Twinning and the ductility of magnesium alloys Part II. “Contraction” twins

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work presents part II of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile and compression tests results are reported for common wrought alloys: AZ31, ZK60 and ZM20. These data are combined with EBSD analysis and simple flow stress models to argue the following: (i) that “contraction” double twinning (which enables contraction along the c axis) can decrease the uniform elongation, and (ii) that compression double twinning can also account for shear failure at low strains. The last of these is described as a combined consequence of strain softening of the continuum and the local generation of twin sized voids.

Twinning and the ductility of magnesium alloys Part I: “Tension” twins

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work is part I of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile test results are reported for the common wrought alloy AZ31. These data are employed in conjunction with a simple constitutive model to argue that View the MathML source twinning (which gives extension along the c-axis) can increase the uniform elongation in tensile tests. This effect appears to be similar to that seen in Ti, Zr and Cu–Si and in the so called TWIP phenomenon in steel.

A criterion for conventional dynamic recrystallization: Application to magnesium alloys

An analytical approximation for the steady state dynamic recrystallized grain size is combined with a simple nucleation criterion to assess the propensity for dynamic recrystallization. In line with observation, the criterion predicts dynamic recrystallization in 99.9995% pure Al but not in material 99.5% pure. It also agrees with the observation that zone refined ferrite can display dynamic recrystallization at high temperatures and low strain rates but not at lower hot working temperatures. The criterion is applied here to common wrought magnesium alloys to argue that conventional dynamic recrystallization is expected under "normal" hot working conditions.

Solidification of cast magnesium alloys

A description of the key solidification steps in the formation of the as-cast microstructure of magnesium alloys is presented. The focus is on the two common magnesium alloy groups: Mg-Al alloys and Mg-Zn-rare earth alloys. The key elements described are: nucleation (including grain refinement), growth of the primary phase and the formation of the eutectic phases. In addition the effect of casting process (e.g. high-pressure diecasting and sand casting) on the outcomes from solidification are discussed. This includes consideration of the formation of banded defects during solidification in the dynamic environment of high pressure die casting.

Texture and mechanical anisotropy in three extruded magnesium alloys

One ZM61 alloy (6·2%Zn, 1·2%Mn) and two magnesium alloys containing nominally 3% of neodymium and yttrium respectively have been prepared in the form of hot extruded flat strips. Their textures and microstructures have been quantified and series of mechanical tests were carried out to determine plane stress yield loci in both the solution treated and aged conditions. The ZM61 alloy had a sharp texture and marked anisotropy of strength that could be rationalised in terms of deformation by basal <a> slip and {1012}<1011> twinning. This material was much weaker in compression than in tension. Precipitation hardening on aging caused a greater impedance to twinning than to slip with the result that the anisotropy was somewhat reduced. The Mg–3Nd alloy had a very weak and different texture but nevertheless demonstrated a pronounced anisotropy of strength. Aging increased the yield stress by about 80% and also inhibited twinning to some extent although the degree of anisotropy remained almost unaffected. The Mg–3Y alloy which showed a texture of intermediate strength was different in type from either of the others. Its strength behaviour was close to isotropic; in particular, no difference existed between tensile and compressive loading, and aging produced only a marginal increase in strength. Twins were relatively infrequent in the deformed Mg–3Y alloy but its mechanical behaviour could not be rationalised according to simple models.

Some issues relating to the ductility of wrought magnesium alloys

The present work is concerned with gaining a better understanding of the factors that control the ductility of wrought magnesium alloys. The ultimate aim is to develop alloys with vastly improved room temperature formability. It is shown that 3D tomography of fractured tensile specimens reveals disk shaped voids aligned more or less at 45 deg. to the tensile axis. These voids are consistent with twin induced void formation. It is also shown that the double twins that produce such voids form in contradiction to Schmid predictions. Finally, it is demonstrated that low levels of rare-earth additions leads to vastly improved texture and ductility in extrusions, as they do in rolled sheet.

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.

Synthesis and physical property characterisation of phosphonium ionic liquids based on P(O)2(OR)2− and P(O)2(R)2− anions with potential application for corrosion mitigation of magnesium alloys

Phosphonium cation based ionic liquids (ILs) have become of interest due to their unique chemical and electrochemical stability as well as their promising tribological properties. At the same time, interest has also grown in the use of phosphate and phosphinate based ionic liquids for corrosion protection of reactive metals. In this work we describe the synthesis and characterization of six novel ionic liquids based on the tetraalkylphosponium cation coupled with organophosphate and organophosphinate anions and their sulfur analogues. The conductivity and viscosity of these ILs has been measured and discussed in terms of the nature of the interactions, effect of anion basicity and the extent of ionic character. The reaction of the IL with a ZE41 magnesium aerospace alloy surface is also demonstrated.

Designing microstructures to enhance the plasticity of wrought magnesium alloys

The data examines the design of magnesium alloys for improved ductility by the edition of rare earth elements. These elements, such as cerium and gadolinium modify the texture of wrought products and also refine the grain size.