Mechanical spectroscopy and twin boundary properties in a Ni50.8Ti49.2 alloy

The internal friction (IF), Young's modulus (E) and heat flow (DSC) have be en measured in a Ni50.8Ti49.2 alloy as a function of temperature after both ageing and solubilization followed by water quenching. The IF of aged mate rial measured at kilohertz frequencies, besides hardly detectable peaks ass ociated with the A reversible arrow R and R reversible arrow M transitions, displays two additional non-thermally activated peaks P-TWM and P-TWR, whi ch appear to be due to stress-induced hysteretic motions of twin boundaries within the martensite and the R-phase, respectively. In the solubilized st ate a large peak corresponding to the A reversible arrow M transition is ob served together with a thermally activated relaxation P-d occurring at arou nd 100 K (for f = 1.2 kHz). The activation energy and limit time of this pe ak are 0.11 eV and 10(-10) s, respectively. Peak PTWM was not observed in t he solubilized state. Peak P-d, which is found to increase with the number of thermal cycles through the transition region undergone by the sample, is attributed to stress-induced motions of dislocations around their equilibr ium positions within the B19' structure of the martensite. The absence of a twin boundary peak in the martensite formed from the solubilized austenite , as well as the large difference in the strength of the transformation rel axations in the aged and solubilized states of the material are accounted f or in terms of the different nature of the predominant twin boundaries in t he two different states of the material. (C) 1999 Acta Metallurgica Inc. Pu blished by Elsevier Science Ltd. All rights reserved.