Microstructural anomalies in a W-Ni alloy liquid phase sintered under microgravity conditions
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fluence on the toughness of well-quenched material Loss of toughness is associated with precipitation on both highangle and subgrain boundaries, and the results are qualitatively consistent with fracture energy being inversely dependent on fractional area coverage.
100
Slope ~a~"
.155
.~a ~ _..... a ~
.140
REFERENCES ~ V
10 10
i
20
i
I
I
I
~
a
15%
~7
80%
Recryst.
Ii
50
100
I
200
I
500
QUENCH RATE (C/sec) Fig. 8--Strength-adjusted fracture toughness vs quench rate assuming a relation of the form K l v = A Q " .
boundary precipitates in both cases. At higher quench rates, boundary precipitation is minimized, in which case a nearly equiaxed recrystallized structure is inherently more susceptible to IG fracture when stressed in the longitudinal and long-transverse directions. 15] Since fracture toughness has been theoretically and experimentally related to fractional grain boundary coverage, it may be instructive to estimate the effect of quench rate on Ai using a simple growth model. At homologous temperatures (T/Ts) below 0.9 (Ts = absolute solidus temperature), lengthening of grain boundary allotriomorphs in the AI-Cu system proceeds by a collector plate mechanism involving both volume and grain boundary diffusion.VS.16] The precipitate length (or diameter in the grain boundary plane) is related to (time) TM with a temperature-dependent proportionality constant comprised of concentration, diffusion, and surface energy terms. Under continuous cooling conditions, the principle of "additivity" may apply;VT] i.e., the nucleation sites saturate early and the instantaneous reaction rate depends only on the temperature, not on the thermal path. The grain boundary fractional area coverage is then given by A / = {\ a ftkNdt~l/Z o ,/
[2]
where N is the number of nuclei per unit grain boundary area and k is the aforementioned rate constant. If the cooling rate, Q, i.e., dT/dt, is reasonably linear, then 1
r
1/z
where the integral has a constant (time-independent) value. Since fracture energy, G, is proportional to 1/V~/ and 9 G oc/G, then K t should be proportional to Q 1/8 . As Fiigure 8 shows, strength-adjusted toughness data ( - 1 MPaVm per 7 MPa in yield strength) are reasonably consistent with such a relationship. In summary, slow quench rates and recrystallization reduce strength and fracture toughness of 7050-T6 plate, as expected, and the effects on toughness are much greater than those on strength. Recrystallization has the largest in2484---VOLUME 26A, SEPTEMBER 1995
1. I. Kirman: Metall. Trans., 1971, vol. 2, pp. 1761-70. 2. G.T. Hahn and A.R. Rosenfield: Metall. Trans. A, 1975, vol. 6A, pp. 653-68. 3. J.T. Staley: in Properties Related to Fracture Toughness, ASTM STP 605, 1976, pp. 71-96. 4. J.T. Staley: Fracture Toughness and Microstructure of High Strength Aluminum Alloys, AIME Meeting, Pittsburgh, 1974. 5. D.S. Thompson: Metall. Trans. A, 1975, vol. 6A, pp. 671-83. 6. J.T. Staley: Mater. Sci. Technol., 1987, vol. 3, pp. 923-35. 7. D.S. Thompson, S.B. Subramanya, and S.A. Levy: Me
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