Effect of HIP at 800 and 900 °C on microstructure and properties of extruded Be-Ti composites

Hot isostatic pressing (HIP) has been proposed for manufacturing large hexagonal TiBe12 blocks for neutron multiplication in the new reference design of the DEMO blanket. This paper investigates the effect of HIP at 800 and 900 degrees C on the microstructure and the properties of extruded Be-Ti composites with the main aim of optimizing HIP parameters. Be-Ti composites produced by powder extrusion at 650 degrees C consist exclusively of Be and Ti phases. During HIP, they interact forming titanium beryllides with a volume fraction of 70-79%. These beryllides have a very high microhardness of 1130-1680 HV. X-ray diffraction showed that the main beryllide phase is TiBe12. After HIP at 900 degrees C, it has a very fine-grained microstructure with a mean grain size of 190 nm. Auger electron spectroscopy revealed that the TiBe2 beryllide forms a thin layer surrounding the remaining Ti phase. Ti2Be17 can be found at the prior titanium phase locations in the form of small particles. The titanium and beryllium phase do not dissolve completely during the used HIP process. In addition, the beryllium phase exhibited a higher porosity after HIP. This results in densities as low as 94.4% and 95.9% of the theoretical density of TiBe12 after heat treatment at 800 degrees C and 900 degrees C, respectively. Differential scanning calorimetry showed that beryllides are mainly synthesized in the temperature range of 670-740 degrees C, which can be monitored by the observation of a heat release. To accomplish diffusion processes in the Be-Ti composite, HIP should be carried out at temperatures exceeding 900 degrees C. Heating rates should be less than 10 K/min to avoid excessive overheating.