Strain–temperature behavior of NiTiCu shape memory single crystals

Single crystal specimens of NiTi10Cu alloys were subjected to temperature cycling conditions under constant tensile and compressive stresses and the transformation strains were monitored. The [111] orientation exhibited the highest experimental transformation strains (6.64%) in tension while the [001] provides the highest transformation strains in compression (5.34%). These transformation strain levels are significantly higher than previously reported values on NiTiCu alloys. The theoretical treatment includes both the calculation of the CVP (correspondent variant pair) formation strain incorporating the growth of monoclinic phase from the most favorably oriented orthorhombic variant, and the concomitant detwinning of the monoclinic martensite. The experimental transformation strain values are consistently below the theoretical levels due to two main reasons: the slip deformation in the austenite domains as confirmed with TEM studies, and the incomplete transformation resulting in a mixture of orthorhombic and monoclinic phases as determined from diffraction patterns. The experimental transformation strains are higher in tension compared to compression for most single crystal orientations due to two factors: the additional strain associated with the detwinning of the B19′phase in the final microstructure (such as in [111] case), and the partial completion of the second step of the transformation limiting the compression strains.     

Tribological behaviour of biomedical Ti–Zr-based shape memory alloys

The tribological behaviour of Ti–30Zr, Ti–20Zr–10Nb and Ti–19Zr–10Nb–1Fe alloys was investigated using reciprocating friction and wear tests. X-ray diffraction(XRD) results indicate that Ti–30Zr, Ti–20Zr–10Nb and Ti–19Zr–10Nb–1Fe alloys are composed of hexagonal a'-martensite, orthorhombic a'-martensite and bcc β phases,respectively. Ti–30Zr alloy has the highest hardness of HV(273.1 ± 9.3), while Ti–20Zr–10Nb alloy exhibits the lowest hardness of HV(235.2 ± 20.4) among all the alloys.The tribological results indicate that Ti–30Zr alloy shows the best wear resistance among these alloys, corresponding to the minimum average friction coefficient of 0.052 and the lowest wear rate of 6.4x10~(-4)mm3·N~(-1)·m~(-1). Ti–20Zr–10Nb alloy displays better wear resistance than Ti–19Zr–10Nb–1Fe alloy, because the iron oxide is easy to fall off and less Nb_2O_5 films form on the worn surface of the latter.Delamination and abrasive wear in association with adhesive wear are the main wear mechanism of these alloys.     

Crystallization process and shape memory properties of Ti–Ni–Zr thin films

The crystallization process of as-deposited Ti–Ni–(10.8–29.5)Zr amorphous thin films was investigated. The Ti–Ni–Zr as-deposited films with a low Zr content exhibited a single exothermic peak due to the crystallization of (Ti,Zr)Ni with a B2 structure. In contrast, a two-step crystallization process was observed in the Ti–Ni–Zr thin films with a high Zr content. Shape memory behavior of Ti–Ni–Zr thin films heat treated at 873–1073 K was investigated by thermal cycling tests under various stresses. The martensitic transformation start temperature increased with increasing Zr content until reaching the maximum value, then decreased with further increasing Zr content. The inverse dependence of transformation temperature on Zr content in the thin films with a high Zr content is due to the formation of a NiZr phase during the crystallization heat treatment. The formation of the NiZr phase increased the critical stress for slip but decreased the recovery strain.     

Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

Effects of carbon addition and aging on the shape memory effect of Fe–Mn–Si–Cr–Ni alloys

The effects of carbon content and aging treatment on the microstructures and shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys were studied. It was found that the carbon content possessed significant effects on the shape recovery ratio of Fe–Mn–Si–Cr–Ni alloys. The shape memory effect of alloys containing different carbon amount could be improved through aging.     

The compositional stability of the P-phase in Ni–Ti–Pd shape memory alloys

The precipitation of the P-phase in Ni–Ti–Pd and Ni–Ti–Pt shape memory alloys has been shown to dramatically increase the martensitic transformation temperature and strength in Ni-rich ternary alloys, yet little is known about the phase's compositional stability. Therefore, the compositional limits of the P-phase have been systematically studied by varying the Pd and Ni content while maintaining the general P-phase Ti₁₁(Ni + Pd)₁₃ stoichiometry. Each alloy was solutionized at 1050 °C followed by water quenching, and aging at 400 °C for 100 h. Four distinct phases were identified by electron and x-ray diffraction: Ti₂Pd₃, B2 NiTi, P- and P1-phases. The latter precipitate phases became more stable with increasing Ni at the expense of the Pd content. Atom probe tomography revealed the P-phase composition to be 45.8Ti–29.2Ni–25Pd (at.%) or Ti₁₁(Ni₇Pd₆) as compared to the P1-phase 44.7Ti– 45.8Ni–9.4Pd (at.%) or Ti₅Ni₅Pd.     

Effect of Si on the reversibility of stress-induced martensite in Fe–Mn–Si shape memory alloys

Fe–Mn–Si is a well-characterized ternary shape memory alloy. Research on this alloy has consistently shown that the addition of 5–6 wt.% Si is desirable to enhance the reversibility of stress-induced martensite vis-à-vis shape memory. This paper examines the effect of Si on the morphology and the crystallography of the martensite in the Fe–Mn–Si system. It is concluded that the addition of Si increases the c/a ratio of the martensite, reduces the transformation volume change and decreases the atomic spacing difference between the parallel close-packed directions in the austenite–martensite interface (habit) plane. It is proposed that, in addition to austenite strengthening, Si enhances reversibility by reducing the volume change and the interfacial atomic mismatch between the martensite and the austenite. Although shape memory is improved, transformation reversibility remains limited by the necessary misfit dislocations that accommodate the atomic spacing differences in the interface.     

Internal structures and shape memory properties of sputter-deposited thin films of a Ti–Ni–Cu alloy

Low-temperature heat treatments of the sputter-deposited amorphous films, which were previously proved to be a new method to produce very good shape memory properties for Ti-rich Ti–Ni alloys, have been applied to a ternary Ti–43.0Ni–6.2Cu alloy (at.%). The basically same nanometric structures as in the binary alloy are formed, i.e. the nanometric structures consist of extremely thin plate precipitates of bct structure, which are formed on {100} planes of the parent B2 structure and have the c-axis normal to the habit planes. High-shape recovery stresses of about 500 MPa with recoverable shape strains of 5% are obtained without accompanying any permanent strains. A shape recovery stress of more than 870 MPa is attained if it is allowed to involve about 1% permanent strain. Although these bct precipitates have large tetragonalities, they are perfectly coherent with the parent bcc lattice. The maximum shape recovery stress is nearly twice that of the Ti-rich Ti–Ni binary alloy having a similar nanometric structure. It is suggested that this remarkable increase in recovery stress may be attributed to the change in Burgers vector of dislocations caused by partial disordering in Ti–Ni–Cu alloys. It is emphasized that the shape recovery stress in this ternary alloy is four times that of the Ti₂Ni containing samples and 10 times that of a bulk Ti–45Ni–5Cu alloy.     

Small-angle neutron scattering of precipitates in Ni–Ti shape memory alloys

Small-angle neutron scattering was performed on polycrystals of Ni–(46–49) at.% Ti quenched in ice water from the solid solution. The presence of small precipitates of a radius of about 1 nm was found for Ni–(46, 47 and 48) at.% Ti. Assuming a composition of Ni₄Ti₃ of the precipitates, their volume fraction varies from 7% to 0.3%. No precipitates are found if the Ti content is closer to stoichiometric NiTi. The formation of these precipitates already during quenching seems to suppress the formation of martensite. Ni–(47.9 and 48.5) at.% Ti were further aged for 1 h at 553 K, and small-angle scattering shows a fully established precipitate microstructure. The particles have a radius of about 1.5 nm and a mean interparticle distance of 4.8–5.8 nm. From the integrated small-angle scattering curves, a volume fraction of Ni₄Ti₃ particles of about 20% is obtained.     

Effect of chemical component on shape memory effect of Fe-Mn-Si-Ni-C-RE shape memory alloy

Effect of chemical component on shape memory effect (SME) of Fe-Mn-Si-Ni-C-RE shape memory alloys was studied by bent measurement, thermal cycle training, SEM etc. Results of study indicate that the alloys with high Mn content (25%) appeare better SME, especially in lower strain. SME improves evidently when Si is higher content, especially it's range from 3% up to 4%. But brittleness of Fe-Mn-Si-Ni-C-RE alloy increases by increasing the Si content. SME of the alloy is weakening gradually as carbon content increases under small strain (3%). But in the condition of large strain (above 6%), SME of the alloy whose carbon content ranges from 0.1 % to 0.12% shows small decreasing range, especially of alloy with the addition of compound RE.     

A simulation study of the shape of β′ precipitates in Mg–Y and Mg–Gd alloys

The metastable β′ phase is a key strengthening precipitate phase in a range of Mg–RE (RE: rare-earth elements) based alloys. The morphology of the β′ precipitates changes from a faceted and nearly equiaxed shape in Mg–Y alloys to a truncated lenticular shape in Mg–Gd alloys. In this work, we study effects of interfacial energy and coherency elastic strain energy on the morphology of β′ precipitates in binary Mg–Y and Mg–Gd alloys using a combination of first-principles calculations and phase-field simulations. Without any free-fitting parameters and using the first-principles calculations, CALPHAD databases and experimental characterizations as model inputs (lattice parameters of the β′ phase, elastic constants and chemical free energy of Mg matrix and interfacial energies of the coherent β′/Mg matrix interfaces), the phase-field simulations predict equilibrium shapes of β′ precipitates of different sizes that agree well with experimental observations. Factors causing the difference in the equilibrium shape of β′-Mg₇Y and β′-Mg₇Gd precipitates are identified, and possible approaches to increase the aspect ratio of the β′ precipitates and thus to enhance the strength of Mg–RE alloys are discussed.     

Effect of ternary alloying elements on the shape memory behavior of Ti–Ta alloys

The effect of ternary alloying elements (X = V, Cr, Fe, Zr, Hf, Mo, Sn, Al) on the shape memory behavior of Ti–30Ta–X alloys was investigated. All the alloying elements decreased the martensitic transformation temperatures. The decrease in the martensitic transformation start (M_s) temperature due to alloying was affected by the atomic size and number of valence electrons of the alloying element. A larger number of valence electrons and a smaller atomic radius of an alloying element decreased the M_s more strongly. The effect of the alloying elements on suppressing the aging effect on the shape memory behavior was also investigated. It was found that the additions of Sn and Al to Ti–Ta were effective in suppressing the effect of aging on the shape memory behavior, since they strongly suppress the formation of ω phase during aging treatment. For this reason the Ti–30Ta–1Al and Ti–30Ta–1Sn alloys exhibited a stable high-temperature shape memory effect during thermal cycling.     

Corrosion behavior of Cu–Al–Be shape memory alloys with different compositions and microstructures

The corrosion behavior of Cu–Al–Be shape memory alloys with different microstructures and Be content in a 3.5% NaCl solution was studied by weight loss, cyclic anodic polarization and chronoamperometric measurements. The beryllium has a beneficial effect in β alloys. A pitting potential of −100 mV/SCE was found by anodic polarization tests for all the studied alloys, corresponding to the formation of pits produced by severe dealuminization. Samples with precipitates were more susceptible to pit formation. The corrosion behavior is strongly affected by the alloy microstructural conditions, and the β samples present higher pitting resistance and repassivation ability.     

Stress–strain behavior of porous Ni–Ti shape memory intermetallics synthesized from powder sintering

The microstructure and stress–strain behavior of porous Ni–Ti intermetallics fabricated by powder sintering of Ni, Ti and/or TiH₂ are investigated. The pores are small and well distributed in the present porous Ni–Ti alloys and the phase constituent and stress–strain behavior of porous Ni–Ti alloys are significantly influenced by the sintering conditions (sintering temperature and sintering time) and TiH₂ additions. With increasing sintering temperature, the pseudoelasticity (PE) increases while the hysteretic width decreases. A second compression also leads to an increase of PE, elastic modulus and related deformation resistance. Further, the stress–strain behavior of porous Ni–Ti alloys is different in some way from that of bulk Ni–Ti alloys and other porous materials.     

Direct measurement of large reversible magnetic-field-induced strain in Ni–Co–Mn–In metamagnetic shape memory alloys

Direct measurements of reversible magnetic-field-induced strain (MFIS) on a single crystalline Ni₄₅Co₅Mn_36.5In_13.5 metamagnetic shape memory alloy were attained via magnetic-field-induced martensitic transformation under different stress levels and at various temperatures. This was achieved using a custom-designed micro-magneto-thermo-mechanical testing system capable of applying constant stress while measuring strain and magnetization simultaneously on the samples, which can fit into conventional superconducting magnets. MFIS levels are reported as a function of temperature, magnetic field and external bias stress. It was necessary to apply an external bias stress in these materials to detect a notable MFIS because a magnetic field does not favor a specific martensite variant resulting in no shape change even though magnetic field leads to reversible martensitic transformation. Fully recoverable transformation strains up to 3.10% were detected under repeated field applications in the presence of different compressive stress levels up to 125 MPa. The bias stress opposes the field-induced martensite-to-austenite phase transformation and causes the critical field for the transformation to increase at a given temperature in accordance with the Clausius Clapeyron relationship. The effect of the bias stress on the kinetic arrest of austenite is also explored.     

The ferromagnetic shape memory system Fe–Pd–Cu

A new ferromagnetic shape memory thin film system, Fe–Pd–Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc–fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe–Pd–Cu samples showed an enhanced stability of the disordered transforming Fe₇₀Pd₃₀ phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc–fct transformation temperature.