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

通过拉伸试验研究了在水中爆炸冲击加载的烧结粉末铬（99.8质量%纯度）的韧脆转变行为的变化。目前的结果可以总结如下。(1)在应变速率为1.7 × 10 ~（-1）S ~（-1）和1.7 × 10 ~（-1）S ~（-1）时，退火态试样的韧脆转变温度（DBTT）分别为543和573 K<-5><-4><-1>。这些结果表明，铬的DBTT是非常敏感的变形速度。(2)由于在930和1420 MPa的冲击载荷下，退火试样中观察到的急剧屈服完全消除，屈服应力值显著降低。(3)在930和/或1420 MPa冲击载荷下的试样的转变温度明显低于退火试样的转变温度。例如，在930 MPa下处理的试样在1.7 × 10 - 4和1.7 × 10 - 4 s-4的拉伸试验中的DBTT分别为290和403 K<-5><-4><-1>。(4)变形孪晶是一种塑性变形，在1420 MPa冲击压力下，铬合金中没有形成，而在1000 MPa左右冲击压力下，钢中观察到了变形孪晶。(5)冲击加载试样中DBTT的降低可以用屈服应力σ_c和脆性断裂应力σ_c之间的关系来解释。由于冲击压力处理降低了铬的屈服应力，从而在弹性不连续处周围诱发了自由位错。例如基体和第二相之间的界面，在冲击加载的试样中比在退火态试样中在更低的温度下满足发生韧-脆转变的条件σ 1 = σ 2。

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 1.7X10^<-5> and 1.7X 100^<-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 at 1.7X10^<-5> and 1.7X10^<-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 1 420 MPa, though they were observed in the steel shock loaded at about 1 000 MPa. (5) The decrease of DBTT in the shock-loaded specimen can be explained in terms of a relationship between the yield stress sigmar and the brittle fracture stress sigmac. Since the yield stress of the chromium is lowered by the shock pressure treatment which induces free dislocations around the elastic discontinuities. such as interfaces between the matrix and the second phases, the condition, sigmar=sigmac, for the occurrence of the ductile-brittle transition is satisfied at a lower tempera-ture in the shock-loaded specimen than in the as-annealed specimen.