Steel and martensitic transformation a–Fe ferrite BCC –Fe austenite FCC Martensite: needle crystals of a form aligned in g form C is diluted in a form hard but brittle New phase: Fe3C cementite in a-ferrite hard and ductile Fe – 0.5% C alloy at 950OC has g form quench to room temperature annealing T<800OC sel=1000MPa

Interaction of dislocation with impurities elastic region sL sU DL/L U L plastic deformation work hardening Impurities diffuse to dislocations and form “clouds”-> dislocations are pinned -> higher elastic limit When s exceeds sU dislocations escapes impurities -> stress needed for plastic deformation decreases

Fracture s s sM s Brittle fracture happens in crystals with little plastic deformation In polycrystalline solids fracture surface follows grain boundaries In single crystal fracture follows cleavage plane (e.g. {100} in NaCl) s s x sM xC a a+x s

Grifith cracks stress lines are concentrated fracture at lower at the end of the crack fracture at lower stress crack l r Recipe: composite materials, toughening (superficial quench under compression) glass ductile polymer glass

Ductile fracture s s Metal beyond elastic limit: plastic deformation followed by ductile fracture Necking: region of smaller cross- section has stronger stress; great deformation at neck, even grains are broken Ductile-brittle transition happens in metal below ~ 0OC. Mobility of dislocation falls -> elastic limit increases and becomes higher than fracture stress neck s

Creep e s T<0.4Tm T>0.4Tm strain time failure linear creep Plastic deformation at constant stress (> elastic limit) increases With time when T>0.4Tm Creep originates from increased atomic mobility at high temperature: dislocation climb, elongation of grains, polygonization s elongation of grains polygonization

High temperature alloys against creep Superalloys: Ni, Cr with added Al and Ti no creep even at T>0.4Tm precipitates of added elements grains of metal oxide are incorporated powder metallurgy single crystal in superalloys