Metastable phase evolution in Al 2 O 3 dispersed nanocrystalline NiCr alloys
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P.R. Subramanian Ceramic and Metallurgy Technologies, GE Global Research, Schenectady, New York 12309 (Received 20 June 2006; accepted 24 August 2006)
The high temperature structural stability of nanograined NiCr alloys reinforced with nanoscale yttria and alumina dispersoids, fabricated by electron beam physical vapor deposition (EBPVD), was examined. The yttria particles coarsened very little and also inhibited grain growth in the matrix successfully, whereas the alumina dispersoids coarsened rapidly and were not as effective in restricting matrix grain growth. A hierarchy of phase transformations took place in the Al2O3 particles present as nano dispersoids in a nanograined NiCr matrix (␥Al2O3 → ␥Al2O3 + ␦Al2O3 → ␣Al2O3), on annealing. Coarsening of the alumina particles was accompanied by these phase transitions. The phase evolution is attributed to differences in free energies between the metastable and stable phases and a kinetic hierarchy in nucleation, brought about by structural and hence interfacial energy considerations.
I. INTRODUCTION
Oxide dispersion strengthened (ODS) alloys containing nanosized dispersoids have been well exploited over the past three to four decades for usage in many high temperature structural applications because they exhibit outstanding creep and fatigue strengths at temperatures in excess of 1273 K.1–3 Most of these earlier studies used yttria as the dispersoid of choice, irrespective of the matrix material, owing to its high stability,1–5 apart from some initial work on NiCr alloys with ThO 2 dispersoids.6,7 However, there is considerable evidence to show that the dispersoid actually consists of mixed oxides, such as Y3Al5O12 (YAG), YAlO3 (YAP), YAH etc.,5,8,9 and the yttria particles were not as stable as originally thought. To explore the potential for strengthening by alternate oxides, Al2O3, which is a well-known stable oxide, has been added to the NiCr alloy in the present study. Secondly, in all the earlier studies the matrix typically consists of coarse elongated grains measuring at least a few micrometers in length),1–9 and there have been few studies dealing with nanograined alloys (100–200 nm) reinforced with nanoscale dispersoids. The current work characterizes the high temperature structural coarsening of nanocrystalline NiCr-ODS alloys reinforced with nanoscale yttria and alumina
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0004 68
http://journals.cambridge.org
J. Mater. Res., Vol. 22, No. 1, Jan 2007 Downloaded: 13 Mar 2015
particles. The yttria dispersoids were far more effective in inhibiting matrix grain growth compared with the alumina particles. In contrast, the alumina particles coarsen extensively and undergo a series of phase transformations during coarsening and are found in a variety of metastable phases. The present investigation focuses on the phase transformations seen in the Al2O3 particles, as an outcome of solid state reaction, in the nano-ODS system. II. EXPERIMENTAL PROCEDURE
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