A rationale for the strong dependence of mechanical twinning on grain size

In metals that yield as a consequence of mechanical twinning, the yield stress is a function of the grain size in much the same way as it is for dislocation glide. However, the sensitivity to grain size is typically greater. The intent of the present communication is to show that this can be understood, at least in part, in terms of a size effect that accompanies twinning. Some confirmatory data from a magnesium alloy are presented.

Fine grained AZ31 produced by conventional thermo-mechanical processing

The magnesium alloy AZ31 was processed by severe hot rolling and annealing. This processing was optimised to produce recrystallised grain sizes as small as 2.2 μm. The texture of the processed plate was similar to that of the as-received material, with a strong basal alignment in the normal direction. Tensile testing of the fine grained material showed an increase in the strength and elongation compared to the as-received plate. It is  concluded that the improved mechanical properties of the fine grained material results from a refinement in the grain size and the elimination of shear bands that pre-exist in the as-received material.

Tensile deformation of a ultrafine-grained aluminium alloy: micro shear banding and grain boundary sliding

This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.

Effect of grain size on deformation twinning behaviour of high MN steel

The effect of grain size on the mechanical properties and deformation twinning behaviour in high manganese steel was investigated. In order to generate different grain sizes, the samples were subjected to hot rolling, cold rolling and annealing. Room temperature tensile testing of the steel with different grain sizes (5-50 µm) indicated the occurrence of twinning induced plasticity (TWIP) in all the samples. Also, changes in work-hardening behaviour accompanied changes in the grain size. The results are discussed in terms of the enhanced sensitivity of twinning to the grain size.

Realized volatility and correlation in grain futures markets: testing for spillover effects

Fluctuations in commodity prices are a major concern to many market participants. This paper uses realized volatility methods to calculate daily volatility and correlation estimates for three grain futures prices (corn, soybean, and wheat). The realized volatility estimates exhibit properties consistent with the stylized facts observed in earlier studies. According to daily realized correlations and regression coefficients, the spot returns from the three grain futures are positively related. The realized estimates are then used to evaluate the degree of volatility transmission across grain futures prices. The impulse response analysis is conducted by fitting the vector autoregressive model to realized volatility and correlation estimates, using the bootstrap method for statistical inference. The results indicate that rich dynamic interactions exist among the volatilities and correlations across the grain futures markets.

Role of grain boundary sliding in the anisotropy of magnesium alloys

The plastic anisotropy of magnesium alloy sheet drops rapidly with test temperature. It has previously been suggested that this may be due to an increase in the activity of (c+a) dislocations. The present note points out that the phenomenon may result, instead, from the action of grain boundary sliding. This can explain the strong effect of grain size on anisotropy. Furthermore, it points to a new avenue for alloy development.

Effect of grain size on the performance of extruded magnesium alloy tubes in three-point bending

The performance of extruded AZ31, AZ61 and AM-EX1 tubes was examined in three-point bending. Different extrusion temperatures were used to investigate the effect of grain size on the load-carrying capacity, energy absorption and fracture propensity of the tubes. Results showed that while the peak load increased with a smaller average recrystallised grain size, the retention of large elongated un-recrystallised grains in the microstructure reduced the load. The presence of the large elongated grains also appeared detrimental to the ability of the tube to deform before fracture.

Simultaneously enhanced strength and ductility of titanium via multimodal grain structure

Commercial Ti with a multimodal grain structure was successfully produced using cryorolling, followed by low-temperature annealing. This multimodal grain structure Ti exhibited a combination of high yield strength (926 MPa), a uniform elongation of 11% and a failure elongation of 23%. The strength enhancement was mainly derived from the ultrafine equiaxed grains, while the improved ductility originated from the large fraction of high-angle grain boundaries and the multimodal grain structure.

Influence of grain supplements during winter on liveweight, mohair growth and mohair quality of weaner Angora goats

To identify methods to improve growth and mohair production of weaned Angora goats (mean liveweight 18-20 kg) during their first winter, two supplementary feeding experiments using whole-grain barley and lupins were conducted on a farm in southern New South Wales, in a region where weaner illthrift had been reported. Experiment 1 was a 2×2 + 1 factorial with 16 replicate goats; two feeding levels (115 or 230 g/day of whole-barley grain)× two periods of feeding (4 or 8 weeks) + Control (grazing only). Experiment 2 had five treatments × 13 replicate goats; three treatments fed 230 g/day of whole-barley grain for periods of 2 or 3 months and two treatments fed a 50:50 mixture of lupin and barley grain at 350 g/day for 2 or 4 months. Goats were individually fed and then all returned together for grazing. There were no effects of feeding in Experiment 1 and variations of feeding 230 g/day of barley in Experiment 2 provided no benefit. Feeding lupin/barley for 4 months increased liveweight (gain 59 g/day), mohair production, mohair fibre diameter and the incidence of medullated fibre. About 25% of this ration was not eaten by eight goats, reducing treatment average intake to 295 g/day. By the end of spring, there was no difference in treatment liveweights. Regression constants indicated that for each 1 μm increase in mean fibre diameter, greasy fleece weight increased 35 g and for each 1 kg increase in pre-shearing liveweight, greasy fleece weight increased 26 g. The results show that Angora weaner goats can grow during winter, provided their energy and protein needs for growth are met. Improved pasture management and higher levels of supplementary feeding to weaned Angoras are indicated compared with current practices on farms in Australia.