THE MARTENSITIC TRANSFORMATION IN IRON - 26 - NICKEL - NITRIDE ALLOYS
Abstract
The formation of an ordered (gamma)'-nitride Fe(,4-x)Ni(,x)N was confirmed by nitriding thin Fe-Ni alloys in NH(,3)/H(,2) mixture at 550(DEGREES)C. Homogeneous alloys of Fe-26Ni-N were prepared by nitriding these strips in NH(,3)/H(,2) gas mixtures. The variations in the lattice parameters of austenite and martensite with nitrogen content were studied by X-ray diffraction. The martensitic start temperatures and morphology were studied metallographically. The transformation characteristics were found to be similar to those in Fe-Ni-C alloys. The effect of preaging austenite on the martensite lattice parameters was investigated. Pre-aging of austenite did not result in any changes in lattice parameters of the martensite containing 0.4-0.5N. The results on an 1.2N alloy were inconclusive. TEM studies on samples thinned after preaging or sample preaged in situ in the microscope did not reveal any preprecipitates of (gamma)'-nitrides. From the variations in the lattice parameter and the intensity ratios (I(,110)/I(,101) + I(,011)) of martensites it is concluded that the increased tetragonality of Fe-Ni-X martensites over that of the Fe-C martensites is due to the change in the elastic constants of martensite with Ni additions.
Degree
Ph.D.
Subject Area
Metallurgy
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