Chinese Journal of Material Research, 2017, 31(6): 409-414
doi: 10.11901/1005.3093.2016.267
时效处理对Ti-3Al-8V-6Cr-4Mo-4Zr合金力学性能的影响

Effect of Heat Treatment on Mechanical Properties of Ti-3Al-8V-6Cr-4Mo-4Zr Alloy
王雪萌, 张思倩, 袁子尧, 陈立佳

摘要:

研究了固溶时效处理对Ti-3Al-8V-6Cr-4Mo-4Zr合金拉伸性能的影响。结果表明:在800℃/30 min+500℃/12 h处理后,合金的硬度和抗拉强度达到极大值,其延伸率和断面收缩率没有明显的降低。合金的硬度和强度的提高是ωα析出相共同作用的结果。在合金的热轧态和热处理态的断口都出现了大量的韧窝,表明其为典型的韧性断裂。

关键词: 金属材料 ; 固溶时效处理 ; 拉伸性能 ; 断口

Abstract:

The effects of solution treatment and aging treatment on microstructures and mechanical properties of Ti-3Al-8V-6Cr-4Mo-4Zr alloy bars have been investigated. The results show that the highest hardness and the highest tensile strength can be achieved by the solution and aging treatment (800℃×30 min/AC+510℃×16 h/AC), and an insignificant decline on elongation rate and necking rate is also acquired. The quantity and size of the ω-phases and α-phases result in the increase of hardness and strength of Ti-3Al-8V-6Cr-4Mo-4Zr alloy bars. A lot of dimples exist in the tensile fractures of the hot rolling alloys and heat treatment alloysdemonstrating a typical ductile fracture.

Key words: metal materials ; solution and aging treatment ; tensile properties ; tensile fracture

Ti-3Al-8V-6Cr-4Mo-4Zr是一种可热处理的亚稳β相钛合金,在730±15℃发生βα相变。这种合金具有高强度、低密度、低弹性模量、耐腐蚀和抗疲劳性能,是制作弹簧件的重要候选材料[1-4]。Ti-3Al-8V-6Cr-4Mo-4Zr合金经过固溶+时效处理后其强度可达1400 MPa以上,作为紧固件材料在汽车工业及航空航天领域也受到关注和使用[5]

需要指出的是,目前Ti-3Al-8V-6Cr-4Mo-4Zr合金时效制度的制定主要还是基于强度方面的考虑。随着合金强度的提高其塑性会不断降低,导致其易产生疲劳裂纹和过早疲劳失效。因此,为了提高该合金件的安全使用寿命,必须探索合金的最佳强度/塑性匹配关系,以进一步优化热处理制度。

作为亚稳β相钛合金,Ti-3Al-8V-6Cr-4Mo-4Zr合金在时效和变形过程中形成α相、α′相、α″相、ω相、和β′ 相等析出相[6-10]。这些析出相的种类、含量和形貌,与合金的强度和塑性密切相关[11-14]。鉴于此,本文系统研究Ti-3Al-8V-6Cr-4Mo-4Zr合金在时效和拉伸变形过程中力学性能的变化,以确定合金的最佳强度/塑性匹配条件,为进一步优化该类钛合金提供理论依据和实验数据。

1 实验方法

实验用合金的名义成分(质量分数,%)为Ti-3Al-8V-6Cr-4Mo-4Zr。使用真空自耗熔炼炉三次熔炼,得到直径300 mm的合金铸锭,在850℃将其锻造成直径为55 mm的棒材,表面打磨后在800℃进行热轧变形,轧制出直径为16 mm的棒材。

将棒材切割成长度为60 mm的试棒,将热轧棒在800℃退火处理30 min后空冷,再将退火后的棒材在350~550℃进行时效处理,温度间隔为50℃。为了研究时效时间对合金硬度的影响,分别在每个时效温度进行时效处理4 h,8 h,12 h和24 h。

使用FM-700e洛氏硬度硬度计测试显微硬度,每个样品在不同区域共取15个点进行测试,取平均值作为样品的显微硬度。使用S-3400型扫描电子显微镜观察样品的组织结构和拉伸断口,在观察前将试样置于酒精中进行超声波清洗,以去除表面附着的脏物。使用JEM-2100型透射电子显微镜对不同状态下的拉伸断裂样品进行显微组织分析,工作电压为200 kV。制备透射电镜样品时,用电火花线切割切取厚度为0.5 mm的薄片,用1200#砂纸研磨至40 μm左右,冲成直径为3 mm的圆片,用Gatan-691型离子减薄仪减薄样品,工作电压4~5 V。

在WDW-100型拉伸试验机上进行拉伸实验,拉伸样品的标距直径为5 mm,标距段长12 mm。试验前对标距部分进行机械抛光处理,以减小表面机械加工缺陷的影响。

2 结果与讨论
2.1 合金的硬度

为了更好的研究固溶+时效处理对Ti-3Al-8V-6Cr-4Mo-4Zr合金强度的影响,先测试了合金固溶+时效处理后材料的硬度。图1给出了Ti-3Al-8V-6Cr-4Mo-4Zr合金在800℃/30 min固溶处理后在350~550℃的时效硬度曲线。结果表明,在350℃和400℃合金的硬度随时效时间的变化不大;在450℃合金的硬度随时效时间的延长而增大;在500~550℃合金的时效硬度曲线变化趋势相似,表现出硬度随时效时间的延长先提高而后趋于稳定的趋势。可以看出,经过800℃/30 min+500℃/12 h处理以后材料的硬度达到极大值。由于固溶时效处理后α相析出的数量与形态决定了合金的综合力学性能[15],根据上述合金的硬度变化规律,选取800℃/30 min+500℃/4 h(α相开始析出)及800℃/30 min+500℃/12 h(α相析出最多)两种状态对其进行组织和性能分析。

图1 Ti-3Al-8V-6Cr-4Mo-4Zr合金在350~550℃时效后的硬度

Fig.1 Age-hardening response of Ti-3Al-8V-6Cr-4Mo-4Zr alloy in 350~550℃

2.2 合金的组织

图2给出了Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的微观组织形貌。可以看出,热轧态合金的组织由原始等轴β晶粒和球状初生α相组成(图2a),经过800℃/30 min固溶处理后合金内部的初生α相变小并逐渐转变成β相,初生α相多集中在晶界附近(图2b)。对合金进一步进行500℃/4 h时效处理后初生α相全部转变成β相,少量次生球状α相从β基体中析出(图2c)。当时效时间延长至12 h时α相析出的数量明显增加且转变为棒状,β晶粒细化,从而使合金的强度显著提高(图2d)。

图2 原始态和不同热处理态Ti-3Al-8V-6Cr-4Mo-4Zr合金的SEM照片

Fig.2 SEM images of Ti-3Al-8V-6Cr-4Mo-4Zr alloy in different heat treatmentconditions (a) origin (b) 800℃/30 min (c) 800℃/30 min +500℃/4 h (d) 800℃/30 min +500℃/12 h

2.3 拉伸性能

表1列出了Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的拉伸性能。从表1可以看出,与热轧态和固溶后的合金相比,时效处理后合金的强度明显提高。在500℃时效处理,随着时效时间的延长合金的强度明显提高。同时,时效后合金的延伸率和断面收缩率比原始态有所降低。

在500℃时效4 h,合金的抗拉强度和屈服强度比热轧态提高近200 MPa,时效12 h后提高400 MPa以上,而合金的塑性有所降低。值得注意的是,对比两种时效,与时效4 h相比,时效12 h合金的抗拉强度提高了约250 MPa,而其延伸率和断面收缩率并没有明显的降低。当时效时间延长至24 h合金的强度并没有提高,且塑性略有下降。综合上述结果,经过800℃/30 min+500℃/12 h处理后合金的抗拉强度和屈服强度均达到1400 MPa以上,其延伸率也达到了12%以上,强度和塑性配比适中,强化效果较好。

表1 Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的拉伸性能
Table 1 Tensile properties of Ti-3Al-8V-6Cr-4Mo-4Zr alloy after heat treatments
σb/MPa σ0.2/MPa φ ψ
Hot rolling 1008.9 1002.4 21.96% 39.22%
800℃/30 min 958.0 954.2 16.33% 31.23%
800℃/30min +500℃/4 h 1205.2 1137.8 12.98% 16.94%
800℃/30min +500℃/12 h 1452.1 1436.9 12.75% 15.85%
800℃/30min +500℃/24 h 1440.2 1429.3 11.95% 14.87%

表1 Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的拉伸性能

Table 1 Tensile properties of Ti-3Al-8V-6Cr-4Mo-4Zr alloy after heat treatments

β钛合金中的α相通常作为基体中的硬化沉淀相使用,时效形成的α相的形状、大小和体积分数等均对合金的强度有极大的影响。经过12 h时效后,在Ti-3Al-8V-6Cr-4Mo-4Zr合金的晶界和晶内都析出了α相且数量比时效4 h后的多,因此其强度提高。同时,12 h时效后合金的晶粒有所细化,这是其延伸率和断面收缩率没有明显降低的原因。

2.4 拉伸断口

图3给出了Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的拉伸断口SEM形貌。可以看出,固溶和时效后的合金断口有大量的韧窝,为典型的韧性断裂(图3)。进一步观察发现,热轧态及固溶态合金的断口内含有较多的韧窝,整个断口形貌表现为典型的单轴拉伸产生的等轴韧窝,韧窝内没有明显的形核质点,这是其塑性较好的原因。时效后的合金在塑性变形过程中也形成了等轴韧窝,且韧窝深度比热轧态及固溶态样品浅,同时出现了一些冰糖状断口,降低了合金的塑性。

图3 Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下的拉伸断口照片

Fig.3 SEM images of tensile fracture in different heat treatment conditions (a) origin, (b) 800℃/30 min, (c) 800℃/30 min+500℃/4 h, (d) 800℃/30 min+500℃/12 h

2.5 讨论

作为β钛合金,Ti-3Al-8V-6Cr-4Mo-4Zr合金的淬透性较好。由于该合金的相变组织复杂,不同的热处理制度对其力学性能有较大的影响。在固溶和时效处理条件下六方结构的α相从β相中脱溶析出,形成α相和β相混合组织,从而使合金的强度达到一个很高的水平。一般认为,β相钛合金在一定的温度下时效时微观组织在形成α+β的平衡相前会形成ωβ′ 等相的沉淀。β′ 相不能提高合金的强度,而ω相虽然能显著提高合金的强度,但是会降低合金的韧性。许多β钛合金在固溶及时效过程中都形成了ω[16-18]。Ng等[19]在研究Ti-10V-2Cu合金的时效处理过程中发现,合金在水淬后形成了尺寸约3 nm的ω相,且其显微硬度为272 HV,而在空冷后形成了尺寸约24 nm的ω相,其显微硬度为507 HV。这表明,随着ω相析出尺寸的增大合金的硬度提高。在β钛合金的固溶时效过程中,ω相及α相的析出数量和尺寸都对合金的硬度和强度有极大的影响。ω相可使合金的强度大幅提高,但是其韧性降低。由前文可知,Ti-3Al-8V-6Cr-4Mo-4Zr合金经800℃/30 min+500℃/12 h处理后硬度达到极大值。合金在该热处理制度下拉伸后强度较高,而延伸率和断面收缩率没有明显降低。这与合金中ω相的形核和长大有密切的关系。由图4可见,合金在时效8 h后出现了少量的ω相(约2 nm)和α相;随着时效时间延长至12 h,ω相逐渐增多但尺寸不变但是α相长大(图4c和d),这时合金的强度达到极大值;当时效时间延长至24 h时ω相的数量和尺寸基本不变而α相的尺寸略有增大,此时的硬度与12 h合金的硬度相当(图4e和f)。这表明,在时效过程中合金硬度和强度的增长与ω相及α相的析出有直接关系。ω相和α相的数量和尺寸决定着合金硬度和强度的升高或降低,时效处理后合金硬度、强度的提高是二者共同作用的结果。

图4 Ti-3Al-8V-6Cr-4Mo-4Zr合金在不同状态下拉伸变形后的显微组织

Fig.4 TEM images of microstructure in different heat treatment conditions (a), (b) 800℃/30 min+500℃/8 h; (c), (d) 800℃/30 min+500℃/12 h;(e), (f) 800℃/30 min+500℃/24 h

另一方面,经过12 h时效后,Ti-3Al-8V-6Cr-4Mo-4Zr合金的强度达到极大值而延伸率和断面收缩率并没有明显降低。这与合金时效后β晶粒细化及拉伸变形中的位错运动有密切的关系。由图5可知,经过12 h处理的合金在拉伸变形后在晶粒内部出现了大量亚晶界,细化了晶内组织。晶粒越细小单位体积内的晶粒越多,形变变形可以分布在更多的晶粒中,产生比较均匀的形变,从而降低局部应力的集中,推迟裂纹的产生和扩展。同时,晶界又是裂纹扩展的主要阻力,因此晶粒的细化提高了合金的塑性。其次,Orowan强化效应的减弱也使合金的塑性提高。在合金时效初期β基体中逐渐析出α相,时效初期析出的α相较小且数量较少,使位错难以从α相间弓出。随着时效时间的延长β基体中α相逐渐长大,以至于发生拉伸变形时位错在α相间弓出的可能性大大增加,使Orowan强化效应减弱。同时α相的析出和长大使β基体中溶质原子浓度降低,对基体的固溶强化作用减弱。因此,强化效应的减弱也使合金的塑性有所提高。

图5 Ti-3Al-8V-6Cr-4Mo-4Zr合金经800℃/30 min+500℃/12 h处理后拉伸试样中位错的形貌

Fig.5 TEM images of dislocation in the heat treatment conditions 800℃/30 min+500℃/12 h after tensile deformation

3 结论

(1) 经过800℃/30 min +500℃/12 h处理后合金的硬度和抗拉强度达到极大值,与800℃/30 min+500℃/24 h处理相比其延伸率和断面收缩率没有明显的降低。

(2) 热轧态和固溶态合金的断口内含有较多的韧窝,断口形貌为典型的韧性断裂;在时效后合金的塑性变形过程中出现了等轴韧窝及沿晶断裂质点,表现为脆性和韧性相结合的混合型断裂方式。

(3) 在时效过程中合金硬度和强度的提高与ω相及α相的析出有直接关系,ω相和α相的数量和尺寸决定着合金硬度和强度的升高或降低,时效处理后合金硬度、强度的提高是二者共同作用的结果。合金经800℃/30 min+500℃/12 h处理后塑性没有明显的降低,与合金时效后晶粒细化、拉伸变形中出现的亚晶界及位错运动有着密切的关系。

The authors have declared that no competing interests exist.

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The microstructures, mechanical properties and biocompatibility of low cost β-type Ti-(6-18)Mn alloys were investigated after solution treatment. Ti–9Mn exhibits the best combination of tensile strength and elongation among the fabricated alloys, and its performance is comparable to or superior to those of Ti–6Al–4V ELI (Ti-64 ELI) in terms of every parameter evaluated. A hardness of 338 HV, a Young’s modulus of 94GPa, a 0.2% proof stress of 1023MPa, an ultimate tensile strength of 1048MPa and elongation of 19% were obtained for Ti–9Mn. Furthermore, the cell viability and metallic ion release ratios are comparable to those of commercially pure titanium, making this alloy promising for biomedical applications. The Young’s modulus is also lower than that of Ti-64 ELI (110GPa), which can possibly reduce the stress shielding effect in implanted patients. This study evaluates mechanical and biological performance of low cost solution treated β-type Ti-(6, 9, 13 and 18mass%)Mn alloys. It includes alloys containing a Mn content range higher than most previously published works (which is around or lower than 8mass%). Furthermore, the effects of the ω phase and the β phase stability of the alloys over some mechanical properties and microstructures are discussed. Ion release behavior under simulated body fluids and cell viability are also evaluated. For the case of the Ti–9Mn, a mechanical and biological performance that is comparable to or superior than that of the widely used Ti–6Al–4V ELI and commercially pure Ti was observed.
DOI:10.1016/j.actbio.2015.08.015      PMID:26283166      URL     [本文引用:]
[8] Fan J K, Li J S, Kou H C, et al.Influence of solutiontreatment on microstructure and mechanicalproperties of a nearβtitanium alloy Ti-7333[J]. Mater. Des., 2015, (83): 499
The effect of solution treatment on the microstructure and mechanical properties of Ti-7333, a newly developed near β titanium alloy, was investigated. Compared to Ti-5553 and Ti-1023, Ti-7333 possesses the slowest α to β dissolution rate, allowing a wider temperature window for processing. The rate of β grain growth decreases with the increase of soaking time and increases with the increase of solution temperature. The β grain growth exponents (n) are 0.30, 0.31, 0.32 and 0.33 for solution treatment temperature of 860°C, 910°C, 960°C and 1010°C, respectively. The activation energy (Qg) for β grain growth is 395.6kJ/mol. Water cooling or air cooling after solution treatment have no significant influence on microstructure, which offers large heat treatment cooling window. However, under furnace cooling, the fraction of α phase increases sharply. α phase maintains strictly the Burgers orientation relation with β phase ({0001}α//{110}β and 〈116120〉α//〈111〉β), except the αp particles formed during forging. The tensile strength decreases with the increase of the solution temperature when only solution treatment is applied, whereas the ductility increases gradually. When aging is applied subsequently, the tensile strength increases with the increase of the solution temperature and the ductility decreases gradually.
DOI:10.1016/j.matdes.2015.06.015      URL     [本文引用:]
[9] Guo S, Zhang J S, Cheng X N, et al.A metastable β-type Ti-Nb binary alloy with low modulus and highstrength[J]. J. Alloys Compd., 2015, (644): 411
With the assistance of thermo-mechanical treatment, an attempt was made to fabricate a simple binary Ti–38Nb (wt.%) alloy with low modulus and high strength. The martensitic transformation from β to α″ occurred in the solution-treated Ti–38Nb alloy, resulting in low yield strength. Upon a cold rolling plus annealing at 673K for 40min, a lot of dislocations and grain boundaries were introduced in the Ti–38Nb alloy, which played key roles in inhibiting the martensitic transformation and retarding the precipitation of ω. Therefore, overwhelming β phase with low content of β-stabilizers can survive at room temperature, giving rise to a low modulus of 56GPa. Meanwhile, the Ti–38Nb alloy was remarkably reinforced, with a yield strength of 850MPa and a ultimate tensile strength of 1020MPa, because of the inhibition of α″ martensitic transformation. As a result, the cold rolled plus annealed Ti–38Nb alloy can exhibit higher strength-to-modulus ratio than those of the current implant materials and thereby has high potential applications in hard tissue replacements.
DOI:10.1016/j.jallcom.2015.05.071      URL     [本文引用:]
[10] Li L, Li M Q, Luo J.Mechanism in the b phase evolution during hot deformationof Ti-5Al-2Sn-2Zr-4Mo-4Cr with a transformed microstructure[J]. Acta Mater., 2015, (94): 36
URL     [本文引用:1]
[11] Cho K, Niinomi M, Nakai M, et al.Improvement in mechanical strength of low-cost β-type Ti-Mn alloysfabricated by metal injection molding through cold rolling[J]. J. Alloys Compd., 2016, (644): 272
61β-type Ti–13Mn was fabricated by metal injection molding as a low-cost Ti alloy.61The strength of solutionized Ti–13Mn (88802MPa) is the same as that of Ti–6Al–4V ELI.61The strength of Ti–13Mn is increased to 208% (185202MPa) after 90% cold-rolling.61This increase is caused by changes in dislocation density, grain size, and porosity.61Moreover, the formation of a deformation-induced ω phase improves the strength.
DOI:10.1016/j.jallcom.2015.12.200      URL     [本文引用:1]
[12] Dai S J, Wang Y, Chen F, et al.Effects of cold deformation on microstructure and mechanical properties of Ti-35Nb-9Zr-6Mo-4Sn alloy for biomedical applications[J]. Mater. Sci. Eng., A. 2013, (575): 35
The effects of cold deformation on the microstructure, texture evolution and mechanical properties of Ti–35Nb–9Zr–6Mo–4Sn alloy during cold rolling were investigated. The results showed that the grains of Ti–35Nb–9Zr–6Mo–4Sn alloy were refined and no transformation of stress-induced martensite phase occurred after cold rolling. The dominant {111}〈110〉 and {001}〈110〉 textures formed after cold deformation and the intensities enhanced with the increase in reduction. The strength increased with the increase of cold reduction due to the effects of the increased dislocation density, microstrain and the grain refinement caused by cold deformation. The grain refinement, the increased dislocation density and the formation of 〈110〉 orientation textures resulted in the decrease of Young's modulus after cold rolling.
DOI:10.1016/j.msea.2013.03.032      URL     [本文引用:]
[13] Yi R W, Liu H Q, Yi D Q, et al.Precipitation hardening and microstructure evolution of the Ti-7Nb-10Mo alloy during aging[J]. Mater. Sci. Eng., C. 2016, (63): 577
61The ωathin quenched alloy transformed to ωisoor α at different temperatures.61ST alloy with ωath+β microstructure showed 80GPa modulus and 20% elongation.61The alloy was highly embrittled by the ωisoand α phases formed during aging.61The ST alloy showed better corrosion resistance than Ti-6Al-4V alloy.
DOI:10.1016/j.msec.2016.03.030      PMID:27040253      URL     [本文引用:]
[14] Li C L, Mi X J, Ye W J, et al.Microstructural evolution and age hardening behavior of a new metastable beta Ti-2Al-9.2Mo-2Fe alloy[J]. Mater. Sci. Eng., A. 2015, (645): 225
A study on microstructural evolution and age hardening behavior of a new metastable beta titanium alloy Ti–2Al–9.2Mo–2Fe was undertaken by microscopic observation, Vickers hardness and tensile tests in this research. The result showed that the athermal ω phase was formed in the beta matrix after solution treatment and followed by water quench, but it seemed that the athermal ω phase did not result in a considerable hardening (300HV in hardness and 670MPa in YS in ST condition). However, the isothermal ω phases with 10–40nm and nano-scaled α platelets with 30–100nm were observed in the alloy aged at lower temperatures (300–450°C). The nano-scaled ω and α phase led to an attractive hardening effect (400–500HV in hardness and above 1500MPa in YS). However, micro-scaled α phase with 0.5–3μm obtained in the samples aged at temperatures (500–600°C) showed a moderate hardening (350–450HV in hardness and 1100–1500MPa in YS). The hardening went through an under-aging, peak-aging and over-aging due to the continuation of nucleation and growth of the α phases and subsequent coarsening. The coarsen α phases (3–5μm) and grain boundary α layers (0.1–0.5μm in thickness) obtained at high temperatures aging (650–750°C) showed a poor hardening or even a softening (around 300HV in hardness and below 1000MPa in YS). The Ti–2Al–9.2Mo–2Fe alloy had a fast aging response, and can be age hardened to high hardness above 400HV within 30min at a broad aging temperature region. The age hardening curves can also provide a basic criterion for selecting aging treatment for a given stress level.
DOI:10.1016/j.msea.2015.08.028      URL     [本文引用:1]
[15] Xu T W, Zhang S S, Zhang F S, et al.Effect of ω-assistedprecipitationon β-α transformationandtensile properties of Ti-15Mo-2.7Nb-3Al-0.2Si alloy[J]. Mater. Sci. Eng., A. 2016, (654): 249
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[16] Song L, Xu X J, You L, et al.Ordered ω phase transformations in Ti-45Al-8.5Nb-0.2B alloy[J]. Intermetallics., 2015, (65): 22
61The βo phase in as-cast high Nb containing TiAl alloys is composed of numerous ordered ω particles.61The βo(ω) area decomposed into large B82-ωo grains and small γ particles after annealing at 85002°C.61The direct α2.→ωo transformation in the lamellar colonies is experimentally observed.61The βo phase is substituted by α2 after annealing at 125002°C through coarsening of the α laths.
DOI:10.1016/j.intermet.2015.05.009      URL     [本文引用:1]
[17] Liu H H, Niinomi M, Nakai M, et al.Athermal and deformation-induced ω-phase transformations in biomedical beta-type alloy Ti-9Cr-0.2O[J]. Acta Mater., 2016, (106): 162
The alloy Ti–9Cr–0.2O has been developed as a potential material for implant rods used in spinal fixation applications, since it exhibits good mechanical properties and a remarkably “changeable Young's modulus”, which is achieved by suppressing the athermal ω-phase formed upon quenching and enhancing the deformation-induced ω-phase transformation. In this study, athermal and deformation-induced ω-phase transformations in Ti–9Cr–0.2O were investigated systematically by transmission electron microscopy. This was done in order to understand the nature of these ω-phase transformations, as well as the specific functionality—the “changeable Young's modulus”—resulting from them. In solution-treated alloy samples, in addition to ideal ω-structures, structures considered as initial ω-structures associated with incommensurate ω-phase were observed. This might be attributed to the composition heterogeneity, heterogeneity of oxygen distribution, and/or the inhomogeneous distribution of defects such as vacancies and locally strained areas. Following cold rolling, some of the selected area electron diffraction patterns of the alloy showed that the reflections of one ω-variant had increased significantly in intensity while those of the other ω-variant had decreased sharply. This vanishing of one type of variant ω-structures is attributable to two possible mechanisms: (i) a reversal mechanism, under which the particular <111> partial dislocations transform the corresponding ω-variants back into β-phase or (ii) a re-orientation mechanism, according to which the ω-variants unfavorable with regard to the loading direction re-orient and turn into the preferred ω-variants.
DOI:10.1016/j.actamat.2016.01.008      URL     [本文引用:]
[18] Zain Y A, Kim H Y, Koyano T, et al.A comparative study on the effects of the ω and α phases on the temperaturedependence of shape memory behavior of a Ti-27Nb alloy[J]. Scr. Mater., 2015, (103): 37
While the stress for inducing the martensitic transformation in a Ti–27Nb alloy changed only slightly on aging, the stress at which the martensite phase starts to plastically deform and the ultimate tensile strength increased on aging due to the formation of thermal ω phase or α phase. An anomalous temperature dependence of the shape memory behavior was also observed and explained by the fact that the thermal ω phase encouraged the formation of the athermal ω phase during cooling.
DOI:10.1016/j.scriptamat.2015.02.032      URL     [本文引用:1]
[19] Ng H P, Douguet E, Bettles C J, et al.Age-hardening behaviour of two metastable beta-titanium alloys[J]. Mater. Sci. Eng., A. 2010, (527): 701
Two novel β-titanium alloys, of compositions Ti–10V–2Ni and Ti–10V–6Cu (wt.%) and with distinct potential for athermal ω-phase formation, have been selected and the sensitivity of the age-hardening responses to the cooling rate post solution treatment investigated. Upon rapid water quenching, the solution-treated Ti–10V–2Ni alloy contained a significantly higher volume fraction of ω-phase than the Ti–10V–6Cu alloy. It was observed that the Ti–10V–6Cu alloy is remarkably insensitive to cooling rates in the range of water quenching to air cooling, in terms of both microstructural evolution and age-hardening behaviour, when aged isothermally at 350 °C and 500 °C. The microstructure is thermally stable and the hardness remains approximately constant for extended aging (48 h) at both temperatures, implying that it may have potential as a deep-hardenable alloy. In comparison, the aging response of the Ti–10V–2Ni alloy was marginally influenced by cooling rate when aged at 500 °C, the effect being ascribed to autocatalytic formation of α-phase in the course of slow air cooling. The results are discussed with respect to the β-phase stability and the time-temperature-transformation (TTT) characteristics of metastable beta-titanium alloys.
DOI:10.1016/j.msea.2010.07.055      URL     [本文引用:1]
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关键词(key words)
金属材料
固溶时效处理
拉伸性能
断口

metal materials
solution and aging treatm...
tensile properties
tensile fracture

作者
王雪萌
张思倩
袁子尧
陈立佳

WANG Xuemeng
ZHANG Siqian
YUAN Ziyao
CHEN Lijia