Atomic structure of interphase boundary enclosing bcc precipitate formed in fcc matrix in a Ni-Cr alloy
- PDF / 2,311,891 Bytes
- 8 Pages / 598 x 778 pts Page_size
- 86 Downloads / 222 Views
I.
INTRODUCTION
A precipitate formed within a matrix grain has a specific orientation relationship with respect to the matrix. Such a precipitate grows by means of the ledge mechanism[~,21 when the crystal structures of the matrix and the precipitate phases are significantly different [e.g., face-centered cubic (fcc)/body-centered cubic (bcc), bcc/hexagonal closepacked (hcp) and fcc/hcp]. It has been repeatedly shown that the ledge mechanism is operative during the migration of matrix/product interphase boundaries in various alloy systems.~2.3m In ledgewise growth processes, it is assumed that the area of the interface (the risers or kinks on the risers of growth ledges) at which atomic attachment occurs from the matrix to the product has an incoherent (or disordered) structure across which there is a lack of continuity of atomic rows and planes.t1.5] Even now,t6] it is considered that there is local atomic disorder at such a growing interface. The change of crystal structure takes place by the poorly coordinated random jumps of the atoms across such interfaces, which are biased by gradients of chemical potential. However, diffusional phase transformations that accompany the stacking sequence change, often exhibit surface relief ef-
T. FURUHARA, Research Associate, and T. MAKI, Professor, are with the Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-01, Japan. K. WADA, formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan, is presently with Fukuyama Works, NKK Corporation, Fukuyama, 721, Japan. This article is based upon a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18~2, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALSTRANSACTIONS A
fects[7,81very similar to those in the diffusionless, displacive transformations (martensitic transformation). To produce a surface relief, it is considered that atomic attachment across the growing interface is highly coordinated. Christian[5] proposed that surface relief effects are essential to the product formed by the migration of coherent interfaces because lattice (or atomic) correspondence is maintained between the matrix and the product phases. It is difficult to imagine the presence of such a correspondence at an incoherent (or disordered) portion of the growing interface, at which atomic attachment should occur in an appreciably random manner. Recently, Howe [9] proposed that continuity of atomic planes across the growing interface leads to an atomic site correspondence in diffusional transformations, resulting in a surface relief effect. To examine the generality of atomic site correspondence, it is important to clarify atomic structures on growing interfaces of the products in various types of diffusional phase transformations. Transformation strain generates under an atomic site correspondence. Such a strain is accommodated by loss
Data Loading...
