The Influence of Thermal History on the Multistage Transformation of NiTi Shape-Memory Alloys

Abstract

The multistage martensitic phase transformation of a polycrystalline NiTi shape-memory alloy (50.3 at. %Ni-49.7 at. % Ti) has been studied by means of calorimetric measurements. After a conventional thermal treatment followed by successive thermal cycles, the initial two-step forward transformation splits into four-overlapping stages. However, the reverse martensitic transformation maintains the initial two-step sequence, usually assigned to the B19 →R→B2 transformation. The correlation between the forward and reverse steps has been established by means of selected thermal cycles together with an estimation of their enthalpy and thermal hysteresis. These results have also provided information about the storage of the elastic strain energy and the frictional works associated with the variants' nucleation. Moreover, the study around the forward transformation temperature range by means of uncompleted thermal cycles undoubtedly shows the presence of temperature memory effects in both stages.

Modeling of partial transformation cycles of SMAs with a modified hardening function

“…Although a number of recent studies has been published on the exploration of PT through DSC [38][39][40][41] and other experimental tests such as scanning electron microscopy (SEM) [42], in order to associate the observed kinetic stops with temperature arrests (the so-called temperature memory effect-TME) and the evolution of multistage martensitic transformation [43], yet there is limited work reported regarding updated or newly developed constitutive models to simulate this behaviour. Most constitutive equations have been derived before 00ʼs, while the most recent and accurate models are mainly focused on compensating the exhibited PT hysteretic response for achieving better control of SMAs in actuator applications without implication of internal state variables.…”

A Critical Review on Nickel–Titanium Thin-Film Shape Memory Alloy Fabricated by Magnetron Sputtering and Influence of Process Parameters

“…In a binary crystallization NiTi alloy, matrix B2 is directly converted to phase B19'. Thermo-mechanical treatment or the addition of other elements in NiTi binary alloy leads to a two-step process of transformation by formation of R-phase which can be further transformed to martensite phase [19]. The parent phase of NiTi alloy has a B2 structure.…”

Effect of Sn Addition on Phase Transformation Behavior of Equiatomic Ni-Ti Shape Memory Alloy

“…B2→R (austenite to R-phase) phase transformation becomes interesting in many applications, such as actuators, because of the small thermal hysteresis and high fatigue life. Different methods used to produce the phase transformation B2→R in the Ni-Ti SMAs, like adding a third alloying element to an equi-atomic Ni-Ti shape memory alloy (5-7), grain refinement (8), thermo-mechanical treatment (9), and aging of Ni-rich alloys (10), have been developed. It's known that when Al, Mo, Fe, Co, and Cr (6-13) is added to near equi-atomic Ni-Ti shape memory alloys, it will produce R-phase (trigonal crystal structure).…”