Physical Metallurgy - Learn about Point and Line Defects

A: Governing principle: Vacancy concentration increases exponentially with temperature and affects dislocation mobility. At 450°C, ferritic steel accumulates thermal vacancies that assist dislocation climb, reducing yield strength without changing phase. Engineers often pick D because quenched-in vacancies sound familiar, but no rapid cooling occurred here.

A: Governing principle: High-temperature creep is controlled by vacancy diffusion enabling dislocation climb. Edge dislocations require climb to bypass obstacles, so their mobility sets the creep rate in this regime. Option B traps engineers who remember cross-slip but forget it’s less rate-controlling at high homologous temperature.

A: Governing principle: Vacancy fraction equals exp(-Qv / kT). Substituting values gives exp(-0.67 / (8.617e-5 × 500)) ≈ 1.8 × 10^-7. Option B catches those who drop the exponential and linearize the relation.

A: Governing principle: Grain boundaries impede dislocation motion and raise strength. ASTM limits grain size to control dislocation pile-up length and therefore mechanical response. Option B tempts those who blur point defects with microstructural scale effects.