INTERFACE STABILITY IN THE NI-CR-AL SYSTEM .2. MORPHOLOGICAL STABILITY OF BETA-NI50AL VS GAMMA-NI40CR DIFFUSION COUPLE INTERFACES AT 1150-DEGREES-C

The morphological stability of the bond interface in β-Ni50Al vs γ-Ni40Cr solid-solid diffusion couples annealed at 1150°C has been examined as a function of annealing time. Interface breakdown results in a microstructure consisting of an irregular layer of α-Cr, β precipitates within the α layer, and lamellar areas of α+β touching the γ region. A series of experiments was designed specifically to examine the early stages of diffusion, interface motion, and intermediate phase formation. Pronounced vaporization of Al from the β- Ni50Al end-member occurs within the initial heat-up period to 1150°C and during the early stages of bonding. The subsequent deposition of Al onto the faying surface of the γ end-member results in the conversion of the γ surface to a β region. The low mobility of Cr and the inability of the β-γ surfaces to maintain equilibrium causes the formation of an intermediate layer of α-Cr. Moreover, the newly formed β region exhibits a fine-grained structure with significant porosity; the number density of pores increases with increased solid-state diffusion. The grain boundaries and pores result in irregular migration patterns of the constituent elements, causing the formation and growth of β protrusions into the α layer. Lateral rejection of Cr from the protrusion then occurs, leading to the formation of α-Cr regions adjoining the β outgrowth. Sympathetic growth of alternate regions of α and β follows, giving rise to a lamellar microstructure that can be explained on the basis of a low mismatch between the α and β lattice parameters, compatible crystal structures, and a pronounced orientation relationship. In a separate but related experiment, the degenerate α+β morphology observed in the normal diffusion couples was replaced by a layer of agglomerated α-Cr when the β-γ couples were annealed under applied high pressure.