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Prof W J Clegg

Whatever the primary function, it is generally the onset of flow and failure processes that limit the lifetime and application of a material. Such processes are usually treated in terms of constants, such as a flow or failure stresses, where time is unimportant. However, these simplifications do not apply in hard, creep-resistant materials, even when deforming at room temperature. The aim of this course is to show how existing ideas can be extended to allow an understanding of why different types of materials behave as they do.

Before the course begins, you might like to read chapter 2 of "Deformation Mechanism Maps" by Frost and Ashby, which gives a good overview of different deformation processes, including the lattice resistance. You should also read chapter 17, which is concerned with how materials fall into groups with similar properties. Another book to read is J.J. Gilman, "Electronic Basis of the Strength of Materials".

This lecture course will cover:

  • Why do dislocations form? Is there a resistance to their movement? Misfit energy of a dislocation: in-plane strains and misalignments. Relation of misalignment to interatom potential. Why do dislocations form. The work required to move a dislocation: concept of the lattice resistance. How does the misfit energy change as the dislocation moves.
  • How do the predictions compare with observations? What controls ease of dislocation motion. Effect of the dislocation width. The lattice resistance in some typical materials. Diamond (also Si, GaAs), glide and shuffle planes; TiC, TiN; b.c.c. metals, e.g. Fe; c.c.p. metals. Overall comparison with observations. Effect of crystal structure and bonding.
  • How is dislocation motion possible below the Peierls stress? Deformation as a thermally activated process. The rate of flow and the dislocation velocity. Magnitude of the activation energy. Effect of τ on the activation energy. Effect of applied stress on dislocation velocity. Temperature dependence of yield stress.
  • How important is temperature when other obstacles control dislocation motion? When is deformation thermally activated. Other obstacles to dislocation motion: forest dislocations. Comparison of magnitude of effect of forest dislocations and lattice resistance. Estimate of energy required to overcome forest dislocation obstacles. Comparison with observed behaviour in Fe and Ni. Logarithmic creep and limited thermal activation.
  • Why does jerky flow occur? Yield drops and Lüders bands. Criteria for yield drop behaviour in crystals. Change in dislocation density during yielding, e.g. Cu. Effect of strain-rate exponent, e.g. Fe. Solutes. Lüders bands and forming, e.g. nylon.
  • How can deformation occur without work hardening? Recovery, its importance in processes at high temperatures. Rate of recovery. Friedel equation, e.g. LiF. Glide and recovery together: steady-state dislocation creep. Predictions and observations of creep rates. Role of diffusion. Comparison with experimental observations.
  • Why limit glide if diffusion is rate controlling? Dislocation glide in particle hardened systems. Effects of increasing temperature on glide. Particle-hardened alloys.
  • How else might creep occur? Diffusional flow. Effect of applied stress on atom potential at grain boundaries. Rate of flow. Importance of grain-size. Need for grain-boundary sliding.
  • Can we replace nickel-based superalloys? Advantages of present systems. Uses & requirements. Possibilities: elements, compounds & intermetallics. Complications. Platinum group alloys. Refractory metal alloys. Others.
  • Stability against failure. Inhomogeneous flow in solid materials. Superplasticity. The onset of tensile failure and necking. Observations of flow in superplastic alloys. Interacting flow processes. Mechanisms of superplastic flow. Superplastic materials and forming, e.g. Ti, SiC.