Papers by Keyword: Martensitic Phase Transformation

Abstract: The mechanical properties and microstructural evolution of a medium-entropy alloy Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₅ (at%) in a low temperature range (including the record low temperatures region down to 0.5 K) were investigated. It has been established that low-temperature plastic deformation initiates martensitic phase transformations in this alloy, and the values of the dynamic modulus of elasticity correlate with the degree of phase transformations.

Abstract: In this study, the effect of oxidation on the transformation temperatures and microstructure of Ti-Ta₂₀-Al₅ high temperature shape memory alloys (Ti-Ta₂₀-Al₅ HTSMAs) is studied. The alloy was studied under two conditions, as cast and after cold rolling and recrystallizing (CR +RE). The microstructure, phase composition, and phase transformation temperature of Ti-Ta₂₀-Al₅ HTSMAs before oxidation were studied. Thermogravimetric analysis (TGA) combination with microstructural investigations were performed after oxidation at temperatures range from 25°C up to 1000°C. For both conditions, the alloy consisted mostly of β – BCC and martensite ʺα – orthorhombic phase before oxidation. Ti-Ta₂₀-Al₅HTSMAs as (CR +RE) shows martensitic transformation temperature higher than 400°C. The oxidation behaviour was linear until 650°C, while parabolic at elevated temperatures. For both conditions, the alloy consisted of β – BCC phase (Ta – rich) and α-hexagonal phase (Ti – rich) with not strong precipitation of ω – phase after oxidation. The results show that the formation of multi – layer oxide, which consists of Al₂O₃, TiO₂, and Ta₂O₅. For both conditions, oxidation can suppress martensitic phase transformation by precipitation of α-hexagonal phase (Ti – rich), Al₂O₃, TiO₂, and Ta₂O₅, and ω phase, would result in degradation of the martensitic phase transformation.

Abstract: The experimental observation of the microstructural deformation behavior of a metastable austenitic stainless steel tested at the real VHCF limit indicates that plastic deformation is localized and accumulated in shear bands and martensite formation occurs at grain boundaries and intersecting shear bands. Based on these observations a microstructure-sensitive model is proposed that accounts for the accumulation of plastic deformation in shear bands (allowing irreversible plastic sliding deformation) and considers nucleation and growth of deformation-induced martensite at intersecting shear bands. The model is numerically solved using the two-dimensional (2-D) boundary element method. By using this method, real simulated 2-D microstructures can be reproduced and the microstructural deformation behavior can be investigated within the microstructural morphology. Results show that simulation of shear band evolution is in good agreement with experimental observations and that prediction of sites of deformation-induced martensite formation is possible in many cases. The analysis of simulated shear stresses in most critical slip systems under the influence of plastic deformation due to microstructural changes contributes to a better understanding of the interaction of plastic deformation in shear bands with deformation-induced martensitic phase transformation in the VHCF regime.

Abstract: Aim of this study is to describe the ductile damage of metastable austenitic steels which show TRansformation Induced Plasticity (TRIP). Therefore, a criterion for the austenite to martensite transformation, the caused additional hardening and evolution equations for the TRIP-strain are incorporated into the damage model of Rousselier. As a first approach, the model is calibrated against unit cell simulations of the porous material for different stress triaxialities.

Abstract: Metal Matrix Composites (MMC) based on a TRIP (TRansformation Induced Plasticity)- or TWIP (TWinning Induced Plasticity)-steel matrix reinforced with MgO-partially stabilized zirconia (Mg-PSZ) are an interesting research field as both components exhibit a deformation-induced or stress-assisted martensitic phase transformation and twinning, respectively. The present work deals with the fatigue characteristics of a reinforced CrMnNi-steel as a function of the ceramic particle size. Therefore, the particles were classified into three grades (grade 1: <10 μm; grade 2: 10-30 μm; grade 3: 30-50 μm) whereas the volume fraction concerning the composite material was kept constant at 10 vol.%. The composites were produced using the hot pressing technique. The tests were performed under total strain control in a range of 0.2% ≤ Δε_t ≤ 1.2%. The microstructure of fatigued specimens was examined using scanning electron microscopy.

Abstract: The aim of this work is to understand the evolutions of the β₁ metastable austenite phase of a CuAlBe Shape Memory Alloy at macroscopic and microscopic scales under mechanical solicitation by neutron diffraction. The tensile specimen, taken in the raw material is subjected to superelastic cycles at room temperature on SMARTS diffractometer. Before loading, the mater ial is fully austenitic. During loading, after elastic deformation of austenite, phase transformation starts, martensite variants appear. The material follows a law of pseudo elastic behavior. At the end of the first mechanical cycle after unloading, the macroscopic curve does not fully return into its original point. A macroscopic deformation is observed. The evolution of first order microdeformations during mechanical cycles shows a large deformation of {400} plane family. This deformation is linked to the presence of <001> partial fibber characterizing the crystallographic texture of the material after elaboration. The FWHM of the (400) diffraction peak is also largely increased during loading. This increase is the signature of the generation of stacking faults during the transformation of β₁ metastable austenite into β₁ martensite.

Abstract: The study explores the vibration sensing effect of Ni-Mn-Ga shape memory alloy, based on the experimental results, researched the characteristics of this alloy applied in mechanical vibration signal sensors, and describes the feasibility of this alloy used for vibration measurements.

Abstract: Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.

Abstract: Shape memory alloys (SMA) are materials that can remember their original shapes. Even if they are subjected to high deformations, they can return back to their original shapes when loading is removed. Their deformation mechanism is based on martensitic phase transformations (PT). In this study, uniaxial material response of polycrystalline SMA is inspected using finite element (FE) method. The material model developed by Levitas and Ozsoy (2009) is used and stress-strain diagrams and volume fractions of austenite and martensite during external loading are determined. The results are found to be in agreement with the experimental results. Moreover, increase in athermal friction, which prevents the interface propagation, delays the PT and for different friction values, different hysteresis is obtained. It is also shown that material response determined by experiments can be obtained using this model.

Abstract: This article is dedicated to the estimation of the relative stability for B2, B19`, B33 and Cm phase in ZrCu-ZrNi-ZrCo intermetallic compound row through the ab-initio electronic structure calculations and subsequent crystal structure Rietveld refinement. The information about electronic and crystal structure of phases in Zr-based intermetallics will allow selecting for this high temperature shape memory alloy such alloying elements that will significantly improve shape memory behavior through definite structural changes.