Microstructure tailoring and fatigue crack resistance in precipitation-strengthened TB9 titanium alloy
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摘要: 研究了不同热处理制度对TB9（Ti-3Al-8V-6Cr-4Mo-4Zr）钛合金alpha（α）相和力学性能的影响。结果表明，当固溶温度从800 ℃上升至810 ℃，后续采用相同时效工艺时，虽然beta（β）晶粒尺寸略微长大，但其抗拉强度显著降低。采用相同固溶工艺时，500 ℃时效8 h出现了无α相析出区域，随着时效时间的延长，α析出相分布特点由非均匀逐渐转变为弥散分布，抗拉强度在时效16 h出现峰值。根据微观组织演变规律，采用800 ℃-0.5 h, 自然空气冷却（AC）+500 ℃-16 h, AC与810 ℃-0.5 h, AC+520 ℃-8 h, AC两种热处理制度时，获得相似微观组织，但前者的拉伸和疲劳性能明显优于后者。虽然两种合金的疲劳裂纹扩展机制均表现为粗糙度诱导裂纹闭合效应，但是通过对疲劳数据进行归一化处理，其结果进一步证实了α相尺寸和分布均匀性对力学性能影响显著。Abstract: The effect of microstructure on mechanical properties in TB9 (Ti-3Al-8V-6Cr-4Mo-4Zr) titanium alloy subject to different heat-treatment processes had been investigated. The results show when the solid solution temperature increases from 800 ℃to 810 ℃, and β grain size in the obtained alloyed after complete heat treatment slightly increases, while tensile strength decreases significantly. When the solid solution process remains same, the occurrence of precipitates free zones can be observed within β grain after aging at 500 ℃ for 8 h. The distribution of α precipitates becomes homogeneous with prolonged aging time, and there is a peak-value of tensile strength at aging temperature of 500 ℃ for 16 h. Based on the microstructure observations, both heat treatment processes of 800 ℃-0.5 h, AC+500 ℃-16 h, AC and 810 ℃-0.5 h, AC+520 ℃-8 h, generate comparable microstructures, while the former process can achieve superior tensile and fatigue properties. Although roughness-induced crack closure effect is main fatigue crack resistance behavior in both alloys after different heat treatment processed, fatigue data normalized by tensile strength verifies that the size and distribution of α precipitates impact mechanical properties significantly.
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Key words:
- TB9 alloy /
- precipitation strengthening /
- fatigue properties /
- tensile strength /
- dislocation
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图 1 试样几何尺寸（单位：mm）
(a) 拉伸试样; (b) 疲劳试样
Figure 1. Specimen geometry for tensile and fatigue tests
图 2 不同热处理制度调控后的TB9工程应力--应变拉伸曲线
Figure 2. Engineering stress-strain curves for TB9 alloy after different heat treatment regimes
图 3 不同固溶温度热处理后光学显微镜成像下的微观组织
(a) 800 ℃-0.5 h, AC; (b) 810 ℃-0.5 h, AC
Figure 3. Optical images for microstructures after different solution temperature
图 4 不同热处理后的扫描电镜成像下的微观组织
Figure 4. SEM images for microstructures obtained by different heat treatment regimes
(a) 800 ℃-0.5 h, AC; (b) 800 ℃-0.5 h, AC+500 ℃-8 h, AC; (c) 800 ℃-0.5 h, AC+500 ℃-16 h, AC; (d) 800 ℃-0.5 h, AC+600 ℃-8 h, AC; (e) 810 ℃-0.5 h, AC; (f) 810 ℃-0.5 h, AC+520 ℃-8 h, AC
图 6 2# 热处理制度疲劳试样在800 MPa下的断口形貌和局部放大
(a) 断口整体形貌; (b) 疲劳源（箭头所示)附近形貌; (c) 裂纹扩展区形貌; (d) 瞬断区附近形貌; (e) 疲劳源附近形貌; (f) 裂纹扩展区形貌
Figure 6. Fracture surface of TB9 alloy with 2# heat treatment after fatigue test at 800 MPa
图 7 6# 热处理制度疲劳试样在750 MPa下的断口形貌和局部放大
（a）疲劳源多发（箭头所示）断口整体形貌；（b）疲劳裂纹扩展联合形成断崖形貌；（c）疲劳源附近形貌；（d）裂纹扩展区形貌；（e）裂纹扩展区放大；（f）瞬断区附近形貌
Figure 7. Fracture surface of TB9 alloy with 6# heat treatment after fatigue test at 750 MPa
表 1 TB9合金化学成分
Table 1. Chemical composition of TB9 titanium alloy
% Ti Al V Cr Mo Zr Fe O N H Bal. 3.34 7.92 5.90 3.96 4.10 0.059 0.126 0.013 0.00026 
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表 2 TB9合金热处理制度
Table 2. Heat treatment processes for TB9 titanium alloy
序号 固溶处理 时效处理 温度/℃ 时间/h 方式 温度/℃ 时间/h 方式 1# 800 0.5 AC 500 8 AC 2# 500 16 AC 3# 500 20 AC 4# 600 8 AC 5# 810 0.5 AC 500 8 AC 6# 520 8 AC 7# 830 0.5 AC 500 8 AC AC：自然空气冷却
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