Recent Developments in Chromium and Chromium Alloys

Abstract

Recent developments in chromium pertinent to its potential use in advanced jet engines are reviewed. Although chromium has a strength-to-density advantage over nickel, its inherent brittleness and further embrittlement by nitrogen during high-temperature exposure are serious limitations to its use. Twelve chromium alloys are currently under development, the strongest showing a potential 150' F (83 K) service temperature advantage over nickel-base superalloys. However, a better balance between solution strengthening and precipitate strengthening must be achieved in order to improve the low temperature ductility of these alloys. Improved coatings for protection against nitrogen embrittlement must be developed. Lewis Research Center S U M~R YChromium-base alIoys, which have been under study now for slightly more than two decades, are attractive as potential competitors for nickel alloys for high temperature applications in advanced jet engines. Although chromium has a strength-to-density advantage over nickel, it is normally brittle at room temperature and is further embrittled by nitrogen during high temperature air exposure.The high temperature strength of chromium can be increased threefold to fourfold by solution strengthening with elements such as tantalum, columbium, tungsten, molybdenum, and rhenium. Solution strengthening, however, involves a significant increase in the ductile-brittle transition temperature. Similar or larger improvements in the strength of chromium can be achieved by precipitate strengthening with borides, carbides, and nitrides of the Groups IVa and Va elements. These precipitates do not embrittle chromium to the same extent as do solution strengthening additions and may even be ductilizing. Coarsening rate estimates suggest that carbides at least should retain sufficiently fine sizes as to be strengthening for times of 1000 hours o r longer at 2000' to 2200' F (1366 to 1478 K).Nitrogen is particularly deleterious to the ductility of chromium because of its high solubility at elevated temperatures and the nature of the CrZN precipitate. Nitrogen embrittlement of chromium during high temperature exposure can be reduced by rare earth alloying, but the effectiveness of these additions is reduced on further alloying for high temperature strength. rently under development. These include five from the United States, three from Australia, and four from the Soviet Union. The strongest of these alloys offer up to a 1 5 0 ' F (83 K) temperature advantage over nickel alloys but the impact ductile-brittle transition temperatures are high.surface alloying techniques to reduce nitrogen embrittlement and emphasis on dispersion strengthening to achieve a better balance between high temperature strength and low temperature ductility.

Evidence for a Cr metastable phase as a tracer in DLI-MOCVD chromium hard coatings usable in high temperature environment

“…By combining the relative intensity of free C and carbide components with the total C content determined by EPMA (4 at%), the amount of C in the carbide form is estimated at about 0.8 at%. This is significantly higher than C solubility in Cr which was reported to be <0.1 at% at about 750 K [31].…”

Fundamental Electronic Structure Characteristics and Mechanical Behavior of Aerospace Materials

“…Among them, chromium alloys have advantages due to low density, high creep and oxidation resistance [31], and molybdenum alloys attract special attention due to their potential for aerospace applications. Unfortunately, the lack of ductility at low temperatures has severely limited the use of bcc refractory metals, whose room temperature brittleness is caused by extrinsic factors -extremely low solubility of interstitial impurities (C, N, 0) resulting in the formation of brittle carbides, nitrides and oxides.…”

Microstructural evolution and mechanical properties of Cr-Ru alloys

“…Its low density (20 pct less than that of most Ni-base alloys) and high thermal conductivity (2 to 4 times higher than that of most Ni-base superalloys) are also attractive since they may result in increased efficiency. [1][2][3][4][5][6] However, the implementation of Cr-rich alloys as a viable substitute for Ni-base alloys has been impeded by their poor ductility at ambient temperature, or high ductile-to-brittle transition temperature (DBTT), which for unalloyed recrystallized Cr with commercial purity is approximately 150 °C in tension. [7,8,9] Below the DBTT, Cr has virtually no ductility.…”