快速多重旋转碾压诱导Ti-6Al-4V表面纳米晶及性能研究

目的 使Ti-6Al-4V能更好地应用于海洋领域.方法 采用快速多重旋转碾压技术(FMRR)对Ti-6Al-4V表面进行冷变形处理,研究其力学性能.然后,对其进行低温等离子渗氮,渗氮温度为550℃,保温时间4 h.利用高分辨透射电子显微镜(HRTEM)、透射电子显微镜(TEM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和电子显微硬度计等测试设备,对FMRR处理后的Ti-6Al-4V表层组织结构和性能以及等离子渗氮处理后表层组织结构进行表征.结果 经15、30、45 min的FMRR处理后,Ti-6Al-4V表层晶粒细化并获得了纳米结构,对应的平均晶粒尺寸为65～90、45～70和20～40 nm.此时,晶界明显增多,局部存在孪晶、小角度晶界和高密度位错等结构缺陷,电子衍射环连续.同时,Ti-6Al-4V表层结构未出现新相,晶粒细化导致衍射峰略有变宽,也提高了Ti-6Al-4V表面的显微硬度,显微硬度为325～453HV,比处理前提高了约41％.FMRR预处理的试样经过低温等离子渗氮后,基体表面的渗氮层主要包括白色化合物层和过渡层两个区域.结论 FMRR处理为后续低温渗氮提供了能量条件和结构条件,提高了渗氮的速度,最终的渗氮层厚度约为100μm.

Study on Microstructure and Properties of Ti-6Al-4V Alloy Nanocrystallized by Fast Multiple Rotation Rolling

This work aims to improve the application of Ti-6Al-4V alloy in maritime field. Fast multiple rotation rolling (FMRR) technique was applied to create a nanostructure layer on Ti-6Al-4V surface. The mechanical properties were also investigated. Then the FMRR samples were plasma nitrided at a temperature as low as 550 °C with the duration of 4 h. The microstructure and properties of FMRR samples and nitrided samples were characterized by high resolution transmission electron microscopy (HRTEM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testers and optical microscopy (OM). Results indicated that FMRR can cause surface nanocrystallization with the average grain size of 65~90 nm, 45~70 nm and 20~40 nm, when the duration was 15 min, 30 min and 45 min respectively. Twins, low angle grain boundaries and high-density dislocations were observed in the surface of FMRR samples. The SAED pattern was composed of continuous diffraction rings. No new phase was induced in the top surface of Ti-6Al-4V by FMRR treatment. The diffraction peaks of FMRR samples were broader compared with the original samples. The microhardness was improved from 325HV in the matrix to about 453HV in the top surface layer. The nitrided layer in the top surface of FMRR sample after plasma nitriding included a bright compound layer and a transition layer. Conclusion:FMRR treatment provides additional energy and structures for the subsequent nitriding at lower temperature, thus leading to an increase in nitriding rate and a roughly 100 µm thick.