Coarsening of the y' precipitate in binary
Ni-Al alloys containing 6.35 and 6.50 wt.% Al has been studied by measuring the ferromagnetic Curie temperature as a function of ageing time at 625 and 715°C. The concentration of Al is linearly related to t-1'3 at long ageing times, in agreement with theory. Analysis of the kinetics has provided values of c,, the concentration of Al in equilibrium with a y' particle of infinite size. The values of c, are about 10% larger than the literature values of the solubility at the relevant temperatures. This suggests that the present values of c, represent the coherent solubilities. The results of this investigation, combined with extant data describing the coarsening of y' in terms of average particle size measurements, are used to exemplify a recently proposed method (A. J. ARDELL, Acta Met. 15, 1772 (1967) for obtaining reliable values of the particle-matrix interfacial free energy, y, and the "effective" diffusivity of the solute in the matrix, Dell. As a result of the proposed method, the value of y has been revised to approximately 14 erg/cm2 from a previous estimate of 30 erg/om2. The value of D,fr is greater than the (extrapolated) coefficient of diffusion of Al in very dilute Ni by a factor cf about two. Derr is also about a factor of two greater than that predicted by the application of irreversible thermodynamics to the process of volume-constrained growth and ungrowth of coherent precipitates during coarsening. UNE APPLICATION DE LA THEORIE DU GROSSISSEMENT DES PARTICULES LE PRECIPITE y' DANS DES ALLIAGES Ni-Al L'auteur a Btudib le grossissement du prbcipitb y' dans des alliages binaires Ni-AI contenant 6,35 et 6,50% en poids d'A1, au moyen de mesures de la temperature du point de Curie ferromagnirtique et en fonctlon du temps de vieillissement & 625 et 715°C. La concentration en Al est relibe linbairement B t-1/3 pour les temps de vieillissement longs, ce qui concorde avec la thborie. L'analyse de la cinbtique de la reaction a permis d'atteindre les valeurs de c,, concentration d'Al en Bquilibre aveo une partioule y' de taille infinie. Ces valeurs de c, sont environ 10% plus &levees que les valeurs de la solubilitb don&es par la litterature pour les temperatures correspondantes. Ceci suggere l'idbe que les pr&.entes valeurs de c, reprbsentent les solubilit& cohhrentes. Les rbsultats de cette recherche, combin& avec les don&es existantes dbcrivant le grossissement de y' d'aprks des mesures de la taille moyenne des particules, sont utili&s pour illustrer une m&hode proposbe rbcement (A. J. ARDELL, Acta Met. 15, 1772 (1957)) ayant pour but d'obtenir des valeurs correctes de 1'6nergie libre y de l'interface particule/matrice