Final answer:
Metals crack under stress when impurities contaminate grain boundaries, as these impurities hinder the sliding mechanism that imparts ductility and malleability to metals. An example is the manufacturing of metal implants, where mismatches in the expansion coefficients of coatings and the base metal can lead to cracking due to thermal stress.
Step-by-step explanation:
A metal is susceptible to cracking under stress when its grain boundaries are contaminated with impurities because the presence of such impurities can interfere with the metal's ability to deform plastically. Metals have free-flowing electrons that allow ions to slide past one another, endowing the metal with ductility and malleability. However, impurities at the grain boundaries prevent this sliding mechanism, leading to stress concentration and, consequently, cracking under stress.
For example, in the manufacturing process of metal implants for hip and knee replacements, researchers seek coatings with expansion coefficients similar to the base metal to prevent thermal stress. This stress can cause cracks to form at the interface of the coating and metal, if expansion coefficients differ significantly. This is equally important for other applications where temperature changes can induce stress, potentially leading to failure of the material due to contamination or mismatched properties.