Atomic Structure of Interphase Boundary Enclosing Bcc Precipitate Formed in Fcc Matrix in a Ni-Cr Alloy T. FURUHARA, K. WADA, and T. MAKI The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane -(112)fcJ/(23T)bcc ) contains a regular array of structural ledges whose terrace is formed by the (111)fcJ/(110)bcc planes. A structural ledge has an effective Burgers vector corresponding to an a/12[121]rco transformation dislocation in the fcc --~ bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fcc to bcc. The second set is sessile misfit dislocations ( - 1 0 nm apart) whose Burgers vector is a/3[111]ec~ = a/2[110]b~. These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface. 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 of coherency, resulting in the introduction of dislocations on interfaces. Aaronson et aL E6] published an overview of nucleation and growth mechanisms of the product phase formed by diffusional and shear processes. It was proposed that the product formed by diffusional processes contains sessile interfacial dislocations whose Burgers vectors lie in the planes of growing interface. On the other hand, in the cases of shear processes, glissile dislocations whose Burgers vectors are not parallel to the growing interface in the case of edge (or mixed) dislocations or whose Burgers vectors lie in the interface in the case of screw dislocations should be present on the growing interface of the product phase. This proposal was based upon many studies, utilizing both conventional and high-resolution transmission VOLUME 26A, AUGUST 1995--1971 diffusional transformations that are accompanied by a stacking sequence change, however, the accommodation mechanism is not yet clarified. In fcc/bcc systems, Luo and Weatherlytlu studied the interfacial structure of intragranular bcc laths formed in the fcc matrix of a Ni-Cr alloy. They showed that growth ledges exist on the broad face of the laths, and that the side facet plane contains a regular array of misfit dislocations accommodating the dilatational strain normal to the parallel close-packed planes. However, the atomic structure of the interphase boundaries was not clarified in detail. The present study aims to examine by means of HRTEM the atomic structure of the interphase boundaries enclosing bcc laths precipitated from the fcc matrix of a Ni-Cr alloy and to deduce the growth mechanism of these precipitates. II. Fig. 1--Bright-field micrograph of a bcc lath formed in the specimen aged at 1273 K for 18 ks: (a) bright-feld micrograph, (b) corresponding selected area diffraction (SAD) patterns (the incident beam direction is [11%//[111]h), and its key diagram. EXPERIMENTAL PROCEDURE The alloy used is Ni-44.8 mass pct Cr with 40 ppm of carbon, 163 ppm of oxygen, 64 ppm of nitrogen, and 6 ppm of hydrogen contained as impurities. A button of this alloy was produced by plasma arc melting. The button was homogenized at 1473 K for 86.4 ks after being sealed in vacuum. After hot-rolling to a plate 4-mm thick, specimens were cut and solution-treated at 1473 K for 3.6 ks, followed by water-quenching to obtain the fcc single-phase structure. Subsequently, the specimens were aged at 1273 K for 18 ks and water-quenched. Transmission electron microscopy samples were prepared by ion milling. Conventional TEM (CTEM) and HRTEM observations were performed using a JEM200CX operated at 200 kV and a JEM4000EX operated at 400 kV, respectively. IIL RESULTS A. Atomic Structure on the Broad Face o f Precipitate Laths Fig. 2--Dark-field micrograph of the broad face of a bcc lath. Growth ledges are pointed to by the arrows. electron microscopy (HRTEM), on the interfacial structure produced during various phase transformations. However, if an atomic site correspondence is maintained during growth in a diffusional transformation, a large transformation strain will be built up when the product grows in the dimension normal to the plane whose stacking sequence changes during transformation. For the product to grow further, such transformation strains should be accommodated not only in a dilatational component but also in a shear component during growth. To date, the atomic structure of the growing interface has been studied only in fcc/hcp and derivative transformations. In fcc/hcp transformation, the passage of a/ 6fcc Shockley partials on every other (O001)hcp// (lll)fcc plane accomplishes the lattice change. Howe et aL E~~ showed by HRTEM that self-accommodation by the operation of three different partials on (111)fcc planes results in zero net shear strain. It is consistent with the deduction made by ChristianTM for fcc/hcp transformation. In other 1972 VOLUME26A, AUGUST 1995 Figure 1 is a transmission electron micrograph of a bcc lath formed in the fcc matrix. The diffraction pattern and its key diagram (Figure 1(b)) show that the bcc lath has the Kurdjumov-Sachs orientation relationship,El21 (111)j.// (110)b, [1]-0]//[]-1 l]b, and [112]//[1T2]b, with respect to its fcc matrix*. *Hereafter, planes and directions in fcc and bcc lattices are described with the subscripts of " f " and " b , " respectively. In Figure 1, the broad face of the bcc lath is parallel to the incident beam. Thus, the habit plane of the broad faces of laths was found to be approximately (112)//(231)b by trace analysis. The dark-field micrograph of Figure 2 shows that growth ledges (indicated by the arrows) are present on the broad face of the lath. The height of these ledges is determined to be approximately 1 to 2 nm from the edge-on image. These results are consistent with the CTEM study by Luo and Weatherly.EH~ Figure 3 is a HRTEM image showing an edgewise view of the atomic structure of the broad face of a bcc lath. The incident beam direction is [1]0]//[]-1 lib. The macroscopic (112)s habit plane is composed of regularly arranged fine steps with ( l l l ) / / ( l l 0 ) b terraces. Each step is one atom plane high (-0.25 nm), and the average spacing between METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 3--HRTEM image of the broad face of a bcc lath. The position of the fcc/bcc interface is indicated by the white line. The incident beam direction is [1 lO]i//[111]~. (a) ( 1 1 1 ) 0 = f//(110) 5166 b....... ~ . . . . . . . . . . . . . . . m ~ l + o o , o + o e o r +o+o| I| 9 ~ o o
Purchase answer to see full attachment