Abstract
The structural evolution of the (Nb, Ti)C and its effect on the mechanical behaviours of Nb–(21, 35)Ti–4C and Nb–25Ti–8C (at-) alloys have been comprehensively investigated. The alloys consist of Nb solid solution (Nbss) and (Nb, Ti)C both for the as cast and the heat treated samples; however, the secondary Nbss precipitates within the (Nb, Ti)C after heat treatment and the composition of the secondary Nbss is mainly the Nb. The driving force of Nbss precipitation from (Nb, Ti)C is summarised as two aspects: the lack of C atoms in (Nb, Ti)C induced by the fast cooling rate; and the difference of thermodynamic stability of TiC and NbC. The small sized long strip secondary Nbss, which adopts a well defined orientation relationship between the (Nb, Ti)C, is obtained in the case of low driving force. On the condition of high driving force, the microstructure of the Nbss precipitations becomes large sized massive and layered. The brittleness of (Nb, Ti)C is reduced by the secondary Nbss precipitations proved by nanoindentation tests.
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