Fine grain size will normally improve the yield strength of metal materials. For example, the yield stress of a sample of brass with a large grain size was 20MNm2. The yield stress of an otherwise identical sample with a grain size of 4m was 120MNm2. a) Why did the yield stress increases in this way? [0.5 Marks] b) What is the value of in Equation 2? [0.5 Marks]