UC Riverside

Refinement to the ultrafine-grain (UFG) regime increases yield strength and improves ductility in magnesium, and high pressure torsion (HPT) enables Mg to be processed to this regime without failure. It is generally agreed that the rate of microstructural refinement scales with applied pressure during HPT, but direct comparisons of pressure’s effects during HPT in Mg alloys are scarce. A dilute Mg-3Y (wt%) alloy was processed to five turns under nominal pressures of 1 GPa, 3 GPa, & 5 GPa. The microstructure was investigated as a function of radius, and thus equivalent strain (ε), by scanning electron microscopy, revealing a shear band-dominated refinement mechanism. Vickers hardness as a function of ε agreed with literature, showing increased refinement rate with pressure, up to a maximum of approximately 90HV at all pressures investigated. TEM was utilized for the nano-scale microstructural investigation at a strain of ε=57 for each pressure, revealing an ultrafine grain structure for the highest pressure and shear bands in the 1 & 3 GPa conditions. APT identifies yttrium segregation to boundaries within the shear band of the 3 GPa ε=57 specimen, and nano-indentation finds that a majority of the hardening comes from within the core of the shear bands, where the microstructure is in the UFG condition. Comparison to the broader The solute segregation as a function of strain was further investigated by APT. In the solutionized condition, no obvious clustering of yttrium is observed and a majority of the solute is in solid solution. At relatively low strain of ε=3, very small clusters of Y are seen segregated at a twin boundary and as the strain is increased to ε=16, the amount of solute segregated and the size of the clusters increases. At a strain of ε=57, the segregation is sizable, with localized solute depletion of the matrix. Finally, the UFG sizes and solute segregation observed in the Mg3Y are considered in the broader context of the extant literature and the role of solutes is investigated. Solute segregation during deformation stabilizes the fine grains and alters the preferred deformation mechanism, opposing the idea of a true Hall-Petch inversion.