Papers by Author: Gordon W. Lorimer

Abstract: The effects of composition and temperature on the ageing response and the microstructural development during ageing treatment of a series of dilute 6xxx series alloys have been investigated. The alloys contained between 0.22 and 0.79 wt% Si and 0.20 and 0.51 wt% Mg. Some of the alloys were copper-free, 0.001-0.002 wt% Cu, while others contained additions of 0.1 wt% Cu. Some of the alloys were ‘balanced’ while others contained excess Si (ExSi). The effects of solution treatment temperature and artificial ageing (T6) on the precipitation process were investigated using various techniques, including differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and analytical transmission electron microscopy (ATEM). MT DATA has been used to predict the phase relationships as a function of temperature and the MT DATA predictions have been compared with the phases observed by DSC and ATEM. The morphology and crystal structures of the precipitates formed were monitored by TEM. The results showed a correlation between the composition and the ageing response of the alloys.

Abstract: The as-cast microstructures of two dilute Mg-Zr and three dilute Mg-Mn alloys were studied using optical microscopy and scanning electron microscopy, including EDX analysis. The results show that the as-cast microstructure of the Mg-Zr alloys was composed of non-dendritic, equiaxed Mg grains, with a few Zr particles within the Mg grains and along grain boundaries. The grain size of the Mg-Zr alloy was significantly reduced by the Zr addition and a fine grain structure was achieved when the zirconium concentration was above 0.4wt. %. The as-cast structure of the Mg- Mn alloys contained columnar, dendritic grains. Two types of Mn particles (equiaxed large particles and rod-like or plate-like small particles) were observed in the as-cast Mg-Mn alloys. The volume fraction of particles and the size differences between the large and small particles increased with an increase of Mn concentration.

Abstract: The microstructure of three dilute Mg-Mn and two dilute Mg-Zr alloys which had been heat treated at different temperatures and times were studied using optical microscopy and various electron optical techniques, including FEGSEM and TEM. It was found that the Mg-Zr alloys homogenized at 500°C and annealed at 350°C contained a fine dispersion of Zr-containing particles at grain boundaries and within grains. In contrast, annealing Mg-Zr alloys at 300°C for up to 3 h led to little modification of the as-cast structures. When the Mn content was less than 0.9 weight percent, homogenization of the Mg-Mn alloys at 550 to 600°C resulted in the dissolution of small rod-like and needle-shape particles, which then grown up as fine particles when aged at 300°C. Mg- 0.6Mn and Mg-0.9Mn alloys annealed at 300 to 400°C without solution treatment contained a large volume fraction of nano-sized precipitates.

Abstract: The deformation and recrystallization behavior of two magnesium alloys, AZ31 and WE43, have been investigated. The cast alloys were heat treated to produce various distributions of second phase particles and deformed in a channel die at a strain rate of 10-4 s-1 at temperatures between 523 and 673 K. The alloys were subsequently annealed at temperatures between 708 and 798 K.The AZ31 and WE43 were extruded at 555 and 633 K, respectively. The microstructures were compared to those developed during channel die deformation. The tensile and compressive strengths and the texture of the alloys were determined in the as-extruded and recrystallised conditions.Optical microscopy and electron backscattered diffraction (EBSD) were used to characterize the microstructures of the alloys. The EBSD technique was used to determine the texture of the deformed and annealed samples.

Abstract: The deformation and recrystallization behaviour of two magnesium alloys, WE43 and an experimental “Alloy A1” containing a large amount of second phase, have been investigated. Optical microscopy and electron backscattered diffraction (EBSD) were used to observe each alloy after deformation in a channel die at 553 and 558 K and subsequent recrystallization during annealing at 673 K. Alloy A1 dynamically recrystallised during deformation at 553 and 558 K. In WE43 the particle distribution affected the onset of dynamic recrystallization. Both alloys exhibited evidence of particle stimulated nucleation (PSN) of recrystallization at second phase particles as well as recrystallization at pre-existing grain boundaries. The A1 and WE43 alloys were extruded at temperatures of 573 K and 633 and 663 K, respectively. The extruded microstructures were compared to those that developed during channel die deformation.