Final answer:
Heat treatment can alter a gold alloy's hardness, proportional limit, and percentage elongation. The correct answer is D. All of the above.
Step-by-step explanation:
Heat treatment can alter a gold alloy's hardness, proportional limit, and percentage elongation. Heating the alloy to high temperatures and then cooling it can change the crystalline structure, which in turn affects these mechanical properties. For example, heating and quenching an alloy can increase its hardness, making it stronger and more resistant to deformation.
One example of heat treatment is annealing, which involves heating a metal to a specific temperature and then slowly cooling it, allowing the atoms to rearrange themselves in a more ordered manner. This process can soften the metal and improve its ductility.
Therefore, the correct answer is D. All of the above.