TABLE OF CONTENTS The behavior of diffusion coatings is strongly dependent on the composition of the substrate alloy because the alloy participates in the formation of the coating. These coatings are based on the ? NiAl phase of the Ni-Al alloy system. NiAl has poor solubility of other elements. As a result, these coatings do not offer wide flexibility for the incorporation of minor elements. In order to address this limitation, a new class called "overlay" coatings has been developed with minimal direct contribution of the substrate alloy. The overlay coatings have a typical composition represented by MCrAlX, where M stands for Ni, Co, and occasionally Fe, and X represents oxygen-reactive elements such as Zr, Hf, Si, and Y. The composition is so selected that the microstructure consists of varying amounts of ? phase in a ? matrix. Phase stability, resistance to oxidation and corrosion, and susceptibility to cracking are controlled by adjusting the composition and the ?/ ? ratio. One of the requirements in selecting the composition of the overlay coatings is to avoid the detrimental phase transformation ? + ? = aCr + ??, as explained in the next section.
