Crystallography of Martensitic transformation in TRIP steel & shape memory alloy

Abstract

Martensitic transformation (MT) in various metallic materials has been investigated for more than 100 years. Though great advances in the knowledge of martensite have been made by many researchers, fundamental understanding looks like being far away. Especially, complex microstructures of martensite in different alloys, which are the first subjects to catch up for comprehensive understanding of martensite and MT, are still puzzle. In a similar vein, the first and second parts of this thesis, respectively, deal with martensitic microstructures of the TRIP steel and Ti-based shape memory alloy (SMA) with crystallographic theories of MT. In the last part, as an effort to apply MT to real use, elastocaloric effect in V-added NiTi SMA was examined. In first part, variant selection of mechanically induced MT in TRIP steel was investigated. A metastable austenite (γ) grain was nanoindented to introduce strain-induced MT, and multiple pop-ins, which imply transformation of the γ into martensite (α), were detected in the nanoindentaion load-displacement curve. After the nanoindenation, cross-section of the indented region was observed using transmission electron microscope (TEM) equipped with an ASTAR system. Several martensite blocks with different variants maintaining the Kurdjumov-Sachs orientation relationship between the ex-austenite within 5 degrees were confirmed. From a finite element calculation combined with phenomenological theory for MT, it was confirmed that each variants corresponded to those for which the transformation strain effectively accommodates external stress. In second part, self-accommodation in Ti-V-Al SMA was studied. Exact phase information of the parent austenite and product martensite were extracted by Rietveld refinement using X-ray diffraction followed by microstructure observations using optic microscopy (OM), electron back scattered diffraction (EBSD) and TEM. The observed features of martensite microstructure including twinning mode of habit plane variants, invariant shear plane, variant clustering etc. were described with the geometrically nonlinear theory of martensite. In third part, elastocaloric (EC) effect in polycrystalline Ni-Ti-V SMA alloy was examined. EC effect means thermal response of material respect to external uniaxial stress and have gathered much attention for the use of next-generation refrigeration systems, recently. By addition of a small amount of V (4.7 at.%), Ni-Ti-V SMA showed twice larger efficiency values such as EC cooling strength (= 50 °C GPa-1) and coefficient of performance (= 22.5) than the conventional pure Ni-Ti. In addition to the efficiency improvements, the present alloy demonstrated the better functional fatigue property compared to that of pure Ni-Ti. These result can be considered as a significant progress in finding suitable materials for an application of EC. From the present study, crystallographic aspects of MT in TRIP and SMA are detailed and described well with solid theoretical backgrounds. Moreover, it is suggested that the newly developed alloy here can be a promising candidate for real application.