Jia-Lun Gu

Variant selection and its effect on phase transformation textures in cold rolled titanium sheet

Abstract The recrystallization and phase transformation textures in cold rolled titanium sheets with different rolling reductions have been investigated by means of three dimensional crystallite orientation distribution function (CODF) analysis, the high temperature X-ray diffraction (HTXRD) and transmission electron microscopy (TEM) technique. The formation of phase transformation texture is discussed in terms of orientation variant selection between α and β phases. It is found that at a low rolling reduction, for example 30%, the transformation texture is composed of a more randomly distributed orientations without orientation variant selection occurs. After titanium sheet cold rolled 80% and subsequently α→β→α phase transformation, the main transformation textures is characterized by [ 2 110]‖ ND fibre texture due to orientation variant selection.

Texture control and the anisotropy of mechanical properties in titanium sheet

By means of three-dimensional crystallite orientation distribution function (CODF) analysis, the relationship between texture control processes and mechanical properties anisotropy in titanium sheet has been investigated. The formation and transition of cold-rolling texture, recrystallization texture and phase transformation texture are also discussed. The results show that, after being cold rolled unidirectionally and annealed, titanium sheet exhibits a strong anisotropy of mechanical properties due to the pyramidal textures (2 1 1 5) [0 1 1 ] and (1 0 1 3) [1 2 1 0]. After a cyclic phase transformation process coupled with cold rolling and annealing treatment, the recrystallization texture component (1 0 1 3) [1 2 1 0] is suppressed and [h k i l] 9∥ND transformation fibre texture (where ND is the sheet normal direction) are developed, which produce a well improved mechanical anisotropy. The cross-rolling process can create a basal type texture (0 0 0 2) 〈u v t w〉 or a near basal type texture, which leads to low level planar mechanical properties anisotropy but to relatively high normal plastic anisotropy (R value).

Serration behaviours in metallic glasses with different plasticity

Abstract We present a statistical study of serration behaviours in Pd77.5Cu6Si16.5, Ti41Zr25Be26Ag8, Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 and Fe50Ni30P13C7 metallic glasses with different plasticity. The four samples show similar serration patterns in the beginning of yielding, and different patterns during later loading. These results indicate that the shear band initiation process in metallic glasses follow some similar dynamics. And the later serration process follows different dynamics and will lead to different plasticity. Here we interpret these serration behaviours from a perspective of inhomogeneity. The different serration patterns and shear band dynamics could be reasonably understood. The serration pattern of the Fe-based sample suggests that the brittleness of metallic glasses might result from a lower degree of inhomogeneity, and a less tendency of forming shear band intersections. This study might provide new experimental evidences for different micro-structures (or inhomogeneity) and dynamic behaviours in metallic glasses with different plasticity.

Serration Behavior of a Zr-Based Metallic Glass Under Different Constrained Loading Conditions

To understand the plastic behavior and shear band dynamics of metallic glasses (MGs) being tuned by the external constraint, uniaxial compression tests were performed on Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 MG samples with aspect ratios of 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, and 3:1. Better plasticity was observed for the samples with smaller aspect ratio (under higher constraint degree). In the beginning of yielding, increasing serration (jerky stress drop) size on the loading curves was noticed for all samples. Statistical analysis of the serration patterns indicated that the small stress-drop serrations and large stress-drop serrations follow self-organized critical and chaotic dynamics, respectively. Under constrained loading, the large stress-drop serrations are depressed, while the small stress-drop serrations are less affected. When changing the external constraint level by varying the sample aspect ratio, the serration pattern, shear band dynamics, and plastic behavior will change accordingly. This study provides a perspective from tuning shear band dynamics to understand the plastic behavior of MGs under different external constraint.

Understanding the effects of Poisson’s ratio on the shear band behavior and plasticity of metallic glasses

In metallic glasses, a high Poisson’s ratio often corresponds to a large plasticity and ductility. Yet the physics underpinning such a connection is still poorly understood. Here through finite element simulations, we reveal that a high Poisson’s ratio could promote the inhomogeneous stress distribution in metallic glasses. The inhomogeneous stress field could cause earlier nucleation and easier arrest of shear bands, and therefore better plasticity. Experimental results also show a trend of decreasing shear limit with Poisson’s ratio. These findings suggest that the stress inhomogeneity might be a key to understand the effects of Poisson’s ratio and loading condition on the plastic deformation behaviors of metallic glasses.

Modelling superplastic deformation of materials with a nonequiaxed microstructure

The superplastic deformation behaviour of a two-dimensional nonequiaxed microstructure is investigated on the basis of the grain-rolling mechanism proposed by Paidar and Takeuchi for an equiaxed microstructure. Analysis shows that not only the deformation geometry but also the dynamics of grain boundary dislocation activity will be altered if grains are elongated rather than equiaxed. Constitutive equations thus derived indicate that the grain aspect ratio can impose a remarkable influence on the superplastic stress-strain rate relationship and such an influence can be quite different if the orientation of the applied stress lies over an angle with respect to the longer axis of the grains. The complexity of modelling the superplastic deformation of engineering materials is also discussed.