Deformation modes and anisotropy in magnesium alloy AZ31

A strongly textured sheet of magnesium alloy AZ31 has been subjected to tensile testing at temperatures between ambient and 300&deg;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 &lt;c + a&gt; 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 (I 0-3 s-1) used in the mechanical tests.<br /

Origins of texture memory in steels

When steels are temperature cycled from ferrite to austenite and back again the texture is usually much weakened but in some cases such as Mn-alloyed IF steels the texture reverts almost exactly to the original state. Experiments have been carried out to investigate this so-called texture memory in two low carbon Mn-alloyed steels and the results are discussed in terms of models that have been previously proposed. It is concluded that texture memory arises from variant selection controlled by nucleation of ferrite on specific austenite grain boundaries that are created during the prior transformation on heating. A necessary criterion is that the cooling transformation involves a displacive mechanism so that the favoured nuclei are also capable of continued growth. The present viewpoint accommodates the roles of alloying elements and cooling rate that are observed

Yielding behaviour of martensite in steel

Although martensite is recognised as a very strong phase in carbon steels, its initial yielding commences at low stresses and the tensile stress-strain curve shows a smooth, rounded form. Evidence is presented from x-ray diffraction to show that this behaviour is due to the presence of intra-granular stresses that are residues after the shear transformation from austenite to martensite. These internal stresses are reduced in magnitude by plastic deformation and also by tempering. Reduction of internal stress due to plasticity is shown by a decrease in XRD line broadening after deformation. A simple model is presented in which the stress-strain behaviour is controlled by relaxation of the internal stresses almost up to the point of the ultimate tensile strength. It demonstrates that only a very small fraction of the material remaining in a purely elastic state provides a large stabilising effect resisting necking. A corollary of this is that the uniform elongation of martensitic steel actually increases with increase in the strength level. Effects of heat treatment are also reproduced in the model, including the increase in conventional yield stress (Rp0.2) that occurs after low temperature tempering

Application of Laser-Ultrasonics for Evaluating Textures and Anisotropy

Various approaches are reviewed for determining elastic anisotropy and its coupling to crystallographic texture, with special reference to ultrasonic measurements. Two new methods are described for measuring the anisotropy of P-wave velocity using laser-ultrasonics. Making measurements across the diameter of a cylindrical specimen as it is rotated makes it possible to maintain a very constant known path length. This permits extremely accurate measurements with a precision of better than 0.01%. Results on 316 stainless steel in different conditions are compared with calculated values obtained from EBSD textures together with measured densities and crystalline coefficients from the literature. Excellent agreement is obtained when applying the Hill geometrical average procedure. A similar approach is adopted to measure the variation of wave velocity in a martensitic steel, after tempering at a range of temperatures. Changes in the anisotropy associated with thermal softening are discussed. The second method uses Galvano mirrors to steer the generating laser to different positions over a sheet surface, allowing wave velocities to be determined along different directions in the anisotropic material

Development of boundary misorientations during grain growth in silicon steels

Changes in grain size, texture and misorientation distributions have been monitored during extensive normal grain growth in 3%Si steels. The boundary misorientation distributions deviate significantly from the Mackenzie relationship. Comparisons of correlated and uncorrelated distributions show large excesses of low angle boundaries. However, these are not a result of low energy boundaries being favoured during grain growth since the deviation diminishes as growth proceeds. The effect originates in the nucleation of grains in colonies of similar orientation during primary recrystallisation. A slight tendency for promotion of 60&ordm; boundaries may indicate some preference for the retention of lower energy twin boundaries during grain growth in silicon steel