Papers by Keyword: Variant Selection

Abstract: Residual stresses in welded joints must be quantified in order to carry out structural integrity assessments on critical nuclear components. This usually requires the application of finite element models for components with wall thicknesses exceeding 50 mm. In ferritic steels, the development of residual stresses is made more complex by the strains associated with the solid-state phase transformations that occur during heating and cooling. Finite element models often do not account for factors that contribute to anisotropy in the transformation strains, such as Greenwood-Johnson plasticity and variant selection. In this work, we search for evidence that might reveal which mechanism (s) contributes to this anisotropy. Coupons of SA508 steel were subjected to simulated welding thermal cycles, with and without external loading, and in-situ X-ray diffraction was used to track changes in crystal structure. The results were checked for evidence of plastic deformation in austenite and variant selection in its daughter phases.

Abstract: Grain boundary texture evolution in case of two of the Zr based alloys (Zircaloy-4 and Zr-2.5\%Nb) was studied. In case of Zircaloy-4, grain boundary texture evolution during $\beta$ $\to$ $\alpha$ phase transformation was monitored. Direct evidence of variant selection during this transformation is presented. In case of Zr-2.5\%Nb alloy, considerable increase in $\alpha/\beta$ interfaces following Burger's orientation relationship was noticed with increasing annealing time at 700 \textdegree{}C.

Abstract: There has recently been significant interest in the problem of variant selection in the strain-induced transformation of austenite to α’-martensite in metastable austenitic stainless steels. Previous work has highlighted our poor understanding of the mechanisms leading to this transformation, in particular the role that the macroscopic stress plays in the transformation. In this work, we have sought to perform detailed experiments aimed at developing a statistical grain level view of variant selection in one particular grade of austenitic stainless steel. EBSD measurements made over a large number of grains as well as macroscopic texture measurements made at different levels of imposed plastic strain allow for comparison against various approaches for predicting variant selection based on the Patel-Cohen interaction energy.

Abstract: A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Abstract: The effect of the metallurgical state of austenite (undeformed vs. deformed vs. deformed + recrystallised) on the properties of the austenite to bainite transformation were investigated thanks to thermal (Gleeble simulations) and thermomechanical (hot torsion) treatments. No obvious influence of the state of austenite was found, using electron backscatter diffraction, on the resulting microtexture. Advantages and drawbacks of using misorientation angle histograms vs. axis-angle pair distribution are discussed regarding investigations of local variant selection. For an austenite grain size higher than about 50 µm, a strong effect of the transformation temperature was evidenced, bainite formed at lower temperature (530°C) exhibiting a microtexture close to that of lath martensite in the same steel.

Abstract: A micromechanical model was developed to account for the particular microtexture of upper bainite in low alloy steels, i.e. the non-random spatial distribution of variants within a given former austenite grain. A self-consistent scheme and an Eshelby approach, considering both transformation shape strain and viscoplastic strain as eigenstrains, was applied to estimate coupling between parent austenite and two or more bainite variants without any applied stress. Model predictions concerning self-accommodation between variants are sensitive to the plane of the first “lattice invariant shear” in the crystallographic model used to determine the shape strain. No obvious effect of the constitutive equations of phases and of the other model parameters was found.

Abstract: Austenitic steels can exhibit a complex transformation sequence during deformation. Indeed, the austenitic phase transforms first into bands of ε (HCP) martensite. This transformation is then followed by the formation of α’ (BCC) martensite. In this study, the crystallography of the transformation together with the occurrence of variant selection is studied at the scale of individual austenite grains. About ten prior austenite grains deformed at different strain levels in uniaxial tension were analysed by means of EBSD techniques. One of the classical approaches to predict the variant selection phenomenon is based on the calculation of the interaction energy between the macroscopic stress and the shape deformation associated with the formation of the product phase. The formation of the α’ variants was observed to lead to a very strong variant selection that cannot be fully explained by energetic criterion. It is suggested that the crystallography of the transformation sequence can account for the unexpected variants.

Abstract: Morphology of phase-separated microstructure consisting of cubic disordered A1 and ordered L1₂ phases and tetragonal ordered D0₂₂ phase in Ni-V-X (X=Al, Si) alloys has been investigated by transmission electron microscopy. Ternary Ni-V-Al alloy showed the lamellar structure of D0₂₂ phase with spherical L1₂ particles, while Ni-V-Si ternary alloy indicated the anisotropic microstructure consisting of plate- or diamond-shaped D0₂₂ and plate-shaped L1₂ phases. When a part of element Al is substituted by Si, the morphology of three-phase microstructure (A1/L1₂/D0₂₂) varied depending on the amount of element substitution. When Ni-16.1V-4.0Al-2.8Si (at.%) alloy was aged at 1173K for long time, D0₂₂ plate-like particle aligned along <110> direction and L1₂ particles formed between them in the shape of sphere. In the case of Ni-16.6V-2.8Al-4.0Si (at%) alloy more substituted for Al by Si, cuboidal L1₂ phases were first formed aligning along <100> direction and then plate-shaped D0₂₂ phases precipitated along the <100> direction in the channel of cuboidal L1₂ particles. Such morphological changes with the substitution of the third elements X by Al or Si were able to be explained based on lattice mismatch.

Abstract: The deformation process can induce the precipitation of martensite in austenitic stainless steels. When shear stress is applied at temperatures near Ms, displacive transformation (martensitic transformation) mode is activated. When external stresses are applied, the work done contributes to a change in free energy either raising or lowering the Ms-temperature. Orientation relationships during austenite to martensite phase transformation were investigated in an austenitic stainless steel samples deformed by cold rolling and deformed in a tension test. EBSD (electron backscatter diffraction) and X-ray diffraction techniques were used to evaluate parent austenite texture and martensite texture after transformation. The observed orientation relationship between austenite and martensite was compared with the predicted orientation relationship by the phenomenological theory of martensite crystallography (PMTC). Aspects related to variant selection were discussed based on the criterion for the action of applied stress in the martensitic transformation postulated by Patel and Cohen. Results showed a very good agreement between measured and calculated results.

Abstract: Rapid cooling of TiAl-based alloy from α phase (disordered hexagonal, A3) generates  phase (ordered tetragonal, L1o) grains through massive transformation nucleating mostly over the α/α grain boundaries. This current work deals with the identification and the validation of different nucleation mechanisms during  massive transformation in TiAl-based alloys. Special attention has been given to the variant selection criteria for the nucleation of the massive structures along different types of α/α grain boundaries. The  massive domains formed along the grain boundaries were analysed using high resolution electron backscattered diffraction (EBSD). Statistical studies were made on different nucleation sites and different mechanisms are proposed. Two–dimensional studies of the nucleation mechanism suggest that the minimization of the interfacial energy could be the predominant criteria during the grain boundary nucleation. In order to verify this nucleation criterion in three-dimensions, serial sections were made and EBSD maps were taken and analysed in each section. The variant selection observed during the nucleation and the growth of the  massive grains is further discussed after getting a broader view under three-dimensional investigations.