High temperature back-stress strengthening by heterogeneous nano TiBw in TC4 alloy

Back-stress strengthening has been proven to be an effective strategy in breaking the room temperature strength–ductility dilemma in hetero-structural metals matrix composite materials, which is difficult to achieve at high-temperature because of grain boundaries softening, dislocation annihilation and grain rotation. Hence, we induced back-stress strengthening to improve the high-temperature tensile properties of Ti alloy by the heterogeneous distribution of nano TiB whisker (TiBw). In this work, electron beam powder bed fusion (EB-PBF) with rapid solidification technology was employed to in-situ generate heterogeneous nano TiBw at both grain boundaries (GBs) and grain interiors. Grains can be significantly strengthening by high temperature back-stress due to the effective blockage of dislocation movement, grain boundary migration and grain boundary slip by nano-TiBw on GBs. Remarkably, intragranular nano TiBw (IG nano-TiBw) dramatically enhances the dislocation storage density. The heterogeneous distribution of nano TiBw achieves a 50°C increase in the service temperature of TC4 alloy.

背应力强化（Back-stress strengthening）已被证实为破解异质结构金属基复合材料室温强塑性两难困境的有效策略，但由于高温下晶界软化、位错湮灭与晶粒转动，该策略难以在高温环境中实现。为此，本研究通过纳米TiB晶须（nano TiB whisker, TiBw）的异质分布，引入背应力强化以改善钛合金的高温拉伸性能。本研究采用电子束粉末床熔融（electron beam powder bed fusion, EB-PBF）结合快速凝固技术，在晶界（grain boundaries, GBs）与晶粒内部原位生成异质纳米TiB晶须。晶界处的纳米TiB可有效阻碍位错运动、晶界迁移与晶界滑移，借助高温背应力实现对晶粒的显著强化。值得注意的是，晶内纳米TiB晶须可大幅提升位错存储密度。纳米TiB晶须的异质分布使TC4合金的使用温度提升了50℃。