Elevated temperature deformation behavior of a dispersion-strengthened Al-Fe,V,Si alloy
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I.
INTRODUCTION
RECENT advances in rapid solidification processing (RSP) have resulted in the development of a new family of dispersion-strengthened Al alloys based on the Al-Fe-Si system with quaternary additions of vanadium.[1–4] Upon rapid cooling, the ternary Al-Fe-Si alloys solidify with a dispersion of metastable Al12Fe3Si particles, which decompose into monoclinic Al13Fe4 and hexagonal Al8Fe2Si upon subsequent processing.[5] The addition of V to the melt results in the preferential formation of coherent aAl12(Fe,V)3Si particles (a cubic phase), which are much more resistant to coarsening than the incoherent, noncubic phases that result from the decomposition of Al12Fe3Si. Furthermore, the randomness of the dispersed particles and the fact that the particle sizes were found not to scale with the Al cells (i.e., the matrix) in the microcellular microstructures obtained via different cooling rates has led some investigators[6] to postulate that the dispersoids are formed directly from the melt as primary phases. The exact composition of the dispersoids has recently been shown to vary between body-centered cubic (bcc)-Al13.37(Fe,V)3Si1.11 and primitive cubic-Al12.86(Fe,V)3Si2.1.[7] These intermetallic dispersoids also have been found to be extremely resistant to elevated-temperature coarsening and room-temperature hardness of these alloys was found to remain relatively unchanged even after prolonged exposure at temperatures greater than 500 7C.[8,9] Thus, these alloys are excellent candidates for elevated-temperature use. Some reports on the mechanical properties of the Al-Fe, V, Si alloys with dispersoid volume fractions (Vf ) up to ;37 pct are available in the literature.[10–15] These data emSHANTANU MITRA, Manufacturing Development Engineer, is with the Components Group, Hewlett-Packard Co., 3175 Bowers, Santa Clara, CA 95054. Manuscript submitted May 26, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
phasized the phenomenon of dynamic strain aging (DSA)[10–13] which was reported at temperatures ;150 7C and strain rates between 1022 and 1026/s. Strain-rate sensitivity of flow stress, shoulders in the yield-strength/temperature plots and minima in the ductility/temperature plots were used to identify DSA. Some data on the strain-hardening behavior of these alloys have also been obtained using constant crosshead-speed testing.[13,14,15] However, few studies on the creep properties of the Al-Fe, V, Si alloys, and especially those with large Vf(;37 pct) have been reported.[16] Creep studies[17–22] on various RSP Al alloys with submicron-sized grains have tended to analyze the behavior in terms of one of the following approaches: i) the inclusion of a threshold stress (s Th ) in the semiempirical power-law equation:[23]
@
#
n
DGb s 2 s Th εz 5 A kBT G
[1]
where G 5 shear modulus, D 5 bulk self diffusivity, n 5 stress exponent, A 5 pre-exponential constant, b 5 Burgers vector, kB 5 Boltzmann constant (1.38 3 10223 J/K), and T 5 temperature; ii) a substructure invariant law proposed by Sherby and co-worker
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