a) Describe the microscopic mechanisms that are responsible for dislocation creep in metallic solids. How would you optimize the microstructure of the metal to resist dislocation creep? [10 marks] b) George Hapompwe was given instructions by the design engineer to be tightening bolts of a turbine casing to a prescribed torque after every 48 hours. After every forty eight hours a signal was given indicating the level of the relaxed stress. This prescribed relaxed stress was set at two thirds (3/4) of the original stress. Hapompwe decided to do a fast one and over-tighten the bofts so that it would take longer for the bolts to relax to this prescribed torque. To his surprise the bolts were relaxing faster than the prescribed 48 hours and so he was required to be tightening more often. Describing all terms, derive an expression to determine the time, Tr, taken for the stress to relax to 3/4 of the original value. Using the derived expression explain to Hapompwe why over-tightening does not help. [10 marks]