Kikuaki Tanaka

Thermomechanics of transformation pseudoelasticity and shape memory effect in alloys

Abstract Transformation pseudoelasticity and shape memory effect of alloy materials are investigated from the thermomechanical point of view. The thermomechanical constitutive equations and the kinetics of transformation established by the theory are applied to explain the stress-strain-temperature behavior of the material. Numerical illustrations for the uniaxial stress state are given.

Phenomenological analysis on subloops and cyclic behavior in shape memory alloys under mechanical and/or thermal loads

Abstract A theoretical framework is presented, from the phenomenological point of view, for the cyclic uniaxial deformation in shape memory alloys subjected to the thermal and/or mechanical loads by introducing three internal variables; the local residual stress and strain and the volume fraction of the martensic phase accumulated during cyclic forward and reverse martensitic transformations. The cyclic effect on the stress-strain and strain-temperature hysteresis loops is discussed. The subloops due to incomplete transformations are also analyzed by assuming the transformation starting stress or temperature which depends on the preloading. Numerical results explain qualitatively well the observations on the thermomechanical behaviors of shape memory alloys.

Transformation start lines in TiNi and Fe-based shape memory alloys after incomplete transformations induced by mechanical and/or thermal loads

Abstract The transformation start condition in the stress-strain or strain-temperature planes is examined in a TiNi polycrystalline shape memory alloy and an Fe-based polycrystalline shape memory alloy under mechanical and/or thermal loads. The martensite start and austenite start stresses in the TiNi alloy are revealed to be almost constant during isothermal loading at the pseudoelastic temperature range, being independent of the extent of prior transformations. In the Fe-based alloy, however, both the martensite start stress and the austenite start temperature are strongly dependent on the extent of prior transformations. The alloy performance is fully contrary to the prediction of the thermodynamic theory of 1. Miller and co-workers (I. Mu¨ller, 1989, Cont. Mech. Thermodyn. 1, 125; I. Mu¨ller and H.B. Xu, 1991, Acta Metall. Mater. 36, 263) and B. Raniecki et al. (1992, Arch. Mech. 44, 261)

Phenomenological analysis of plateaus on stress-strain hysteresis in TiNi shape memory alloy wires

Abstract The formation of the steps, the plateaus, on the stress-strain curves in TiNi shape memory alloy wires is discussed from the continuum mechanical point of view. The two metallurgical processes are considered to be essential: The phases, the parent phase and the martensite phase, are separated by a phase interface which propagates along the wire when the wire is subjected to straining. The stress-strain hysteresis loop shifts both to the higher strain side and the lower stress side during strain controlled cyclic loading. The latter process is, following the metallurgical observation, characterized by introducing three internal variables; the residual stress and strain which are accumulated during cycling and the martensite phase which is also accumulated and takes no part in the subsequent transformations. The numerical simulation well explains the formation of the plateaus in the stress-strain loops during cycling.

On T-T-T and C-C-T diagrams of steels: a phenomenological approach to transformation kinetics

SummaryA new transformation kinetics for the solid-solid transformation, which includes the temperature-rate as one of the variables, is proposed from the phenomenological point of view, and the procedure to construct the T-T-T and C-C-T diagrams is discussed. As an application of the theory the diagrams for the eutectoid and bainitic transformations of two typical carbon steels are drawn. The results clearly prove that the theory proposed could work well in practical situations in the engineering field.ÜbersichtEs wird eine neue phänomenologische Theorie zur Umwandlungskinetik vorgeschlagen. Dabei wird auch die Konstruktion von ZTU-Schaubildern und von Umwandlungsdiagrammen für kontinuierliche Abkühlung diskutiert. Zur Anwendung der Theorie werden die Diagramme für die eutectoidische und die bainitische Umwandlung zweier unlegierter Stähle gezeichnet. Die Ergebnisse zeigen, daß die Theorie auch praktische Anwendung finden kann.

On analysis of transformation superplastic deformation in pure iron

Abstract Transformation superplastic deformation in pure iron during the temperature cycling is analyzed. The results can fully explain the contradictory data about the dependence of the increment of superplastic strain in a cycle on the temperature range as well as on the temperature-rate of cycling, which have been presented by different research groups.