Three types of in-situ TiC(5 vol%,10 vol%and 15 vol%)reinforced high entropy alloy CoCrFeNi matrix composites were produced by vacuum induction smelting.The effect of two extreme cooling conditions(i.e.,slow cooling in fu rnace and rapid cooling in copper crucible)upon the microstructure and mechanical properties was examined.In the case of slow cooling in the furnace,TiC was found to form mostly along the grain boundaries for the 5 vol%samples.With the increase of TiC reinforcements,fibrous TiC appeared and extended into the matrix,leading to an increase in hardness.The ultimate tensile strength of the composites shows a marked variation with increasing TiC content;that is,425.6 MPa(matrix),372.8 MPa(5 vol%),550.4 MPa(10 vol%)and 334.3 MPa(15 vol%),while the elongation-to-failure(i.e.,ductility)decreases.The fracture pattern was found to transit from the ductile to cleavage fracture,as the TiC content increased.When the samples cooled rapidly in copper crucible,the TiC particles formed both along the grain boundaries and within the grains.With the increase of TiC volume fraction,both the hardness and ultimate tensile strength of the resulting composites improved steadily while the elongation-to-failure declined.Therefore,the fast cooling can be used to drastically improve the strength of in-situ TiC reinforced CoCrFeNi.For example,for the 15 vol%TiC/CoCrFeNi composite cooled in the copper crucible,the hardness and ultimate tensile strength can reach as high as 595 HV and 941.7 MPa,respectively.
Titanium matrix composites reinforced with a-Al_2O_3 and TiB_2 particles were fabricated by in situ synthesis from a Ti-Al-B_2O_3 system. The reaction processes and microstructure were analyzed by using differential scanning calorimetry(DSC), scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results showed that the reactions in the Ti-Al-B_2O_3 system can occur spontaneously and consist of three steps: 1) 15 Al + 7B_2O_3 → 7α-Al_2O_3 + AlB_(12) + 2B; 2) 14 B + 2Al → AlB_(12) + AlB_2 and 3) 7Ti + AlB_(12) + AlB_2 → 7TiB_2 + 2Al. The final reinforcements were composed of a-Al_2O_3 and TiB_2 particles, which were uniformly distributed in the titanium matrix.
