Microstructure and mechanical properties of a nanostructured Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr supersaturated solid solution prepared by high pressure torsion

Abstract

In this study, a solution-treated Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy was processed by high pressure torsion (HPT) at room temperature, with the aim of producing a material with exceptional mechanical properties. Transmission electron microscopy and X-ray diffraction line broadening analysis reveal that a nanostructured supersaturated solid solution with a mean grain size of ~48 nm and high dislocation density of ~4.7×10 14 m-2 is achieved by HPT deformation. After HPT processing, the present solution-treated samples show finer grains and higher dislocation densities as compared to as-cast samples of this alloy, other solution-treated Mg-RE alloys and conventional Mg alloys deformed using similar HPT processing conditions. This could be attributed to retardation of the dislocation annihilation through enhanced solutedislocation and / or dislocation-dislocation interactions. The higher dislocation density and nanosized grains within supersaturated solid solution cause the microhardness to increase to ~126 HV, which is higher than that of the as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr (wt.%) alloy processed by HPT, and is also higher than that of any conventional Mg-rich alloy deformed using HPT.

Evolution of long-period stacking ordered structure and hardness of Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy during processing by high pressure torsion

“…These suggestions are consistent with XRD data which show line broadening for the LPSO phase and the TEM observations (see Fig. 15(a), the hardness obtained in present case is larger than that obtained by HPT processing of the same alloy in an as-cast condition [21], but it is similar to that obtained by HPT processing of the same alloy in a solution-treated condition [43]. In Fig.…”

“…As-cast followed by HPT [21] Water quenching followed by HPT [43] Furnace cooling followed by HPT (this work) [57]. We measured ρ=5.06×10 14 m -2 (Table 1), which provides a hardness increment of ~30 HV.…”

“…Thirdly, when LPSO lamellae have dissolved the high solute content in the matrix further contributes to retardation of dislocation annihilation through the enhanced solute-dislocation and / or dislocation-dislocationinteraction[43].The crystallite sizes estimated from XRD line broadening analysis are slightly smaller than those measured by TEM observations, because the former represents the coherent diffraction domain size that is related to the subgrain or dislocation cell size[33,48] whereas the latter is obtained by the linear intercept method[31][32][33]. The lattice axial ratio c/a of the Mg-rich hexagonal closedpacked (HCP) phase can be used to evaluate the degree of lattice distortion induced by solute atoms[49]: both first-principles calculations and experiments[50][51][52] indicate that c/a decreases with increase of RE solutes in the Mg-rich phase.The measured c/a of our HPT-processed samples is smaller than that of pure Mg (1.623) andreduces with increasing HPT deformation, with c/a for the HPT20 sample being quite similar to that of the Mg-Gd-Y-Zn-Zr supersaturated solid solution prepared by HPT starting from an assolid solution treated condition[43]. These observations indicate that the concentration of RE solute atoms in the Mg-rich phase increases due to the HPT-induced LPSO decomposition.…”

An investigation of the thermal stability of an Mg Dy alloy after processing by high-pressure torsion

“…A C C E P T E D M A N U S C R I P T 22 5. HPT processing and an annealing treatment leads to significant decomposition of second particles by the decreasing Dy element in the different types of particles that are present in this alloy.…”

“…Severe plastic deformation (SPD) techniques such as equal-channel angular pressing (ECAP) and especially high-pressure torsion (HPT) have proven effective in producing ultrafine-grained (UFG) microstructure down to the sub-micrometer level in several Mgbased alloys [1][2][3][4][5][6][7][8][9]. Furthermore, HPT processing generally produces smaller grains sizes than ECAP processing due to the imposition of higher strains [10].…”