Synthesis of in situ TiB 2 /TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends
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resulted in the phase composition and microstructure similar to those of the previously discussed 4-mrn-thick sampies. For the BN:2Ti:Ni blend (10-mm-thick samples), the final composition after thermal explosion under pressure (scheme in Figure 1) was expected to contain TiB2, TiNo.66, and Ni according to the reaction BN + 2Ti + Ni ~ 0.5TiB 2 + 1.5TIN0.60 + Ni
[5]
The experimental results, however, are somewhat different (Figure 11). It can be seen that there are no Ni peaks in the XRD pattern. Instead, the major Ni-containing phase is Ni3Ti; some equiatomic NiTi might also be present. We assume, therefore, that the reaction that takes place is, rather, BN + 2Ti + Ni ---) 0.5TiB2 + TiN1 x + mNi3Ti + nNiTi
[6]
The farther the composition of TiNl_x from the stoichiometric TiN, the less the amount of NiTi phase that will be in the final material. Unfortunately, it is practically impossible to predict what phase composition is thermodynamically the most favorable, since there are no thermochemical data available for nonstoichiometric TIN,_ x. All the samples synthesized via thermal explosion under pressure were fully dense, and no residual porosity was detected in the final microstructure. C. Mechanical Properties of the Synthesized Composites
Mechanical properties of in situ composites synthesized from the 2BN:3Ti: 1.5Ni powder blend are presented in Table I along with the densities and phase compositions of the materials. It can be seen that owing to their practically full density, the mechanical properties of composites synthesized via pressure-assisted thermal explosion are significantly higher than those of the displacement reaction synthesized materials. The high value of microhardness measured in the thermal explosion synthesized samples, - 1 8 GPa, is comparable to that of pressureless sintered (2100 ~ or hot pressed (1900 ~ 21 MPa) TiB2 and 0.7TiC/0.3ZrC ceramicsY ~ Comparing two types of samples subjected to thermal explosion, the 10-mm-thick samples containing a ductile Ni phase possess noticeably higher bending strength, O'TRS,and fracture toughness, K~c,than the 2078--VOLUME 27A, AUGUST 1996
Vickers Hardness, P = 20 kg (GPa)
Phase Composition (XRD) TiBz, TiN, Ni
partially reacted 4-mm-thick samples containing, instead of Ni, a relatively brittle Ni3Ti intermetallic phase. The high 20.5 MPa~/m fracture toughness value of the in situ Nitoughened TiB2/TiN ceramic matrix composite obtained in the present work is within the range of the highest fracture toughness numbers measured in tough ceramics with metal dispersions, t31j Thus, the finely dispersed Ni phase appears to be effective in dissipating the energy of cracks propagating in the ceramic matrix.
IV.
SUMMARY AND CONCLUSIONS
The main thrust of the present research was to fabricate near-net-shape TiB2/TiN and TiB2/TiN/Ni composites by reactive synthesis of cold-sintered BN/Ti and BN/Ti/Ni powder blends. This goal was successfully accomplished via two different processing routes: pressureless displacement reaction synthesis
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