The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA
- PDF / 2,596,102 Bytes
- 10 Pages / 630 x 792 pts Page_size
- 98 Downloads / 208 Views
I.
INTRODUCTION
P W A 1480 is a single-crystal nickel-base superalloy with well-documented microstructures and mechanical properties.[l,2] In an earlier communication, t31 the authors reported observing a surprisingly high density of superlattice stacking faults after low-temperature deformation and noted a relationship between the temperature dependence of these faults and the unique mechanical behavior of this alloyY j Since it was apparent that the faults played a major role in controlling the strength of the alloy, a detailed study of the faults and the mechanisms of fault formation was undertaken. In this article, the temperature dependence of these faults and the associated bounding partials are analyzed in detail, and in a forthcoming article, the relationships between the faults and the mechanical properties will be modeled. In the L12 lattice, the a(110) unit superdislocation can dissociate by a number of well-known mechanisms. Three of these dissociations are particularly relevant to the present work. The dissociation which is most commonly
WALTER W. MILLIGAN, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, Michigan Technological University, Houghton, MI 49931. STEPHEN D. ANTOLOVICH, Professor and Director, is with the School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA 303320245. Manuscript submitted October 30, 1990. METALLURGICAL TRANSACTIONS A
observed in superalloys is that which creates an antiphase boundary (APB). If the precursor superdislocation is in the primary octahedral slip system, the dissociation on the (111) plane is as follows: a
_
a
_
a[i01] --~ 2 [101] + APB + - [101] 2.
[1]
The a / 2 ( l 1 0 ) partials may further dissociate into Shockleys if the complex stacking fault (CSF) energy is not too high. Alternatively, the a[i01] superdislocation may dissociate by the following reaction, creating a superlattice-intrinsic stacking fault (SISF) instead of an APB: a
a[i01] ~ -
_
a
__
[2111 + SISF + - [112] 3 3
[2]
The a/3(112) partials may further dissociate by a number of schemes. Finally, if the dissociation occurs via Reaction [1], the resultant a / 2 ( l 1 0 ) partial can dissociate by the following reaction instead of the Shockley reaction, yielding an SISF instead of a CSF: a. _ a _ a 2 [1011--* 3 [2111 + SISF + g [1211
[31
Note that a dissociation via Reaction [3] will result in an SISF adjacent to an APB. VOLUME 22A, OCTOBER 1991 --2309
II.
EXPERIMENTAL PROCEDURES
As part of a large project, t41 a variety of mechanical tests (tension, creep, fatigue) were conducted on (001) and (123) oriented single crystals in the temperature range from 20 ~ to 1100 ~ For the tensile and fatigue tests, two strain rates were used, 8.33 • 10 -5 s -1 (0.5 pct/min) and 8.33 • 1 0 - 3 s -3 ( 5 0 pct/min). Thin foils were prepared by electropolishing discs in a solution of 5 pct perchloric acid, 35 pct butyl cellusolve, and 60 pct methanol at - 2 5 ~ and 9 to 12 V. Foils were studied on three different JEOL microscopes:
Data Loading...
