Reactive Sintering of Cu-Al, Ni-Al and Ni-Ti Powder Mixtures

Abstract: Intermetallic compounds are a distinct group of materials having attractive properties and wide applications. In the present work, the formation of intermetallic compounds from elemental components in Cu-Al, Ni-Al and Ni-Ti systems has been studied. Metallic powders were mixed in stoichiometric ratios corresponding to CuAl2, NiAl and NiTi. Calorimetric studies were made using compacted disks as well as loose, uncompacted samples. Samples were heated in a DSC unit where the chamber was continuously flushed with a reducing gas (Ar-4%H2) during heating in order to protect the sample from oxidation. The influence of heating rate (7.5,15°C per minute) and particle sizes of the components on the combustion behavior of the samples were studied. Onset temperatures for the combustion reactions were observed near the lowest eutectic melting temperatures. In compacted samples, a pre-combustion stage was observed prior to the ignition. At low heating rates, the compacted samples showed an eutectic melting peak in their DSC graphs. This confirmed the formation of some intermediate phases through solid-state interdiffusion of components. It was observed that the ignition temperature could be lowered by increasing the heating rate. Uncompacted samples as well as those containing large powder particles showed relatively higher onset temperatures for the combustion reaction. Microstructural studies and X-ray analyses indicated the formation of multiphase products in most of the samples. Fine powder particles and higher heating rates favor the formation of single phase products. Further, an attempt was made to carry out an in-situ study to follow the morphological changes occurring during the reactive sintering of NiTi compacted powder mixtures. The fracture surface of a broken compacted sample was monitored continuously during heating in a hot stage inside an environmental scanning electron microscope. A reducing gas atmosphere was maintained near the sample by intermittent flushing of Ar-4%H2 gas mixture during the experiment. The results indicated the formation of a needle-shaped phase on the surface of titanium particles at temperatures above 800°C. This temperature was lowered by the use of fine nickel particles. At higher temperatures, this phase covered the sample surface completely. Microstructural and X-ray studies confirmed the formation of NiTi2 in the samples. For purposes of comparison, the same experiment was repeated for pure titanium sample. No needle phase was observed in this case. This observation suggests the formation of NiTi2 and its oxidation to Ni2Ti4Ox which appeared as the needle-shaped phase on the sample surface.