RECOVERY AND RECRYSTALLIZATION OF A NICKEL-ALUMINUM ALLOY WITH ORDERED AND DISORDERED PHASES

Annealing experiments are performed with rolled sheet of an "advanced nickel aluminide", code IC-218, containing Cr, Zr, and B as alloying elements. This alloy consists of almost-equal-to 80% ordered gamma' and almost-equal-to 20% disordered gamma lamellae, in parallel orientation and perfect coherency. About half the hardness increment due to the 20% deformation is recovered on annealing for a few minutes at 675 to 775-degrees-C; line shape analyses of the diffraction lines and measurements of line intensities indicate that most of this softening is due to dislocation rearrangement, but some is attributable to reordering of the gamma' phase partially disordered by rolling. Electron micrography establishes that a coherent gamma/gamma' interface acts as a one-way valve, allowing dislocations to move freely from gamma' into gamma but not vice versa. Consequently, the recovered alloy is rich in dislocations in the gamma phase, whereas the gamma' phase is almost clear of dislocations. Recrystallization is retarded in comparison to single-phase gamma' but takes place readily at 1000-degrees-C. Nuclei form preferentially in gamma' regions; as they grow, at first a coherent gamma/gamma' lamellar structure grows with them; at a later stage, the gamma becomes spheroidized.