Optimisation of compositions and heat treatments of PT-based superalloys
Date
2009-10-19T10:57:56Z
Authors
Shongwe, Mxolisi Brendon
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Abstract
ABSTRACT
This study investigated the optimisation of compositions and heat treatment of Pt-based Pt-
Al-Cr-Ru superalloys. Initially, six alloys of different compositions were selected using an
optimum composition from previous research as a starting point. These alloys were analysed
in both the as-cast and heat treated conditions. The heat treatment comprised two stages, first
at 1500°C for 18 hours and quenched in water, followed by 1100°C for 120 hours and then
air cooled. Microstructural characterisation was carried out in an HR FEI Nova NanoSEM in
BSE mode, also using EDX, and XRD. The as-cast alloys had (Pt) dendrites with a
eutectic/eutectoid of (Pt) + ~Pt3Al, with tetragonal ~Pt3Al precipitated in the dendrites. All
heat treated alloys had precipitates of tetragonal ~Pt3Al in a (Pt) matrix. The volume fraction
of the precipitates varied, and the maximum proportion was 30%. To increase the volume
fraction, more samples were made with 11 at.% Al. All of the 78 at.% Pt alloys had more
~Pt3Al precipitates than the 80 at.% Pt alloys, and the Pt80:Al11:Ru5:Cr4 alloy had no
discernable precipitates. The microstructure was similar to the first batch of samples and
Pt78:Al11:Ru5:Cr6 had the highest precipitate volume fraction.
The hardness of the alloys was also studied in both conditions. Cracking and slip modes
around the hardness indentations were studied to evaluate the toughness. In both conditions,
most of the alloys had high hardnesses and exhibited wavy slip, with no cracking from the
indentation corners. Pt78:Al11:Ru5:Cr6 was the promising alloy because it had the best
microstructure (highest proportion of precipitates), a high Al content for oxidation resistance
and a reasonable low Pt content for both reduction in price and density.