Eutectoid decomposition in Ag-Ga
- PDF / 1,697,247 Bytes
- 7 Pages / 598 x 778 pts Page_size
- 88 Downloads / 238 Views
I.
INTRODUCTION
EUTECTOID decomposition products can be classified as pearlitic or bainitic, based upon their morphologies. Pearlite is a lamellar product that results from the cooperative and synchronized growth of two constituent phases from a parent phase, and bainite is a nonlamellar product that results when the constituent phases grow more or less independently of each other.t~.2] In addition to occurring in steels, both types of eutectoid products have been observed in a wide variety of nonferrous alloys,t3] Recent studies in iron, copper, and titanium alloys by Shiflet and co-workerst4-9] demonstrate that crystallography plays an important role in the growth of pearlite. The habit plane between the two lamellar products generally corresponds to a low-energy interface, the existence of which depends upon the crystal structures of the phases and their orientation relationships. This work describes the crystallography, transformation kinetics, and morphology of pearlite in Ag-Ga alloys. The eutectoid invariant in Ag-Ga occurs at 15.3 wt pet Ga (21.8 at. pet Ga) and 380 *C (Figure lt~0]). The hightemperature phase,/3 (hop, structure type hP2), decomposes at the eutectoid to the silver-rich solid solution, a (fcc, structure type cF4), and an ordered hexagonal phase. The ordered phase, designated /3', is isomorphous with the phase in Ag_Zn,[l~,12]whose structure type is he9. [13,14]* A1*Different designations have been used in the literature for the Ag-Ga hexagonal phases;/3 and/3' are employed here following Ref. 10.
J.K. CHEN, formerly Graduate Student, Materials Science and Engineering Department, Virginia Tech, is Postdoctoral Fellow, Materials Science and Engineering Department, McMaster University, Hamilton, ON, Canada L8S 4L7. C.W. SPENCER, Professor Emeritus, G. CHEN, Research Associate, and W.T. REYNOLDS, JR., Associate Professor, are with the Materials Science and Engineering Department, Virginia Tech, Blacksburg, VA 24061-0237. M.E. EKSTRAND, formerly Undergraduate Student, Materials Science and Engineering Department, Virginia Tech, is Process Engineer, Caparo Steel Co, Fan'ell, PA 16121. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICAL AND MATERIALS TRANSACTIONS A
though the prototype ~" Ag-Zn structure is based upon a stoichiometry of 50 at. pct Zn, eutectoid/3' has only approximately 26 at. pct Ga. The lower Ga concentration is accommodated in/3' by randomly mixing appropriate proportions of Ag and Ga atoms on one of the three sublattices of the hP9 struc~tre.[ ~21 II.
EXPERIMENTAL PROCEDURE
A series of 100-g ingots was prepared by melting highpurity Ag and Ga (99.999 pct purity). One ingot was melted in a covered graphite crucible in an air furnace, but the best control over composition and uniformity was obtained by induction melting the components in a graphite
Data Loading...
