Mechanical Behavior and Microstructure Evolution of Mg-Gd-Y-Zn-Zr Alloy during Multipass Hot Compression Deformation

The multi-pass hot compression deformation behavior of the cast alloy with the composition of Mg-13Gd-4Y-2Zn-0.6Zr, was investigated, and the four-pass compression tests were conducted at the temperatures ranging from 350°C to 500°C and strain rate 0.01 s-1. The experimental results showed that the alloys incurred different degrees of softening by multipass deformation. The microstructure evolution for the deformed alloy was investigated, the influence of the microstructure on the hardness properties of the alloy discussed. The tests reveal that dynamic recrystallization is not the main softening mechanism for this alloy; rather, kink deformation refines the grains to achieve the observed softening effect. The hardness test curve showed that the hardness increased gradually with an increasing number of deformation passes. The improvement of the main mechanical properties related to the strengthening by the grain refinement. In multipass deformation, the misorientation of the kink belt gradually increased,and refined the grains. On the other hand, the grain size of the eutectic phase at the grain boundary decreased with increase of deformation passes. In addition, the mechanical properties were improved by the distribution dispersion of tiny cuboidal particles and acicular-like phases in the matrix.

Investigation of Retained Strains during Multipass Deformation

Double-hit compression tests were carried out at different temperatures and strain rates for a nickel based alloy and a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations. It is shown that at a given total amount of deformation, the retained strain is reduced for the as deformed grains that have not been recrystallized yet, but increased for the recrystallized grains, when comparing double hit with single hit compression tests.

Modelling Strain Induced Precipitation of Niobium Carbonitride during Multipass Deformation of Austenite

A new model of strain induced precipitation in niobium microalloyed austenite is proposed. This is based on the experimental observation of formation of microbands during hot deformation of iron – 30% nickel, which remains austenitic to room temperature. Precipitates are preferentially nucleated on nodes in the dislocation network in the microbands. This geometry enables features of earlier models to be simply explained, and facilitates extension of the model to multipass deformation. It is shown that the model captures all the essential features of previous experimental observations on microalloyed C – Mn steels. Currently, sensitivity analysis of the model and systematic experimental work are being undertaken to quantitatively validate the model.