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Title: Microstructure/property relationships in three high strength wrought magnesium alloys based on elektron 675
Author: Twier, Abdulhamied Moktar
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2011
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The object of the present investigation has been to relate the mechanical properties of a high strength Mg-Y-Gd alloy to alloy composition, extrusion parameters and microstructure. Three alloys with various Y: Gd ratios, of similar total solute content (at %) to Elektron 675, have been investigated in this study:9122: Mg – 6.5 wt % Y – 7.6 wt % Gd – 0.4 wt % Zr 9123: Mg – 8.2 wt % Y – 4.8 wt % Gd – 0.4 wt % Zr9124: Mg – 2.6 wt % Y – 13.1 wt % Gd – 0.4 wt % ZrThe three alloys were extruded at 425 and 475 °C with extrusion ratio 17: 1 to give two samples from each alloy, group a and b. Alloy 9122 was also extruded at 460 and 500 °C with extrusion ratio 10: 1 to give another two samples c and d. The as-cast microstructure of the three alloys comprised equiaxed α-magnesium grains and regions of eutectic decorating some grain boundaries formed during solidification of the ingot. Variation of extrusion parameters has resulted in a dispersion of different volume fractions of second phase particles in different groups of samples. The chemistry of second phase particles was determined by in-situ bulk energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD); magnesium yttrium was identified for near-equiaxed particles and yttrium hydride was proposed as a possible identification for cuboids. The composition of these compounds varied with variation of Y: Gd in the three alloys. Variation of extrusion ratio and extrusion temperature had a clear effect on the evolution of the extruded microstructure and the development of crystallographic texture as characterized by optical microscopy, electron backscattered diffraction (EBSD) and inverse pole figures. The microstructure of the extruded samples was refined during extrusion through dynamic recrystallization (DRX). Samples of groups a, b and c exhibited a microstructure in which bands of fine, equiaxed grains in association with stringers of second phase particles (running along the extrusion direction) were formed between bands of coarse, equiaxed grains. Sample d contained only small volume fraction of second phase particles; only a few alternating bands were formed and considerable grain growth occurred. A random texture was developed during extrusion in samples of groups a, b and c; a wide spread of orientations accompanied by a (new) previously unreported texture component in which basal poles of some grains are aligned nearly parallel to the extrusion direction was developed. The new texture component was weak in sample c and entirely suppressed in sample d. The asymmetry in tensile and compressive yield stress commonly associated with magnesium extrusions was nearly eliminated in samples of group a and b. The new basal texture component was likely to be a result of nucleation of DRX on sites rotating into this orientation. This is proposed to be a result of deformation in those regions in grains was accomodated by basal, prismatic and pyramidal slip. The grains nucleated in this orientation appear to have developed a form of preferred growth which led to strengthening of this new component following solution treatment. The effect of varying solute content (Y & Gd) and different ageing temperatures 150, 200, 250 and 300 °C (T5 & T6) on the ageing response and precipitation reaction were investigated using hardness measurements and transmission electron microscopy (TEM). Alloy 9122 showed the highest ageing response of the three alloys at 150, 200 and 250 °C (T5 & T6); specimens aged in the T5 gave higher hardness than the T6 treatment, a contribution of fine grain size. The three alloys did not respond to ageing at 300 °C. The precipitation reactions that occurred in alloy 9122 (at under, peak and overageing) and alloys 9123 and 9124 (at peak ageing) at 250 °C have been characterized. The precipitation sequence observed in sample 9122a can be described as: . The microstructure of peak aged specimens of alloys 9122 and 9124 were similar; both contained a homogeneous dispersion of precipitates and some metastable precipitates. Alloy 9123 contained only a homogeneous dispersion of precipitates and remnants of precipitates and no precipitates. The enhanced thermal stability of and phases are most likely responsible for the superior elevated temperature properties of Elektron 675. The effect of varying solute content (Y & Gd) and extrusion parameters on the mechanical properties were determined using tensile testing. Alloys 9122 and 9124 exhibited higher 0.2 % proof stress and UTS than alloy 9123 and alloy 9122 was the hardest alloy. Alloy 9122 exhibited the worst ductiliy (T5 and T6) among 9123 and 9124, and this was attributed to the higher volume fraction of second phase particles. The ductility of samples a, b and c in the as-extruded & T5 conditions, particularly in the transverse direction, was limited by stringers of second phase particles, whereas ductility and failure in sample d was governed by grain size and texture. The ductility and failure of all samples aged in the T6 treatment, irrespective of the extrusion history, was controlled by texture and grain size rather than stringers of second phase particles.
Supervisor: Robson, Joseph ; Lorimer, Gordon Sponsor: Libyan Government ; Magnesium Elektron Limited
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Magnesium wrought alloys ; Extrusion ; Texture ; Dynamic recrystallization ; Precipitation ; EBSD ; TEM ; SEM ; EDX ; EPMA