Papers by Keyword: Reactive Hot-Pressing

Abstract: ZrB2-based ultra-high temperature ceramics (UHTCs) were prepared from a mixture powder of Zr/B4C/Si with different ratio via reactive hot pressing. The experimental results showed that the sintering temperature above 1800°C was necessary for enhancing the activity of the powders and thus improving the densification of the product. The sinterability and densification properties of ZrB2-based UHTCs meliorated with the amount of Si increasing. However, many large ZrB2 agglomerates formed when the amount of synthesized SiC in the product reached 25vol%, which led to decrease the mechanical property. The composite had highest mechanical properties when the volume ratio of ZrB2: SiC: ZrC was 73.86:20:6.14, and its flexual strength and the fracture toughness were 645.8MPa and 5.66MPa·m1/2 respectively. The microstructure investigation showed the in-situ formed SiC and ZrC were located in the triple point of ZrB2 grains with a size less than 3μm.

Abstract: The two (Al2O3+TiB2+Al3Ti)/Al composites were fabricated from Al-B2O3-TiO2 and Al-B-TiO2 raw powders by reactive hot pressing, respectively. The microstructure of in situ two composites was analyzed by OM, SEM and TEM. The results showed that coarse Al3Ti blocks with several tens of micrometers size were formed during hot pressing. The equiaxed Al2O3 particulates and hexagonal TiB2 particulates with finer sizes were formed in the composites simultaneously. The microstructure formation mechanism of (Al2O3+TiB2+Al3Ti)/Al composites were discussed. The results showed that Al2O3 reinforcements were formed on the surface of TiO2 or B2O3 powder and TiB2 particles were formed on B or B2O3 powders. The formation of coarse Al3Ti block is result from continuous diffusion of Ti in liquid Al during reactive hot pressing. In addition, there are fine Al3Ti precipitates exist in the composite fabricated from Al-B-TiO2 powders. This contributes to the improved mechanical properties in terms of yield and ultimate stresses and Young’s modulus of the composite.

Abstract: MoSi2-SiC precursor powder has been synthesized via the mechanical alloy method with the elements Mo, Si and C powder. The SiC/MoSi2 composites with different SiC volume fraction have been prepared by reactive hot-pressing the precursor powder at 1350 °C. There is a significant increase in the fracture toughness due to addition of SiC reinforcement. However, the intermediary ternary phase exists in this system—namely, Mo5Si3C, Nowotny phase, which has negative effect on the fracture toughness, and can be eliminated through high temperature treatment at 1600 °C for 2 hrs.

Abstract: Based on low-temperature hot-press sintering and rapid thermit reaction, heat-resistant metal matrix composites with nano-ceramic reinforcement were prepared via reactive hot pressing. According to XRD, the composites comprised predominantly of (fcc) Cr0.19Fe0.7Ni0.11, (fcc) Fe-Cr and alumina at 700°C through the highly-exothermic thermit reaction between the starting powders. Three-point bending strength, fracture toughness, Vickers hardness and relative density increased with the increase of hot-press sintering temperature and holding time. The improving mechanical properties may be explained by increasing of content of (fcc) Cr0.19Fe0.7Ni0.11. SEM analysis showed a microstructure consisting of equiaxial granules at 700°C for 1 h and a uniformly dispersed network of very fine grains at 700°C for 2 h. It is considered that, in the reactive hot-pressing process, Al atoms diffused into the metal matrix (Fe2O3, Cr, Ni) sites and formed Al2O3 and Fe-Cr-Ni matrix. Such a technique offers the possibility of synthesizing heat-resistant metal matrix composites with nano-ceramic reinforcement materials at considerably lower temperature.

Abstract: In the frame of the development of new nuclear reactors (Gen-IV project), materials able to withstand very high temperatures are required. Among the components of the cores, the transition metal carbides could be favourably considered as fission products barriers or inert matrix for the fuel. They are highly refractory, have good neutron properties and fission products retention. Unfortunately, first they are brittle and second the elaboration process of bulk materials requires drastical conditions (temperature, pressure). We have then undertaken the comparison of different materials to investigate the potentialities of nano-sized microstructures for the improvement of the mechanical properties and to determine the impact of different elaboration routes to obtain high density articles. The elaboration process of the nano-sized powders is presented elsewhere [M. Dollé et al., this meeting]. Here, we compare different sintering processes (uniaxial hot-pressing, spark plasma sintering) and we present a new process able to produce high density zirconium carbide articles with soft conditions. We show that the sintering of co-ground zirconium and graphite powders lead to high density, homogeneous, low grain-size materials at temperatures as low as 1500°C. At the contrary, the nano-powders appear to be more difficult to densify that classical micro-sized powders.