高温高静水圧力下におけるFe-Cr合金（σ相）のぜい性破壊応力

Abstract

In order to clarify the effect of high hydrostatic pressure on the brittle fracture stress, three point bending tests have been carried out for an Fe-Cr alloy (σ-phase) containing about 47%Cr and 0.7%Si at atmospheric pressure and under 800 MPa at temperatures up to 1200 K. In the temperature range from room temperature to 1140 K, the specimens remained in the σ-phase and transgranular cleavage fracture occurred under both pressures. By scanning electron microscopy, the fracture surface consisted of river patterns and Wallner lines. The fracture stress at atmospheric pressure increased gradually with increasing temperature to about 1000 K and increased rapidly at higher temperatures. The same tendency was observed in the tests under 800 MPa but the tracture stress under 800 MPa was always higher than that at atmospheric pressure, and the difference between these stresses became larger at higher temperatures than about 1000 K. From a consideration of the stress condition around a crack, a formula σFP=σF0+P⁄Kσ was proposed to represent the influence of hydrostatic pressure on the brittle fracture stress at low temperatures, where σF0, σFP are the fracture stresses at atmospheric pressure and under high pressure, P is the pressure in positive sign and Kσ is the stress concentration factor at the crack. At higher temperatures, Kσ was replaced by Neuber’s plastic stress concentration factor Kσ*.