Effects ofC, O, and N on the property ofvanadium are a long-standing research subject. However, research into the effects of C, O, and N on V-4Cr-4Ti is limited.

Research with model V-4Cr-4Ti alloys doped with O and N provided information on the partition­ing of O and N into the precipitates and matrix. The density of the blocky precipitates and thin pre­cipitates increased with N and O levels, respectively. Figure 9 shows hardness as a function of N and O levels in V-4Cr-4Ti after melting and annealing at 1373 K for 1 h.2 Hardness after annealing at 1373 K, where only the blocky precipitates were observed in the matrix, increased to a certain extent with O level (~4.5 Hv/100 wppm O), but only very weakly with N level (~0.9 Hv/100 wppm N). These data suggest that, after the annealing, most of the N is included in the blocky precipitates and stable to ~-1373 K. On the other hand, O exists in the matrix, the blocky and the thin precipitates, and the partition­ing changes with the heat treatment. Thus, for the purpose of the property control of V-4Cr-4Ti, the level of N before the heat treatment is not so impor­tant but that of O is crucial. It is to be noted, however, that N contamination during the operation can influ­ence the properties of vanadium alloys seriously.

Fundamental information on the impurity dis­tribution and interaction with solutes and dislocations is obtained by serrated flow in tensile deformation as shown in Figure 10. Temperature and stain rate depen­dence of the flow showed that the serrated flow above 673 K is related to C and O and above 773 K to N. Small serration height at 673 K for NIFS-HEAT-1 (1-3 MPa) relative to that of US-832665 (^9MPa) was observed and attributed to the difference in O level.23