Effect of Heat Treatment on Microstructure and Mechanical Behavior of Nb-35Ti-4C Alloy

The phase composition, interface structure, microstructure evolution and mechanical behavior of the Nb-35Ti-4C alloy before and after heat-treatment were comprehensively investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and Nano Indenter'XP. The results show that as cast and heat treated Nb-35Ti-4C alloy consists of Niobium solid solution (Nbss) and carbide of (Nb, Ti)C. The numerous secondary (Nb, Ti)C with several nanometers in size precipitates from Nbss matrix during heat treatment. Furthermore, the secondary Nbss with a reticular structure distribution also precipitates from carbide, the phase transformation of the carbide can be summarized as (Nb, Ti)C→(Nb, Ti)C+Nbss. The secondary Nbss nucleates along the close- packed surface (111) of fcc (Nb, Ti)C at the early state of heat-treatment, and the strip secondary Nbss and the (Nb, Ti)C matrix adopt a well-defined orientation relationship (OR):[011]_(Nb,Ti)C//[001]_Nbss, (111)_(Nb,Ti)C//(110)_Nbss. The hard carbide has a strong bonding with the Nbss, and the eutectoid transformation of the large sized carbide after heat treatment leads to less possibility for the cracks formation on the carbide surface, which in turn improves the toughness.