Cooling Rates and Fine Microstructures of RSR and Argon Atomized Superalloy Powders
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COOLING RATES AND FINE MICROSTRUCTURES OF RSR AND ARGON ATOMIZED SUPERALLOY POWDERS
F. Cosandey, R.D. Kissinger and J.K. Tien Henry Krumb School of Mines, Columbia University, New York, NY 10027
USA
ABSTRACT Superalloy powders of an IN 792 plus Hf type alloy produced from Rapid Solidification (RSR) and Argon Atomized process (AA) techniques are analyzed and compared according to particle size. Characterization of powders include surface and interior morphology, particle size and void distribution, cooling microstructures, microcrystallinity, microsegretation and especially dendrite arm spacing and cooling rates. It is found that, for the alloy studied, RSR cooling rates are a factor of only two to four greater than those for AA, regardless of particle size.
INTRODUCTION There have been many studies characterizing nickel-base superalloy powders produced by either argon atomization (AA) or rapid solidification rate (RSR) techniques [1-4]. Questions have been raised as to how AA and RSR powders compare with respect to powder surface and interior morphology, particle size distribution, macro- and microporosity and, especially, cooling rate. Unfortunately, these comparative studies usually involve dissimilar alloy compositions. This paper presents the summary of a comparative study of AA and RSR nickel-base superalloy powders produced from the same alloy. Alloy C101, an IN 792 plus Hf type alloy whose composition is shown in Table I, was chosen for this study because it is a very strong wrought alloy suitable for powder processing [1,3]. Vacuum atomized (VA) IN 100 plus Hf powder was also studied, but owing to space limitations the results are not included in this summary report. The VA study, as well as a detailed report of AA and RSR C101 powders, will be submitted for publication to Metallurgical Transactions at a later date. RESULTS AND DISCUSSION Surface morphology The surface morphology of particles produced by RSR and AA is shown in Fig. la and lb, respectively. The RSR powders are smooth with occasional fractures caused by collision between individual particles and between particles and apparatus walls, while AA powder surfaces are connected to many small satellite particles. Occasionally both RSR and AA powders are enveloped by shells formed by splat cooling of a liquid droplet against an already solidified particle. The occurrence frequency of these shells is larger for RSR than for AA powders. TABLE I Nominal composition of alloy Cl01 (w/o) Ni
Cr
Co
Mo
W
Ta
Al
Ti
C
B
Hf
59.8
12.0
8.9
1.9
4.3
4.5
3.4
4.0
0.002
0.017
1.10
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Particle size distribution The particle size distribution for RSR C101 is narrower than for AA, with an accumulated weight fraction of 92% and 65%, respectively, for powders below 140 Um. Porosity Both powders contain large pores created during the liquid drop formation. The frequency of pore formation decreases as particle size decreases and is lower for RSR than for AA particles, as summarized in Table II. The variation in pore fractions can be attributed
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