October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 5 1 S hape Memory and Super- elasticity: Advances in Sci- enceandTechnology ( SMJ ) is the official journal of the International Organization on Shape Memory and Superelas- tic Technologies (SMST), an affiliate society of ASM Inter- national. The journal publishes original peer-reviewed papers that focus on shape memory materials research with contributions from materials science, experimental and theoretical mechanics, physics with cogni- zance of the chemistry, underlying phases, and crystallogra- phy. It also provides a forum for researchers, scientists, and engineers of varied disciplines to access information about shape memory materials. The best paper award winning arti- cles from 2015, 2016, and 2017, as selected by Shape Memory Editor-in-Chief Huseyin Sehitoglu, are highlighted here. SMJ is available through springerlink.com. For more information, vis- it asminternational.org/web/smst. Fig. 1 — Temperature dependences of isobaric specific heat, cP, in a cP/T–T2 coordinate determined for M (49.98, 50.55, 50.92, and 51.09 Ni) and P (51.54 and 51.75 Ni) phases. COMPOSITION DEPENDENCES OF ENTROPY CHANGE AND TRANSFORMATION TEMPERATURES IN NI-RICH TI-NI SYSTEM K. Niitsu, Y. Kimura, X. Xu, and R. Kainuma For Ni-rich Ti–Ni alloys, physical properties such as spe- cific heat and electric resistance were systematically investi- gated. The B2/B19′ martensitic transformation temperatures ranging from 180 to 373 K were determined for Ni contents of 49.98–51.09%, and a sudden disappearance of martensitic transformation was confirmed for Ni contents greater than 51.23%, which has also been well reported in the literature. The entropy change was also evaluated fromdifferential scan- ning calorimeter measurement, and it was clarified that the entropy change plotted to T 0 temperature shows an S-shaped curve, starting to drastically decrease at about 300 K. Thermo- dynamic approaches were then carried out attempting to de- termine the reason for the disappearance of transformation. The entropy change estimated from direct measurements of specific heats for 51.75 Ni (B2) and 50.92 Ni (B19′) was found to bemore consistentwith the experimental data, rather than the calculated curve based on the Debye model for vibration spe- cific heat. It was proposed that the equilibrium between the parent and martensite phases obeys the Clausius–Clapeyron relationship in the composition–temperature system. Using the constructed composition–temperature diagram, the dis- appearance of martensitic transformation in the Ti–Ni system can be well understood as being due to the drastic increase of hysteresis at low temperature (Fig. 1). CRITICAL STRESSES FOR TWINNING, SLIP, AND TRANS- FORMATION IN TI-BASED SHAPE MEMORY ALLOYS Avinesh Ojha and Huseyin Sehitoglu We investigate the effect of Nb and Ta contents on the (i) critical resolved shear stress (CRSS) for the β − α ″ transformation, (ii) the CRSS for austenite slip, and (iii) the CRSS for twin nucleation in martensite ( α ″ phase) that govern shape memory and superelasticity in Ti-based alloys. The crit- ical stresses for slip and twinning are achieved with a modi- fied Peierls Nabarro formalism utilizing generalized stacking fault energy and the generalized planar fault energy (GPFE), respectively, obtained from first-principles density functional theory (DFT) calculations. During the calculation of the twin- ning stress, we show the importance of the shuffling process in stabilizing and lowering the GPFE curve. Similarly, the transformation stress is obtainedwith heterogeneousmarten- site nucleation mechanism incorporating the energy barriers associated with the transformation process. Here, we point 1 3 SMJ HIGHLIGHTS Fig. 2 — Stress–strain response showing shape memory effect, superelasticity, martensite slip, and austenite slip as a function of temperature in Ti–Nb–Ta alloy.