Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Application of severe plastic deformation by torsion to form amorphous and nanocrystalline states in large-size TiNi alloy sample

Results of investigations of the initial structure of large-size Ti_49.4Ni_50.6 samples subjected to severe plastic deformation by torsion under a high pressure (HPT) are reported. The study was performed using transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and measurements of mechanical properties. Under an applied pressure of 6 GPa, the alloy was found to undergo a martensitic B2 → B19′ transformation. Even after HPT using a single revolution of anvils, the granular structure of titanium nickelide is refined so that there is formed a nanocrystalline state of B2 austenite (i.e., the reverse martensitic B19′ → B2 transformation occurs) and amorphization of the alloy begins. The HPT with a high number of revolutions leads to the almost complete amorphization of the alloy, which is explained by a high degree of shear deformation. In this case, all nanocrystalline inclusions in the amorphous matrix have an ordered B2 structure.     

Effect of heat and thermomechanical treatments on the structure and physical and mechanical properties of the N30K10T3 invar

Invar alloy N30K10T3, whose austenite is metastable with respect to the martensitic γ → α transformation that occurs upon cooling below the martensitic point (M _s = ?80°C), has been studied. The following six ways of the alloy strengthening have been tested: (1) aging (a) in a temperature range of ΔT _a = 20–700°C; (2) liquid-nitrogen cooling (lnc) of the material preliminarily hardened by aging under the aforementioned conditions (route 1) (a + lnc); (3) preliminary phase-transformation-induced hardening (ph) (γ → α → γ_ph) and aging in the temperature range of ΔT _a (ph + a); (4) liquid-nitrogen cooling of the material preliminary hardened via route 3 (ph + a + lnc); (5) preliminary cold deformation (to 30%) at room temperature and aging in a temperature range of ΔT _a (cd + a); and (6) liquid-nitrogen cooling of the material preliminary hardened via route 5 (cd + a + lnc). The six ways of hardening were found to affect the hardness, electrical conductivity, magnetic permeability, and temperature dependence of the thermal expansion coefficient.     

Mechanisms of martensitic phase transformations in body-centered cubic structural metals and alloys: Molecular dynamics simulations

Two different mechanisms of the stress-induced martensitic phase transformation at the crack tip in body-centered cubic (bcc) structural metals and alloys have been studied by molecular dynamics simulations. For cracks with 〈1 0 0〉 crack fronts, the bcc (B2) to face-centered cubic (fcc) (L1₀) phase transformation along the Bain stretch occurs. Whereas for cracks with 〈1 1 0〉 crack fronts, either the bcc (B2) to fcc (L1₀) or the bcc (B2) to hexagonal close-packed (hcp) transformation is the candidate. We have found that the combination of local stress and crystal orientation plays an important role in the mechanism of the martensitic transformation. Thus a simple way to determine the mechanism of the martensitic transformation is developed. The complicated deformation behaviors at the crack tip in bcc iron and B2 NiAl are discussed in terms of this method.     

The microstructure and properties of rapidly quenched Ti-rich Ti-Ni binary alloys with shape memory effects

The structure, phase composition, and martensitic transformations in binary titanium-rich Ti-Ni alloys with shape memory effects, produced by ultrarapid quenching using melt jet spinning, have been studied using electron microscopy, X-ray diffraction, and measurements of some physicomechanical properties in a wide temperature range. The alloys with a Ti content that exceeded the stoichiometric composition by 5% and more can be produced in an amorphous state. The alloys with a smaller deviation from the stoichiometry, as well as the Ti₅₀Ni₅₀ alloy, are crystallized in a submicrocrystalline state and undergo a B2 → B19’ martensitic transformation at temperatures above room temperature. They have high strength and plastic properties and demonstrate narrow-hysteresis shape-memory effects.     

Effect of preliminary plastic deformation on the structure and physicomechanical properties of aging invar alloy N30K10T3

The invar alloy N30K10T3 after water quenching from 1150°C (austenite, γ phase) has the temperature of the start of martensitic transformation M _s ≈ ?80°C and the Curie temperature T _C ≈ 200°C. The effect of aging-induced phase decomposition in a deformed supersaturated solid solution on its hardness HV, electrical conductivity σ, magnetic permeability μ, and linear expansion coefficient β has been studied. It has been shown that cold plastic deformation of the alloy (at 20°C) to 30–50% increases its hardness, virtually does not change the conductivity, and decreases permeability. Aging of the deformed invar results in increasing HV and σ and decreasing μ. At room temperature, the deformed invar has a low linear expansion coefficient; its magnitude grows the faster, the greater the aging temperature T _a. Plastic deformation increases the density of dislocations, which form a banded substructure in austenitic grains. Besides, a metastable martensitic phase has been observed, which undergoes a reverse martensitic transformation into austenite upon heating in the temperature range from 550°C to 650°C. This transformation causes a decrease in the linear expansion coefficient β(T) of the deformed material. In samples aged at T _a = 700°C (after deformation), an athermal aging-induced martensite (α_a phase) appears after cooling them to 20°C. The appearance of the α_a phase is due to an increase in the temperature of the start of the martensitic transformation to above the room temperature caused by aging. In the samples containing the α_a phase, there is observed a decrease in β in the temperature range from 350 to 670°C, which is due to the reverse transformation of the aging-induced martensite into austenite (α_a → γ).     

Martensitic transformation and magnetic properties in Ni–Fe–Ga–Co magnetic shape memory alloys

Ni–Fe–Ga–Co is a promising system for magnetic shape memory alloy applications, due to its good ductility, mobile twin boundaries and high transformation temperatures. Unlike previous studies which focused on compositions with a Ga content of 27 at.%, here the martensitic transformation and magnetic properties over a large composition range of Ni_54−xFe₂₀Ga₂₆Co_x, Ni_54−xFe₁₉Ga₂₇Co_x, Ni_56−xFe₁₇Ga₂₇Co_x and Ni_54−xFe₁₈Ga₂₈Co_x (x = 0, 2, 4) are investigated. The martensitic transformation temperature T_m and the Curie temperature T_c can be tailored in a wide range by changing composition and heat treatment. A coupling of martensitic and magnetic transformations at ∼90 °C is found for Ni₅₂Fe₁₇Ga₂₇Co₄. Additionally, the effect of thermal cycling on the martensitic transformation of single- and two-phase Ni–Fe–Ga–Co alloys is discussed. Furthermore, an intermediate face-centered cubic phase induced by powderization and transformed into a body-centered cubic phase by aging is reported. The saturation magnetization is significantly decreased by powderization, while recovered by the subsequent aging.     

Microstructure and martensitic transformation behavior of Ni56-xMn25FexGa19 shape memory alloys简

Microstructure, martensitic transformation behavior, mechanical and shape memory properties of Ni56-x Mn25 Fex Ga19(x = 0, 2, 4, 6, 8) shape memory alloys were investigated using optical microscopy(OM), X-ray diffraction analysis(XRD), differential scanning calorimeter(DSC), and compressive test. It is found that these alloys are composed of single non-modulated martensite phase with tetragonal structure at room temperature, which means substituting Fe for Ni in Ni56 Mn25 Ga19 alloy has no effect on phase structure. These alloys all exhibit a thermoelastic martensitic transformation between the cubic parent phase and the tetragonal martensite phase. With the increase of Fe content, the martensitic transformation peak temperature(Mp) decreases from 356 °C for x = 0 to 20 °C for x = 8, which is contributed to the depressed electron concentration and tetragonality of martensite. Fe addition remarkably reduces the transformation hysteresis of Ni–Mn–Ga alloys. Substituting Fe for Ni in Ni56 Mn25 Ga19 alloy can decrease the strength of the alloys and almost has no influence on the ductility and shape memory property.     

Low Young’s modulus of Ti–Nb–Ta–Zr alloys caused by softening in shear moduli c′ and c44 near lower limit of body-centered cubic phase stability

The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti–Nb–Ta–Zr β-phase alloys with a body-centered cubic structure, developed for biomedical applications, were investigated using their single crystals, in order to clarify the origin of the low Young’s modulus in polycrystals. Transmission electron microscopy observations clarified that α″ martensitic transformation occurred in a temperature range that depended on the β-phase stability below room temperature. Electromagnetic acoustic resonance measurements clarified that the shear moduli c′ and c₄₄ of single crystals softened upon cooling from room temperature and became rather low near the martensitic transformation start temperature, i.e. the lower limit of β-phase stability. An analysis by the Hill approximation indicates that low c′ and c₄₄ caused the low Young’s modulus, and thus it is probable that the softening in c′ and c₄₄ is the origin of the low Young’s modulus.     

Site preference,electronic structure and possible martensitic transformation in Heusler alloys Ni2CoZ (Z = Al,Ga, In,Si, Ge,Sn, Sb)

The site preference, electronic structure, magnetic properties and martensitic transformation in Heusler alloys Ni₂CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) have been investigated by first-principles calculations. Different from literature, it is found that these Ni₂CoZ alloys tend to form XA structure (Hg₂CuTi-type), except for Ni₂CoIn, in which L2₁ (Cu₂MnAl-type) structure is preferable. Theoretical calculation reveals that the tetragonal martensitic phase has a lower total energy compared with the cubic austenitic phase, therefore, a structural transition from cubic to tetragonal is likely to happen in these Ni₂CoZ alloys. The largest energy difference is observed in Ni₂CoIn. It is interesting that Ni₂CoSi is paramagnetic in austenitic state, while is ferromagnetic in martensitic state. This leads to a large change in the total moment, which is meaningful for the realization of magnetic field-induced martensitic transformation in this alloy.     

Effect of heat treatment on structural and phase transformations in the Ti49.5Ni50.5 alloy amorphized by high-pressure torsion

Results are presented for a study of the structural and phase transformations that occur in the titanium-nickelide shape-memory alloy Ti_49.5Ni_50.5 subjected to heat treatment after deformation-induced amorphization by megaplastic high-pressure torsion (HPT) using five or ten revolutions of Bridgman anvils. The investigations were performed using transmission and scanning electron microscopy, X-ray diffraction, and measurements of the temperature dependences of electrical resistivity and magnetic susceptibility. It has been established that the crystallization of the alloy already occurs upon low-temperature treatment, beginning with ～500 K. The evolution of the structure and the stage character of the development of crystallization and recrystallization processes depending on temperature have been determined. It has been shown that the annealing of the amorphized alloy makes it possible to obtain highly homogeneous nanostructured, submicrocrystalline, or bimodal states in the B2 austenite. A complete diagram of thermoelastic martensitic transformations of the B2 austenite has been constructed in the region from a nanostructured to a conventional polycrystalline state (with a grain size of 20–50 μm). The effect of size on the stabilization of austenite has been revealed and its specific features have been studied for the B2 → R and B2(R) → B19′ martensitic transformations depending on the structural state of the alloy.     

Size dependence of martensite transformation temperature in nanostructured Ni-Mn-Sn ferromagnetic shape memory alloy thin films

In the present study, we report the influence of grain size on structural and phase transformation behaviour of nanostructured Ni-Mn-Sn ferromagnetic shape memory alloy thin films synthesized by dc magnetron sputtering. With increase in substrate temperature, the structural phase changes from austenite with L2₁ cubic crystal structure to martensite with monoclinic structure. In addition, field-induced martensite-austenite transformation is observed in magnetization studies using superconducting quantum interference device magnetometer. The martensitic transformation behaviour of these films depends critically on the microstructure and dimensional constraint. Both, the martensite start temperature (M_s) and austenite finish temperature (A_f) of these nanostructured films decreases with decreasing grain size. The excess free volume associated with grain boundaries has been observed to increase with decrease in grain size which in turn leads to an increase in the number of grain boundaries. It has been proposed that the grain boundaries impose constraints on the growth of the martensite and confine the transformed volume fraction in nanocrystalline structure. A martensite phase nucleated within a grain will be stopped at the grain boundaries acting as obstacles for martensite growth. The investigations revealed that below a critical grain size of 10.8 nm, the austenite phase is observed to be more stable than the martensite phase which leads to the complete suppression of martensitic transformation in these films.     

Investigation of the martensitic transformation and the damping behavior of a superelastic Ti–Ta–Nb alloy

In this study the α″ stress-induced martensitic transformation and damping behaviour of the superelastic β-Ti–25Ta–25Nb alloy are investigated by tensile tests at room temperature and by dynamic mechanical analysis (DMA) in tensile mode for different applied stresses. Tensile tests show a fully non-linear elastic domain and, consequently, a specific method is proposed to determine the elastic modulus. Due to the wide range of temperature over which the martensitic transformation occurs in this class of alloys, the martensitic start temperature, M_s, is not a relevant parameter to characterize the transformation and the temperature M_max corresponding to the temperature of maximum transformation is used. The important gap between these two temperatures explains the fully non-linear elastic behaviour of this alloy during conventional tensile tests. It is observed that two main damping sources occur in this alloy: friction at austenite/martensite interfaces during the martensitic transformation and friction at martensite/martensite interfaces at lower temperature. However, a third unexpected damping peak is also observed at high stress. Its origin is discussed with respect to the orientation of the applied stress and with regard to the most favourably oriented martensite variants determined by Schmid factor analysis.     

Influence of annealing on magnetic field-induced structural transformation and magnetocaloric effect in Ni–Mn–In–Co ribbons

A study of the magnetic field-induced martensitic transformation and magnetocaloric effect in Ni₄₅Mn₃₇In₁₃Co₅ and Ni₄₆Mn₃₅In₁₄Co₅ ribbons prepared by melt-spinning was carried out. Annealing significantly increases the degree of ordering in the austenite phase, reduces the critical field and hysteresis of the magnetically induced martensitic transformation and increases the magnetically induced shift of martensitic transformation temperatures. For the most In-rich sample, Ni₄₆Mn₃₅In₁₄Co₅, annealing at 900 °C for 2 h leads to the formation of Co-rich face-centered cubic γ precipitates. The Curie temperature of the γ phase is about 370 K. The formation of the second phase has little impact on the hysteresis, but broadens the transformation interval and reduces the magnetic entropy change.     

Magnetic-field-induced martensitic transformations in Ni47 − x Mn42 + x In11 alloys (with 0 ≤ x ≤ 2)

Magnetic properties and martensitic transformations in the Ni_{47 ? x} Mn_{42 + x} In₁₁ alloys (with 0 ≤ x ≤ 2) have been studied. The magnetic-field-induced martensitic transformation was found to be observed for all the alloys. The critical temperatures of magnetic and structural phase transformations, temperature dependences of spontaneous magnetization of austenite and martensite, and the critical field, at which the martensitic transformation occurs, have been determined based on magnetic measurements performed for the alloys under study. The spontaneous magnetization of the alloys in the martensitic state has been shown to be lower than that in the magnetic-field-induced austenitic state by a factor of six.     

Accumulation of Residual Strain in TiNi Alloy During Thermal Cycling

Residual strain accumulation during thermal cycling of the Ti₅₀Ni₅₀ alloy under a constant stress of 200 MPa through the temperature range of complete and incomplete forward martensitic transformation was studied. The temperature range of the forward martensitic transformation during thermal cycling was chosen as 25, 50, 75, or 100% of the M _s-M _f interval measured in the first cycle. It was shown that intensive accumulation of residual strain took place in the last stage of the forward transformation. It was observed that resistivity increased more rapidly with an increase of the fraction of the temperature range of forward martensitic transformation.     

Martensitic transformation and magnetic properties in ferromagnetic shape memory alloy Ni43Mn46Sn11−xSix

This study investigated the effects of Si addition and heat treatment on the martensitic transformation and magnetic properties of Ni₄₃Mn₄₆Sn_11?xSi_x (x = 1, 2, 3) alloys. The martensitic transformation temperatures were found to increase with increasing Si content in the alloy. A magnetic field-induced martensite-to-austenite transformation was found in Ni₄₃Mn₄₆Sn₁₀Si₁. Ageing of the Ni₄₃Mn₄₆Sn₁₀Si₁ alloy at 673 K resulted in the formation of a (Mn,Ni)–SiSn precipitate. The precipitate contains very low Sn content, causing the increase of the Sn content in the matrix phase and a decrease of martensitic transformation temperature. Ageing at 573 K is found to increase the Curie temperature and the saturation magnetization. This is attributed to the increase of atomic ordering of the matrix. Solution treatment of the aged samples at 1073 K was effective to restore the original transformation behaviour and magnetic properties.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.