Effect of Shock Pressure on the Ductile-Brittle Transition Behavior of Sintered Chromium.

Abstract

The change in the ductile-brittle transition behavior of sintered powder chromium (99.8 mass % purity) explosively shock loaded in water was examined by means of a tension test. The present results can be summarized as follows. ( 1 ) The ductile-brittle transition temperature (DBTT) of an as-annealed specimen was found to be 543 and 573 K in tests at strain rates of 1.7×10-5 and 1.7×10-4 s-1, respectively. These findings show that the DBTT of the chromium is very sensitive to the deformation speed. ( 2 ) Due to the shock loading at 930 and 1420 MPa, the sharp yield observed in the as-annealed specimen was completely eliminated and the yield stress value was considerably decreased. ( 3 ) The transition temperature of the specimen shock loaded at 930 and/or 1420 MPa was markedly lower than that of the as-annealed specimen. For example, the DBTT of the specimen treated at 930 MPa was 290 and 403 K in the tension tests at1.7×10-5 and 1.7×10-4 s-1, respectively.( 4 ) Deformation twins, a kind of plastic deformation, were not formed in the present chromium subjected to shock pressures up to 1420 MPa, though they were observed in the steel shock loaded at about 1000 MPa. ( 5 ) The decrease of DBTT in the shock-loaded specimen can be explained in terms of a relationship between the yield stressσr and the brittle fracture stressσc. Since the yield stress of the chromium is lowered by the shock pressure treatment which induces free dislocations around the e1astic discontinuities, such as interfaces between the matrix and the second phases, the condition, σr = σc, for the occurrence of the ductile-brittle transition is satisfied at a lower temperature in the shock-loaded specimen than in the as-annealed specimen.