Carbon enriched defect and the associated deformation cracking behaviors in a titanium alloy
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摘要: 针对Ti-6Al-4V钛合金锻棒中超声波探伤出现异常杂波的问题，通过光学显微镜和扫描电镜金相分析、显微硬度、能谱、电子探针、背散射电子衍射等表征方法，在锻棒中出现异常杂波的位置发现了不常见的富碳缺陷及其导致的裂纹缺陷。首先，在微观组织中观察到了一种被α相稳定区域环绕的斑块特征。然后，通过化学成分和晶体学分析，确认了这种斑块是具有有序FCC晶格的Ti2C相，而环绕的α相稳定区域则是由硬α相组成。根据化学元素分布特点，推定缺陷来源是受到富含碳氮等元素的异物污染的原材料。在锻造变形过程中，微裂纹在Ti2C/α相界形成，并向α相内扩展。微裂纹最终突破Ti2C晶粒的裂尖桥接作用，汇合形成宏观裂纹缺陷。Abstract: Since the contacting ultrasonic detection results indicated some defects existed in a bar of Ti-6Al-4V alloy, researching work had been carried out using optical microscope, scanning electron microscope metallographic analysis, microhardness, energy spectrum, electron probe, backscatter electron diffraction and other characterization methods. And uncommon carbon enriched defect had been figured out. Flecks surrounded with α stable area were observed in the microstructure. Then, it was figured out that flecks were Ti2C with ordered FCC crystal and the α stable area was hard α phase based on the chemical composition and crystallology analysis. According to the distribution characteristics of chemical elements, the defect source was presumed to be raw materials contaminated by elements rich in carbon and nitrogen. Micro cracks initiated at Ti2C/α boundaries and grew into the α side in the hard α and Ti2C regions during the forging. Micro cracks would break through the Ti2C crystal bridges and merged into ultrasonically detectable macro crack defects at last.
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Key words:
- carbon enriched defect /
- hard α phase /
- Ti2C /
- grain bridge
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图 1 锻棒探伤异常位置切片取样上的宏观缺陷
Figure 1. Macro defects on the slice sample in abnormal detect position from the forging bar
图 2 P0和P1位置的扫描电镜形貌
（a） 正常的微观组织；（b）斑块上的裂纹；（c）斑块周围的α相稳定区域；（d）微裂纹和微孔洞；（e）α相稳定区域的变形带；（f）斑块中的细小析出相
Figure 2. SEM images at P0 and P1
图 4 碳化物、邻近区域、正常区域的显微硬度
Figure 4. Micro hardness measured in carbide fleck area, neighborhood area and normal microstructure area
图 5 化学成分的电子探针定量分析
（a）未进行腐蚀的金相试样上的电子探针分析的位置；（b）化学元素的质量百分数-位置曲线
Figure 5. Quantitative analysis of chemical composition by electron probe
图 6 斑块组织的电子背散射衍射分析结果
（a）菊池带衬度（Band contast）；（b）物相分布
Figure 6. Electron backscatter diffraction analysis of plaque tissue
图 7 缺陷区域的晶体学位向分析
（a）αP、（b）Ti2C、（c）Ti3O的y轴方向反极图（IPF）位向图，（d）αP、（e）Ti2C、（f）Ti3O的极图（PF），（g）反极图位向图中晶向的颜色标尺
Figure 7. Crystal orientation analysis of defect regions
图 8 缺陷区域热加工过程中的组织演变机制
Figure 8. Microstructure evolution mechanism of the defect area during hot deformation
图 9 图5（a）中虚线标记区域的能谱面扫描分析结果
（a）能谱面扫描图；（b）αA区域的亮度-距离曲线
Figure 9. EDS mapping test results within the dash marked area in Fig. 5(a)
图 10 热变形过程中碳化物引起的开裂行为分析
（a）钛合金双相微观组织承受15%下压量的等效应变场仿真[13]；（b）局部位向差图（KAM）；（c）小角度晶界比例-角度曲线；（d）Ti2C斑块区域中的裂纹
Figure 10. Analysis of cracking behavior caused by carbide during thermal deformation
表 1 在斑块区域、邻近区域、正常区域通过电子探针分析获得的化学成分
Table 1. EPMA results at Fleck, the neighborhood, and the normal area
% 位置 Ti Al V Fe O N H 总计 斑块 88.7 0.16 0.77 0.006 0.160* 89.796 邻近区域 92.6 4.46 1.96 0.005 0.160* 99.185 正常区域P0 89.5 6.03 4.12 0.173 0.160* 99.983 宏观化学成分 6.10 4.06 0.190 0.160 0.0064 0.0009 * 由于电子探针无法准确测定轻气体元素的含量，氧元素的微区含量参照宏观化学成分检测结果。 
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表 2 常见合金元素影响钛合金相变点的经验系数[11−12]
Table 2. The empirical coefficients influencing the phase transition point for commonly titanium alloying elements[11−12]
元素 w/% 系数 α稳定元素 Al** 0.0～2.0 14.5① 2.0～7.0 23.0① N 0.0～0.5 5.5② O 0.0～1.0 2.0② C 0.0～0.15 2.0② β稳定元素 H 0.0～5.0 −5.5② Si 0.0～0.45 −1.0③ Mo 0.0～5.0 −5.5① V 0.0～10.0 −14.0① Fe 0.0～15.0 −16.5① Cu 0.0～7.0 −12.0① Nb 0.0～10.0 −8.5① Cr 0.0～7.0 −15.5① 中性元素 Zr 0.0～10.0 −2.0① Sn 0.0～18.0 −1.0① ** 铝元素对相变点的影响的计算方法为分段计算并累积结果；①为每变化1.00%的结果；②为每变化0.01%的结果；③为每变化0.10%的结果。
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