Effect of structure on the heat resistance of a niobium alloy

1. The creep rate of alloy NM20T3Ts decreases with increasing grain size, reaches a low point, and then increases. 2. The variations of the width of line (321), electrical resistivity, hardness, and grains size of alloy NM20T3Ts are similar to the changes in the creep rate and point to changes in the structure of the grains themselves. 3. In the process of high-temperature annealing (at 1500–1800°) a network of subgrain boundaries is formed and at some stage of development it has an effect equivalent to that of grain refining, which in conformity with concepts of the high-temperature mechanisms of deformation leads to an increase of the creep rate. 4. For the highest heat resistance of alloy NM20T3Ts the grain size must be fairly large (up to 120 μm) but not so large that coarsening of the substructure can occur.