Laser Melting of Zirconium Carbide: Determination of Phase Transitions in Refractory Ceramic Systems
Laser Melting of Zirconium Carbide: Determination of Phase Transitions in Refractory Ceramic Systems
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DOI：
10.1111/j.1551-2916.2011.04560.x
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发表时间：
2011-10-01
影响因子：
3.9
通讯作者：
Lee, William E.
中科院分区：
文献类型：
--
作者：
Jackson, Heather F.;Jayaseelan, Daniel D.;Lee, William E.
利用脉冲激光加热和光学测温技术产生的时间-温度热图数据适用于测定Zr-C系陶瓷的极高温(>；43000 K)固相线、液相线和共晶转变。相变温度与相界和之前发表的个别测量结果有很好的相关性。熔融样品的显微组织和衍射分析证实，ZRC存在于液相中，并分解为ZRC或ZRC+石墨共晶。相变温度对激光脉冲时间尺度和重复熔化不敏感，熔化表面的微观结构与相图一致，表明局部达到热力学平衡。亚表面非平衡微观结构归因于冻结过程中随深度变化的温度梯度和溶质分配。本工作表明，脉冲激光加热是产生平衡组织的一种可行技术，这是精确测量相变温度的先决条件。报告温度不确定的主要来源是ZRC发射率的估计。在固-液相变过程中持续观察到的不连续温度下降表明，液体的发射率相对于固体的发射率有所降低。根据固体ZRC的发射率估计为0.6，液体ZRC的发射率估计为0.44-0.58。
Pulsed laser heating and optical pyrometry were used to generate time-temperature thermogram data suitable for the determination of extremely high-temperature (>43000 K) solidus, liquidus, and eutectic transitions for ceramics in the Zr-C system. Transition temperatures correlated well with phase boundaries and individual measurements published previously. Microstructural and diffraction analysis of melted specimens confirmed that ZrC existed in the liquid phase and resolidified to ZrC or a ZrC+graphite eutectic. Transition temperatures were insensitive to laser pulse timescale and repeated melting, and microstructures of melted surfaces were consistent with the phase diagram, indicating the local attainment of thermodynamic equilibrium. Subsurface nonequilibrium microstructures were attributed to thermal gradients with depth and solute partitioning during freezing. The present work indicates that pulsed laser heating is a viable technique for producing equilibrium microstructures in ZrC as a prerequisite for precision measurement of phase transition temperatures. The main source of uncertainty in reported temperatures was the estimation of ZrC emittance. A consistently observed discontinuous temperature decrease upon the solid-liquid phase transition indicated a decrease in the emittance of liquid ZrC with respect to solid ZrC. Based on an estimated emittance of solid ZrC of 0.6, emittance of liquid ZrC was estimated at 0.44-0.58.