S. Kustov

Entropy change and effect of magnetic field on martensitic transformation in a metamagnetic Ni–Co–Mn–In shape memory alloy

We studied the entropy change and the shift of the martensitic transformation temperatures with magnetic field in samples of a polycrystalline Ni–Co–Mn–In alloy having different degrees of long-range atomic order due to different heat treatments. We found, for the samples of the same composition, strong variations of the entropy change with the degree of atomic order, mediated by the difference between the Curie and martensitic transformation temperatures. Calculations of the field-induced shift of the transformation using data of entropy variations show good agreement with experimental results.

Influence of martensite stabilization on the low-temperature non-linear anelasticity in Cu-Zn-Al shape memory alloys

Abstract The advanced acoustic technique has been used to investigate the mobility of partial dislocations/intervariant boundaries in the β 1 ′ martensite of a Cu-Zn-Al alloy subjected to the martensite stabilization and to the β-phase ageing, suppressing the stabilization effect. The non-linear anelasticity has been studied for frequencies of about 100 kHz and strain amplitudes 2×10 −7 –2×10 −4 over the temperature range 300–8 K. Measurements at low temperatures, below approximately 70 K, allowed us to eliminate anelastic effects associated with the motion of quenched-in defects, which are ‘frozen’ for these temperatures, and to assess the intrinsic mobility of partial dislocation/intervariant boundaries. The results obtained for stabilized samples are compared with those for β-phase aged samples, and with the previously reported data for the Cu-Al-Ni alloy, which is not prone to the stabilization at ambient temperatures. We suggest distinguishing mechanisms of stabilization according to their localization: a homogeneous and a heterogeneous component. Namely, short-range reordering occurring in the bulk of the crystal is responsible for the homogeneous component of the stabilization. The local rearrangement of the martensite structure in the vicinity of lattice defects (pinning of partial dislocations/intervariant boundaries by quenched-in defects and more intense than in the bulk localized re-ordering) is assumed to be responsible for the heterogeneous component of the stabilization process. The acoustic technique is shown to be able to distinguish and to study details of various effects associated with the heterogeneous and homogeneous changes in the structure of martensite, induced by the stabilization and different heat treatments.

Two-stage reverse transformation in hyperstabilized β1′ martensite

Abstract A Cu–Al–Be alloy is hyperstabilized by quenching into β 1 ′ martensite: only a fraction of martensite is retransformable to bulk β-phase at temperatures approximately 100 K above the nominal transformation temperatures. The rest of the hyperstabilized martensite undergoes the second stage of reverse transformation at much higher temperatures by means of the nucleation of lamellar β-phase.

Entropy Changes in Ferromagnetic Shape Memory Alloys

Several features of the entropy change S related to the martensitic transformation (MT) in metamagnetic alloys are discussed. In these alloys a change in magnetic order is concomitant with the MT, as it occurs between ferromagnetic austenite and non-magnetic (weakly magnetic) martensite. In this case it has been shown that S strongly decreases as the MT temperature range go far below the Curie temperature of austenite. The behavior of S can be understood considering the different signs of the lattice and magnetic contributions to the total entropy change. It has been shown that the so called kinetic arrest of the MT is directly related to the decrease of S, which in the limit S → 0 leads to the suppression of the driving force for the transformation.

Fundamental Development on Utilizing the R-phase Transformation in NiTi Shape Memory Alloys

In near equiatomic NiTi alloys, the reversible thermoelastic transformation between B2-structured austenite phase and the R-phase is attracting increasing interest for practical applications. However, the following two issues limit the widespread utilization of the R-phase transformation: (1) there is no effective approach to control the R-phase transformation temperatures; (2) it is not easy to largely separate the temperature domain of the R-phase and the B19′ martensite phase transformation, especially in the presence of an external force. This article reviews concisely the work of the present authors on solving the above two problems. The effect of grain size on the aging microstructure and related transformation behavior is first discussed. Inspired by these findings, an approach to solve the above two problems has been developed by introducing nanoscaled Ni4Ti3 precipitates in the samples with micron-sized grains. The performance of alloys associated with the R-phase transformation, which shows controllable transformation temperatures, is summarized.

Mechanomagnetic spectroscopy of phase transitions in ferromagnetic shape memory alloys

A technique for direct experimental investigations of coupling between applied elastic stress and magnetic response in ferromagnetic shape memory alloys over the temperature ranges of paramagnetic to ferromagnetic and martensitic transformations has been developed, based on the inverse magnetostriction (Villary) effect. Primary characteristics available from the experiment are elastic and anelastic properties and, the most important, signal related to stress-induced magnetization.

Transient internal friction during thermal cycling of Cu–Al–Ni single crystals in β1′ martensitic phase

Pronounced transient internal friction, accompanied by shear modulus defect and reversible torsional deformation, has been revealed during thermal cycling of Cu–Al–Ni single crystals in the β1′ martensitic phase. These phenomena are associated with microplastic straining of the martensitic phase due to anisotropy of thermal expansion of the martensitic variants.

Damping Properties of SMA

This chapter analyzes applicability of different models of anelasticity to damping capacity of shape memory alloys both in the martensitic state and during the martensitic transformation. The chapter focuses mainly on recent observations made in Cu-based and NiTi alloys. From the latest works it is evident that the high damping capacity can not only be related to the hysteretic mobility of interfaces between martensitic variants but may be associated as well with internal defects of variants.

Stress-induced magnetization in polycrystalline Ni-Fe-Ga ferromagnetic shape memory alloy

Results are reported for simultaneous experimental studies of the stress-induced magnetic induction and of anelasticity in a polycrystalline Ni–Fe–Ga ferromagnetic shape memory alloy. The authors found in the martensitic state a linear relationship between mechanically induced induction and stress amplitude, in contrast to a strong nonlinearity of the anelastic strain. This difference between elastic and magnetic responses to applied stress points to inefficient coupling between the motion of elastic twins/domain boundaries and magnetic domain walls. They argue that, in acoustic experiments, magnetic domain walls in Ni–Fe–Ga polycrystals perform only linear displacements within local potential minima, characteristic for the microeddy current magnetomechanical damping.

Influence of partial shape memory deformation on the burst character of its recovery in heated Ni–Fe–Ga–Co alloy crystals

Room-temperature stress–strain curves of Ni49Fe18Ga27Co6 alloy single crystals possessing shape memory (SM) have been studied. Specific features of these diagrams are revealed upon compressive loading of these single crystals in the [110]A direction. The influence of preliminary SM deformation on the process of its recovery during the reverse martensite transformation has been studied. It is established that SM deformation above 4.2% leads to a sharp increase in the shape recovery on heating and the process exhibits a burst character, involving motion of the entire crystal. The experimental data are analyzed and stress–strain curves are simulated in the framework of the theory of diffuse martensitic transitions.