Rapid fabrication of function-structure-integrated NiTi alloys: Towards a combination of excellent superelasticity and favorable bioactivity

Porous NiTi has brought new expectations to the field of orthopaedic implants due to its excellent mechanical properties such as high strength and superelasticity together with good biocompatibility. In order to facilitate the surrounding bone tissue ingrowth into the implanted porous alloy, reasonably large sized pores and a high amount of porosity are required. There is, however, a major challenge for clinical applications: the higher the porosity, the worse are the mechanical properties and the superelasticity. In this work, therefore, function-structure-integrated NiTi alloys consisting of a central solid and an outer porous layer were fabricated by spark plasma sintering (SPS). When sintered at 750 °C, the NiTi alloy with 14% porosity in the inner part and 49% porosity as well as 350 μm average pore size in the outer layer exhibits an exceptionally high compressive strength (∼1375 MPa), together with an excellent superelastic recovery strain (>4%) and favorable cellular affinity (ROS1728 osteoblasts). Altogether, this work provides a strategy to design materials with function-structure integration and suggests that properly designed function-structure integrated NiTi alloys may be promising as advanced bone implants.     

NiTi with 3D-interconnected microchannels produced by liquid phase sintering and electrochemical dissolution of steel tubes

A process was developed for fabricating 3D fully interconnected microchannels in superelastic NiTi-Nb for bone implant applications by combining spaceholder powder metallurgy and liquid-phase sintering. Prealloyed NiTi powders were blended with 3.1 at.% Nb and cold-pressed around a 3D scaffold of carburized steel tubes acting as space-holders. The tubes were then electrochemically dissolved to form orthogonally interconnected microchannels with 400 μm diameter and ∼34% volume fraction. Finally, the powder preform was heated to 1185 °C to form a quasi-binary NiTi-Nb eutectic liquid, which liquid-phase-sintered the NiTi powders without filling the microchannels. The resulting continuously bonded matrix contains an additional 16% porosity, for a total structure porosity of ∼50%. NiTi-Nb micro-architectured structures have excellent potential as bone implant scaffolds due to the high versatility in channel size, fraction, and spatial arrangement. Fully interconnected 3D microchannels also increase fluid transport within the scaffold, assisting in nutrient delivery and waste transport to and from cells deep within the scaffold.     

Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α₂-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α₂/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.     

Hierarchical nanoporous metal/BMG composite rods with excellent mechanical properties

Hierarchical nanoporous structured NPC/BMG composite rods (NPC/BMG: nanoporous copper/bulk metallic glass) were facilely fabricated by one-pot chemical dealloying the Cu₅₀Zr₄₅Al₅ BMG rods in 0.05 M HF solution for 24 h. The cross-sectional SEM images illustrate that the NPC/BMG composite rods exhibit the perfect combination of inner rod-shaped Cu-Zr-Al amorphous phase core and outer tube-shaped NPC layer with a thickness of 85 μm. As compared to the reported NPC composites, the new composite rods demonstrate remarkable enhanced mechanical properties with an ultrahigh strength of 1500 MPa and a large compression strain of 2.9%. Additionally, the increase in the compression strain is attributed to the formation of the buffer deformation zone resulting from the existence of the outer nanoporous copper tube.     

Thermoelectric properties of ScCoSb, ScNi0.86Sb and MgNiSb compounds

Blends of 90 wt.% Ti and 10 wt.% W powders were consolidated by powder metallurgy, using an initial W powder size that was very fine (0.7 and 2 μm) or very coarse (<250 μm). Dissolution of W powders in the Ti matrix during consolidation was almost complete for the former blends (thus forming Ti–10W “alloys”) but very limited for the latter blend (thus forming a Ti–10W “composite”). The Ti–10W alloys exhibit much higher yield and tensile strengths than the Ti–10W composite, indicating that tungsten strengthens titanium more efficiently as a solute atom (solid-solution strengthening) than as a second phase (composite strengthening by load transfer). The Ti–10W alloys also exhibit much higher ductility than the Ti–10W composite, whose brittle W particles exhibit fracture or pull-out from the matrix.     

Chemical and mechanical treatments to improve the surface properties of shape memory NiTi wires

In this paper the results of an experimental study concerning the effect of different surface treatments on NiTi shape memory alloy wires are presented. These treatments were conducted in order to improve the adhesion properties between the NiTi wires and an epoxy resin, acting as the matrix of a composite material.Mechanical and chemical surface treatments (immersion in acid and alkaline solutions), and different combinations of the above surface preparation procedures were studied.For the characterisation of the resulting alloy surface conditions electrochemical impedance spectroscopy, polarisation curves and potential versus time measurements were carried out.The alloy wire/epoxy matrix adhesion was characterised through pull out tests. The results proved that all adopted treatments can remarkably influence the electrochemical properties of the wires. The acid treatments favour the formation of a surface passivation layer, while the alkaline treatments are effective in producing a rougher surface morphology. Moreover, these basic treatments significantly reduce corrosion resistance of the alloys, another material property that has been incidentally investigated in the present context. The main effect of the mechanical surface treatment, consisting in abrading the alloy wires using an emery paper, was to increase the homogeneity of surface roughness.From the experimental results clear indications on the most promising surface treatments can be inferred.     

High carrier concentration in Mg2Si1−xSbx (0 ≤ x ≤ 0.10) prepared by a combination of liquid-solid reaction,ball milling,and spark plasma sintering

Ternary compounds of Mg₂Si_1−xSb_x (0 ≤ x ≤ 0.10) are prepared by a combination of liquid-solid reaction, ball milling, and spark plasma sintering. The carrier concentration of Mg₂Si_1−xSb_x increases with the Sb content x and reaches 1.1 × 10²¹ cm⁻³ at x = 0.10, which is approximately ten times higher than that previously reported. The high carrier concentration is attributed to the facilitation of Sb-doping by ball milling and the suppression of Mg vacancy formation by short-time sintering. No decrease in the carrier concentration of Mg₂Si_0.90Sb_0.10 is observed after annealing at 773 K for 100 h in a semi-closed system, which suggests that the compound is stable at 773 K under a high partial pressure of Mg.     

Microstructure and mechanical properties of equimolar FeCoCrNi high entropy alloy prepared via powder extrusion

An equiatomic FeCoCrNi high alloy (HEA) with both high tensile strength and ductility was produced by a powder metallurgy (P/M) method. The P/M process includes a gas atomization and a hot extrusion of pre-alloyed HEA powder. Microstructures and mechanical properties were characterized using optical microscopy (OP), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and tensile tests. The results show that the P/M FeCoCrNi HEA has a single face centered cubic (fcc) structure and an equiaxed microstructure. No obvious porosity and brittle intermetallic phase was found. The as-extruded alloy exhibits a very high tensile strength of 712.5 MPa, and still maintains an elongation as high as 56%. The improvement of the tensile properties is caused by the solid solution strengthening, grain boundary strengthening and homogenous microstructure. Therefore, the powder hot extrusion can be considered as a promising way for preparing large-sized HEAs with high mechanical properties.     

Analysis of density and mechanical properties of high velocity compacted iron powder

A new method for producing higher density PM parts, high velocity compaction (HVC), was presented in the paper. Using water atomized pure iron powder without lubricant admixed as the staring material, ring samples were compacted by the technique. Scanning electron microscopy (SEM) and a computer controlled universal testing machine were used to investigate the morphologies and the mechanical properties of samples, respectively. The relationships among the impact velocity, the green density, the sintered density, the bending strength and the tensile strength were discussed. The results show that with increasing impact velocity, the green density and the bending strength increase gradually, so the sintered density does. In addition, the tensile strength of sintered material is improved continuously with the sintered density enhancing. In the study, the sintered density of 7.545~g/cm³ and the tensile strength of 190~MPa are achieved at the optimal impact velocity of 9.8 m/s.     

Nanocrystalline Al3Ni2 alloy with high hardness produced by mechanical alloying and high-pressure hot-pressing consolidation

An elemental powder mixture corresponding to Al₃Ni₂ phase stoichiometry was subjected to mechanical alloying. A metastable nanocrystalline AlNi intermetallic phase with the mean crystallite size of 12 nm was formed upon milling. Heating of the synthesised powder in a calorimeter up to 720 °C caused phase transformation into an equilibrium Al₃Ni₂ intermetallic phase with the mean crystallite size of 41 nm. The product of mechanical alloying was consolidated at 1000 °C under the pressure of 5 GPa and 7.7 GPa. During consolidation, a phase transformation analogous with the one observed in the course of heating in the calorimeter took place. Both bulk materials have nanocrystalline structure with mean crystallite size of 67 nm and 58 nm, the smaller one in the sample consolidated under the higher pressure. The hardness of the produced Al₃Ni₂ intermetallic is 8.81 GPa (898 HV1) and 8.72 GPa (887 HV1), while the specific yield strength, estimated using the Tabor relation, is 624 kNm/kg and 617 kNm/kg for the sample hot-pressed under 5 GPa and 7.7 GPa respectively. On the basis of the obtained results, we can assume that the quality of consolidation with preserving a nanocrystalline structure is satisfactory and the hardness as well as the estimated specific yield strength of the produced materials are relatively high.     

The high temperature flow behavior modeling of NiTi shape memory alloy employing phenomenological and physical based constitutive models: A comparative study

The present study was conducted to examine the constitutive behavior of a near equiatomic Nickel Titanium shape memory alloy through modified Zerilli–Armstrong and Arrhenius type models. The acquired flow stress data from isothermal hot compression tests in a temperature range of 700–1100 °C and under the strain rates of 0.001–0.1 s⁻¹ was employed to calculate the material constants thereby properly establishing the corresponding constitutive equations. Furthermore, a comparative study has been made on the capability of the aforementioned models to predict the high temperature flow behavior of the adapted shape memory alloy. This was performed by comparing the prediction relative errors, average absolute relative error and correlation coefficient. The results show that Arrhenius type equation predicts the flow behavior more accurately than the Zerilli–Armstrong one. However, the latter requires lesser material constants and running time to be established.     

Microstructure and mechanical properties of magnesium composites prepared by spark plasma sintering technology

Spark plasma sintering (SPS) technology was used to determine the appropriate conditions for SPS sintering of commercially pure magnesium as well as the magnesium alloy AZ31. It was found that the sintering temperatures of 585 °C and 552 °C were the most suitable sintering temperatures for the magnesium and the AZ31 alloy, respectively. Magnesium matrix and AZ31 alloy matrix composites reinforced with SiC particles were then successfully fabricated by the SPS method at sintering temperatures of 585 °C and 552 °C, respectively. A uniform distribution of SiC particles was observed along the boundary between matrix particles. The mechanical properties, i.e. hardness and tensile strength increased with increasing SiC content up to 10 wt%. However, when the SiC content was larger than 10 wt%, the tensile strength decreased due to the agglomeration of SiC particles. The agglomeration of SiC particles was found to lead to the degradation of the interfacial bonding strength between matrix and reinforcement.     

Correlation between microstructure and mechanical properties of severely deformed metals

There is a correlation between the microstructure and the mechanical behavior of ultrafine-grained face centered cubic (fcc) metals processed by equal-channel angular pressing (ECAP). It is shown that the saturation yield strength is related to the maximum dislocation density according to the Taylor equation and, in addition, the value of the parameter α in the Taylor equation is strongly affected by the stacking fault energy because of different geometrical arrangements of dislocations within the grains. It is also demonstrated that the ductility of Cu processed by ECAP decreases with increasing strain but at extremely high strains the ductility is partially restored due to the recovery of the microstructure.     

粉末冶金Ti-Al-Mo-V-Ag合金的显微组织与力学性能

通过粉末冶金元素混合法,制备含α及β相的Ti-Al-Mo-V-Ag合金。通过X射线衍射、金相观察、扫描电镜观察及力学性能测试,研究Ag的添加及烧结温度对Ti-5Al-4Mo-4V合金的组织与性能影响。结果表明：Ag的添加能提高粉末冶金Ti-5Al-4Mo-4V合金的的相对密度,改善合金的力学性能;在1250℃下烧结4h后,Ti-5Al-4Mo-4V-5Ag合金的相对密度及抗压缩强度分别达到96.3%和1656MPa。     

Microstructure and mechanical properties of in-situ MC-carbide particulates-reinforced refractory high-entropy Mo0.5NbHf0.5ZrTi matrix alloy composite

Refractory high-entropy Mo_0.5NbHf_0.5ZrTi alloy matrix composite with MC-carbide particulates-reinforced was prepared by arc melting. It is found that Mo_0.5NbHf_0.5ZrTiC_0.1 and Mo_0.5NbHf_0.5ZrTiC_0.3 alloys consist of one disordered body-centered cubic (BCC) solid solution phase as the matrix phase and MC carbide phase. MC carbide is enriched with Zr and Hf due to the higher binding strength and without any Mo. Noticeable strengthening from the carbide is not observed for C_0.1 and C_0.3 alloys while the compressive plasticity is improved slightly due to the decrease of solution strengthening for the matrix BCC disordered solid solution phase.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

Mechanical synthesis and rapid consolidation of a nanocrystalline 3.3Fe0.6Cr0.3Al0.1-Al2O3 composite by high frequency induction heating

Nanopowders of 3.3Fe_0.6Cr_0.3Al_0.1 and Al₂O₃ were synthesized from Fe₂O₃, Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe_0.6Cr_0.3Al_0.1-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1. The average grain sizes of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1 were 84 and 32 nm, respectively.     

Influence of Sb on damping capacity and mechanical properties of Mg2Si/Mg–9Al composite materials

The influence of Sb addition on mechanical properties and damping capacity of Mg₂Si/Mg–9Al composite materials were investigated. Due to modification of Sb, Mg₂Si intermetallic compound exhibits refinement polygonal type, and the grain size of Mg matrix reduces. Such improved microstructure of the modified materials results in the large improvement in tensile properties and damping capacity.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.