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Title: Thermomechanical processing of magnesium alloy Elektron 43
Author: Brownsmith, Tomas
ISNI:       0000 0004 7233 0869
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2018
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Elektron 43 (WE43C) is a modern magnesium rare earth alloy (Mg-RE) with potential light-weighting applications in wrought civil aerospace components. Rare earth elements are known to improve mechanical properties and weaken texture of wrought Mg-RE alloys, but for Elektron 43 the parameters of thermomechanical processing (TMP) that produce optimum microstructure development are not well understood. A large data-set of Elektron 43 flow stress data was collected in an extensive range of hot compression tests at typical TMP temperatures (350-500°C) and strain rates (0.001-100\s). Friction parameters were determined in a ring compression study. Material data was corrected for friction, strain rate and temperature variations. Parameters were fit for a sinh constitutive equation. The corrected material data and used to develop an initial finite element model in the commercial software package QForm. Further work to calibrate the heat transfer parameters is required. The effect of two extrusion parameters (temperature and ram speed) on microstructure and mechanical properties was explored. Extrusion of 60 mm diameter Elektron 43 billets to 20 mm diameter rods (ER=9.92) was conducted at three temperatures (380°C, 420°C and 460°C) and a range of ram speeds (~0.1-15 mm/s). An approximate extrusion limit diagram was formed from empirical relations for extrusion load and hot cracking. Extruded microstructures were bi-modal consisting of dynamically recrystallised (DRX) grains and elongated deformed grains. The typical `prismatic' deformation texture (extrusion direction ED||) was weakened by increasing DRX fraction, concomitant of increased ram speed and temperature. Small DRX grains at low temperature/ram speed had very weak preferences for the 'RE component' with ED||. Increase in ram speed/temperature resulted in a rarely reported 'c-axis' RX texture (ED||) becoming increasingly prominent. This texture dominated at high temperatures and speeds. C-axis grains were larger than those of the RE and prismatic orientations, with the relative difference increasing with DRX fraction. Thus a growth advantage of c-axis grains has been demonstrated: it is postulated these grain boundaries have higher boundary mobility (considering the 90° misoreintation with the deformation texture) and driving pressure (as they are not well oriented for basal slip). Suppression of all RE texture modification followed extrusion at 460°C, 0.16 mm/s. The RX texture was typical of non-RE Mg alloys: ED||. It is argued that at this condition solute segregation is suppressed. Extrusions showed low ambient yield asymmetry. The highest measured yield stresses were correlated with partial RX. These two observations can be explained by a balance of texture, Hall-Petch strengthening and work hardening. One unexplained observation of note is that the maximum observed yields generally correspond to a reversed yield asymmetry.
Supervisor: Robson, Joseph ; Quinta Da Fonseca, Joao Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Recrystallisation ; Recrystallization ; Magnesium ; Asymmetry ; Constitutive ; EBSD ; Rare Earth ; Extrusion ; Texture