Grain size and high-temperature yield strength of polycrystalline copper

Abstract

Polycrystalline, electrolytic tough pitch copper was prepared in a series of stable grain structures by deforming at 540 and 600°C at strain rates from 7.5 × 10⁻⁴ to 8 × 10⁻² S⁻¹. By applying a prestrain of 0.4 and annealing isothermally, grain sizes ranging from 0.056 to 0.810 mm were produced. The prepared grain sizes decreased with the amount and strain rate of the prestrain, and increased with the prestraining and annealing temperature. After working and sufficient annealing to permit full recrystallization, the yield strengths were determined at 540°C (0.6T _m) and at four strain rates ranging from 7.5 × 10⁻⁴ to 8 × l0⁻² S⁻¹. The high-temperature yield strength increased linearly with the inverse square root of the grain size, as it does at room temperature. However, the grain boundary strength coefficient (slope of the Hall-Petch plot) increased with strain rate, indicating that the grain boundaries serve as both local and long-range obstacles to flow at elevated temperatures.