The diffusivity of Ni in Fe-Ni and Fe-Ni-P martensites
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addition the parent phase will contain the dislocation structure of the original a 2. Several investigators have measured the diffusivity of Ni in pure a iron, 5,6 D~', and in a Fe-Ni 7 containing P above 600 ~ The diffusivity of Ni in a 2, D,~', however, has not been measured. In a c o m p a n i o n study Romig and Goldstein 8 have determined the Ni solubility limits of a and 3' phases in the Fe-Ni and Fe-Ni-P systems. In that study a and 3, phases were produced by tempering a 2 at temperatures from 300 to 700 ~ The amount of 3,-austenite phase growth was much greater than that which was calculated if the growth was controlled by diffusion in the a phase. This result arzues that the diffusivity of a, m a y be significantly higher than that of a. It is the purpose of this study to measure DN ' and to investigate the effect of dislocation structure on diffusivity. EXPERIMENTAL PROCEDURE For volume diffusion controlled growth, the size of the precipitate and the concentration gradient in the matrix phase are a function of four factors: the bulk alloy composition, the solubility limits of the equilibrium phases, the length of the diffusion field which controls impingement effects and the diffusivity of the components in the matrix phase. If the first three factors are known, the diffusivity in the matrix phase can be determined by matching calculated and experimentally measured concentration profiles. The following sections describe the mathematical model for calculating the concentration profiles, the experimental techniques used to produce the appropriate heat treated samples and the matching procedures employed to obtain D N' Mathematical Model The diffusion model employed for 3, growth was a direct modification of the Fe-Ni-P pseudobinary for-
ISSN 0360-213318110211-0243500.75/0 METALLURGICAL TRANSACTIONSA 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME12A, FEBRUARY 1981--243
mulation of Moren and Goldstein. 9 The Moren and Goldstein model simulated the nonisothermal growth of a in 7 for the Fe-Ni-P system and calculated Ni gradients in both the a and 7 phases. Modifications were made so that growth occurred isothermally and that y grew in a matrix of a. The Crank-Nicholson 1~ numerical technique was used and a Murray-Landis t' gridspace transformation was employed to allow for the a/7 interface movement. The a/7 interface veolicty was determined by either a flux balance or a total mass balance technique. 9J2 The following assumptions are made for the model: 1) The diffusion coefficient in the a or a 2 phase is not a function of composition, 2) The a/'y interface is planar with 7 growth normal to the interface, and 3) Local equilibrium occurs at the a/'~ interface. The planarity of the a/7 interface and the presence of local equilibrium at the a/3' interface was demonstrated for the experimental time periods employed by Romig and Goldstein. 8 However there is no data available on the effect of Ni content on the diffusivity of the bcc F e N i phase. The boundary conditions for the
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