Ch. Hartig

Deformation modes in γ-TiAl as derived from the single crystal yield surface

Based on a detailed analysis of the topography of the single crystal yield surface, a deformation mode map for slip and twinning of L10 ordered γ-TiAl is derived. Various different deformation systems have been considered, namely, glide of ordinary and superdislocations as well as shear contributions by ordered twins. In the framework of the Taylor model for all combinations of critical resolved shear stress, parameters for polycrystal deformation are determined such as Taylor factors, slip distribution among the various deformation modes and internal stresses set up between grains. The comparison of theoretical with experimental textures, and of predicted with observed anisotropy of plastic flow, is utilized to determine the slip geometry and the relative strength levels of the different deformation systems. The results are used for a general discussion of deformation processes observed in TiAl and how these are affected by work hardening characteristics. It is concluded that superdislocations of one kind or another are necessary deformation modes, and that all modes have similar CRSS values at large strains.

Plastic flow of γ-TiAl-based polysynthetically twinned crystals: micromechanical modeling and experimental validation

Abstract A micromechanical model for the calculation of the plastic behavior of a lamellar structure is presented. This model is based on a rate-sensitive approach to describe the plasticity at the single crystal (lamella) level and on the relaxed constraints theory to account for the influence of the lamellar morphology on the overall plastic response of the structure. The equations for the cases of 2- and N -lamellae structures undergoing states of applied stress or strain rate are presented. The model is applied to a lamellar matrix–twin pair which is a simplified representation of a γ -TiAl polysynthetically twinned (PST) crystal. For this case, a morphology-based classification of the critical stresses of the γ -TiAl deformation systems is also presented. This model for PST plasticity is successfully validated by comparison with available experimental data.

Micromechanical interaction in two-phase iron-copper polycrystals

Abstract The plastic deformation of two-phase iron–copper polycrystals was studied experimentally and modelled in a semi-analytical approach, taking into account work-hardening behaviour, initial texture, slip processes and volume fractions of the phases. Iron–copper polycrystals including the single-phase materials were produced by powder metallurgy in various compositions of iron and copper. The two-phase materials had microstructures ranging between interpenetrating networks and matrix/inclusion type. Samples were deformed by rolling and compression at room temperature. Besides stress vs. strain during compression the texture and the microhardness distribution were measured before and after the deformation. The determined quantities (stress, strain, texture) were compared with model calculations performed with a viscoplastic self-consistent (VPSC) model from R. Lebensohn and C. Tome. The best predictions of this model were found in the case of copper inclusions in an iron matrix whereas for interpenetrating networks a viscoplastic Taylor model was in better accordance with experiment.

Influence of the deformation conditions on the texture evolution in γ-TiAl

After rolling sheets of TiAl a modified cube component occurs in the texture. The c-axes of the tetragonal unit cells are aligned with the transverse direction. A strong mechanical anisotropy is observed which depends on the temperature. The modified cube texture is stable during tensile deformation and gets sharper even under superplastic conditions. Textures measured after compression up to temperatures of 950°C exhibit a shift of the 〈110〉 and 〈101〉-fibre components. This is caused by mechanical twinning. At higher temperatures (T>1100°C) the highly twinned grains preferently recrystallize and a texture component around 〈302〉 becomes stable. A bulging mechanism is observed as a special form of the dynamic recrystallization.

Deformation Mechanisms of AZ31 Magnesium alloy

A detailed investigation of the deformation mechanisms plays an important role for a better understanding of texture evolution and anisotropic behavior of magnesium wrought alloys. Therefore, room temperature deformation tests of AZ31 hot rolled sheets and extruded bars have been performed. The experimental tensile textures of the rolled sheets show a reorientation resulting in a {0001}〈101;¯0〉 main component.