Changes in Crystal Structure and Microhardness During Thermal Treatment of a NiCoCrAlY Alloy.
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CHANGES IN CRYSTAL STRUCTURE AND MICROHARDNESS DURING THERMAL TREATMENT OF A NiCoCrAlY ALLOY. Thomas Rehfeldt*, Gerhard Schumacher** and Hellmuth Klingelhoeffer** *Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D-14109 Berlin, Germany; **Bundesanstalt fuer Materialforschung und -pruefung, Unter den Eichen 87, D-12205 Berlin, Germany
ABSTRACT The stability of microstructure and the microhardness of a NiCoCrAlY alloy was studied after thermal treatment at high temperatures and subsequent quenching into ice water. The alloy revealed mainly two ordered phases, a γ’ phase with L12 crystal structure and a β phase with B2 structure. The γ’ phase is shown to become unstable in the temperature range between 1073 K and 1373 K where it undergoes an order-disorder phase transformation. The low transformation temperature compared to pure Ni3Al is ascribed to deviations from the Ni3Al stoichiometry and to the additional alloying elements Co and Cr. The γ’-γ order-disorder transformation is shown to be attended by a decrease in microhardness of about 30 %.
INTRODUCTION For the further increase of temperature in the combustion chambers of gas turbines thermal barrier coatings (TBC) on turbine blades are presently being tested. Due to the different thermal expansion coefficients of superalloy and TBC large stresses are induced at the metal – ceramic interface. In order to reduce these stresses, bond coat alloys are deposited between the superalloy and the TBC. High plasticity of the bond coat alloys is, therefore, required in order to adapt the stresses induced by the different thermal expansion coefficients of superalloy and TBC. The high porosity of the TBC also requires protection of the superalloy by the bond coat alloy against oxidation. NiCoCrAlY alloys are candidate materials for application as bond coat material deposited between superalloys and ceramic TBCs on turbine blades [1,2,3]. Some NiCoCrAlY alloys were shown to exhibit changes in the coefficient of thermal expansion at high temperatures which are poorly understood [3,4]. The microstructure of such NiCoCrAlY alloys generally consist of mainly two phases, a γ’ phase with ordered L12 crystal structure (Cu3Au type structure) and a β phase with ordered B2 crystal structure (CsCl type structure). Many ordered alloys are, however, known to become unstable at high temperatures and transform into the corresponding disordered phase leading to large changes in heat flow near the phase transition temperature and, therefore, to large changes in thermal expansion [5]. This work presents results of investigation of microstructure and microhardness of a NiCoCrAlY alloy at high temperature. The composition of the alloy (see Table 1) is close to the ones investigated by Clemens and coworkers [3] and by Brindley [4].
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EXPERIMENTAL Microstructure was examined by means of a scanning electron microscope (SEM) at 30 kV and by means of a transmission electron microscope (TEM) at 120 kV and 300 kV. The composition of the phases was measured by means of energy dispers
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