U-Nb合金中温失塑现象的微观机制研究

U-Nb alloys are important structural and functional material in nuclear engineering. However, U-Nb alloys exhibit intermediate temperature ductility-dip phenomenon during high temperature tensile test, which make the machining of U-Nb alloys harder at intermediate temperatures, as a result the production and application of U-Nb alloys were limited. Currently, the micro mechanism of intermediate temperature ductility-dip phenomenon in U-Nb alloy is still unknown. The possible factors for intermediate temperature ductility-dip phenomenon, such as precipitates, crystal twin, phase transformation, were analyzed. Combined with previous experimental results, it is considered that phase transformation is the main factor affecting intermediate temperature ductility-dip phenomenon in U-Nb alloys. U-5.7Nb alloy will be used in this study, which will be prepared by casting. Through heat treatment experiment and high temperature pre tension test at intermediate temperature, the microstructure of the specimen will be restore and hold to room temperature after cracking, immediately. The intermediate temperature ductility-dip sensitivity and phase transformation at intermediate temperature will be analyzed to study effects of phase transformation on intermediate temperature ductility-dip phenomenon in U-5.7Nb alloy. Research results is very important for understanding the mechanism of intermediate temperature ductility-dip phenomenon，which could improve the hot workability of U-Nb alloy. Some research results will be used directly to guide the actual production of U-5.7Nb alloy.

U-Nb合金是核工程中重要的结构和功能材料。U-Nb合金在高温拉伸时存在中温失塑现象，增加了合金在中温区间的加工难度，限制了合金的生产和应用。目前，U-Nb合金中温失塑现象的微观机制仍不明确。我们对U-Nb合金中温失塑现象的可能影响因素（析出相、孪晶、相转变等）进行分析，结合前期的实验研究结果，初步认为相转变是U-Nb合金中温失塑现象主要影响因素。本研究以U-5.7Nb合金为研究载体，通过铸造法制备合金样品，通过热处理实验和中温区的高温预拉伸实验，还原和保存试样开裂时刻的微观组织，分析U-5.7Nb合金中温失塑敏感性和中温区间的相转变，研究相转变对U-5.7Nb合金中温失塑现象的影响机制。研究结果对理解U-Nb合金中温失塑现象的形成机制具有重要意义，对提高U-Nb合金热加工性能具有指导意义，部分研究结果将直接用于指导U-5.7Nb合金实际生产。

U-Nb合金是核工程中重要的结构和功能材料。U-Nb合金在高温拉伸时存在中温失塑现象，降低了合金在中温区间的可加工性，限制了合金的生产和应用。目前，U-Nb合金中温失塑现象的微观机制仍不明确。我们通过热处理制备了400-600℃不同温度和不同热处理时间的热处理样品，对其微观组织结构进行了表征，对U-Nb合金中温失塑现象的影响因素（析出相、孪晶、相转变等）进行分析，认为相转变是U-Nb合金中温失塑现象主要影响因素。结果显示：U-Nb合金中在400℃高温拉伸时观察到中温脆性现象，从金相分析和扫描电镜分析来看在发生脆性的温度区间（400℃）未观察到α″相的分解，但XRD峰位有展宽，通过TEM观察，XRD峰位有展宽是由于纳米级析出相析出而导致的。淬火态U-Nb的变形为分为孪生加滑移两方面，从晶体结构来看，单斜α″相U-Nb对称性低，滑移系少，理论塑性很低，孪生变形在塑性变形中起到了重要作用。孪生对提高塑性有直接和间接两方面的作用，①孪生变形直接提供塑性变形（~10%），②孪生可以改变晶体的取向，使晶体的滑移系由原先难滑动的取向转到易于滑动的取向，间接提高滑移变形能力。从断口分析看，400℃时材料变形方式为局部滑移，纳米级析出相导致其孪生变形受阻，从而导致其导致塑性降低。本项目的研究工作为U-Nb合金失塑裂纹控制和提高U-Nb合金热加工性能提供了理论依据。基于此，设计了两种U-Nb合金丝材轧制和热处理方案，实现了两种U-Nb合金丝材的轧制变形与热处理组织调控，成功制备出U-Nb合金丝材，为U-Nb合金的焊接、修复等提供了候选的原材料。