Mechanism of Less Ductility in Nickel-Base Super Alloys at Intermediate Temperatures

采用真空拉伸试验研究了纯镍、80%Ni-20%Cr合金、50%Ni-50%Cr合金和50%Ni-50%Cr合金雾化粉末烧结材料的高温塑性加工性。所有试样在1100K左右的温度下都失去了延展性。这种低延展性，即中温脆性是由晶界韧性断裂引起的。纯镍在应变速率大于10^<-1> s^<-1>时，中间脆性消失。随着应变速率的增加，镍铬合金的脆性变弱。试样的脆性行为与铜及其合金的脆性行为非常相似。镍铬合金（即镍基合金的基本体系）的延展性较差，最终被认为是由于晶体晶界的粘性。并对镍铬粉烧结材料的高温加工性进行了研究。在700 ~ 900K的温度范围内发现了良好的和易性。然而，在温度低于700-900K和高于700-900K时，延展性很差。低温下延展性差的原因是铬颗粒的脆性断裂。另一方面，在较高的温度下，镍和铬颗粒之间的界面处的拉应力，由于两种颗粒的热膨胀系数差异较大，导致延展性降低。

High temperature plastic workability of pure nickel, 80%Ni-20%Cr alloy, 50%Ni-50%Cr alloy and a sintered material made from gas atomized powder of 50%Ni-50%Cr alloy were investigated in vacuum by tension teal. All specimens lost their ductility at temperatures around about 1100K. This less ductility, i.e., the intermediate temperature embrittlement was led by the grain boundary ductile fracture.The intermediate brittleness disappeared in the pure nickel deformed at a strain rate higher than about 10^<-1> s^<-1>. Also the brittleness in the nickel-chromium alloys became slight with increasing strain rate. The brittleness behavior in the present specimens was quite similar to the behavior found in copper and its alloys.The less ductility occurring in the nickel-chromium alloy, i.e., a fundamental system of nickel-base alloys, was conclusively considered to be due to viscous nature of crystal grain boundaries.High temperature workability of sintered material made from nickel-and chrolnium-powder was also examined. Good workability was found in a temperature range of 700 to 900K. However, the ductility at temperatures lower and higher than 700-900K was very poor. The poor ductility at the lower temperatures came from brittle fracture in the chromium particles. At the higher temperatures, on the other hand, the tensile stress at the interface between nickel and chromium particles, originated from a large difference between thermal expansion coefficients of both particles, led to less ductility.