Phase relations in the Nb–Ni–Cr system at 1100 °C

Authors: Alexander A Kodentsov Frans J J van Loo

Publish Date: 2012/08/25

Volume: 143, Issue: 9, Pages: 1309-1314

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Abstract

The isothermal cross section through the ternary phase diagram Nb–Ni–Cr at 1100 °C was constructed by means of diffusion couples and equilibrated alloys It was found that nearly 28 at of Cr can be dissolved in the μ phase Nb7Ni6 at this temperature and the solubility of chromium in NbNi3 is approximately 5 at Under these circumstances the lowtemperature cubic modification of the NbCr2 Laves phase can dissolve up to 6 at of nickel but further increase of the Ni content up to approximately 10 at stabilizes the hexagonal hightemperature modification of the Laves phase The presence of this pseudoternary compound which is in equilibrium with all binary intermetallics and bodycentred cubic BCC Nb and Crbased solid solutions largely determines the topology of the isotherm at 1100 °C The formation of this phase was also observed in the reaction zone between Nb and Ni–Cr solid solution when chromium concentration exceeded 15 atThe practical interest in the Nb–Ni–Cr system is generated mainly by the fact that Ni–Crbased alloys are important corrosionresistant hightemperature materials Their application in combination with the Nbbased alloys having a very high specific strength at elevated temperatures might lead to a significant improvement in the hightemperature performance of various structural components To optimize the fabrication procedure of such composite structures it is paramount to be able to predict and control the phases which are formed at the interfaces between niobium and Ni–Cr alloys upon solidstate bonding and under service conditions It was repeatedly demonstrated that the optimal starting point for research on any metal–metal interactions is the investigation of the phase equilibria and reactive phase formation in relevant materials systemsThe Nb–Ni–Cr system has been studied extensively within a limited composition domain bounded by the binary NbNi3 and NbCr2 intermetallics and Ni and Crbased solid solutions 1 2 3 4 5 6 7 Most of the cited work was performed almost 50 years ago In the investigated composition region the partial liquidus projections several isothermal sections in the temperature range 1100–1200 °C and several isopleths have been established For more details and more complete bibliography the reader should access a rather exhaustive compilation by Gupta 8On the contrary for the higher Nbcontaining regions of this system only very limited experimental information about phase relations is available 9 Therefore the present investigation was designed to establish the phase relations in the Nb–Ni–Cr system at 1100 °C below the liquidus temperature 4 and to gain insight into the morphological evolution of the interfacial region between niobium and Ni–Cr solid solution during the reaction at this temperatureThe isothermal cross section through the ternary diagram was constructed by the traditional method of equilibrated alloys and diffusion couple technique Diffusion couples with twophase endmembers were used The efficiency of this technique in constructing isothermal cross sections through ternary phase diagrams is higher compared with that when singlephase alloys are used as endmembers of the couples because in this case the chance to “hit” interfaces at which three phases are in equilibrium is much larger Further details concerning the use of diffusion couple techniques in studying phase diagrams can be found in Ref 10Backscattered electron image BEI showing the morphology of the diffusion zone developed between chromium and twophase alloy with the nominal composition Ni60Nb40 after annealing in vacuum at 1100 °C for 196 h Note that the NbNi3 domains within the microstructure of the twophase endmember exhibit a white contrastBEIs of the reaction zones developed in the annealed 1100 °C 196 h diffusion couples based on twophase alloy with the nominal composition Ni50Cr50 and a the twophase alloy Ni60Nb40 and b pure Nb Note that domains of the Crbased solid solution Cr ss present within the microstructure of the Ni50Cr50 alloy exhibit a dark contrast the various phases on the micrographs are denoted by their binary formulae and T is the pseudoternary phase

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