B2 Parent Phase Research Articles

Metastable β-Phase Ti–Nb Alloys Fabricated by Powder Metallurgy: Effect of Nb on Superelasticity and Deformation Behavior

This study investigates the effect of Nb concentration on the mechanical properties, superelasticity, as well as deformation behavior of metastable β-phase Ti–Nb alloys produced via powder metallurgy. The alloys were fabricated through mechanical alloying, followed by consolidation using hot pressing. The resulting microstructure comprises fine β-phase grains with TiC carbide precipitates at the grain boundaries. The study reveals non-linear variations in the values of yield strength for the manufactured materials, which were attributed to the occurrence of various deformation mechanisms activated during the loading. It was found that the mechanisms change with the increasing concentration of Nb in the manner: stress-induced martensitic transformation, twinning, slip. However, all these mechanisms were activated at a reduced concentration of Nb compared to the materials obtained by casting technology previously reported in the literature. This is most probably associated with the elevated oxygen content, which affects the stability of the parent β-phase. The study revealed that superelasticity in Ti–Nb-based alloys prepared using powder metallurgy may be achieved by reducing the content of β-stabilizing elements compared to alloys obtained by conventional technologies. In this study, the Ti–14Nb (at. pct) alloy exhibited the best superelasticity, whereas conventionally fabricated Ti–Nb alloys displayed superelasticity at an Nb concentration of approximately 26 at. pct. The developed material exhibited a non-conventional, one-stage yielding behavior, resulting in a superelastic response at significantly higher stresses compared to conventionally fabricated Ti–Nb alloys.

Study on phase transformation, internal stress and texture evolution of cold drawn Nb nanowire-NiTi composite wires upon annealing

Phase transformation, internal stress and texture evolution of cold drawn Nb nanowire-NiTi composite wires upon annealing were investigated. Nb nanowires in the composite wires exhibited strong <110> fiber texture and internal stress anisotropy, and the internal stress in the annealed composite wires gradually decreased as the annealing temperature increased above 500 °C, and Nb nanowires partly spheroidized into nanoparticles after annealed at 800 °C. Different temperatures annealed composite wires exhibited significantly different phase transformation behaviors, and NiTi matrix in these composites is in different phases at room temperature. With the increase of annealing temperature, the phase structure of the NiTi matrix underwent the evolution of oriented B19′ martensite, B2 parent phase, oriented R phase and self-accommodated B19′ martensite phase. Meanwhile, the reoriented textures of B19′ martensite and R phases are induced by the internal stress in NiTi matrix.

In-situ synthesis of TiNiTa coating by laser cladding with orthogonal test method

In this paper, we focus on the optimized process parameters of the TiNiTa shape memory alloy functional coatings prepared by laser cladding technology on the Ti6Al4V (TC4) alloy, and studied properties. On the basis of orthogonal experiment, the optimum process parameters of TiNiTa coatings were determined as follows: the laser power was 100 W, the scanning rate was 2.5 mm/s, the defocusing amount was 10 mm, the pulse width was 5 ms, and the frequency was 10 Hz. The influence of laser power, scanning speed and defocus amount on the hardness of cladding layer was evaluated by the range analysis from the experimental data. The results show that the range of defocusing amount, scanning speed and laser power were R1 = 34, R2 = 14.334, R3 = 44.334, respectively. Therefore, the degree of influence on the process parameters on the quality of the prepared TiNiTa coating was determined in the following order: defocus amount, laser power, scanning speed. When the energy density was 1698 J/mm2, the microstructure of the cladding layer was compact and fine, and the metallurgical bond with the matrix was formed. In addition, the polarization potential and the electrochemical impedance spectrum (EIS) radius of the cladding layer were firstly increased and then reduced with the increase of the energy density. It can be seen that the corrosion resistance of TC4 titanium alloy surface was significantly improved. XRD results shows that the cladding layer was mainly composed of B2 parent phase, B19′ martensite phase and Ti2Ni phase. The results indicate that energy density was an important factor affecting the structure and properties of cladding layers. Through the above analysis, the combination of orthogonal test and range analysis provides an efficient and reliable method to select and optimize the technological parameters of laser cladding.

B2 to ordered omega transformation during isothermal annealing of refractory high entropy alloys: Implications for high temperature phase stability

Al containing refractory high entropy alloys or complex concentrated alloys (RHEAs or RCCAs) often exhibit a patterned BCC + B2 microstructure, resembling the FCC + L12 microstructure typically observed in many Ni/Co base superalloys. There is a rapidly growing interest in these BCC+B2 microstructures, especially due to their promising balance of elevated temperature mechanical properties. Unfortunately, the stability of such microstructures on long-term exposure to elevated temperatures is not well-understood and prompts the motivation of the present study. The current study traces the phase transformation pathway during the intermediate temperature annealing of such microstructures for extended time periods, and more specifically the formation of ordered omega type phases. In the case of a high strength RHEA, Al0.5Mo0.5NbTa0.5TiZr the relationships between the two structures strongly suggest a strong possibility that such ordered omega phases are essentially derived from a parent B2 phase. At the early stages of annealing, the solutionized condition of this alloy consists of a co-continuous mixture of BCC and B2 phases which transforms into a continuous B2 matrix with discrete BCC precipitates presumably via spinodal decomposition concomitant with ordering. With increasing annealing time at 800 °C, isolated islands of BCC phase embedded with discrete, fine-scale pockets of the B2 and hP18 (P63/mcm) phase develop, where the hP18 phase is an ordered derivative of the omega phase, commonly found in titanium and zirconium base alloys. The transformation of the parent B2 phase to the hP18 ordered omega type phase can be rationalized via a combined replacive and displacive transformation, including simultaneous rejection and enrichment of specific alloying elements to achieve the desired stoichuometry. First-principles based DFT calculations were also carried out to compare the stability of the B2 versus hP18 phases, as a function of composition. While long term annealing and CALPHAD based calculations both indicate towards an equilibrium phase field comprising of hP18, B2 and BCC phases, hP18 does not directly nucleate homogeneously within the BCC grains. Instead, it appears to form heterogeneously along grain boundaries and more importantly within the grains with B2 phase acting as an intermediate phase, both of which indicates a high nucleation barrier for its homogeneous nucleation. The universality of this phenomenon has been demonstrated using examples from other RHEAs, and the resultant impact on the high temperature stability of the BCC+B2 microstructure has been discussed.

High damping performances over wide temperature range in the B doped Ti-Ni shape memory alloys

• The Ti-Ni alloys with B addition undergo B2 ↔ B19′ martensitic transformation. • Two high internal friction peaks are observed in the B doped Ti-Ni alloys. • One internal friction is derived from reversible martensitic transformation. • The other originates from the interactions between boron and twinning boundaries. In present work, the microstructure, martensitic transformation behavior and damping properties of the B doped Ti-Ni shape memory alloys were investigated. The B2 parent phase and B19′ martensite phase are both detected in the whole samples, irrespective of B contents. In addition, the TiB whiskers are observed in the alloy with high boron contents. The martensitic transformation temperatures increase firstly and then decreases with the increasing of the B contents. The Ti-Ni alloys with B addition exhibit two high internal friction peaks, which are derived from the B2 ↔ B19′ martensitic transformation and interactions between boron and twinning boundaries, respectively. The tanδ of low temperature relaxation peak can be up to approximately 0.074 in the alloy with 0.7 at.% B contents. The introduction of proper interstitial atoms is considered as promising method to improve the damping performances.

Investigations on the Phase Transformations, Equilibria and Athermal ω in Ni-Ga-Cr Ternary System.

In the present work, the phase equilibria of the Ni-Ga-Cr ternary system at 850, 1000 and 1150 °C were experimentally investigated to provide the essential data for developing the high-entropy shape memory alloys (HESMAs) containing Ni, Ga and Cr. At 850 °C, in the Ni-rich portion, the B2 phase shows equilibrium with the L12 phase when the Cr content is less than 10.49 at. %, while displaying the equilibrium with L12 and BCC phases when the Cr content increases. The B2 + L12 + BCC changes into B2 + FCC + BCC three-phase equilibria from 850 to 1150 °C, as the L12 phase region becomes narrow with rising temperature. The two-phase equilibrium, B2 + BCC, was found at all the isothermal sections investigated. Other three-phase equilibria were also discovered: B2 + α-Cr3Ga + BCC and Ni2Ga3 + α-Cr3Ga + L at 850 °C, and B2 + α-Cr3Ga + L at 1000 °C. Significantly, an athermal ω intermetallic compound with the space group of P3¯m1 was observed distributing at the B2 phase in the quenched Ni45.98-Ga25.50-Cr28.52, Ni42.23-Ga15.70-Cr42.07 and Ni16.54-Ga13.63-Cr69.83 (at. %) alloys after being annealed at 1150 °C for 10 days. The high-resolution transmission electron microscopy (HRTEM) results reveal that the ω shows a crystallographic orientation of [11¯0]B2//[112¯0]ω; (111)B2//(0001)ω with the B2 parent phase.

TiB whisker induced variant selection in titanium matrix composites

Considerable studies confirmed the existence of multiple orientation relationships (ORs) between TiB whiskers and α precipitates in titanium matrix composites (TMCs), but the role of TiB on α variant selection has not been demonstrated. In this study, three preferred ORs between TiB and α variants were obtained from β heat-treated TMCs by analyzing the misorientation angle/axis distributions, thereby confirming the selection of TiB whiskers for α variants. It was found that the grain refinement and equiaxed effect of TiB whiskers on the matrix microstructure are mainly reflected in the selected α variants. The dependence of the variant-selection degree on the cooling rate was investigated and the strongest variant selection of TiB whisker occurred at a cooling rate of 1°C/s. In addition, reconstruction of the parent β-phase revealed that the selected α variants heterogeneously nucleate from TiB and do not exhibit the Burgers relationship with the parent β phase.

Precipitation Behavior of O Phase during Continuous Cooling of Ti-22Al-25Nb Alloy

The microstructure evolution and formation mechanism of the O phase in a Ti-22Al-25Nb (at.%) orthorhombic alloy resulting from different cooling rates were investigated. The results show that the morphology of the precipitated O phase is significantly affected by the cooling rate. As the cooling rate decreases, the floccular O, composed of many fine acicular O phases, gradually grows into the lamellar O phase. When the alloy is cooled from the B2 phase region, the grain boundary O (OGB) preferentially nucleates at the triple junctions and grain boundaries and forms the flat and the zig-zag OGB according to different cooling rates. The OGB consists of separated, flat OGB parts and unconnected, zig-zag OGB composed of multiple short, separated, flat OGB at a higher cooling rate. The zig-zag OGB presents a connected state due to the sufficient diffusion time at a lower cooling rate. When the alloy is cooled from the (B2 + α2) phase region, the increase of the phase boundary provides favorable conditions for the nucleation of the O phase due to the presence of α2 particles. The precipitated rim O phase appears on the periphery of the α2 particles at lower cooling rates. The analysis indicates that the Widmanstätten intragranular O (OWI) precipitated directly from the B2 phase maintains the plane relation with the parent B2 phase, and the Widmanstätten grain boundary O (OWGB) holds the specific orientation relationship with one of the two adjacent B2 grains. The OGB keeps the specific orientation relationship with one of the B2 grains as much as possible. When it cannot maintain the specific orientation relationship with one of the B2 grains, the OGB maintains a near-orientation relationship with B2 grains on both sides to reduce the nucleation activation energy. Moreover, there can be more than one nucleation site for the O phase on a single B2 grain boundary to form the OGB. The rim O phase formed through a decomposition reaction of α2→α2 (Nb-lean) + O (Nb-rich) is controlled by a diffusional mechanism and maintains a specific orientation relationship, i.e., {001} O//{0001} α2 and &lt;110&gt;O//&lt;112¯0&gt; α2, with the parent α2 particles.

Design and Characterization of Al-Cr-Nb-Ti-V-Zr High-Entropy Alloys for High-Temperature Applications

High-entropy alloys (HEAs) are a new class of metallic alloys without a principal component. These materials are attractive because of their unique structures and properties, including mechanical ones. Some of HEAs based on refractory metals are considered as advanced high-temperature materials. However, the development of new applicable compositions is complicated by poor understanding of relationships between the chemical and phase composition of HEAs and their effect on mechanical performance. Here we report the results of comprehensive studies of Al-Cr-Nb-Ti-V-Zr high-entropy alloys, particularly AlCrxNbTiVZry (х = 0.00, 0.25, 0.50, 1.00, 1.50 at y = 0 and y = 0.0, 0.25, 0.50, 1.00, 1.50 at x = 0). The strength and ductility of such alloys are shown to have a complex relationship with the composition and order of the parent B2 phase, and the amount and nature of secondary phases (C14 Laves phase and zirconium aluminides). Some of the investigated alloys demonstrate high specific strength at T ≤ 800°C. AlNbTiVZr0.25 alloy also has a stable microstructure and higher creep resistance at 600°C compared to existing creep-resistant alloys. Based on calculations of equilibrium and nonequilibrium phase diagrams by the CALPHAD method, we propose new Al-Cr-Nb-Ti-Zr alloy compositions with unique lamellar eutectic structure composed of the B2 and C14 Laves phases and with enhanced strength properties. The structure/property relationships and high-temperature deformation mechanisms of refractory HEAs with eutectic microstructure are analyzed.

Microstructure transition gradients in titanium dissimilar alloy (Ti-5Al-5V-5Mo-3Cr/Ti-6Al-4V) tailored wire-arc additively manufactured components

The nature of the chemical mixing and microstructure gradients that occur across the interface transition, when manufacturing tailored components with the two high-performance dissimilar titanium alloys (Ti-6Al-4V (Ti-64) and Ti-5Al-5V-5Mo-3Cr (Ti-5553)) by the wire-arc additive manufacturing (WAAM) process, are reported. It has been shown that a relatively long-range chemical gradient occurs during the transition between layers produced with the two different titanium alloys, due to convective mixing in the melt pool between the substrate layers and new alloy wire. This resulted in a stepwise exponential decay composition profile normal to the layers, the width of which can be described by a simple dilution law, with steep local composition gradients seen within the boundary layers at the fusion boundary of each individual layer. The alloy-alloy composition gradients had little effect on the β-grain structure. However, they strongly influenced the transformation microstructure, due to their effect on the parent β-phase stability and the β → α transformation kinetics and reaction sequence. The microstructure gradient seen on transitioning from Ti-64 → Ti-5553 was significantly more abrupt, compared to when depositing the two alloys in the reverse order. Under WAAM thermal conditions, Ti-64 appears to be more sensitive to the effect of adding β-stabilising elements than when Ti-5553 is diluted by Ti-64, because at high cooling rates, stabilisation of the β phase readily suppresses α nucleation when cooling through the β transus, and the normal Ti-64 lamellar transformation microstructure is abruptly replaced by finer scale α laths generated by precipitation during subsequent reheating cycles.

Influence of Batch Mass on Formation of NiTi Shape Memory Alloy Produced by High-Energy Ball Milling

A high-energy ball milling technique was used for production of the equiatomic NiTi alloy. The grinding batch was prepared in two quantities of 10 and 20 g. The alloy was produced using various grinding times. Scanning electron microscopy, X-ray diffraction, hardness measurement and differential scanning calorimetry were used for materials characterization at various milling stages. The produced alloy was studied by means of microstructure, chemical and phase composition, average grain and crystallite size, crystal lattice parameters and microstrains. Increasing the batch mass to 20 g and extending the grinding time to 140 h caused the increase in the average size of the agglomerates to 700 µm while the average crystallites size was reduced to a few nanometers. Microstrains were also reduced following elongation of milling time. Moreover, when the grinding time is extended, the amount of the monoclinic phase increases at the expense of the body-centered cubic one—precursors of crystalline, the B2 parent phase and the B19′ martensite. Crystallization takes place as a multistage process, however, at temperatures below 600 °C. After crystallization, the reversible martensitic transformation occurred with the highest enthalpy value—4 or 5 J/g after 120 and 140 h milling, respectively.

Electronic Mechanism of Martensitic Transformation in Nb-doped NiTi Alloys: A First-Principles Investigation.

The effect of Nb on the crystal structures and electronic mechanism of martensitic transformation in Ni50Ti50–xNbx alloys is investigated by first principles. The lattice parameters, the formation energy, the middle eigenvalue of the transformation stretch tensor (λ2), and the energy difference between the parent and martensite (ΔE) as a function of Nb content x (x = 0, 2.08, 6.25, 8.33, 10.42, 12.5, 18.75) are calculated. Lattice parameters increase with the increase of Nb content. The formation energies of the parent B2 phase, martensite orthorhombic B19, and monoclinic B19′ increase with the increase of Nb content. It is also found that at ≤10.42 at. % Nb, the martensite stable phase is monoclinic structure B19′; at >10.42 at. % Nb, the orthorhombic crystal structure B19 is formed. The energy difference between the parent and martensite means that the transformation temperature decreases with increasing Nb concentration at Nb ≤ 10.42 at. % and increases at >10.42 at. % Nb. The λ2 of the NiTiNb alloys have the same value of about 0.95 with low Nb content. Furthermore, the electronic structure mechanisms behind the martensitic transformations are discussed in detail based on the density of states.

Martensitic transformation behaviors of all-&amp;lt;italic&amp;gt;d&amp;lt;/italic&amp;gt;-metal Heusler Mn&amp;lt;sub&amp;gt;50&amp;lt;/sub&amp;gt;Ni&amp;lt;sub&amp;gt;50&amp;minus;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;minus;&amp;lt;/sub&amp;gt;&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;y&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;Fe&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;x&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt;Ti&amp;lt;sub&amp;gt;&amp;lt;italic&amp;gt;y&amp;lt;/italic&amp;gt;&amp;lt;/sub&amp;gt; alloys

<p indent="0mm">A series of Mn₅₀Ni₅₀₋_{<italic>x</italic>}₋_{<italic>y</italic>}Fe_{<italic>x</italic>}Ti_{<italic>y</italic>} all-<italic>d</italic>-metal Heusler alloys ribbon samples were fabricated, and the effect of dual element Fe- and Ti-doping on the Martensitic transformation (MT) has been studied systemically. For the alloys with same Fe content, the increase of Ti content would stabilize the B2 parent phase and then reduce the MT temperature. In addition, the MT temperature also decreases with the increase of Fe in the Ti8 alloys, suggesting that the substitution of Fe for Ni atoms could stabilize the B2 parent phase. Meanwhile, part of 5M martensite transforms into L1₀ martensite with increasing Fe content higher than 2 in the Ti8 alloys, causing the decrease of thermal hysteresis. This result reveals that the driving force of the MT between L10 martensite and parent phase is lower than that between 5M martensite and parent phase. On the contrary, the MT temperature increases slightly with increasing Fe content for the <italic>y</italic>&gt;8 alloys, which is due to the possible disordered occupation of Fe atoms. The introduction of Fe has a ferromagnetic activation to establish the weak ferromagnetic ordering and increase the Curie temperature. A large change of resistivity is observed during the MT transition, which is due to the electron scattering effect of the distinct lattice distortion in martensite. Furthermore, proper annealing treatment could eliminate the effect of disordered occupation of Fe atoms and stabilize the B2 parent phase, resulting in the reduction of MT temperature for all Ti-doped samples. The present work suggests that a series of MT transitions could be obtained by dual element Fe- and Ti-doping, and it provides the candidates for future research on magnetocaloric and elastocaloric materials.

H-phase precipitation and its effects on martensitic transformation in NiTi-Hf high-temperature shape memory alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Self-accommodation and morphological characteristics of the B33 martensite in Zr–Co–Pd alloys

Thermoelastic martensitic transformation (MT) plays a vital role in shape memory effects and superelasticity of various alloys. To understand the self-accommodation mechanism of the MT of Zr–Co–Pd alloys, which demonstrate typical MT from B2 to B33 structure, the configuration of the alloy’s martensite variants and their crystallographic relationship were investigated by electron backscatter diffraction, transmission electron microscopy, and high-angle annular dark-field scanning transmission electron microscopy. Results suggest that two habit plane variants (HPVs) bounded by {021}B33 compound twinning are formed around each B2 axes of the B2 parent phase. It is determined that the minimum self-accommodation unit, to relax the strain energy accompanying MT, is a pair of trapezoid- or triangle-shaped HPVs. No lattice invariant shear exists in the martensite variants with a trapezoid- or triangle-shape. In the latter stage of MT, each HPV pair may contact and impinge on {110}B2 plane of B2 parent phase, which is shear and shuffling plane on MT. The combination of the four variants forms the roof-type morphology.

A sandwich-structured Nb/NiTi composite with good bio-compatibility, near-linear-elastic deformation and large elastic admissible strain

A combination of good bio-compatibility and large near-linear-elastic deformability was achieved in a novel sandwich-structured Nb/NiTi composite. This Nb/NiTi composite comprises of exterior β-phase Nb layers and interior NiTi layer containing the (001) compound twined B19′-martensite and trace amount of B2-parent phase. The exterior nontoxic Nb layers offer the good bio-compatibility for Nb/NiTi composite by isolating the surface of the interior NiTi layer from cells. In situ synchrotron X-ray diffraction measurement reveals that besides the intrinsic elastic deformation, a slight and recoverable B2↔B19′ stress-induced martensitic (SIM) transformation occurs in a continuous mode, which produces the extra phase transformation strain and thus contributes to the large elastic admissible strain of the Nb/NiTi composite. The large near-linear-elastic deformability in the Nb/NiTi composite is mainly due to the intrinsic elasticity, the recoverable SIM transformation proceeding in a continuous mode as well as the (001) compound twins within B19′ martensite. These results may contribute to design and develop novel biomedical composites that have good bio-compatibility and large near-linear-elastic deformability.

A multiscale study of hot-extruded CoNiGa ferromagnetic shape-memory alloys

Ferromagnetic shape-memory CoNiGa alloys have attracted much scientific interest due to their potential alternative use as high-temperature shape-memory alloys, bearing a high prospect for actuation and damping applications at elevated temperatures. Yet, polycrystalline CoNiGa, due to strong orientation dependence of transformation strains, suffers from intergranular fracture. Here, two multi-grain CoNiGa samples were prepared by a novel hot extrusion process that can promote favourable grain-boundary orientation distribution and improve the material's mechanical behaviour. The samples were investigated by multiple methods and their microstructural, magnetic, and mechanical properties are reported. It is found that a post-extrusion solutionising heat treatment leads to the formation of a two-phase oligocrystalline homogeneous microstructure consisting of an austenitic parent B2 phase and γ-CoNiGa precipitates. Reconstruction of the full 3D grain morphology revealed large, nearly spherical grains with no low-angle grain boundaries throughout the entire sample volume. The presence of γ precipitation affects the transformation behaviour of the samples, by lowering the martensitic transformation temperature, while, in conjunction with the oligocrystalline microstructure, it improves the ductility. Controlling the composition of the B2 matrix, as well as the phase fraction of the γ phase, is thus crucial for the optimal behaviour of the alloys.

H-phase Precipitation and its Effects on Martensitic Transformation in NiTi-Hf High-Temperature Shape Memory Alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Design and fabrication of a Nb/NiTi superelastic composite with high critical stress for inducing martensitic transformation and large recoverable strain for biomedical applications.

A novel Nb/NiTi superelastic composite with a shell-core structure was designed and fabricated to achieve a combination of biocompatibility and superelasticity (large recoverable strain ε accompanied by high critical stress for inducing martensitic transformation σSIM). The good biocompatibility is mainly attributed to the outer non-cytotoxic Nb shell that prevents inner NiTi core from direct contact with cells. Meanwhile, the inner NiTi core endows the composite with superelasticity through a fully reversible stress-induced martensitic transformation between B2 parent phase and B19' martensite. These results might shed some light on design and development of novel superelastic composites for biomedical applications.

Shape Change and Crystal Orientation of B19 Martensite in Equiatomic TiPd Alloy by Isobaric Test

We investigated the texture and the shape change in the equiatomic TiPd alloy, and discussed the relationship between the shape change and the atomic movements associated with martensitic transformation. Thermomechanical analyzer tests indicate that the direction of the shape change was different between the 0° and 90° samples, cutting out parallel and perpendicular to the hearth side of button ingot, respectively. In the 0° sample, shrinking and expansion were observed during the reverse and forward martensitic transformations, respectively, whereas the opposite tendency was confirmed in the 90° sample compared to the 0° sample. During the isobaric test, the martensitic variants were oriented to a (010) plane with compressive loading, and the B2 parent phase crystals also became coarse. There is a close relationship between the shape change due to the crystal orientation by the isobaric test and the shear-shuffling direction due to martensitic transformation.