Design of a Creep-Resistant Nickel-base Superalloy for Power Plant Applications

Part I - Mechanical Properties Modelling

F. Tancret, H. K. D. H. Bhadeshia and D. J. C. MacKay

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1, Gaussian processes are used to model the tensile and creep rupture properties of superalloys as a function of their composition and processing parameters, making use of large databases on existing alloys. The models are able to estimate the actual influence of alloying elements on the mechanical properties over a wide range of temperature and stress. They have been used, in conjunction with general metallurgical concepts and industrial requirements, as a basis for the design of a new Ni - Cr - W - Al- Ti - Fe - Si - C - B superalloy with desirable properties. It is estimated that the proposed forgeable and weldable alloy should have a creep rupture life at 750oC of 100 000 h under a stress of 100 MPa, witlt a huge reduction in price compared to existing commercial alloys with similar properties. In a following paper, Part 2, an attempt is made to design against the formation of undesirable phases and chemical segregation, using phase diagram calculations. Preliminary experimental results are presented in Part 3.

Materials Science and Technology, Vol. 19 (2003)283-290.

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Part II - Phase Diagram and Segregation Simulation

F. Tancret and H. K. D. H. Bhadeshia

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1 Gaussian processes were used to model the mechanical properties of superalloys, and have been used as a basis for the design of a new Ni - Cr - W - AI- Ti - Fe - Si - C - B superalloy with desirable properties. In this part, an attempt has been made to design against the formation of undesirable phases, and to calculate the solidification range, forging window and heat treatment, using phase diagram calculations. The potential for chemical segregation has also been estimated following Scheil's assumptions, and remains acceptable.

Materials Science and Technology, Vol. 19 (2003)291-296.

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Part III - Experimental Results

F. Tancret, T. Sourmail, M. A. Yescas, R. W. Evans, C. McAleese, L. Singh, T. Smeeton and H. K. D. H. Bhadeshia

A semi-industrial scale rolled bar of the creep resistant Ni base superalloy designed in Parts 1 and 2 has been fabricated. The influence of heat treatment on microstructure and phase fonnation has been investigated using both optical and electron microscopy, and X-ray diffraction. Mechanical properties at ambient and high temperature have been measured by hardness testing, compression testing, and tensile creep testing. These experimental results are in good agreement with the predictions of Parts 1 and 2 concerning phase formation, yield stress and creep rupture stress. The target of a lifetime of 100000 h at 750oC under 100 MPa seems attainable. The design procedure is therefore mainly validated, and results in a promising new alloy for power plant applications.

Materials Science and Technology, Vol. 19 (2003)297-302.

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Part IV - An Affordable, Creep-Resistant, Nickel-base Superalloy for Power Plant

F. Tancret and H. K. D. H. Bhadeshia

An affordable, forgeable, weldable, creep-resistant and corrosion-resistant nickel-base superalloy was designed using modern computing approaches (Gaussian processes modelling of mechanical properties, thermodynamical simulation). Both microstructure and mechanical properties are consistent with predictions; in particular, creep tests suggest that the alloy will match the engineering requirement of a lifetime of 100 000 h at 750oC under 100 MPa.

Preliminary modelling and experimental investigations were performed concerning processing issues. The primary solidification segregation behaviour was succesfully modelled using Scheil's approximation, and concentration profiles within dendrites could be predicted, which is important in view of forging and welding the alloy. Also, the precipitation hardening behaviour during ageing was described by an approach combining a diffusion-controlled growth model, a thermodynamical simulation software and a simple strengthening model. This is useful in view of optimising the heat-treatment.

These studies need to be completed by further experimental investigations and models refinements -composition of the interdendritic medium, complex dendrite geometries, aeging at different temperatures, etc. Further work will also include investigations about high temperature ductility and forgeability, recrystallisation, welding, etc.

Proceedings of the 6th International Charles Parsons Turbine Conference, Engineering Issues in Turbine Machinery, Power Plant and Renewables, eds A. Strang, R. D. Conroy, W. M. Banks, M. Blackler, J. Leggett, G. M. McColvin, S. Simplson, M. Smith, F. Starr and R. W. Vanstone, Institute of Materials, London, 2003, 525-535

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Nickel-base Superalloys

Powerpoint conference-poster 2001

Powerpoint conference-poster 2002

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