خصوصیات اصطکاکی و پوشیده از آلیاژ دیرگداز بر پایه کریستال نیکل Friction and Wear Characteristics of Single Crystal Ni-Based Superalloys at Elevated Temperatures

 Introduction Nickel-based superalloys are widely used as structural components in demanding environments due to their excellent stability (i.e., resistance to mechanical and chemical degradation) at elevated temperatures [1–10]. These superalloys were primarily developed to meet the demand of jet engine and industrial gas turbine engine blades operating at temperature in excess of 980 °C [1, 6, 11]. Such alloys consist of nickel-based solid solution, referred to as γ-matrix with a dispersion of precipitates, generally referred to as γ′. The precipitates are hard intermetallic Ni3(Al, X) compounds, where X is typically Ti, Nb or Ta. The γ-matrix (i.e., FCC crystal structure) is largely coherent with the γ′ precipitates (i.e., L12 crystal structure), which allows the precipitationhardened microstructure to remain very stable at high temperatures [1, 6]. This is one of the primary reasons such alloys have been very successful for high-temperature application. The early generation of such alloys contained about 40% by volume of the precipitate, and the most commonly used Waspaloy® is a typical example of that. However, with increasing temperature demand, it was realized that alloys with 65% precipitate by volume were optimum for achieving a balance of high-temperature creep resistance and tensile strength [1, 6]. The early generation of such alloys was polycrystalline and typically used in cast form using investment casting techniques to achieve complex shapes. Subsequently, it was recognized that creep resistance of such alloys can be further improved by eliminating the grain boundaries (i.e., high difusion paths) normal to the principal stress. Consequently, this led to the development of directional solidifcation of columnar grain alloys and eventually single grain or single crystal castings [1, 10]. The development of single crystal technology by Pratt & Whitney allowed addition of refractory alloying elements as Ta and Re with further improvement in temperature performance. More details on various aspects of superalloys can be found elsewhere [1]. The precipitate microstructure can be naturally formed using a series of heat treatments, and thus, an optimum balance of mechanical properties can be achieved by judiciously varying the heat treatment sequence.