MATERIALS SCIENCE AND ENGINEERING

LOW-TEMPERATURE CARBURIZATION OF NI-BASE ALLOYS

Ni-base alloys strengthened by particles of the intermetallic phase γ'-Ni₃Al ("superalloys") are widely utilized for high-temperature structural applications, e.g. gas turbine blades. Certain alloying elements enable a good combination of mechanical properties and corrosion resistance at room temperature as well as high temperatures and extend the usefulness of these materials to e.g. petrochemical and medical applications.

Present research is concerned with further increasing the performance and the lifetime of these alloys under service conditions. Mainly, this requires to optimize the surface properties. One of the more recent approaches to this problem is surface alloying with carbon, which increases the surface hardness, improves phase stability at higher temperatures, and/or provides better corrosion resistance. However, conventional carburization – conducted at high temperature to maximize carbon solubility – usually degrades the mechanical properties by promoting coarsening of the γ'-Ni₃Al or the formation of other, deleterious phases.

A promising new approach to solve these problems is carburization at low temperature [1,2]. Although the solubility limit for carbon at low temperature is smaller than at high-temperature, corresponding research on austenitic stainless steels has shown that low-temperature carburization can result in a non-equilibrium state in which carbon concentrations vastly exceeding the the equilibrium solubility limit are retained in solid solution. For Ni-base alloys, such low-temperature colossal supersaturation (LTCSS) with carbon has great advantages over the conventional, high-temperature carburization method. In addition to providing much higher carbon contents, the lower processing temperature will minimize the degradation of mechanical properties by coarsening of the microstructure.

We presently study the effect of LTCSS on different Ni-base alloys. The surface, microstructure, and fundamental properties of these materials are being characterized in by various experimental methods in order to understand the microscopic mechanisms involved in low-temperature carburization. In next step, large-scale industrial applications will be developed for each material, based on its individual applications and required properties.

Fig. 1. Microstructrure of superalloy Rene-N6 (scanning electron microscopy image).