Effect of stress state on the stress-induced martensitic transformation in polycrystalline Ni-Ti alloy
- PDF / 1,233,516 Bytes
- 8 Pages / 597.28 x 777.28 pts Page_size
- 6 Downloads / 225 Views
I.
INTRODUCTION
T O successfully model and utilize the mechanical behavior of Ni-Ti, the governing stress-induced martensitic transformation must be thoroughly understood. The present investigation has sought to further the current understanding of the stress-induced martensitic transformation in NiTi by applying unique triaxial stress states to polycrystalline Ni-Ti and measuring the resulting pseudoelastic transformation strains. The results presented in this work are currently being used in the development of new models for the thermomechanical behavior of materials exhibiting stress-induced martensitic transformations. A number of previous investigations have considered the effect of stress state on the stress-induced martensitic transformation. A large portion of these investigations have focused on the effect of hydrostatic compression on the martensitic start (Me) and austenite start (A~) temperatures. In the study of martensites exhibiting a negative volumetric transformation strain, i.e., A V 'r = Vm -- Vo < 0, linear increases in transformation temperature with increasing hydrostatic pressure have been observed in Au-Cd,tl~ Cu-A1-Ni,I2] and the R phase to M phase transformation in Ni-Ti. [31 For the austenite to R phase transformation in NiTi, which exhibits no volume change (A V'r = 0), transformation temperatures have been found to remain constant with increasing hydrostatic pressureJ 3) Finally, for the positive volumetric change (A p r > 0) transformation observed in Fe-Ni-Co-Ti, transformation temperatures have been found to decrease with increasing hydrostatic pressure3 3] The results of these studies indicate that the effect of pressure on martensitic transformation is linked to the volumetric change (AV'r = V m - V,O associated with the austenite (parent) to martensite transformation, i.e., hydrostatic pressure favors transformations that result in a neg-
KURT JACOBUS and MARK BALZER, Graduate Research Assistants, and HUSEYIN SEHITOGLU, Professor and Associate Head, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urban~Champaign, Urbana, IL 61801. Manuscript submitted February 27, 1996. 3066- VOLUME27A, OCTOBER 1996
ative volumetric change and hinders those that result in a positive volumetric change. Research efforts have also focused on stress-induced transformations that occur when an effective stress component is present. The influence of uniaxial stress sense (tension v s compression) on the pseudoelastic stress-induced martensitic transformation has been examined to a limited extent. Although some treatments have found strong symmetry between tension and compression behavior for Au-Cd single crystals, t4,51the majority of the treatments find at least some degree of asymmetry between tensile and compressive behavior. Burkart and Readt6J studied the stress-induced transformation in In-T1 single crystals and found that at a given temperature, 20 pct more compressive stress than tensile stress was needed to begin the transformation. Sakamoto e t al
Data Loading...
