Ryuichi Tomoshige

Abstract:

Our research group has developed a novel welding method that is capable of bonding ceramics, intermetallics, and metal. The welding method is composed of both combustion synthesis and explosive welding. That is, explosive welding is performed just after finishing combustion synthesis of ceramics (for example, TiB₂ or TiC⁺Al₂O₃) and intermetallics (for example, TiNi) on a metal plate. Here, combustion synthesis generates high temperature of around several thousand degrees Celsius due to their exothermic reaction. We have reported that fabricated layered-composites were well-joined among TiB₂ ceramics, TiNi intermetallics, and metal layers, and performed well in resistant-to-thermal shock tests. This is tested by heating the layered material to 550°C and then rapidly cooling it by dropping it into water, due to their high joining strength and pseudo-elastic effect of TiNi layer [1]. In this study, we have fabricated another layered-composite system, that is TiB²⁺TiN/TiNi/steel, by the hot explosive welding technique, and have observed interlayer structures of the obtained layered-composite, especially between ceramics and intermetallics, with electron microscopes. As a result, scanning electron microscopy (SEM) revealed that TiB2 and TiN crystal grains in ceramic layer closely bonded each other. According to observations with transmission electron microscope (TEM), they showed a characteristic crystal orientation relationship, i.e. Blackburn orientation relationship, which is the same as the matter we have revealed it previously in TiB₂+TiN composites prepared by hot dynamic compaction technique [2]. Further detailed scanning TEM (STEM) observations and energy dispersive spectroscopy (EDS) revealed a component of nickel infiltrated from TiNi layer to intergranular regions of TiB2+TiN ceramic layer, which seemed like the capillary action. It was speculated that the phenomena must have generated after finishing the combustion synthesis of ceramic layer, because TiB₂ and TiN phases in the ceramics layer has completely formed. Also, this phenomenon resulted in a decrease in Vickers hardness value in the ceramics layer (around 10 GPa), even though TiB₂ and TiN phases have essentially hardness values of about 33 and 20 GPa, respectively. Here, it is well-known that mechanical properties, including the hardness values, are affected by an aspect of grain boundaries. Therefore, the decrease in hardness must have been caused by the decrease in intergranular bonding strength, as a consequence of the infiltration of metal nickel into boundaries between TiB2 and TiN grains.