Effects of microstructural condition on the constitutive deformation behaviour of magnesium AZ80 between 296 and 693 K.

摘要

This work reviews the status of the AZ (aluminum - zinc) series of magnesium (Mg) alloys as a forging material with promising weight saving potential in wrought form for the automotive industry, but inadequate ductility in forging components. Knowledge of the physical basis for the ductility and, in particular, the role of the alloying elements on the elevated-temperature deformation behaviour are shown to be limited and complicated by the many possible variations in as-cast, and heat treated microstructures. Experiments are proposed based upon six unique starting microstructures representative of potential as-cast forging blanks, which are developed from three separate DC cast Mg AZ80 billets. Specialized elevated temperature tensile testing, using infrared heating between 296 and 693 K coupled with a step-ramp technique to control strain rate, is used to determine the temperature dependent constitutive deformation behaviour and precise strain rate sensitivity of the flow stress for the starting microstructures. The results show that the presence of second phase gamma-Mg17Al 12 is detrimental to ductility as brittle failure initiated by interface decohesion is observed independent of deformation temperature. At elevated temperatures ductility is enhanced and brittle failure eliminated by pre-dissolving the gamma phase and homogenizing the resulting super-saturated solid solution. An anomalous peak in yield stress observed at 423 K is attributed to either solute induced competition between basal and prismatic glide, or a solute drag effect. Haasen plot analysis shows that at room temperature, deformation obeys the Cottrell-Stokes relation (CSR) to failure, and that strain rate sensitivity increases with temperature independent of starting microstructure. A deviation from the CSR at 423 K is attributed to the increased glide resistance of dislocation interactions with Al and Zn solute atoms. The deformation mechanism responsible for yield at room temperature is shown to be independent of grain size and alloying content. Constitutive representation of the elevated temperature stress-strain data reveals that the mean slip distance can be used as an internal constitutive parameter capturing the evolution of the material structure during deformation.