The age hardening characteristics of Nb-Base alloys containing carbon and/or silicon: Part I. (Nb-15 At. Pct Hf)

Abstract

A study was made to investigate solutioning and aging reactions, and their effects on microstructure and microhardness, in fifteen experimental Nb-base alloys, involving Nb-C, Nb-Si-C, Nb-Si, Nb-15 Hf, Nb-15 Hf-C, Nb-15 Hf-C-Si, and Nb-15 Hf-Si* systems. Alloys were solution treated, quenched, then aged for 2 h at temperatures ranging from 800 to 1600°C, to examine the effect of temperature, and aged at 900 to 1100°C isothermally for times varying from 2 to 20 h. Hardness provided the means of evaluating structural effects. Microstructural changes were determined by optical and electron microscopy, and the phases were chemically extracted and identified by X-ray diffraction or electron diffraction. In general, all of the alloys containing hafnium exhibited age hardening characteristics. This age hardening appears to be due to precipitation of the complex monocarbide (Nb, Hf) (C, O, N) phase. Replacement of some of the carbon by silicon in the Nb-15 Hf-C alloys raises the overaging temperature from 800 to 1000°C and leads to strength retention for longer times as compared to the alloys which contain carbon only. Out of several alloys studied, the Nb-15 Hf-0.46 C-2.32 Si and Nb-15Hf-l.31C-0.96 Si showed the best aging characteristics.