Matthew Barnett

Influence of deformation conditions and texture on the high temperature flow stress of magnesium AZ31

The evolution of hot working flow stress with strain is examined in torsion, uniaxial compression and channel die compression. The flow stress was found to be strongly dependent on texture and deformation mode. At low strains this dependency accounted for a difference in flow stress of up to a factor of two. At higher strains the influence of texture and deformation mode was less marked. The stresses corresponding to an equivalent strain of 0.5 were modelled using a power law expression with an activation energy of 147 kJ/mol and a strain rate exponent of 0.15. The influence of texture and deformation mode on flow stress is rationalised in terms of the influence of prismatic slip, twinning and dynamic recrystallisation on deformation stress and structure.

A semianalytical Sachs model for the flow stress of a magnesium alloy

A semianalytical Sachs-type equation for the flow stress of magnesium-base alloys is developed using the Schmid law, power law hardening, and a sigmoidal increase in the twinning volume fraction with strain. Average Schmid factors were estimated from electron backscattered diffraction (EBSD) data. With these, the equation provides a reasonable description of the flow curves obtained in compression and tension for samples of Mg-3Al-1Zn cut in different orientations from rolled plate. The model illustrates the general importance of basal slip and twinning in magnesium alloys. The significance of prismatic slip in room temperature tension testing is also highlighted. This is supported with EBSD slip line trace analysis and rationalized in terms of a possible sensitivity of the critical resolved shear stress for prismatic (cross) slip to the stress on the basal plane.

The generation of new high-angle boundaries in aluminium during hot torsion

The crystallographic rotation field for deformation in torsion is such that it is possible for orientations close to stable orientations to rotate away from the stable orientation. A Taylor type model was used to demonstrate that this phenomenon has the potential to transform randomly generated low-angle boundaries into high-angle boundaries. After imposing an equivalent strain of 1.2, up to 40% of the simulated boundaries displayed a disorientation in excess of 15°. These high-angle boundaries were characterised by a disorientation axis close to parallel with the sample radial direction. A series of hot torsion tests was carried out on 1050 aluminium to seek evidence for boundaries formed by this mechanism. A number of deformation-induced high-angle boundaries were identified. Many of these boundaries showed disorientation axes and rotation senses similar to those seen in the simulations. Between 10% and 25% of all the high-angle boundary present in samples twisted to equivalent strains between 2 and 7 could be attributed to the present mechanism.

Double twinning mechanisms in magnesium alloys via dissociation of lattice dislocations

In this work, we propose dislocation mechanisms for the formation of double twin structures in hexagonal close packed (HCP) crystals through the nucleation of secondary twins within primary twin domains. The model considers that secondary twins associated with the most commonly observed double twin variants (i.e. type 1 and type 2) nucleate and thicken by a sequence of three distinct dissociation reactions of mixed basal dislocations. Provided that the less frequently observed double twin variants (i.e. type 3 and type 4) also form by a dislocation-based mechanism, we show that their development must proceed by a separate set of dissociation reactions involving pyramidal ⟨c+a⟩ slip dislocations. Mechanistic, crystallographic and energetic considerations indicate that the type 1 variant should be the most prevalent. The mechanisms proposed here would also apply to the analysis of compound twins and HCP metals other than Mg that exhibit double twinning, such as titanium.

Predicting the critical strain for dynamic recrystallization using the kinetics of static recrystallization

School of Engineering and Technology, Deakin University, Pigdons Rd, Geelong, VIC 3217,Australia(Received November 12, 1999)(Accepted March 9, 2000)Keywords: Steels-austenite; Dynamic recrystallization; Recrystallization1. IntroductionOne view of dynamic recrystallization is that it comprises static recrystallization occurring in the timescale of deformation. While this is clearly not true for geometric dynamic recrystallization [1] orcontinuous dynamic recrystallization [1], there is some metallographic support for it with respect to thebeginning of conventional dynamic recrystallization. The microstructure after a few volume percent ofrecrystallization by this mechanism appears very similar to that seen after a small degree of staticrecrystallization [2,3].A number of workers have constructed mathematical equations for the beginning of conventionaldynamic recrystallization (DRX) using a formulation based on the nucleation mechanisms of staticrecrystallization (SRX) [2,4,5]. These models fit the observed trends in DRX quite well. However, theydo not allow any firm conclusions to be drawn with respect to the relative kinetics of the two processes.In the present work, conventional equations for the kinetics of SRX are modified to allow “SRX” tobegin prior to the end of deformation. In this manner a critical strain for the beginning of “SRX” duringdeformation is derived. This value is then compared with observations of the critical strain required forthe initiation of DRX in steel.2. BackgroundThe static recrystallization kinetics following the hot deformation of steel are often expressed in termsof the time taken after deformation for 50% recrystallization (t

Deformation modes and anisotropy in magnesium alloy AZ31

Abstract A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300 °C. Structures have been examined by optical and transmission electron microscopy and also by atomic force microscopy to quantify surface displacements seen at grain boundaries. Plastic anisotropy varies strongly with test temperature as was observed previously by Agnew and Duygulu. The present findings do not support the view that crystallographic becomes a major contributor to deformation at higher temperatures. Rather, the material behaviour reflects an increasing contribution from grain boundary sliding despite the relatively high strain rate (10– 3 s– 1) used in the mechanical tests.

Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys

Asymmetric yield behavior in tension and compression is a common and usually undesirable feature of wrought magnesium alloys. To prevent yield asymmetry, it is necessary to favor slip over twinning, as it is the unidirectional nature of twinning combined with the strong textures produced in wrought magnesium alloys that produce yield asymmetry. In this article, the potential to use precipitates to strengthen selectively against twin growth is discussed. The effect of precipitates shape and habit on strengthening of slip and twinning is calculated using simple Orowan-based models. It is shown that basal plate precipitates, although being poor strengtheners against basal slip, are good strengtheners against twin growth. This is because they produce the maximum unrelaxed back-stress when they remain unsheared inside the twin. The predictions of the model have been validated against experiments for two alloys that form different precipitate types: AZ91 (basal plates). and Z5 (c-axis rods). Crystal plasticity modeling has been used to predict that an optimized distribution of basal plate precipitates is expected to strongly reduce yield asymmetry, even in strongly textured magnesium alloy.

Microstructural evolution during hot deformation of duplex stainless steel

Abstract The microstructure evolution during hot deformation of a 23Cr–5Ni–3Mo duplex stainless steel was investigated in torsion. The presence of a soft δ ferrite phase in the vicinity of austenite caused strain partitioning, with accommodation of more strain in the δ ferrite. Furthermore, owing to the limited number of austenite/austenite grain boundaries, the kinetics of dynamic recrystallisation (DRX) in austenite was very slow. The first DRX grains in the austenite phase formed at a strain beyond the peak and proceeded to <15% of the microstructure at the rupture strain of the sample. On the other hand, the microstructure evolution in δ ferrite started by formation of low angle grain boundaries at low strains and the density of these boundaries increased with increasing strain. There was clear evidence of continuous dynamic recrystallisation in this phase at strains beyond the peak. However, in the δ ferrite phase at high strains, most grains consisted of δ/δ and δ/γ boundaries.

Mechanical properties of atomically thin boron nitride and the role of interlayer interactions

Atomically thin boron nitride (BN) nanosheets are important two-dimensional nanomaterials with many unique properties distinct from those of graphene, but investigation into their mechanical properties remains incomplete. Here we report that high-quality single-crystalline mono- and few-layer BN nanosheets are one of the strongest electrically insulating materials. More intriguingly, few-layer BN shows mechanical behaviours quite different from those of few-layer graphene under indentation. In striking contrast to graphene, whose strength decreases by more than 30% when the number of layers increases from 1 to 8, the mechanical strength of BN nanosheets is not sensitive to increasing thickness. We attribute this difference to the distinct interlayer interactions and hence sliding tendencies in these two materials under indentation. The significantly better interlayer integrity of BN nanosheets makes them a more attractive candidate than graphene for several applications, for example, as mechanical reinforcements.

Recrystallization During and Following Hot Working of Magnesium Alloy AZ31

The microstructures of magnesium AZ31 are examined following hot compression testing and annealing. The grain size, fraction dynamically recrystallized and, in a couple of cases, the crystallographic texture are reported. It was found that the progress of dynamic recrystallization is strongly sensitive to processing conditions but that the dynamically recrystallized grain size was less sensitive to stress than in other metals. It was also found that, for structures containing between 80 and 95 % dynamic recrystallization, abnormal grain growth occurs during annealing. The crystallographic texture produced is also sensitive to the deformation conditions.