Tensile Characteristics of Warm-Swaged Beryllium

Abstract

Cast-billet extruded SR and CR grade beryllium were warm-swaged at about 550°C and annealed mainly at 700 to 800°C. Then they were tensile-tested at 25 to 550°C. The results obtained are as follows:
(1) Ultimate tensile strength (UTS), 0.2% proof stress (PS) and percent elongation (E) at 25°C are propotional to d^−1⁄2, where d is the average grain size in units of mm. UTS and PS at 300 to 550°C are also propotional to d^−1⁄2. In the case of PS, however, values of d should be corrected with those of subgrains instead of those of normal grain size measured by optical microscopy.
(2) Each of true stress (σ)-true plastic strain (ε_p) curves is divided into three stages: The first is associated with the yield point phenomenon. Second and third stages are expressed by equation σ=σ₀+K_εp^m, where σ₀, K and m are constants. The value of m in second stage is about 0.6 at 25°C and about 0.3 at 200 to 500°C. The value of K in the stage decreases with increase in temperature.
(3) The yield point phenomenon is most clearly observed in the SR annealed at 750°C and stretched at 500°C, whose structure consists of fine subgrains. It is deduced that this structure is a major factor contributory to the phenomenon.
(4) At 25 to 200°C, most fractures are brittle fractures. River pattern, twin formation and separation at grain boundary are observed in the fractured surfaces. At elevated temperatures, however, void formation at triple points of grain boundaries prevails and is frequently associated with transcrystalline cracks. Connection of voids leads the specimen to fracture and well-developed dimples of the order of grain size are observed at the fractured surface.