Mechanical properties and structural evolution during deformation of fine grain magnesium and aluminum alloys.

摘要

Grain refinement improves the formability and the strength of wrought Mg and Al alloys. Ultrafine grain Mg is produced by a new process for severe plastic deformation, called Alternate Biaxial Reverse Corrugation (ABRC). Fine grain structure in Al is produced by creating a new composition capable of precipitating dispersed intermetallics in the alloy.; Slip and twinning subdivide an initial bimodal grain structure of Mg alloy during processing. Dynamic recovery and recrystallization lead to the formation of nearly uniform ultrafine microstructure of average grain size 1.4mum, containing many submicron grains. In Mg, twinning causes grain refinement in the early stages, but it is inhibited when grain size becomes finer. A strong basal texture is created after several corrugation and flattening steps, but eventually weakened as grain size becomes finer. Grain rotation and possible dynamic recrystallization are believed to cause a drop in the intensity of basal texture.; At room temperature, grain refinement causes a considerable increase in strain rate sensitivity of flow stress (m) leading to the enhancement of post-uniform elongation. Yield strength increases, and becomes more isotropic due to the inhibition of twinning in fine grain Mg alloy, compared to coarse grain alloy. Normal anisotropy ratio (R value) for fine grain Mg at room temperature is higher than that for coarse grain alloy.; At warm temperatures, formability is significantly increased due to an increase in strain rate sensitivity of flow stress and diffuse quasistable flow in fine grain Mg, as compared with coarse grain alloy. At 200°C and strain rates below 2x10-4s-1, the fine grain alloy demonstrates a high rate of strain hardening up to a true strain of 0.6 in addition to its high strain rate sensitivity (m ∼ 0.4-0.5), leading to a high elongation of 300-400%. There is competition between dynamic grain growth and grain refinement during straining at warm temperature. Mg exhibits isotropic deformation behavior (R ∼ 1.0) at elevated temperatures in sharp contrast with its room temperature behavior, i.e. textural effects are minimized.; For Al-based alloy containing low Mg (Al-3wt% Mg-1.3wt% Zn-1wt% Cu-0.5wt% Sc-0.2wt% Zr), the effects of different thermomechanical treatments on recrystallization, superplastic response, and age-hardening response are investigated. This alloy produces tensile elongations over 400% at a strain rate of 10-3 s-1 and 500--525°C. During superplastic deformation, dynamic recrystallization and strain-induced grain growth occur to cause a bimodal microstructure to transform into a uniform structure with a stable grain size. Room temperature yield strength of this new alloy is 235MPa, considerably higher than conventional Al-Mg alloys containing higher levels of Mg.