The Process of Tungsten-Nitride Precipitation upon Nitriding Ferritic Fe- 0.5 at.% W Alloy

摘要

The precipitation of tungsten nitride upon internal nitriding of ferritic Fe- 0.5 at.% W alloy was investigated at 610°C in a flowing NH_3/H_2 gas mixture. Different tungsten nitrides developed successively; the thermodynamically stable hexagonal δ-WN could not be detected. The state of deformation of the surface plays an important role for the development of tungsten nitride at the surface. The morphologies of the tungsten nitrides developed at the surface and those precipitated at some depth in the specimen are different. The nitride particles at the surface exhibit mostly an equiaxed morphology (with the size of the order 0.5 μm) and have a crystal structure which can be described as a superstructure derived from hexagonal δ-WN. These nitride particles show a strong preferred orientation with respect to the specimen frame of reference but have no relation with the crystal orientation of the surrounding ferrite matrix. In the bulk, nano-sized and finely dispersed platelet-like precipitates grow preferentially along {100}a-Fe. It is unclear whether these precipitates consist of binary iron nitride a"-Fe16N2 or of a ternary Fe-W-N. Additionally to the finely dispersed particles, bigger nitrides at ferrite grain boundaries develop exhibiting platelet-type morphology and possessing a crystal structure which can be also described as a superstructure derived from hexagonal δ-WN. Upon prolonged nitriding assumed discontinuous precipitation of the initially precipitated finely dispersed nitrides starts from the ferrite-grain boundaries resulting in lamellas consisting of alternate ferrite and hexagonal nitride lamellas, whereas the nitride lamellas having a Pitsch-Schrader orientation relationship with the surrounding ferrite matrix. The nitrides precipitated upon nitriding in the bulk were found to be unstable during H2 reduction at 470°C. Remarkably, upon such low temperature dissolution of the nitrides took place but only the nitrogen from the nitride particles could diffuse out of the nitride platelets and the specimen, leaving W-rich regions (W-"clusters") at the location of the original precipitates.