Martensitic Transformation Effect Research Articles

A two-scale thermo-mechanically coupled model for anomalous martensite transformation and elastocaloric switching effect of shape memory alloy

Conventional shape memory alloys (SMAs) exhibit an approximately linear relationship between the critical stresses of martensite transformation (MT) and ambient temperature (Clausius-Clapeyron relationship). Meanwhile, depending on the sign of entropy difference between the parent and martensite phases, these alloys behavior either a conventional or an inverse elastocaloric effect which originates from the latent heat release/absorption during stress-induced MT. Recently, MT with a non-monotonic Clausius-Clapeyron relationship (denotes as anomalous MT) and an elastocaloric switching effect (changes from the inverse elastocaloric effect at relatively low temperature to the conventional elastocaloric effect at relatively high temperature) were reported in CoCr-based Heusler-type SMAs. In this paper, to understand and quantitatively describe these new phenomena, a two-scale thermo-mechanically coupled constitutive model is constructed. In the mesoscopic scale, the fine twinned structure and the potential correspondent variant pairs of martensite phase are analyzed based on the crystallographic symmetry and the geometrically nonlinear theory of MT. The classical Debye's model is employed to describe the lattice and electronic heat capacities of the parent and martensite phases; while, a Monte Carlo simulation for the three-dimensional (3D) Ising's model is performed and a scaling law for the magnetic heat capacity of parent phase is proposed. Then, a new constitutive model is established based on the fundamental laws of irreversible thermodynamics to describe the non-isothermal stress-induced MT between parent and martensite phases. To consider the thermo-mechanical interactions between the parent phase and correspondent martensite variant pairs, each variant pair is regarded as an inhomogeneous inclusion with a mobile interface and embedded in the parent phase. With the help of interfacial operator, the thermodynamic driving force of MT is derived from the constructed Helmholtz free energy and dissipative inequality. Internal heat generation originated from the thermo-elasticity, inelastic deformation dissipation and latent heat is derived from the conservation law of energy. In the macroscopic scale, the thermo-mechanical response of SMA specimen is obtained by solving the equations of force balance, deformation compatibility, and thermodynamic equilibrium through the full-field calculation and mean-field approximation. Finally, the proposed model is verified by comparing the predictions with the experiments. It can be found that the anomalous MT and elastocaloric switching effect of CoCrAlSi SMA can be well captured by the proposed model.

Finite element analysis on the effect of martensitic transformation and plastic deformation on the stress concentration factor in a thin notched superelastic NiTi ribbon

The severe nonlinear behavior caused by the martensitic transformation (MT) and subsequent plastic deformation (PD) of detwinned martensite leads to a complex local stress redistribution at the location of stress risers of superelastic shape memory alloy (SMA) components. Nevertheless, in the literature, the simple linear elastic fracture mechanics (LEFM) equations are widely used in the evaluation of the fracture response of superelastic components which has resulted in obvious conflicts between the conclusions regarding the effect of MT on the fracture parameters, i.e. stress intensity factor (SIF) and material toughness. Furthermore, the linear elasticity method is frequently used in the literature to calculate the stress intensity range ([Formula: see text]) when the fatigue crack growth rate dependence on [Formula: see text] ([Formula: see text]) is being evaluated. Moreover, the PD followed by MT is poorly considered in the fracture mechanics of SMAs. This paper presents a numerical investigation on the role of both MT and PD, as well as the notch acuity, on the evolution of notch-tip stresses and strains and stress concentration factor ([Formula: see text]) upon the incremental application of the macroscopic tensile load on a thin NiTi notched superelastic ribbon, to mimic the effects of MT and PD on the SIF of superelastic parts. It is revealed that MT results in drastic deviations of the notch-tip stress, as well as the stress concentration factor ([Formula: see text]), from that obtained in LEFM. Due to the heterogeneous evolution of MT, the trend of the deviations is not regular and unique upon monotonic external loading. Accordingly, the results represent the ineffectiveness of the LEFM method in the evolution of the stress concentration factor (hence, the SIF) and toughness in monotonic loading, as well as the stress intensity range ([Formula: see text]) under fatigue loading in SMA components.

Effects of martensitic transformation on residual stress of P91 welded joint

The new method considering thermo-metallurgical and thermo-mechanical relationship of P91 welded joint was proposed in this paper by using the ABAQUS. In this new method, we developed a whole set of programs by using subroutines to do the calculations and consider the martensitic transformation (MT) effects. Firstly, the effect of MT latent heat on temperature field was considered. Then on the basis of the temperature field, the residual stress was calculated by considering the effects of volume expansion, the yield strength change and the transformation plasticity due to MT. The results show that the MT latent heat makes the temperature increase, which is consistent with the conclusion in the literature. The martensite microstructure is concentrated in weld bead and heat affected zone. The residual stress distribution is M-shape when the effects of MT are considered, which is consistent with the experimental results. The volume expansion makes the residual stress decrease a lot, the yield strength change makes the residual stress increase a little and the transformation plasticity has the ‘relaxation effect’. The volume expansion, the yield strength change and the transformation plasticity’s effects on residual stress decrease in turn.

Martensitic transformation and magnetocaloric properties in Ni40.4Mn46.5Sn10.9Sb2.2 ribbons

The Ni40.4Mn46.5Sn10.9Sb2.2 ribbons were prepared by melt-spun method. The martensitic transformation (MT) and magnetocaloric effect in melt-spun and annealed Ni40.4Mn46.5Sn10.9Sb2.2 ribbons were investigated. After the heat treatment, the MT temperature increases obviously in the annealed ribbons. The large values of magnetic entropy changes and the effective refrigerant capacity around the MT and Curie temperature of the austenite are found to be 30.9 and −2.2 J/kg K, 70.6 and 132.6 J/kg, respectively, under the field change of 30 kOe for the annealed ribbons. The annealing effect on the MT and magnetocaloric effect, together with the origin of the large magnetic entropy changes, has been discussed in this paper.

Contributions to the Transformation Entropy Change and Influencing Factors in Metamagnetic Ni-Co-Mn-Ga Shape Memory Alloys

Ni-Co-Mn-Ga ferromagnetic shape memory alloys show metamagnetic transition from ferromagnetic austenite to paramagnetic (or weak-magnetic) martensite for a limited range of Co contents. The temperatures of the structural and magnetic transitions depend strongly on composition and atomic order degree, in such a way that combined composition and thermal treatment allows obtaining martensitic transformation (MT) between any magnetic state of austenite and martensite. The entropy change ΔS measured in the magnetostructural transition comprises a magnetic contribution which depends on the type and degree of magnetic order of the related phases. Consequently, both the magnetization jump across the MT (ΔM) and ΔS are composition and atomic order dependent. Both ΔS and ΔM determine the effect of applied magnetic fields on the MT, hence knowledge and understanding of their behavior can help to approach the best conditions for magnetic field induced MT and related effects. In previous papers, we have reported findings regarding the behavior of the transformation entropy in relation to composition and atomic order in Ni50−xCoxMn25+yGa25−y (x = 3–8, y = 5–7) alloys. In the present paper we will review our recent results, summarizing the key findings and drawing general conclusions regarding the magnetic contribution to ΔS and the effect of different factors on the magnetic and structural properties of these metamagnetic alloys.

Transformation Volume Effects on Shape Memory Alloys

It is generally accepted that the martensitic transformations (MTs) in the shape memory alloys (SMAs) are mainly characterized by the shear deformation of the crystal lattice that arises in the course of MT, while a comparatively small volume change during MT is considered as the secondary effect, which can be disregarded when the basic characteristics of MTs and functional properties of SMAs are analyzed. This point of view is a subject to change nowadays due to the new experimental and theoretical findings. The present article elucidates (i) the newly observed physical phenomena in different SMAs in their relation to the volume effect of MT; (ii) the theoretical analysis of the aforementioned volume-related phenomena.