c) Above are representations of copper samples that have undergone different pro

Question

c) Above are representations of copper samples that have undergone different processing steps. The symbol represents a dislocation. Both samples are viewed at the same magnification. Which of the two samples would you expect to be stronger? Explain why. (Don't just state the pertinent difference(s) between the samples, explain WHY that difference affects strength.) d) THIS QUESTION IS OPTIONAL. If you are happy with your performance on the in-class writing assignment, skip this question and I will use 2x your in-class score for this question. (The in-class was graded out of 3 points, so doubling its score will make it worth). During each step of the annealing process (recovery, recrystallization, and grain growth) the internal energy of a material decreases. For each of the 3 steps, briefly describe how the internal energy lowered during that step. List the processing steps you could use (in the correct order) to transform sample 1 into Sample 2. Be as specific as possible.

Explanation / Answer

ANS c): Sample2 seems to have more strength as compared to Sample 1.

Explanation:

Ductile metals (like copper ) become stronger when they are deformed plastically at temperatures below their melting point. The reason for strain hardening is the increase of dislocation density with plastic deformation. The average distance between dislocations decreases and dislocations start blocking the motion of each other.

The ability of a metal to plastically deform depends on the ability of dislocations to move. Since hardness and strength (both yield and tensile) are related to the ease with which plastic deformation can be made to occur, by reducing the mobility of dislocations, the mechanical strength can be increased and hence greater mechanical forces will be required to initiate plastic deformation. The more unconstrained the dislocation motion, the greater is the facility with which a metal may deform, and the softer and weaker it becomes.

Copper in Sample 1 has undergone less cold work and therefore it has dislocations which can move easily. But Sample 2 seems like to have more cold work. As it contains higher dislocation density, the resistance to dislocation motion by other dislocations becomes more pronounced. Thus the value of imposed stress necessary to deform the copper and therefore strength is more for sample 2 than sample 1.

ANS d):

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