Stability Of Martensite Phase Research Articles

First-principles phonon-based model and theory of martensitic phase transformation in NiTi shape memory alloy

Remarkable shape memory property of Nitinol (NiTi) originates from a diffusion-free reversible phase transition it exhibits upon cooling from the cubic austenite to a low-symmetry martensite structure. Atomistic mechanisms of this martensitic transformation (MT) and consequent emergence of its various low-symmetry structures are not understood yet. Starting with first-principles density functional theoretical calculations, we present here a phonon-based model and its statistical mechanical analysis to obtain atomistic insights into martensitic phases and transformation in NiTi. We uncover seven order parameters that are relevant to the MT in NiTi. From Monte Carlo simulations of an effective model Hamiltonian derived to capture its low energy landscape, we determine its soft phonons and establish the cell-doubling M5′ mode as the primary order parameter. Using Landau theoretical analysis, we show that relative strengths of its third-order coupling with secondary order parameters (e.g. strain) determine the symmetry of low-T structures emerging at its MT. These couplings can be used as the descriptors of stability of martensitic phases that will guide the strategies to improve the shape memory of NiTi through substitutional alloying.

First-principles study of Zn-doping effects on phase stability and magnetic anisotropy of Ni-Mn-Ga alloys

The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature TM and Curie temperature TC were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔEA−NM, and energy differences between paramagnetic and ferromagnetic state, ΔEPM−FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in TM. TC is affected only weakly or slightly decreases, because ΔEPM−FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both TM and TC. Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in TC but also in smaller increase in TM. Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing TC can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.

Macroscopic modeling of strain-rate dependent energy dissipation of superelastic SMA dampers considering destabilization of martensitic lattice

Superelastic shape-memory alloys (SMAs) are unique smart materials with a considerable energy dissipation potential for dynamic loadings with varying strain-rates. The energy dissipation arises from a hysteretic phase transformation of the polycrystalline atomic grid structure. In fact, the nucleation from austenite to martensite phase and vice versa exhibits a strong thermomechanical coupling. The hysteresis depends on the latent heat generated by the austenitic-martensitic transformation and the convection of that heat. Due to the thermomechanical coupling, the martensitic nucleation stress level and thus the propagation of martensitic phase fronts strongly depends on the material temperature. High strain-rate interferes with the release of the latent heat to the environment and determines quantity, position and propagation of martensitic transformation bands. Lastly, high strain-rate reduces the hysteresis surface. The propagation velocity and quantity of martensitic bands have an impact on martensitic phase stability. The degree of atomic disorder and accordingly the change in entropy influences the reverse phase transformation from martensite to austenite. In other words, the stability of the martensitic state affects the stress level of the reverse transformation. However, in phenomenological superelastic SMA models, which are used to simulate energy dissipation behavior, the effects of the rate-dependent entropy change are not considered in particular. To incorporate the rate-dependent entropy change, we improved a one-dimensional numerical model by introducing an additional control variable in the free energy formulation for solid-solid phase transformation of superelastic SMAs. In this model, the observed effects of the strain-rate on the reverse transformation are taken into account by calculating the rate-dependent entropy change. A comparison of the numerical results with the experimental data shows that the model calculates the dynamic superelastic hysteresis of SMAs more accurately.

Site occupancy, composition and magnetic structure dependencies of martensitic transformation in Mn2Ni1 + xSn1−x

A delicate balance between various factors such as site occupancy, composition and magnetic ordering seems to affect the stability of the martensitic phase in . Using first-principles DFT calculations, we explore the impacts of each one of these factors on the martensitic stability of this system. Our results on total energies, magnetic moments and electronic structures upon changes in the composition, the magnetic configurations and the site occupancies show that the occupancies at the 4d sites in the inverse Heusler crystal structure play the most crucial role. The presence of Mn at the 4d sites originally occupied by Sn and its interaction with the Mn atoms at other sites decide the stability of the martensitic phases. This explains the discrepancy between the experiments and earlier DFT calculations regarding phase stability in NiSn. Our results qualitatively explain the trends observed experimentally with regard to martensitic phase stability and the magnetisations in Ni-excess, Sn-deficient NiSn system.

Residual stress induced stabilization of martensite phase and its effect on the magnetostructural transition in Mn-rich Ni-Mn-In/Ga magnetic shape-memory alloys

The irreversibility of the martensite transition in magnetic shape memory alloys (MSMAs) with respect to external magnetic field is one of the biggest challenges that limits their application as giant caloric materials. This transition is a magneto-structural transition that is accompanied with a steep drop in magnetization (i.e., 'delta M') around the martensite start temperature (Ms) due to the lower magnetization of the martensite phase. In this communication, we show that 'delta M' around Ms in Mn rich Ni-Mn based MSMAs gets suppressed by two orders of magnitude in crushed powders due to the stabilization of the martensite phase at temperatures well above the Ms and the austenite finish (Af) temperatures due to residual stresses. Analysis of the intensities and the FWHM of the x-ray powder diffraction patterns reveals stabilized martensite phase fractions as 97, 75 and 90% with corresponding residual microstrains as 5.4, 5.6 and 3% in crushed powders of the three different Mn rich Ni-Mn alloys, namely, Mn1.8Ni1.8In0.4, Mn1.75Ni1.25Ga and Mn1.9Ni1.1Ga, respectively. Even after annealing at 773 K, the residual stress stabilised martensite phase does not fully revert to the equilibrium cubic austenite phase as the magneto-structural transition is only partially restored with reduced value of 'delta M'. Our results have very significant bearing on application of such alloys as inverse magnetocaloric and barocaloric materials.

Effect of nano Al2O3 addition on mechanical properties and wear behavior of NiTi intermetallic

It has been found that the high wear resistance of NiTi alloy is mainly attributed to its pseudoelasticity which is only effective within a small temperature range. It is believed that pseudoelasticity becomes ineffective by applying high-load wear condition which yields plastic deformation and temperature increment during wear test. Therefore, the enhanced wear resistance can be obtained from the improvement of mechanical property of the alloy without much reduction of pseudoelasticity. In this study, a low weight percentage of hard Al2O3 nanoparticles were added to NiTi atomized powders. The resultant powder mixture was homogenized by ball milling and sintered in a vacuum furnace in order to improve the wear property of the composite in comparison with the NiTi alloy. The results demonstrated that the addition of nanoparticles increased the stability of martensite phase. Nanoindentation test results showed that both hardness and elastic modulus were considerably increased in the composite samples in comparison with pure NiTi, while the pseudoelasticity was not significantly reduced. The worn surfaces were investigated using a scanning electron microscope equipped with energy dispersive X-ray. The wear test results confirmed the improved wear performance of NiTi matrix after the addition of nanoparticles under both low and high loads can be mainly attributed to superior mechanical properties combined with pseudoelasticity effect of the composite samples.

Effect of thermal and thermo-mechanical cycling on the microstructure of Ni-rich NiTi shape memory alloys

Microstructural changes of Ni-rich NiTi shape memory alloy during thermal and thermo-mechanical cycling have been investigated using Electron Back Scattered Diffraction. A strong dependence of the orientation of the prior austenite grain on the misorientation development has been observed during thermal cycling and thermo-mechanical cycling. This effect is more pronounced at the grain boundaries compared to grain interior. At a larger applied strain, the volume fraction of stabilized martensite phase increases with increase in the number of cycling. Deformation within the martensite leads to stabilization of martensitic phase even at temperatures slightly above the austenite finish temperature. Modulus variation with respect to temperature has been explained on the basis of martensitic transformation.

Influence of impurities on phase stability of martensites in titanium

The influence of H, C, N and O impurities on the phase stability of titanium was studied by first principles total energy calculations. The occupation energies of the impurities were estimated to identify their site preferences. All impurities considered prefer to occupy the octahedral site except for H, which tends to occupy the tetrahedral site in the β phase. Electronic structures were analyzed to clarify the intrinsic influence mechanisms of impurity on the stability of martensitic phases. It was found that the density of states around the Fermi energy, which was affected dramatically by impurity, and the bonding interactions between impurity and titanium were connected to the phase stability of Ti. Elastic constants of impurity-containing systems were estimated from curves of energy against strain to evaluate the mechanical stability of these systems. It was shown that the α″ phase can not be stabilized by impurities considered here, while the α′ phase (regardless of the impurities) and H- and C-containing β phase are thermodynamically stable and also satisfy the mechanical stability criteria.

Martensite aging effects on the dynamic properties of Au–Cd shape memory alloys: Characteristics and modeling

We revisit the martensite aging effect in Au–Cd alloys, by studying the evolution of dynamic properties (i.e. storage modulus and internal friction) with aging time. We found that the storage modulus shows a gradual increase while the internal friction shows a gradual decrease with increasing aging time. Moreover, the evolution of dynamic properties with aging time obeys a simple relaxation function with activation energy of 0.46 eV in AuCd 49.5 alloy. The dependence of such evolution on temperature, defect concentration, frequency and displacement amplitude was also well characterized. All the observed characteristics are understood by the gradual stabilization of martensite phase during aging, driven by a symmetry-conforming short-range ordering tendency of point defects. Furthermore, we proposed a homogenous Landau-type model to describe the response of storage modulus during aging by introducing a time-dependent variable coupled with the order parameter. Such systematic investigation on the dynamic properties during martensite aging is crucial for applications under mechanical vibrations.

A thermodynamic explanation for martensitic phase stability of nanostructured Fe–Ni and Co metallic materials

In order to explain the experimental facts that start temperatures of reverse martensite transformation (As) for nanostructured Fe–Ni and Co are lower than those for their coarse-grained counterparts, a thermodynamic model was established in this paper. The thermodynamic analysis shows that the decrease of As for nanostructured Fe–Ni and Co can be attributed to the difference of surface free energy for martensite and austenite. The correlation between the decrease of As and start temperature of martensitic transformation (Ms) in nanostructured material is further proposed.

Evaluation of Magnetostriction of the Single-Variant Ni-Mn-Ga Martensite

The temperature dependence of ordinary magnetostriction of the axially compressed Ni–Mn–Ga alloy with the low values of shear elastic modulus C'(T) ~ 1 – 10 GPa has been evaluated theoretically in the framework of Landau theory. The computations showed that the compression with 50 MPa stress reduces the ordinary magnetostriction by factor 3 at room temperature. Nevertheless, the magnetostriction of compressed alloy exceeds the value of 10–4 in the whole temperature range of martensitic phase stability, strongly depends on the temperature in the vicinity of martensitic transformation (MT), and is practically constant well below MT temperature. Therefore, the purposeful search for the alloy with the low value of shear elastic modulus and high MT temperature (well above 300 K) may result in the discovery of good magnetostrictive material. This material will posses the temperature-independent magnetostriction value about of 10–4 –10–3 and rather low electric conductivity enabling the technical applications of this material in dynamic regimes.

Two important aging effects on the martensite phase in CuZnAl alloys. Rubber effect and stabilization of martensite

Two well-known aging effects on the martensite phase, rubber effect and stabilization of martensite, are closely related to each other. They appear in the same specimen after the same aging treatment. It has been shown that the time constants for these effects are quite similar. However, the causes to bring about these effects may not be the same. For the rubber effect, a short-range-order model has been proposed (K.Marukawa and K.Tsuchiya, Scripta Met. Mater., 32, 77, (1995)). Although somemodifications on the model has been suggested, the original one seems to be most appropriate. In this report, it is shown that the principal cause for the stabilization is the change in the long-range-order of the alloy, while the long-range-order has no relation to the rubber effect. There are two cases for the stabilization in Cu-Zn-Al alloys. In one case, the parent phase has B2 order and the B2 order disintegrates to the disordered state during the aging after the martensitic transformation. In the other case, the matrix phase has L21 structure and the ordered structure transforms to a different ordered structure, D022. Both these changes in the long-range order result in a stabilization of martensite phase. The relation between these effects and the change in the long-range or short-range order is examined quantitatively with the help of the Monte Carlo simulation. It is shown that the transformation temperature is quite sensitive to the change in the long range order, so that a few percent change is enough to give rise to a change in the transformation temperature of tens of degrees. It is also shown that the changing rates of the long range order and the short range order are comparable with each other in most cases. These results are compared with recent experimental results.

Electronic structure and stability of intermetallic compounds in the Ti–Ni System

Abstract For the systematical understanding of the stability of martensitic phases and precipitates which appear in Ti–Ni shape memory alloys, we made a first principle electronic structure calculation of them by using the tight-binding linear muffin-tin orbital method in the atomic sphere approximation (TB-LMTO-ASA). The obtained results are the following: (1)the total electronic density of state (DOS) at the Fermi energy D ( e F ) of TiNi decreases as the successive B2→R→B19′ transformation proceeds; (2) when the number of valence electrons increases, D ( e F ) of the R-phase increases but that of the B19-phase decreases; (3) D ( e F ) of Ti 3 Ni 4 decreases as the number of valence electrons decreases and that of TiNi 2 decreases as the number of valence electrons increases. By comparing these results with experimentally obtained results, we derived a criterion that phases appearing in Ti–Ni system tend to become stable at 0 K as D( e F ) decreases.

Opportunities in Martensite Theory

A workshop has explored interactions of materials science, applied mechanics, physics and mathematics in understanding fundamentals of martensitic transformations. System theory offers a framework for addressing realistic complexity. Theory of invariant-plane kinematics has been extended to multivariant plate groups and hierarchical structures. Electronic total energy calculations explore the origins of martensitic phase stability, and Landau-Ginzberg models for transformations with and without shuffles provide nonlocal continuum theories for treatment of interfacial structure and mobility as well as the competition between classical and nonclassical transformation mechanisms. Quantitative kinetic theory incorporates defect distributions, but requires further analysis of the evolution of mean particle volume. Kinetic theory provides the basis for constitutive relations for transformation plasticity.