Pseudoelasticity and Cyclic Stability in Co49Ni21Ga30 Shape-Memory Alloy Single Crystals at Ambient Temperature

Abstract

As-grown Co 49 Ni 21 Ga 30 [001]-and [123]-oriented single crystals were subjected to cyclic compression loading at room temperature above the austenite finish temperature of 15°C. Straincontrolled experiments were performed using both incremental strain steps and constant strain amplitudes. Cyclic deformation with a maximum strain amplitude of 2.5 pct resulted in rapid accumulation of irrecoverable strains in the [123]-oriented crystals. However, after a few cycles, the samples demonstrated cyclic stability with fully recoverable transformation. By contrast, the [001]-oriented crystals displayed excellent cyclic stability with hardly any change in stress-strain characteristics. In-situ optical microscopy and electron backscattered diffraction analysis were employed to clarify the events that take place at different stages of a typical loading-unloading history. The in-situ observations also revealed that the initiation and growth characteristics of stress-induced martensite (SIM) are heterogeneous on the microscopic scale in CoNiGa alloys. In addition, theoretical transformation and detwinning strains, and resolved shear stress factors (RSSFs), were calculated based on the energy minimization theory and are compared to the experimentally obtained orientation-dependent transformation stress and strain levels. It is shown that the selection of an appropriate orientation is one of the key criteria to optimize the pseudoelastic (PE) response and cyclic stability of CoNiGa alloys.

Magnetic Field‐Induced Phase Transformation in NiMnCoIn Magnetic Shape‐Memory Alloys—A New Actuation Mechanism with Large Work Output

“…The origin of this orientation dependence is mainly the crystallographic relation between the applied stress direction and possible crystallographic systems (transformation shear plane-also known as habit plane-and direction) that can be activated during parent to martensite transformation. Using ''energy minimization theory'', [30][31][32][33][34][35] it is possible to determine the habit plane and direction as well as the twinning shear and direction for given lattice parameters and structures of transforming phases. Below, we use this theory to calculate the phase transformation strain for the present NiMnCoIn alloy as a function of orientation.…”

“…For more detailed information on the theoretical framework, please refer to Refs. [30][31][32][33][34][35].…”

“…Once the habit plane normal and transformation shear direction are determined, it is possible to calculate the transformation strain (e tr ) assuming the formation of the most favorable CVP. [31,35] Furthermore, the detwinning strain (e dt ) can be calculated assuming the growth of the variant with the larger volume fraction in expense of the one with the smaller volume fraction within the CVP. [31,35] The contour plots of the calculated theoretical e tr and e tr max (¼e tr þ e dt ) values for tension and compression loading are shown in Figure 14 as a function of crystallographic orientation.…”

“…Brewer [38] reported that the thermal hysteresis along the [123] and [100] orientations is similar, which might also be the case for the [111] orientation and for the orientation dependence of the stress hysteresis. However, the latter argument needs to be validated experimentally since it has been reported in other SMAs [30,31,35,36,[39][40][41][42] that stress hysteresis can be a function of orientation and stress state.…”

Transformation-induced plasticity in high-temperature shape memory alloys: a one-dimensional continuum model

“…The coupling of plasticity and transformation in SMAs produces irrecoverable strain and reduces the work output under cyclic loading. The poor stability of SMAs under thermal cyclic loading is experimentally illustrated in [1,2], and the instability during superelasticity is reported in [3,4]. Furthermore, the creep mechanism is usually activated in HTSMAs because the martensitic transformation temperatures lie between 0.3 and 0.5 of HTSMA melting temperatures where visco-plastic behavior in metallic alloys occurs.…”