TB and the variation of local stress state by precipitates. The ultimate strength of reverse tensile test is found to increase more substantially in the pre-compressed Mg alloys in comparison to pure Mg, especially in the AZ91 alloy, which is attributed to the hardening effect caused by the formation of many low-angle and secondary TBs and also for the reduced Schmid factor for the basal slip in Mg alloys during detwinning.

This study investigates the influence of pre-fatigue on the bending behavior of needle-punctured 2.5D C/C composites with density between 1.75 and 1.80 g/cm3 prepared by chemical vapor deposition (CVD). Three-point bending pre-fatigue tests are carried out under applied stress/static flexural strength ratio of 60%, 70%, 80% and 90% up to one million cycles. The results show that except for the 60% sample, flexural bending strength of most samples subjected to pre-fatigue is improved significantly compared with that without pre-fatigue. Detailed investigation indicates that the subsequent bending behavior of sample is closely related to both stress level in pre-fatigue and the resultant microstructure. Under low stress level (60–70%) during pre-fatigue, the composite is characterized by a plastic deformation and a slight enhancement of bending strength ascribed to the weakened fiber/matrix interface after fatigue. After pre-fatigue with high stress level (80–90%), the composite demonstrates brittle failure mode but much higher residual strengths during subsequent bending test. The reason for this is ascribed to the variation of the microstructure in fiber/matrix interface during the pre-fatigue, which strongly affects the subsequent mechanical responses.

In this study, biocompatibility of Co-29Cr-5Mo (CCM), 316L steel (316L) and Ti-6Al-4V (TC4) alloys after surface plastic deformation under the condition comparable to the human ankle activities were investigated in details. Biocompatibility of all alloys decreases after surface deformation, while it is most significantly observed in CCM alloy. The different responses of biocompatibility are related to the corresponding microstructure evolution during surface deformation: martensitic phase transformation, dislocation slipping and mechanical twinning in CCM alloy result in the extremely localized microstructure, giving rise to the obviously decreased corrosion resistance or biocompatibility; quite homogenous microstructure after surface deformation leads to the slightly decreased corrosion resistance or biocompatibility in both 316L and TC4 alloys, since the surface deformation is dominated by slipping in 316L and by both slipping and a few mechanical twinning in TC4.

Microstructure and mechanical properties of novel Ni-20Co-12Cr superalloys, with and without Nb addition, were systematically studied during long-term thermal exposure. With increased exposure time, the average diameter of the γ′ precipitates increased in both alloys in succession, this is more obviously observed in alloy containing 1 wt% Nb (1Nb). It is suggested that Nb increased the γ′ coarsening rate by accelerating the diffusion of Al and Nb in γ matrix. In addition, the γ′ phase fraction is increased by about 4% in 1Nb compared to the alloy without Nb (0Nb). The morphology of the γ′ phase changed from near-spherical to cuboidal shape during exposure in both alloys. Due to the increased γ/γ′ lattice misfit by Nb addition, 1Nb alloy showed an earlier tendency of shape change. Vickers hardness results revealed that the overall hardness decreased with the exposure time because the size increment of the γ′ precipitate weakened the precipitates strengthening and Orowan strengthening.

The tradeoff between the strength and fracture elongation in the high strength Cu-Fe alloys has been a research focus recently. A Cu-10 wt.%Fe-0.5 wt.% Nb alloy was designed and fabricated by casting and thermomechanical treatment to high strength and elongation. Microstructure evolutions and properties variations of the studied alloy during cold-drawing and cold rolling were investigated. After annealed at 400 ℃ for 1 h and further aged at 450 ℃ for 8 h, the ultimate tensile strength, elongation, product of the strength times the elongation (PSE), and electrical conductivity of the cold-drawn studied alloy were 736 MPa, 36 %, 22.82 GPa %, and 48.74 %IACS, respectively. Nanocrystals, Nb precipitates, and Fe fibers were detected in the studied alloy, which contributed to the high strength and the high fracture elongation. These findings provide essential understandings of the effects of the niobium on Cu-Fe system alloys, and the designed Cu-Fe-Nb alloy with high strength and fracture elongation could fulfill some requirements of the electronic and electrical industry.

The dynamic recrystallization (DRX) behavior of the biomedical Co-29Cr-6Mo-0.16N (CCMN) alloy was analyzed in detail by performing compressive tests at strain rates of 1.0 s− 1 and 30 s− 1, with the temperature ranging from 1273 K to 1473 K. It was found that the deformation bands in the non-DRXed region were stacking fault (SF) bands along the {111} planes that intersected each other or with the pre-existing grain boundaries even at low strain levels, and the dominant DRX mechanism was ascribed to the repeated formations of Σ3n boundaries from these dense SF bands. High strain rates were very effective for obtaining a fine and fully DRXed microstructure. The Σ3n boundaries evolve into general high-angle grain boundaries (HAGBs) because of the external stress applied to the sample during compression, and it becomes difficult for these boundaries to form inside DRXed grains at high strains because of the finer grain size obtained as a result of the DRX process.

Dynamic recrystallization (DRX) behaviors of biomedical Co-29Cr-6Mo-0.16N (CCMN) alloy were analyzed in detail in hot compressive tests. At low temperature (≤1050 °C) and low strain rate (≤1 s −1 ), DRXed grains preferentially nucleate along pre-existing grain boundaries or Σ3 n boundaries, with a result of necklace microstructure broadening with increasing strain; while at higher temperature and/or higher strain rate, DRX occurs uniformly in matrix resulting in homogenous and texture-free fine grained microstructure. These two kinds of DRXed microstructures formed in CCMN alloy are related the repeated formation of annealing twin during hot deformation, which was determined by both the temperature and the activity of stacking fault formation. It was also found that the Σ3 n boundaries formed at lower strain level will evolve into general high angle grain boundaries (HAGB) with further straining. A schematic model for DRX mechanism of CCMN alloy was proposed with respect to temperature, strain rate, and strain.

We created high-aspect-ratio sub-micron-sized lamellar porous structure by simple anodic selective dissolution of lamellar Ti-Al alloy in NaCl aqueous solution. The geometries of lamellar structures, which determined those of lamellar porous structure, were controlled by hot forging and subsequent ageing treatment before dissolution process. Although the changes in microstructure affected the selective dissolution behaviors, γ-TiAl phase could be dissolved selectively. As a result, the average sizes of unidirectional units of lamellar pores could be controlled in the range from to 25 μm to 500 μm. The pitches of the lamellar pores were controlled in the wide range from below 100 nm to over 1 μm. Keywords: Aluminum, Titanium, SEM, XPS, Anodic dissolution

Detailed metallurgical investigations have been performed on a used Co-Cr-based metal-on-metal (MoM) hip joint bearing containing a type of liner that is commonly used in such joints. The damage on the metal-liner sliding surface was considerably more severe than that on the metal head counterpart, in terms of wear-scar density and width and microcrack frequency. Cross-sectional transmission electron microscopy revealed that a thick (>3 μm) nanocrystalline layer formed on the sliding surface of the head, whereas the liner had coarse carbides embedded in it and nanocrystals were formed in a very limited region no deeper than 1 μm. Comparative investigation of an unused head and a liner of identical type showed that although the chemical compositions of the liner and head were nearly identical, their microstructures were significantly different. Specifically, the grain size in the liner was larger than that in the head on average, and the grain boundaries of the liner were decorated with coarse carbides. Moreover, X-ray diffraction analysis revealed a large tensile residual stress only in the liner. These differences are possibly responsible for the wear damage on the liner being more serious than that on the head.

In this study, copper-based composites containing nanoscale TiC with high strength and high electrical conductivity (712 MPa and 72% IACS) were produced by a newly developed mechanical milling process. As-milled powder mixtures were investigated by X-ray diffraction (XRD) analysis. The results indicated that the lattice parameters of copper were increased with progress of milling due to the formation of solid solution of Cu (Ti, C). There was no transformation of Ti and C into TiC phase during the high energy milling process. It was found that the TiC particles were firstly formed during the sintering process. The effects of SPS parameters including sintering temperature and pressure on electrical and mechanical properties of sintered samples were systematically investigated. The heat treatment process after SPS was found to increase the electrical conductivity greatly as the proceeding reaction of Ti/C results in an extremely low Ti concentration in Cu matrix. Moreover, an obvious drop in microhardness was observed. The strength was slightly improved by the following hot pressing, while there was no obvious change in electrical conductivity. The microstructure evolution during the entire developed process was analyzed by means of Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The formed TiC particles were homogeneously distributed in copper matrix. Furthermore, the ultrafine-grained (UFG) structure developed by the present process could maintain stable because of the Zener pinning effect caused by nanoscale TiC particles located at grain boundaries.

Corrosion behavior in 5.2 M aqueous hydrofluoric acid solution and tensile properties of Ni-30Co-16Cr-15Mo-6Fe-xCu (x = 0, 0.5, 2, and 6) (mass%) alloys are investigated. The corrosion resistance of alloy can be significantly improved by the slight additions of Cu (e.g. 0–2 mass%) without altering either the microstructure or the tensile properties of alloy. On the contrary, alloy with higher addition of Cu, characterized by Cu-rich phases both at grain boundaries and inside the matrix, demonstrates lower corrosion resistance and lower tensile strength. Interestingly, corrosion resistance in grain boundary of alloy gradually increases with Cu content increasing irrespective of the appearance of Cu-rich phases.

Twin boundary mobility (TBM) has significant influences on the mechanical properties of magnesium (Mg). In the current study, the effects of pre-strain on TBM in pure Mg are systematically investigated by evaluating the change of yield stress during compression and tension, and microstructure observations by in-situ electron back-scattered diffraction. Owing to lower dislocation density and less restriction from other boundaries, smaller twins in pure Mg subjected to 2% compressive strain exhibit higher TBM than larger twins in pre-4% and pre-8% compressed pure Mg. Decreased TBM with increasing pre-strain is primarily because of the larger number of dislocations, which impede twin boundary motion. These findings indicate that altering twin size and dislocation density by pre-strain is an effective way to tailor TBM, providing a basis for developing new industrial Mg alloys.

In situ Ti-6Al-4V/TiB composites with different contents of TiB were prepared by SPS. The flow behavior of the composites deformed at temperatures of 900~1200 °C and strain rates of 0.001 to 10 s−1 was studied. The constitutive equations and processing maps were established. Results show that the composites have fine microstructures, and nano-sized TiB whiskers distribute homogeneously in the α+β titanium matrix. The activation energy of the composites is low due to the fine microstructures, resulting in a good workability. The processing maps suggest that the instable deformation occurs at low temperatures and high strain rates. The flow localization band of Ti matrix, cracking and debonding of TiB whiskers are the reasons for the instable deformation. Dynamic recrystallization has been found in composites deformed at low strain rates, accounting for the stable deformation.

The microstructure with homogeneously distributed grains and less prior particle boundary (PPB) precipitates is always desired for powder metallurgy superalloys after hot isostatic pressing (HIPping). In this work, we studied the effects of HIPping parameters, temperature and pressure on the grain structure in PM superalloy FGH96, by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM) and Time-of-flight secondary ion spectrometry (ToF-SIMS). It was found that temperature and pressure played different roles in controlling PPB precipitation and grain structure during HIPping, the tendency of grain coarsening under high temperature could be inhibited by increasing HIPping pressure which facilitates the recrystallization. In general, relatively high temperature and pressure of HIPping were preferred to obtain an as-HIPped superalloy FGH96 with diminished PPB precipitation and homogeneously refined grains.

A Cu-TiC alloy, with nanoscale TiC particles highly dispersed in the submicron-grained Cu matrix, was manufactured by a self-developed two-step ball-milling process on Cu, Ti and C powders. The thermostability of the composite was evaluated by high-temperature isothermal annealing treatments, with temperatures ranging from 727 to 1273 K. The semicoherent nanoscale TiC particles with Cu matrix, mainly located along the grain boundaries, were found to exhibit the promising trait of blocking grain boundary migrations, which leads to a super-stabilized microstructures up to approximately the melting point of copper (1223 K). Furthermore, the Cu-TiC alloys after annealing at 1323 K showed a slight decrease in Vickers hardness as well as the duplex microstructure due to selective grain growth, which were discussed in terms of hardness contributions from various mechanisms.

Co–28Cr–6Mo–0.23C–0.17N alloy cylindrical rods were fabricated by electron beam melting (EBM) with cylindrical axes along the build direction. The inhomogeneity in microstructures of the as-fabricated rods and heat-treated rods were investigated, along with the creep behavior of the heat-treated rods, focusing on the influence of microstructural inhomogeneity. Although the constituent phase varied along the build direction in the as-EBM-built rod, from single e-hexagonal close-packed (hcp) phase in the bottom to single γ-face-centered cubic (fcc) phase in the top, the γ-fcc phase can be kept in a wide range of build height, i.e. ∼40 mm from the top finishing plane. The as-EBM-built rods consisting of both e phase and γ phase can be transformed into single e-hcp phase by the aging treatment at 800 °C for 24 h. However, the e-hcp grain size in the aged rod was heterogeneous along the build height. The grain size increased along the build height at first, then decreased gradually to the position where the phase transitioned from γ-fcc to e-hcp in the as-EBM-built rod. The grain size was nearly uniform on the top part, which used to be single γ-fcc phase. The values of stress exponent n and apparent activation energy Q were determined to be 5.0 and 365 kJ mol−1, respectively. Intergranular fracture occurred, and fracture nearly always occurred in the homogeneous fine-grain region. The decrease in stacking fault energy with increasing temperature keeps the dislocations expanding and increases the apparent activation energy.

Titanium matrix composites reinforced with TiB whiskers were in situ synthesized by spark plasma sintering (SPS) at a temperature range of 1173­1473K, using a mixture of 6.2mass% TiB2, 4.1mass% Ti, and 89.7mass% Ti6Al4V powders. The in-situ synthesis mechanism and the effect of sintering temperature on the sintering microstructure were investigated. The results show that an increase of the sintering temperature causes the relative densities of the composites to increase and the reaction between Ti and TiB2 to be more complete. Nano-sized TiB reinforcements with a diameter of around 80nm and fine-grained matrix with an average grain size of 12µm are obtained after SPSed at 1373K. Clean and perfect TiB/matrix interfaces without debonding or cracks are obtained during SPS process. Low sintering temperature and short sintering time are believed to be the main reasons for the improved fine-grained microstructure. [doi:10.2320/matertrans.M2014347]

We examined the effects of cold working on the corrosion resistances of Ni–30Co–16Cr–15Mo and Ni–16Cr–15Mo in 5.2 M HF acid solution at 100 °C. After cold working, the Ni–30Co–16Cr–15Mo samples exhibited a more significant increase in hardness compared to Ni–16Cr–15Mo as well as a more localised microstructure that contained a greater number of planar slipping bands and mechanical twinning. Both alloys exhibited a decrease in corrosion resistance, with the decrease being much greater in the case of Ni–30Co–16Cr–15Mo. The difference in the corrosion resistances of the alloys was ascribable to the differences in their microstructures.

TiC-dispersion-strengthened Cu alloys were prepared by a two-step ball-milling (BM) process followed by sparks plasma sintering (SPS), heat treatment and hot rolling in sequence. The two-step BM process is composed of a pre-ball-milling (pre-BM) on both Ti and graphite powders as well as a subsequent homogenizing by BM together with Cu powder. Microstructure evolution analysis was performed to evaluate the effects of BM conditions on the electrical and mechanical properties of Cu-based alloys. The X-ray results revealed that titanium carbide (TiC) formed from Ti and C under high impact energy BM. Moreover, the formation of TiC during the SPS and heat treatment processes was found to more beneficial in enhancing the mechanical properties of alloy. The residual Ti in Cu matrix was found to be the predominant factor lowering the electrical conductivity of Cu–Ti–C alloys.

Cobalt–chromium–molybdenum (CoCrMo) alloys are widely applied as wear-resistant material. This article aims to illustrate the influence of phase constitution and precipitate morphology on the wear behavior of hot-pressed high carbon alloys. Wear behavior of two kinds of CoCrMo alloys, HP (hot-pressed) and HPA (hot-pressed+annealing) with various microstructure, hardness and surface topography were evaluated in detail. The e phase dominant HP alloy with homogenous lamellar precipitates demonstrates higher hardness, consequently lower coefficient of friction and prominent wear resistance with mild sliding wear. In contrary, the γ phase dominant HPA alloy with carbide-free grains was characterized by abrasive wear of micro grooves. The matrix hardness strengthened by e phase is more significant to affect wear behavior than precipitate hardening. Homogenous lamellar precipitates have a stand-out effect to block abrasion and accumulate tribochemical products.

Hot deformation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, subgrain formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deformation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.