J. Dadda, H. J. Maier, University of Paderborn, Paderborn, Germany; I. Karaman, Texas A&M University, College Station, TX
Pseudoelasticity as a function of temperature and crystallographic orientation in Co49Ni21Ga30 single crystals under compression was studied. The pseudoelastic temperature window and its dependency on the Clausius-Clapeyron (CC) slope and the resolved shear stress factor (RSSF) for the phase transformation was also established for each orientation. The <001>, <123> and <235> oriented crystals demonstrate 400, 220 and 180 wide PE ranges and 1.7, 2.5 and 4 MPa/°C as CC-slopes, respectively. Moreover, a high RSSF value results in a low CC-slope. For each orientation, a stress in the range of 600-700 MPa at the temperature Md is critical, as the stress-induced martensite (SIM) is no longer feasible and CC curves deviate from linearity indicating the onset of plasticity. As the <001>-oriented crystal has demonstrated SIM transformation at temperatures as high as 425 °C (Md), the effect of homogenization on crystal with this orientation was also investigated. Homogenization was carried out at 1200 °C for 4 hours followed by water quenching. Homogenized crystals exhibit a PE temperature window of 350 °C with a CC-slope of 2 MPa/°C. This is attributed to the early occurrence of plasticity in the austenite suppressing the transformation at temperatures above 320 °C (Md) because of the soft matrix. As expected, microstructural analysis of the homogenized crystals revealed only β-matrix, whereas that of the as-grown crystals revealed the presence of γ precipitates in the β-matrix. The precipitates increase the matrix strength and inhibit the dislocation motion, and thus a larger PE window is observed in the case of as-grown crystal.
Summary: We found that the CoNiGa alloys are the potential high-temperature shape-memory alloys as they demonstrated stress-induced phase transformations at the temperatures as high as 425 °C. In this study, pseudoelastic (PE) behavior of <100>, <123>, and <235> -oriented single crystals as a function of temperature was studied. These orientations were chosen because the propensity to induce dislocations in the parent phase during the phase transformation is quite different as the {110}<001> is the active slip system. In addition, the ease of transformation in each orientation depends also on the resolved shear stress factor (RSSF) for the transformation.The results obtained demonstrate that the RSSF value and the Schmid factors for slip govern the PE temperature range and the transformation strain. Theoretical calculations for the transformation strains, detwinning and RSSFs were established based on the energy minimization theory in order to comprehend orientation dependent transformation stress and transformation strain levels. Effect of homogenization on the <001>-oriented crystals also investigated, and found that this resulted in lower PE temperature window. This is attributed to the soft matrix of homogenized crystal as expected.
