Order-Strengthening in a nickel-base superalloy (hastelloy alloy S)
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tion-strengthened, nickel-base superalloy developed for applications where oxidation resistance, low thermal expansion and retained ductility after long-time exposure at elevated temperatures are prerequisites. Its typical heat treatment consists of annealing at 1340 K (1950 ~ followed by air cooling to produce an essentially single phase material. When specimens from annealed heats were aged at 810 K (1000 ~ for 1000 to 8000 h and then tensile tested at room temperature, it was found that relative to the annealed condition, the 0.2 pct yield strength had nearly doubled while about 70 pct of the tensile elongation was retained. It is the objective of this note to report on the formation of a long-range ordered phase that caused the observed strengthening. The heat investigated was in the form of a sheet 1.4 m m thick. Its chemical analysis is listed in Table I. Tensile tests were conducted at room temperature on
sheet specimens (gage length 50.8 mm, width 12.7 mm, and thickness 1.4 m m ) at a strain rate of 8 • 10 -5 x - L Thin foils for transmission electron microscopy and diffraction work were prepared by the jet polishing technique in a solution consisting of 1 part HNO3.and 3 parts methanol at about 243 K. All the foils were examined in a Phillips 300 EM operated at 100 kV. Typical r o o m temperature tensile properties of both annealed and aged specimens are listed in Table II. Figure 1 shows a n u m b e r of selected area electron diffraction patterns derived at different tilts from foils of aged specimens. It can be seen that in addition to the characteristic fcc reflections, extra reflections are present at every 1/3 (220) and 1/3 (420) reciprocal lattice vectors. This is a characteristic feature of a Pt~Mo-type superlattice whose crystallography was described by Das and T h o m a s I for the case of Ni2Mo in the Ni-Mo system. Its unit cell is body centered orthorhombic with a = V/2/2 a 0, b = 3 V ~ / 2 a 0 and c = a o where a 0 is the lattice constant of the disordered fcc unit cell. The superlattice unit cell can be derived from the disordered fcc cell by the stacking of atomic layers on either {420} or (220) planes, where every third plane contains all m o l y b d e n u m and in between all nickel atoms, in the case of Ni2Mo, thus generating six crystallographically equivalent variants? Based upon the chemistry of the heat investigated, the observed
Fig. l--Selected area electron diffraction patterns derived from specimens aged 8000 h at 8 l0 K. The fundamental reflections are indexed in terms of fcc notations. (a) [001],
(b) [011], (c) [hE], (d) [114).
H. M. TAWANCY is Engineering Associate, High Technology Division, Cabot Corporation, Kokomo, IN 46901. Manuscript submitted April 2, 1980. 1764--VOLUME l l A , OCTOBER 1980
ISSN 0360-2133/80/1013-1764500.75/0 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
METALLURGICAL TRANSACTIONS A
Table I. Chemical Analysis In Wt Pct Ni
Cr
Mo
Fe
Mn
Si
W
Co
A1
La
C
S
Balance
15.14
14.23
1.00
0.55
0.32
0.25
0.23
0.17
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