β单相区固溶温度对Ti-55531合金组织及性能的影响

研究了β单相区不同的固溶温度对Ti-55531合金片层组织参数及力学性能的影响规律。结果表明,经不同温度固溶处理,再经相同的时效处理后,合金的β晶粒尺寸随固溶温度的改变而改变,进而影响时效析出α片的含量及尺寸,最终导致合金力学性能的差异。当固溶温度在830～900℃之间时,随着固溶温度的升高,原始β晶粒尺寸增大,后续时效析出的α片长、宽及长宽比均先增大后减小,合金强度直线下降,塑性先降低后增加。固溶温度为860℃时,合金对应的强度塑性匹配最好。合金的断裂失效机制为以微孔聚集为主,沿晶开裂和穿晶断裂并存的混合断裂机制。     

β21s钛合金棒材热处理研究

研究固溶时效热处理对β21s钛合金棒材显微组织和力学性能的影响。结果表明：在固溶温度一定时,随着时效温度的升高（从540,550到560℃）,合金的强度下降,而塑性则有所上升;在时效温度一定时,随着固溶温度的升高（从750,770,790到800℃）,合金强度先有所升高（在790℃时达到峰值）,而后又有所降低,而塑性则逐步降低。     

固溶温度对TB10钛合金力学性能的影响

研究了固溶温度对TB10(Ti-5Mo-5V-2Cr-3Al)钛合金在热处理过程中力学性能变化规律。结果表明:当TB10钛合金在740～840℃之间固溶,并在520℃时效时,在相变点以下固溶,随着固溶温度的升高,材料的强度降低、塑性升高,但在800℃时网状晶界α相恶化了材料的塑性,在相变点以上固溶时,材料的强度和塑性随着固溶温度的升高而下降;时效后的TB10钛合金随着固溶温度升高,材料的强度升高、塑性下降;当固溶温度为740℃时,时效态的材料强度和塑性比固溶态略有升高,固溶温度越高,时效强化效果越显著,时效态材料的塑性比固溶态的低,但在800℃时由于晶界α相的网状被破坏使时效态材料的塑性比固溶态的高。     

热处理对新型β钛合金组织与性能的影响

合理的热处理制度能显著影响β钛合金的显微组织和强化行为。通过对一种新型Ti-Al-V-Mo-Cr-Zr-Nb-Fe亚稳β钛合金的固溶时效处理,研究了热处理工艺对该合金组织与力学性能的影响。结果表明:该合金720℃固溶处理后,可以获得单一均匀的等轴β晶粒,为最佳固溶温度;经440~520℃时效处理后,发现时效温度对该新型合金α相析出的形态与尺寸的影响显著:在较低温度440℃时效时β基体上有针状α相析出,平均晶粒尺寸在1~2μm左右;较高温度520℃时效时,α相宽度和片层间距都增大,α相尺寸长大到3~5μm,针状α相向短棒状转化。在实验温度范围内,随着时效温度升高,合金强度降低,塑性增加。720℃固溶较低温度时效合金可获得较好的强度与韧性匹配。该合金理想的热处理工艺参数为720℃/30 min、空冷(AC)+440℃/12 h、空冷(AC),由此可获得到良好的综合性能(抗拉强度UTS=1412.8 MPa,屈服强度YS=1309.4 MPa,延伸率A=8.56%,断面收缩率Z=44.94%)。     

固溶时效对Ti-6Cr-5V-5Mo-4Al-1Nb合金组织和力学性能的影响

研究了固溶温度、时效温度、时效时间对Ti-6Cr-5V-5Mo-4Al-1Nb(Ti-65541)合金显微组织与力学性能的影响。结果表明,在β相变点以上固溶并时效后,合金中析出细小的次生α相,初生α相完全消失;在较低温度固溶并时效后,次生α相和初生α相同时存在。时效温度对合金强度和塑性的影响最为显著,固溶温度次之,时效时间的影响最弱。随着时效温度的升高,合金的抗拉强度和屈服强度降低,塑性提高。随着固溶温度的提高,合金的强度提高,塑性降低。随着时效时间的延长,合金强度和塑性总体呈降低趋势。在740～760℃范围内固溶处理,在540～580℃范围内时效且时效时间在4～6 h内,可获得综合性能优异的Ti-65541合金。     

紧固件用TB2钛合金棒材热处理工艺研究

研究了热处理工艺对原始组织为粗大β晶粒+少量细小α晶粒的紧固件用TB2钛合金棒材组织与力学性能的影响。结果表明：随着固溶温度的升高,棒材组织中α相含量逐渐减少,β晶粒尺寸明显增大,经780℃固溶后强度和塑性匹配最好;固溶+时效处理时,随着时效温度的升高,棒材组织中析出的次生α相体积分数先增加后减少,且棒材强度先升高后降低;经固溶+预拉伸变形+时效处理后,棒材组织中晶粒有一定细化,次生片状α相含量增多,抗拉强度较固溶后直接时效提高了近10%。     

热处理对新型医用Ti-3Zr-2Sn-3Mo-25Nb钛合金显微组织与力学性能的影响

分析了不同热处理制度对新型医用Ti-3Zr-2Sn-3Mo-25Nb合金显微组织与力学性能的影响。结果表明:经(α+β)两相区固溶处理后,合金主要由初生α相和β相组成;初生α相尺寸短小,以颗粒状形式出现在晶粒内部和晶界附近。在实验参数范围内,升高固溶温度导致初生α相的体积分数降低,β相的体积分数增加。经β单相区固溶处理后,合金主要由单一β相组成;随着固溶温度升高,β相晶粒逐渐长大,晶粒尺寸约为25μm。经(α+β)两相区固溶+时效处理后,在β相基体上析出针状α相,尺寸细小,呈交叉排列。经β单相区固溶+时效处理后,在β相晶界附近和晶粒内部析出尺寸大小不一的细针状α相;晶界附近析出的α相具有一定的取向,晶粒内部析出的α相横纵交错,大小不均匀。经固溶处理后,合金获得中等水平的强度和较好的塑性;随着固溶温度的升高,合金的抗拉强度和屈服强度逐渐减小,塑性和弹性模量逐渐增加。经(α+β)两相区固溶处理后,合金的抗拉强度与屈服强度之差(Rm-Rp0.2)较β单相区固溶处理后的大。经(α+β)两相区固溶+时效处理后的合金强度增量高于β单相区固溶+时效处理后的合金强度增量。在相同时效条件下,随着固溶温度升高,合金的强度和模量逐渐减小,延伸率基本保持不变。经780℃/30min固溶空冷+480℃/6h时效空冷处理后合金的弹性模量达最小值(E=64GPa),最接近于人体自然骨骼等硬组织的弹性模量。     

热处理工艺对近α钛合金组织与性能的影响

研究了固溶和时效工艺参数对一种近α钛合金Ti-5Al-2Nb-2Zr-1.5Mo显微组织与力学性能的影响.结果表明:在α+β两相区,随着固溶温度的升高,初生α相含量(体积分数,下同)减少,亚稳相含量增加,冷却过程中分解的针状马氏体α'相使合金的硬度升高;时效温度的变化通过影响α'相分解析出次生α相的尺寸、数量及分布方式影响合金的力学性能,合金经960℃固溶,500℃时效4 h后维氏硬度达到峰值374,屈服强度和抗拉强度分别达到874,982 MPa.     

热处理对VST55531钛合金的组织和拉伸性能的影响

研究了固溶温度和时效温度对VST55531合金组织和拉伸性能的影响。结果表明：在本实验范围内,合金强度分别随固溶温度的提高和时效温度的提高而呈降低趋势,整体塑性较差,延伸率和断面收缩率分别在7．5％和17．0％以下。时效温度为550℃时,合金的极限抗拉强度都在1500MPa以上,尤其经830℃,1h固溶空冷并经550℃,8h时效的合金,其抗拉强度可达到1555MPa。在两相区固溶空冷的合金组织主要为8转变组织和初生。相,初生a相形貌为片状的。束域,时效时析出的次生a相有效地提高了合金的强度。     

Ti-1300F高强钛合金丝材热处理工艺优化

采用正交试验方法,研究了固溶温度、时效温度和时效时间对?6.5 mm Ti-1300F合金丝材室温拉伸性能和显微组织的影响。结果表明:经α+β两相区固溶+时效处理后,合金的显微组织由细小等轴初生α相、弥散针状次生α相和β基体组成。时效温度对合金强度和塑性的影响最为显著,固溶温度次之,时效时间的影响最小。根据试验结果对热处理工艺进行了优化,经(760～790)℃/1 h, WQ+(500～540)℃/4 h, AC处理后,Ti-1300F合金丝材获得强度和塑性的良好匹配。     

热处理制度对Ti-55531合金组织和力学性能的影响

通过采用不同的热处理制度研究了时效温度和β退火温度对Ti-55531合金显微组织和力学性能的影响。结果表明:Ti-55531合金固溶加时效处理后可获得初生α相呈长条或等轴状的组织,β基体上大量析出的次生α相使其获得较高的强度,且强度随时效温度升高而显著降低,延伸率变化不明显,断面收缩率在620℃以上随着时效温度升高有所增加,但该组织状态断裂韧度偏低;β退火后可获得均匀的片状组织,具有较高的断裂韧性,抗拉强度在600~650℃之间随退火温度升高呈线性关系降低,可根据需要很方便地调整强度级别,塑性随退火温度升高变化不太明显。     

Microstructure and Mechanical Properties of a High-Strength Ti-4Al-2Fe-3Cu Alloy Fabricated by Sintering and Hot Extrusion

A Ti-4Al-2Fe-3Cu (wt pct) alloy containing only low-cost alloying elements was fabricated by vacuum sintering a blend of TiH₂, Al, Fe, and Cu powders at 1200 °C for 1 hour followed by hot extrusion at the same temperature. The as-extruded alloy exhibited a microstructure consisting of mainly α/β lamellar colonies and Ti₂Cu as a minor phase. The average colony size and lamella thickness were 118 and 12 µm, respectively, and Fe and Cu were predominantly distributed in the β lamellae. The as-extruded alloy had a high tensile yield strength (YS) and ultimate tensile strength (UTS) of 1248 and 1270 MPa, respectively, but a limited ductility (elongation to fracture: 2.3 pct). Annealing at 750 °C for 4 hour caused the average colony size and lamella thickness of the alloy to increase to 145 and 17 µm, respectively, and the volume fraction of the β phase decreased with the annealing. These microstructural changes resulted in a slight decrease of the YS and UTS to 1221 and 1253 MPa, but a clear increase of the ductility with the elongation to fracture reaching 4 pct. This work demonstrates that a combination of relatively low-temperature vacuum sintering, hot extrusion, and annealing can be effectively utilized to fabricate a low-cost Ti-4Al-2Fe-3Cu alloy with high strength and appreciable tensile ductility.

     

Effect of Pre- and Post-weld Heat Treatments on Linear Friction Welded Ti-5553

Linear friction welding allows solid-state joining of near-beta (β) titanium alloy Ti-5553 (Ti-5Al-5V-5Mo-3Cr). In the as-welded condition, the weld zone (WZ) exhibits β grain refinement and marked softening as compared with Ti-5553 in the solution heat treated and aged condition. The softening of the weldment is attributed to the depletion of the strengthening alpha (α) phase in the WZ and the adjacent thermo-mechanically affected zone (TMAZ). Specifically, in near-β titanium alloys, the strength of the material mainly depends on the shape, size, distribution, and fraction of the primary α and other decomposition products of the β phase. Hence, a combination of pre- and post-weld heat treatments were applied to determine the conditions that allow mitigating the α phase depletion in the WZ and TMAZ of the welds. The mechanical response of the welded samples to the heat treatments was determined by performing microhardness measurements and tensile testing at room temperature with an automated 3D deformation measurement system. It was found that though the joint efficiency in the as-welded condition was high (96 pct), strain localization and failure occurred in the TMAZ. The application of post-weld solution heat treatment with aging was effective in restoring α, increasing the joint efficiency (97 to 99 pct) and inducing strain localization and failure in the parent material region.     

Microstructure and Texture Evolution During Sub-Transus Thermomechanical Processing of Ti-6Al-4V-0.1B Alloy: Part I. Hot Rolling in (α + β) Phase Field

In the current study, the evolution of microstructure and texture has been studied for Ti-6Al-4V-0.1B alloy during sub-transus thermomechanical processing. This part of the work deals with the deformation response of the alloy by rolling in the (α + β) phase field. The (α + β) annealing behavior of the rolled specimen is communicated in part II. Rolled microstructures of the alloys exhibit either kinked or straight α colonies depending on their orientations with respect to the principal rolling directions. The Ti-6Al-4V-0.1B alloy shows an improved rolling response compared with the alloy Ti-6Al-4V because of smaller α lamellae size, coherency of α/β interfaces, and multiple slip due to orientation factors. Accelerated dynamic globularization for this alloy is similarly caused by the intralamellar transverse boundary formation via multiple slip and strain accumulation at TiB particles. The (0002)_α pole figures of rolled Ti-6Al-4V alloy shows “TD splitting” at lower rolling temperatures because of strong initial texture. Substantial β phase mitigates the effect of starting texture at higher temperature so that “RD splitting” characterizes the basal pole figure. Weak starting texture and easy slip transfer for Ti-6Al-4V-0.1B alloy produce simultaneous TD and RD splittings in basal pole figures at all rolling temperatures.     

Effect of α phase on fatigue crack growth of Ti-6242 alloy

Fatigue crack growth as a function ofαphase volume fraction in Ti-6Al-2Sn-4Zr-2Mo(Ti-6242)alloy was investigated using fatigue testing,optical microscopy,scanning electron microscopy,and transmission electron microscopy.Theα+βannealing treatments with different solid solution temperatures and cooling rates were conducted in order to tailor microstructure with differentαphase features in the Ti-6242 alloy,and fatigue crack growth mechanism was discussed after detailed microstructure characterization.The results showed that fatigue crack growth rate of Ti-6242 alloy decreased with the decrease in volume fraction of the primaryαphase(αp).Samples with a large-sizedαgrain microstructure treated at high solid solution temperature and slow cooling rate have lower fatigue crack growth rate.The appearance of secondaryαphase(αs)with the increase of solid solution temperature led to crack deflection.Moreover,a fatigue crack growth transition phenomenon was observed in the Paris regime of Ti-6242 alloy with 29.8% αp(typical bi-modal microstructure)and large-sizedαgrain microstructure,owing to the change of fatigue crack growth mechanism.     

热处理对Ti-5Mo-5V-2Cr-3Al合金组织和性能影响

通过扫描电镜(SEM), 光学显微镜(OM)和X射线衍射分析(XRD)等对Ti-5Mo-5V-2Cr-3Al(Ti5523)合金棒材分别经固溶和固溶时效处理后得到的微观组织, 相含量等进行分析, 结合性能数据, 分析了微观结构对性能的影响. 研究发现, 在720 ℃固溶0.5 h, 并在540 ℃时效6 h后, 获得了一种规则的垂直有序排列的亚结构, 这种网篮状亚结构起到了组织细化的作用, 从而使得断面收缩率非常高. 研究还发现, 两相区固溶后析出相不仅使得强度提高, 对材料的塑性也有贡献.     

Effect of Phase Contiguity and Morphology on the Evolution of Deformation Texture in Two-Phase Alloys

Deformation texture evolution in two-phase xFe-yNi-(100-x-y)Cr model alloys and Ti-13Nb-13Zr alloy was studied during rolling to develop an understanding of micro-mechanisms of deformation in industrially relevant two-phase FCC-BCC steels and HCP-BCC titanium alloys, respectively. It was found that volume fraction and contiguity of phases lead to systematic changes in texture, while morphology affects the strength of texture. There was a characteristic change in texture from typical Brass-type to a weaker Copper-type texture in the austenite phase accompanied with a change from alpha fiber to gamma fiber in ferrite phase for Fe-Ni-Cr alloys with increase in fraction of harder ferrite phase. However, similar characteristic texture evolution was noted in both α and β phase irrespective of the different initial morphologies in Ti-13Nb-13Zr alloy. Viscoplastic self-consistent simulations with two-phase scheme were able to qualitatively predict texture evolution in individual phases. It is proposed that the transition from iso-strain-type behavior for equiaxed microstructure at low strain to iso-stress-type behavior at higher strain is aided by the presence of higher volume fraction of the second phase and increasing aspect ratio of individual phases in two-phase alloys.

     

Al-3B变质对铸造Al-10Si合金显微组织与力学性能的影响

采用Al-3B对铸造Al-10Si合金进行了变质处理,运用非平衡相图和杠杆定律分析了变质处理Al-10Si合金显微组织变化规律,研究了变质处理对合金力学性能的影响。研究表明,Al-3B变质处理使铸造Al-10Si合金的凝固过冷度减小;当变质温度一定时,随着Al-3B加入量增加,铸造Al-10Si合金组织中初生α-Al相体积分数呈现先增加后减少的变化趋势,当加入0.25%Al-3B时,初生α-Al相的体积分数达到最大值42.8%,且合金中的Si相呈细小短片状或短棒状;当变质剂加入量一定时,随着变质温度的增加,初生α-Al相的体积分数逐渐增加。Al-3B变质剂加入量对Al-10Si合金抗拉强度和延伸率影响的规律与其对合金组织中初生α-Al相体积分数影响的规律完全一致,初生α-Al相体积分数和共晶Si尺寸和形态对合金综合力学性能起到决定性的作用。在740℃采用0.25%Al-3B变质处理后,铸造Al-10Si合金的抗拉强度和延伸率分别提高了11.7%和86.3%,该合金综合力学性能得到大幅提高。     

Effects of microstructure on the short fatigue crack initiation and propagation characteristics of biomedical α/β titanium alloys

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical α/β titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary α grain boundaries in an equiaxed α structure, whereas, in a Widmanst?tten α structure, initiation occurs within the α colonies and prior β grains, where α plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed α structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 μm), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanst?tten α structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an α structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of α colonies or prior β grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanst?tten α structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical α/β titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed α structure is changed by the volume fraction of primary α phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary α phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.