Characterization of the Martensitic Transformation in NiPtAl Alloy Using Digital Holographic Imaging
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INTRODUCTION
NIAL and NiPtAl coatings are widely used in the gas turbine industry to protect high-temperature components such as turbine blades from oxidation and corrosion.[1] Repeated in-service cycling from room temperature to 1373 K (1100 C) and above can result in the progressive deformation of the coating, known as rumpling,[2–5] or crack formation,[6,7] leading to component failure. Many degradation mechanisms have been proposed and modeled,[8] and the martensitic transformation occurring during cooling in these alloys is of particular concern.[9–13] It is indeed well-known that when the Ni content in NiAl is above ~ 60 at. pct, a transformation from B2 b-NiAl to L10 martensitic phase will happen upon cooling,[14] with the martensitic transformation starting temperature (Ms) increasing by ~ 124 K for a 1 at. pct increase in Ni concentration.[15] Similar behavior is observed for ternary NiPtAl alloys with Ms temperature highly dependent on Pt concentration.[16] For example, the Ms temperatures in binary 63Ni-37Al, ternary 53Ni-5Pt-37Al, and ternary 53Ni-10Pt-37Al alloys (all compositions in at. pct) were estimated to be ~ 398 K,[14] 342 K, 349 K, 829 K,
BENJAMIN P. THIESING, and CHRISTOPHER J. MANN are with the Northern Arizona University, Flagstaff, AZ 86011-6010. Contact e-mail: [email protected] SEBASTIEN DRYEPONDT, DONOVAN LEONARD, and RALPH B. DINWIDDIE are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6156. Manuscript submitted February 7, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
1014 K (125 C, 69 C, 76 C, 556 C, 741 C), [16] respectively, depending on the cooling rate. The composition of the as-deposited NiPtAl bond coating is typically Ni-5Pt-38-40Al at. pct, plus up to 5 at. pct of Cr and Co depending on the composition of the substrate. If the Al concentration is above ~ 38 at. pct, the martensitic transformation is not observed because the Ms temperature is below room temperature. However, the Al, Ni, and Pt concentration will change over time due to the interdiffusion with the substrate at high temperature and the consumption of Al to form a protective alumina scale. The Ms temperature will therefore increase with time, and Chen et al.[11] estimated the Ms temperature for a NiPtAl bond coat deposited on single crystal superalloy Rene N5 to be ~ 873 K (600 C) after cycling between room temperature and 1423 K (1150 C) for a duration equivalent to ~ 28 pct of the overall coating lifetime in laboratory furnace cycling tests. The same authors calculated the change of volume between the austenite and martensite to be ~ 2 pct, and demonstrated that the transformation would generate significant stress in the coating during thermal cycling.[12] In this paper, the martensitic transformation occurring in a 56.8Ni-5.6Pt-37.6Al model alloy was characterized using conventional bright-field microscopy, thermal imaging, and an advanced digital holographic imaging system.[17] This composition is a simplified representation (without Co, Cr, etc.) of a NiPtAl coating after short
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