Metallurgical Aspects of Deformation Processing

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The research proposed is focused entirely on high temperature deformation, with five sub-components: 1. Dynamic Transformation of Austenite at Temperatures High in the Austenite Phase Field. The following aspects of this newly discovered mechanism will be investigated: i) effect of stress on variant selection; ii) effect of composition on critical strain and ferrite volume fraction; iii) characteristics of interstitial and substitutional partitioning during transformation; iv) coalescence of Widmanstätten ferrite and retransformation to austenite; and v) thermodynamics of dynamic transformation. 2. Dynamic Transformation of Ti and Zr Alloys. Ti and Zr alloys have recently been shown at McGill to be susceptible to dynamic transformation. While in ferritic alloys, deformation raises the Ae3, in the hexagonal materials, deformation lowers the beta transus. This transformation follows the Burgers relation and is thus unlike the displacive mechanism that applies to steel. The mechanisms of dynamic transformation in these materials are unknown and will be determined. These include the thermodynamic background as well as the microstructural, textural and compositional effects. 3. Twinning, Variant Selection and Dynamic Strain Aging in Mg and Ti Alloys. The role of strain accommodation on the variant selection process will continue to be investigated during primary, secondary and tertiary twinning. Of particular interest is the accommodation taking place in the vicinity of shear bands and the effects of negative rate sensitivity on this process. Another aspect concerns the dynamic strain aging associated with both rare earth and conventional solutes. This has the aim of enabling the production of desirable "rare-earth" textures using less expensive alloying elements such as Ca. The mechanisms that permit this to be achieved will be identified and explored. 4. Mechanics of Torsion Testing and Application to Simulation of Hot Rolling. The Hencky strain continues to be employed in the analysis of shear experiments such as torsion and remains a controversial topic. The applicant will contribute to this debate by clarifying the cases where use of the Hencky strain is justified. Simulations will be carried out of both strip (short interpass time) and plate (long interpass time) mill schedules. The role of dynamic transformation in industrial rolling will be clarified together with the need to modify runout table models so as to take the effect of dynamic transformation into account. 5. Continuous and Discontinuous Dynamic Recrystallization. The accurate control of steel processing relies on a detailed understanding of static and dynamic recrystallization. Research will continue to be carried out in this important area, with more detailed examinations of the role of continuous dynamic recrystallization in multi-directional forging and other avenues of severe plastic deformation (SPD).