Influence of heating in vacuum on low-temperature fracture behavior of carburized Mo-Ti alloys

Abstract

In order to deduce the state of carbon and its interaction with titanium in the carburized molybdenum-titanium alloys (target titanium content: 0.1, 0.2, 0.5 and 1.0 mass%), changes of low-temperature fracture strength and ductility after heating in vacuum at 1773 K were investigated. Bend tests were performed at temperatures from liquid nitrogen temperature to room temperature and yield strength, maximum strength and bend angle were calculated in each temperature. In this study the low-temperature fracture strength and ductility of a material were represented by the critical stress and the critical temperature, respectively. Fracture surfaces of the specimens failed at low temperatures were examined by using a scanning electron microscope and crack initiation and propagation characteristics were investigated. The results are summarized as follows. 1. (1) For the alloy with lower titanium content (e.g. Mo-0.1 mass%Ti alloy): the critical stress, the critical temperature and the fracture mode were almost unchanged after heating in vacuum. In this case it is deduced that carbon exists as free carbon and carbides at the grain boundaries. During heating, reduction of free carbon occurred by decarburizing, but it was compensated by the carbon produced by the resolution of carbides in a manner similar to pure molybdenum. Consequently there still remained enough carbon to strengthen the grain boundaries. 2. (2) For the alloy with higher titanium content (e.g. Mo-1.0 mass%Ti alloy): the critical stress remarkably decreased and the critical temperature increased after heating in vacuum. The fracture mode also changed from transgranular to intergranular. In this case it is deduced that carbon exists as free carbon both at the grain boundaries and at the interfaces between titanium-oxides and matrix. During heating, only the reduction of free carbon proceeded and consequently the grain boundaries became short of carbon.