Microstructure and High Temperature Strengthen of Pure Cr and Cr-Cr_2B Eutectic Alloy

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Cr-13.5mol%B eutectic ingots were unidirectionally casted with rates 30 mm/h and 800 mm/h. From a structure observation, it was cleared that Cr-Cr_2B eutectic composite had a unidirectional eutectic structure. Width of Cr and Cr_2B phase are about 3 μm and 1 μm, respectively. And then, it was found that there was unique crystal orientation relation between Cr and Cr_2B phase.Compression test was conducted in temperature range form 1000 ℃ to 1400 ℃ and strain rate from 5 x 10^<-5> s^<-1> to 1 x 10^<-3> s^<-1>, and temperature and strain rate dependences of yield stress were evaluated. From these tests, high temperature strength at 1200 ℃ was about 400 MPa and this stress level was close to the stress of Ni base super alloys. And, a stress exponent of strain rate was about 15. This high exponent was agreed with the exponent of lamellar composites. From temperature dependence of yield stress, an activation energy was evaluated and was about 1000 kJ/mol. This energy was extremely larger than the energy of self-diffusion.From the TEM observation deformed at high temperature, recovery structure, that is dislocation network, was revealed in Cr phase and many typical micro twins in Cr_2B phase.From these findings, it was concluded that the high temperature strength of Cr-Cr_2B eutectic composite affected to the twining mechanism of Cr_2B phase