关于热处理后亚稳β钛合金的组织、强度和塑性，已经有大量的研究工作[10]。Li等[11]发现，经过850℃/1 h固溶处理和500℃/2 h单级时效处理的Ti-2Al-9.2Mo-2Fe合金强度最高，其抗拉强度为1543 MPa，达到了超高强钛合金标准。Zheng等[12]的研究发现，固溶+双级时效处理也能大幅度提高亚稳β钛合金的强度。在预时效阶段析出的ω相在二次时效阶段促进次生α相形核，进而生成更加细小弥散且分布均匀的次生α相，从而使合金的强度提高。马权等[13]对TB8合金进行固溶+双级时效处理后发现，次生α相明显细化使合金抗拉强度的提高超过了17%。Zhou等[14]则发现，与其他热处理工艺相对，固溶+随炉冷却处理也能显著提高合金的力学性能，生成枝晶状生长的α相实现了强度与塑性的良好匹配。这些结果表明，固溶+单级时效处理、固溶+双级时效处理、固溶+随炉冷却处理等不同的热处理工艺，对亚稳β钛合金析出的次生α相及强度和塑性产生不同的影响。
Ti-6Mo-5V-3Al-2Fe-2Zr (%,质量分数)合金是一种新型的亚稳β钛合金，基于Mo当量准则和d-电子成分设计方法设计，其Mo当量为12.15，Bo、Md值分别为2.7823、2.3765。作为一种新型亚稳β钛合金，热处理工艺对其组织、强度和塑性的影响尚不十分清楚。鉴于此，本文对该合金分别进行固溶+单级时效处理、固溶+双级时效处理以及固溶+随炉冷却处理，研究热处理工艺对Ti-6Mo-5V-3Al-2Fe-2Zr合金的次生α相以及强度和塑性的影响。
1 实验方法
实验用材料为亚稳β钛合金Ti-6Mo-5V-3Al-2Fe-2Zr (%，质量分数)。将高纯海绵钛、Al-Mo中间合金、Al-V中间合金、纯铁和海绵锆经过两次真空自耗熔炼，得到直径为12 mm的亚稳β钛合金Ti-6Mo-5V-3Al-2Fe-2Zr (%，质量分数)铸锭，对其在β相区锻造得到合金板材。用电火花线切割在板材上切取实验用试样。
采用公式法和金相法测定该合金的相变点。根据加入不同合金元素后合金相转变温度的变化估算合金的相变点 [15]
式中的882℃为纯钛的相转变温度，fi (xi )为合金中各元素对相变点的影响值。使用
将切取的试样分别进行固溶+单级时效处理(HT1)、固溶+双级时效处理(HT2)和固溶+随炉冷却处理(HT3)，热处理工艺在图1中给出。
图1
图1
合金的热处理工艺
Fig.1
Heat treatment process (a) solution and single-stage aging; (b) solution and two-stage aging; (c) solution and furnace cooling
是用Nordlys Nano型EBSD探测器观察和分析电解抛光后试样的晶粒尺寸和析出相 (抛光液为6%高氯酸+35%正丁醇+59%甲醇)，步长为0.04 μm。使用腐蚀液(氢氟酸∶硝酸∶水=1∶3∶7)腐蚀经过金相砂纸打磨、机械抛光后的合金试样表面，用S-3400N型扫描电子显微镜观察试样的显微组织。使用WDW-100型电子万能试验机分别对三种热处理后的试样进行室温拉伸实验，以测定其拉伸性能，计算其抗拉强度、屈服强度以及断后伸长率。使用S-3400N型扫描电子显微镜观察断口形貌，分析其断裂形式。
2 实验结果
2.1 显微组织
图2给出了热处理前Ti-6Mo-5V-3Al-2Fe-2Zr合金的EBSD图像，可见其组织全部为β相。
图2
图3给出了经过不同工艺的热处理后Ti-6Mo-5V-3Al-2Fe-2Zr合金的显微组织。由图3a和3b可见，经过HT1处理后在β晶界处生成了连续的晶界α相(αgb)；在β晶粒内析出了短棒状次生α相(αi)。由图3c和3d可见，经过HT2处理后在β晶界处也生成了连续的αgb相，但是在β晶粒内析出的αi相数量更多且大部分尺寸较小。其原因是，在预时效阶段生成的ω相在后续高温时效阶段促进αi相的形核从而生成了细小弥散的αi相[18~21]。但是，由图3d可见，一些优先形核的αi相在后续高温时效阶段充分的生长而使部分αi相长大而粗化。由图3e和3f可见，经过HT3处理后在β晶界处生成了由αgb相形核并向晶内平行生长的αwgb相，在β晶粒内析出的αi相由短棒状变为针状，宽度明显减小且间距变窄。
图3
图3
不同热处理后合金的显微组织
Fig.3
Microstructure of the alloy after different heat treatment (a) HT1, grain boundary; (b) HT1, intragranular; (c) HT2, grain boundary; (d) HT2, intragranular; (e) HT3, grain boundary; (f) HT3, intragranular
图4给出了经过不同热处理的合金的EBSD图像。图4a、b、c的解析率分别为96.23%、96.12%和95.56%，三者的解析率较高且差异较小。用EBSD进一步分析了次生α相。结果表明，经过HT1、HT2、HT3处理后合金的β晶粒内析出的αi相尺寸逐渐减小，且在β晶界处均生成了连续的αgb相；与HT1和HT2相比，经过HT3处理后合金的晶界处生成了αwgb相，与图3中合金的显微组织相同。用EBSD统计次生α相(αs)的体积分数φ(αs)并结合图3统计αi相的平均间距λ，结果列于表1。可以看出，不同的热处理使αs相体积分数变化的趋势为，HT2>HT3>HT1；αi相平均间距的变化趋势为，HT1>HT2>HT3。
图4
图4
热处理后合金的EBSD图像
Fig.4
EBSD images of the alloy after different heat treatment (a) HT1; (b) HT2; (c) HT3
表1 热处理工艺对αs相体积分数和αi相平均间距的影响
Table 1
| Heat treatment | φ(αs)/% | λ/nm |
|---|---|---|
| HT1 | 34.8 | 88.75 |
| HT2 | 40.3 | 64.85 |
| HT3 | 37.5 | 47.15 |
2.2 合金的拉伸性能
合金经过不同工艺的热处理前后的屈服强度(Rp0.2)、抗拉强度(Rm)、断后伸长率(A)和强塑积(Psp)，列于表2。可以看出，经过三种工艺的热处理后合金的强度都大幅度提高；与HT1处理相比，HT2热处理使合金的屈服强度和抗拉强度提高，断后伸长率由5.1%降低为4.8%，合金的塑性没有明显的变化；HT3处理后合金的屈服强度和抗拉强度进一步提高，其断后伸长率也明显提高；在三种热处理中，HT3处理后合金的强塑积最高，为10.94 GPa%，其强度与塑性匹配最佳。
表2 热处理对合金拉伸性能的影响
Table 2
| Heat treatment | Rp0.2/MPa | St.dev | Rm/MPa | St.dev | A/% | St.dev | Psp/GPa% |
|---|---|---|---|---|---|---|---|
| Before HT | 784 | 9 | 891 | 10 | 9.1 | 0.22 | 8.11 |
| HT1 | 1099 | 22 | 1196 | 26 | 5.1 | 0.17 | 6.10 |
| HT2 | 1256 | 19 | 1352 | 21 | 4.8 | 0.21 | 5.68 |
| HT3 | 1324 | 13 | 1421 | 11 | 7.7 | 0.13 | 10.94 |
2.3 合金的断口形貌
图5
图5
不同热处理后试样的拉伸断口的形貌
Fig.5
Fracture morphology of tensile specimens after different heat treatment (a) HT1; (b) HT2; (c) HT3
3 讨论
式中Rν为单晶摩擦应力影响项，Rss为合金元素于β基体的固溶强化影响项，Rgb为β晶界强化影响项，Rpb为初生α相与β基体的界面强化影响项，Rpcpt为次生α相析出强化影响项。对显微组织的观察结果表明，在本文的实验中未观察到初生α相，于是
根据
式中Rpcpt, exp为次生α相析出强化影响量，Rp0.2, exp为合金的屈服强度，Rν+Rss+Rgb的值则等于热处理前合金的屈服强度。由此计算出的Rpcpt, exp数值，列于表3。
表3 不同热处理后合金中次生α相的析出强化影响量
Table 3
Heat treatment | Rp0.2,exp /MPa | Rν+Rss+Rgb /MPa | Rpcpt,exp /MPa |
|---|---|---|---|
| HT1 | 1099 | 784 | 315 |
| HT2 | 1256 | 784 | 472 |
| HT3 | 1324 | 784 | 540 |
图6
图6
Rpcpt,exp与φ(αs )和λ的关系
Fig.6
Dependence of Rpcpt,exp on φ(αs ) and λ (a) the dependence of Rpcpt,exp on φ(αs ); (b) the dependence of Rpcpt,exp on λ
式中τ为位错运动施加的应力；τ*为αi/β界面产生的应力场，其值与位错源的位置以及界面能量有关，b为伯格斯矢量。
同时，堆积位错的数量可表示为[30]
式中v为泊松比；G为剪切模量。设位错源位于两个αi相之间，λ/2为位错的运动距离，则根据
式中k0为材料常数。
图7给出了不同工艺热处理后合金的屈服强度与λ-1/2的关系。可以看出，整体呈现出近似线性相关的关系，与
图7
4 结论
(1) 在三种工艺热处理的Ti-6Mo-5V-3Al-2Fe-2Zr合金的晶内析出了αi相，在晶界生成了连续的αgb相；与固溶+单级时效处理与固溶+双级时效处理相比，固溶+随炉冷却处理析出的αi相间距最小，且在晶界处生成了向晶内平行生长的αwgb相。
(2) 与固溶+单级时效处理及固溶+双级时效处理相比，固溶+随炉冷却处理的Ti-6Mo-5V-3Al-2Fe-2Zr合金强度和塑性匹配最佳，其抗拉强度为1421 MPa，屈服强度为1324 MPa，断后伸长率为7.7%。
(3) 经不同工艺的热处理后Ti-6Mo-5V-3Al-2Fe-2Zr合金晶内析出的αi相间距是影响其强度的主要因素，随着αi相间距的减小合金的强度提高；αwgb相的生成，使合金的塑性显著改善。
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[J].The microstructure evolution and phase composition of an α + β titanium alloy, Ti-3Al-5Mo-4.5V (wt.%), have been investigated. Electron probe micro analysis (EPMA) quantitative results manifest that the stability of β phase decreases with increasing quenching temperature, which is influenced by the significant variation of β-stabilizing elements concentration. Detailed microstructure analysis shows that the β → ω phase transformation does occur when quenching at 750 °C and 800 °C. The ω-reflections change from incommensurate ω-spots (750 °C) to ideal ω-spots (800 °C) as the β stability of the alloy decreases. Further the decrease of β phase stability encourages the formation of athermal α′′ martensite, which has the following orientation relationships: [111]β//[110]α′′, [100]β//[100]α′′ and [-110]β//[00-1]α′′ with respect to the β matrix.
Effect of heat treatment on microstructure and mechanical properties of ultra-fine grained Ti-55511 near β titanium alloy
[J].
Effect of long term aging on the fatigue crack propagation in the β titanium alloy Ti 17
[J].
