October_2021_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 2 1 5 1 1 SMJ HIGHLIGHTS June 2021 DYNAMICS OF PHASE FRONTS DURING HIGH- DRIVING-FORCE TRANSFORMATION OF SHAPE MEMORY ALLOY WIRES Asaf Dana, Hiroshi Sekiguchi, Koki Aoyama, Eilon Faran, Klaus-Dieter Liss, and Doron Shilo The reverse martensitic transformation proceeds through several subprocesses at various time and length scales. The authors recently studied the transformation kinetics in the large thermodynamic driving-force regime. They induced a rapid heating pulse in a shape memory alloy wire and tracked its evolution by multiframe time-resolved x-ray diffraction at synchrotron radiation with simultaneous stress measurements. The study identified three stages oc- curring at different times on the microsecond-scale and at different length scales. Specifically, the transformation was shown to occur initially in a thin layer near the surface, and only later in the bulk of the wire. They explain the obtained experimental results by modeling the evolution of the phase transformation using a continuum approach. Theoretical approaches are discussed and model fitting to experimen- tal results provides insight into the kinetic relation between the velocity of the phase front and the driving force. Results support a scenario in which a cylindrical phase front prop- agates inward along the wire radius. The propagation of such a high-specific energy front releases energy faster than low-energy fronts forming under low driving forces. Fig. 2 — Superelastic behavior of Fe–Mn–Al–Ni tensile samples with different grain ori- entations. EBSD orientation map plotted for LD of the near- < 101 > sample (a) and the corresponding stress–strain curve up to 7% applied strain; (b) EBSD orientation map plotted for LD of the near- < 001 > sample; (c) the corresponding stress–strain curve up to 8% applied strain (d). (a) (b) (c) (d) September 2021 EFFECT OF CRYSTALLOGRAPHIC ORIENTATION AND GRAIN BOUNDARIES ON MARTENSITIC TRANSFOR- MATION AND SUPERELASTIC RESPONSE OF OLIGO- CRYSTALLINE Fe−Mn−Al−Ni SHAPE MEMORY ALLOYS A. Bauer, M. Vollmer, and T. Niendorf In situ tensile tests employing digital image correlation were conducted to study the martensitic transformation of oligocrystalline Fe–Mn–Al–Ni shape memory alloys in depth. The influence of different grain orientations, i.e., near-<001> and near-<101>, as well as the influence of different grain boundary misorientations are in focus of the present work. The results reveal that the reversibility of the martensite strongly depends on the type of martensitic evolving, i.e., twinned or detwinned. Furthermore, it is shown that grain boundaries lead to stress concentrations and, thus, to for- mation of unfavored martensite variants. Moreover, some martensite plates seem to penetrate the grain boundaries re- sulting in a high degree of irreversibility in this area. Howev- er, after a stable microstructural configuration is established in direct vicinity of the grain boundary, the transformation begins inside the neighboring grains eventually leading to a sequential transformation of all grains involved (Fig. 2). September 2021 PHASE STABILITY OF THREE Fe−Mn−Al−Ni SUPER- ELASTIC ALLOYS WITH DIFFERENT Al:Ni RATIOS J.M. Vallejos, M.F. Giordana, C.E. Sobrero, and J.A. Malarría Fe–Mn–Al–Ni superelastic alloy is a po- tential candidate for diverse engineering ap- plications due to its outstanding properties and low material costs. Recent studies sug- gest that slight changes in the chemical com- position severely affect superelastic response, phase stability, and grain growth kinetics. In this paper, we found that the Al stabilizes the parent α phase at high temperature and promotes the formation of β precipitation at lower temperature. An alloy with a 3:1 ratio between Al and Ni produces homogeneously distributedβprecipitateswithhighphase frac- tion in the alpha matrix after quenching from 1200°C. The presence of these precipitates stabilizes the α phase, lowers the martensitic transformation temperature and gives the al- loy a fully reversible stress-induced martensi- tic transformation behavior without the need to apply an aging step. In alloys with lower Al content the β precipitation produced during quenching is severely restricted and pseudo- elasticity is impaired. 4 1 5