The grain size dependence of the yield,flow and fracture stress of commercial purity silver

Heavily cold-rolled sheet material of 99.9 pct purity Ag has been recrystallized at varying temperatures to give average grain diameters,l, in the range between 1 and 60 μ. For this material, the yield stress, flow stress at several strain values, and fracture stress follow the Hall-Petch relation: $$sigma _varepsilon = sigma o_varepsilon + k_varepsilon l^{ - 1/2} $$ whereσ _ε is the flow stress at a particular value of strain, ε, ands _{o ε} andk _ε are the experimental constants appropriate to a particular strain value. The range in grain size obtained for this material was sufficiently large to determine that silver can be appreciably strengthened by grain size refinement and that several other relations previously suggested to relate the stress and grain size could be discounted. The finest grain sizes were measured from replicas of etched specimens as viewed with the electron microscope. It is proposed that this type of grain size strengthening may be responsible for the exceptional strength which occurs in certain films of silver fabricated by vapor deposition techniques.