1. The problem statement, all variables and given/known data I am given the trends for yield stress and fracture toughness as functions of temperature for steel. I need to explain the trends 2. Relevant equations none 3. The attempt at a solution I am guessing the yield stress is the stress that the material can withstand before it starts to deform plastically. So that infers that the steel is more elastic at lower temperatues. I cant explain the reasoning for this. Any ideas?