Ti-48Al合金全片层组织的稳定性

研究了Ti 48Al合金全片层组织在 115 0℃时效时的组织稳定性。结果表明 ,140 0℃ ,1h固溶处理后炉冷获得的粗片层组织的稳定性远远优于空冷获得的细片层组织。细片层组织首先在晶界处发生不连续粗化 ,而后粗化片层组织中的α2 相发生溶解并球化 ,从而转变为近γ组织 ;空冷组织中魏氏片层的存在降低了片层组织的稳定性 ,魏氏片层 /基体界面与晶界一同成为片层组织发生分解的起始部位。Ti 48Al 0 .8%B(摩尔分数 )合金的细片层组织因晶界TiB2 相的存在有效抑制了晶界不连续粗化 ,但γ相从TiB2 /基体界面和晶界重新形核生长使片层组织转变为均匀的细晶近γ组织     

Ti—48Al合金片层组织的稳定性

研究了Ｔｉ－４８Ａｌ合金全片层组织在１１５０℃时效时的组织稳定性,结果表明,１４００℃,１ｈ固溶处理后炉冷获得的粗片层组织的稳定性远远优于空冷获得的细片层组织,细片层组织首先在晶界处发生不连续粗化,而后粗化片层组织中的ａ２相发生溶解并球化,从而转变为近γ组织;空冷组织中魏氏片层的存在降低了片层组织的稳定性,魏氏片层／基体界面与晶界一同成为片层组织发生分解的起始部位。Ｔｉ－４８Ａｌ＋０．８％Ｂ（摩尔     

Ti-45Al-5Nb(-0.3Y)合金的连续冷却相变规律

冷却速度对Ti-45Al-5Nb和Ti-45Al-5Nb-0.3Y合金连续冷却相变有较大的影响.炉冷形成全层片组织,空冷下层片形成被α→γm块状反应抑制,油冷形成了极细小的层片组织,水冷主要发生了α→α2有序化转变.空冷导致了羽毛状组织消失和α2相的增加,水冷导致α2晶界的细小层片晶团尺寸较小、数量较多.Y添加对Ti-45Al-5Nb合金连续冷却相变有较小的影响.     

热处理对快速凝固Ti-48Al-4Cr合金组织影响规律研究

采用单辊旋淬快速凝固设备制备了Ti-48Al-4Cr(at.%)薄带,并在真空封装后进行热处理实验,研究热处理对快速凝固Ti-48Al-4Cr合金的组织演变规律。结果表明,快速凝固Ti-48Al-4Cr合金凝固基体为等轴的γ相,基体中含有少量的B2相、α2相颗粒和片层组织;经723℃保温1h空冷后,亚稳的α2相颗粒失稳,但片层组织仍然比较稳定;热处理温度升高至932℃时,片层组织中的杆状α2相开始按照瑞利分解失稳,分解成大量短杆状或者颗粒状的α2相;在995℃保温1h以后,基体中已较难发现片层组织存在,但存在着排列方向相同的棒状α2相,同时在等轴γ相晶界处发现了数量较多的颗粒状B2相。本论文的研究为进一步研究快速凝固条件下的TiAl金属间化合物的组织与相转变提供了基础,丰富了快速凝固理论。     

TiAl 合金中的γ→α析出转变行为

研究了Ti-48Al和Ti-48Al—0．8B合金中的γ→α析出转变，分析了α析出相的生长形态、晶体学特征、生长动力学及α／γ相界面结构。结果显示α相从γ相中析出有两种方式：一种是从γ晶粒内沿{111}，晶面以片状形貌析出，且α相与γ基体保持共格位向关系；另一种则是在γ晶界上通过不连续析出转变，以不规则的块状形貌析出，并向着晶界一侧与之无位向关系的γ晶粒内生长。添加0．8％(摩尔分数)B能显著降低γ晶粒内片状α析出相的形核率，并抑制α相生长。HREM分析表明：片状α相是在γ相的堆垛层错上形核，并通过“台阶-凸起-扭折”机制生长；α／γ相界面上复杂层错的存在及台阶形核率低是片状α相沿厚度方向生长缓慢的主要原因。     

微纳米颗粒增强TiAl基复合材料组织与室温力学性能

原位自生颗粒增强金属基复合材料是提高金属材料强韧性的有效途径,采用放电等离子烧结技术(SPS),以氧化石墨烯、碳化硅、氮化硼为增强体,原位自生制备TiAl基复合材料,研究第二相对TiAl基复合材料显微组织演变及室温性能的影响。结果表明,增强体的改变直接影响了TiAl基复合材料第二相形貌和分布。添加石墨烯在TiAl合金α₂和γ片层界面处弥散析出微纳米尺度第二相Ti₂AlC;添加碳化硅在基体中分别生成微米级晶界相Ti₅Si₃,微纳米片层间相Ti₂AlC;添加氮化硼未能在TiAl合金α₂和γ片层界面处析出微纳米第二相,而是纳米级TiB₂和Ti₂AlN相析出在晶界处与基体形成连续核壳结构;复合添加石墨烯和氮化硼既能在片层间原位析出Ti₂AlC相,又能在晶界处形成核壳结构。TiAl基复合材料的室温压缩性能和摩擦磨损性能均得到有效提高,复合添加石墨烯和氮化硼可获得优异的室温力学性能。TiAl基复合材料的...     

Al含量微调对TiAl合金组织及压缩力学性能的影响

研究了Al含量对TiAl合金微观组织及压缩力学性能的影响,并分析其破坏机理。研究发现,Al含量对TiAl合金微观组织影响显著。通过真空自耗电弧冶炼方法制备的Ti-44.1Al(原子分数,%)合金的组织为全层片组织,层片团粗大,呈现柱状晶特征;而Ti-47.3Al合金的组织为双态组织,三维连通的网状γ相将粗大的铸造组织分割成细小的层片团。力学性能研究发现,与Ti-47.3Al合金相比,无论是在准静态还是动态压缩加载条件下,Ti-44.1Al合金都表现出较高的屈服强度,较低的抗压强度以及较差的塑性变形能力。破坏机理分析表明,准静态压缩加载条件下,在Ti-44.1Al合金中,微孔在γ/α_2层片团的α_2相中萌生并聚集形成裂纹;而在Ti-47.3Al合金中,微孔同时在γ/α2层片团中α_2相中以及三维连通的网状γ相中萌生,微孔聚集形成裂纹并扩展;动态压缩加载条件下,在Ti-44.1Al合金中,在γ/α_2层片团中存在大量的α_2相与γ相的相界,由于加载时间短,在相界处易引起位错塞积而导致应力集中,致使微裂纹在相界处迅速萌生并扩展;而在Ti-47.3Al合金中,微裂纹不仅在γ/α_2层片团中α_2相与γ相的相界处萌生,同时也会在三维连通的网状γ相中迅速萌生并扩展,直至材料破坏。     

高压凝固Ti-48Al合金片层组织失稳机理

采用高压凝固设备制备了Ti-48Al(原子分数,%)合金,并在真空封装后进行热处理试验,研究热处理工艺对高压凝固Ti-48Al合金的片层组织失稳机理。结果表明,在低于共析转变温度进行热处理时,常压凝固Ti-48Al合金组织中较难发现α_2相颗粒,而高压凝固片层组织界面处已开始析出α_2相颗粒。加热至1280℃进一步确定,相比于常压凝固,高压凝固Ti-48Al合金更易分解,且片层组织中γ相优先分解,长杆状的残余α_2相经历球化长大,这为进一步研究高压下组织与相转变提供了基础,丰富了高压凝固理论。     

β相稳定元素对TiAl合金高温变形行为的影响

利用Gleeble-1500D热模拟试验机研究了Ti-44Al、Ti-44Al-6Nb和Ti-44Al-6Nb-1Cr-2V合金在1 100～1 250℃和0. 01 s-1条件下的热变形行为。研究结果表明,添加β相稳定元素可降低TiAl合金的流变应力,在相同变形条件下Ti-44Al-6Nb-1Cr-2V合金具有更低的峰值应力。高温变形时,TiAl合金主要发生片层弯曲和拉长协调变形,以及片层团晶界处动态再结晶和B2相协调变形。动态再结晶程度随着变形温度的升高以及β相稳定元素含量的提高而增加,B2相协调变形和促进动态再结晶是TiAl合金的主要软化方式。添加β相稳定元素和控制B2相含量能有效改善TiAl合金热加工性能。     

碳对粉末冶金Ti-46.6Al-1.4Mn-2Mo的组织细化和力学性能改善

主要由基体γ相和少量α2相组成的TiAl金属间化合物，具有优良的比强度、抗氧化性和抗蠕变性，被用作轻型高温结构材料，主要用于汽车业和航空领域。人们已开发出多种技术，改善和提高TiAl合金的力学性能。比如，通过控制片层团及片层的尺寸和形貌，设计片层间距来优化TiAl合金全片层组织(α2及γ板条交替排列)，达到提高TiAl合金的断裂韧性的目的。在TiAl合金中，间隙原子由于可改变片层间距，从而影响合金性能，而受到广泛关注。韩国学者通过添加碳而细化Ti-46.6Al-1.4Mn-2Mo合金组织来获得最佳力学性能。实验合金名义成分为Ti-46.6…     

Microstructure and fracture toughness of a β phase containing TiAl alloy

Microstructures and fracture toughness of Ti-45Al-2Nb-1.5V-1Mo-0.3Y alloy have been investigated. The alloy exhibits fine nearly lamellar microstructures, consisting mainly of fine lamellar grains, together with mixtures of γ and residual β phases along lamellar colony boundaries. Precipitation of both β and γ phases from α₂ lamellae was found after aging at 950 °C for 48 h. Phase transformations involving β phase both in α₂ laths and along colony boundaries are discussed. This TiAl alloy possesses a higher K_IC value up to 23.5 MPam^1/2 at room temperature, compared with fully lamellar Ti-45Al-5Nb-0.3Y alloy. The toughening mechanism for current alloy is concluded as trans-lamellar fracture, ligament bridges and crack deflection, together with precipitation of β and γ phases. The precipitation of fine β and γ particles is considered as intrinsic toughening mechanism, because α₂/β and α₂/γ interfaces generating due to precipitation can restrict dislocation motion effectively.     

热处理对吸铸TiAl基合金铸件组织的影响

利用金属型底浇式真空吸铸技术和添加合金元素获得TiAl基合金薄板,观察并研究薄板热处理前后的组织。结果表明,在金属型强制冷却和添加合金元素的共同作用下,利用金属型底浇式真空吸铸技术获得的TiAl基合金薄板件铸态组织细小、致密。其中Ti-47Al-5Nb-0.5Si和Ti-47Al-2W-0.5Si合金的铸态平均晶粒多在20μm左右,但是,铸态组织的片层组织不均匀,且在组织内部存在较大的偏析。将试样加热到1300℃,保温5 h,随炉冷却热处理后,TiAl基合金薄板的铸态组织有所长大,但是片层更加平整稳定,偏析也明显得到改善。     

Ti—48Al合金片层组织的连续粗化机制

研究了 115 0℃时效时Ti 48Al合金全片层组织的连续粗化机制。片层组织的连续粗化不仅能通过片层界面缺陷 (如台阶、端部、弯曲的界面等 )迁移来实现 ,而且可以通过γ/γ片层界面迁移或分解的方式来实现 ;在真孪晶、伪孪晶和 12 0°旋转有序型γ/γ界面当中 ,12 0°旋转有序界面的稳定性最低 ,最容易迁移或分解 ;γ片层内的 12 0°旋转有序畴界与片层界面的交汇处易形成热沟(thermalgroove) ,它往往成为片层界面发生分解的起始部位     

Effect of thermal stresses on the microstructure of the continuous cooling TiAl alloys

A series of continuous cooling tests were performed on TiAl alloys using a Gleeble3500 machine to investigate the effect of thermal stresses on the microstructure. The results show that macroscopic thermal stresses promote correlated nucleation of γ lamellae. The trend of the dominance of one twin-group γ variants in local regions is weakened, and the γ/γ interfaces tend to be true twin and pseudotwin boundaries rather than 120° rotational faults under macroscopic thermal stresses. Meanwhile, thermal-induced deformation generated under the effect of both microscopic and macroscopic thermal stresses results in numerous low angle grain boundaries (LAGBs) and dislocations. The LAGBs and dislocations distribute heterogeneously among lamellar colonies and phases. No mechanical twins are observed due to the low strain and low strain rate characteristics of the thermal-induced deformation. These findings could shed light on understanding and preventing the cracking of TiAl components during cooling process.     

Obtaining of a fine near-lamellar microstructure in TiAl alloys by Spark Plasma Sintering

This work presents a study of the Spark Plasma Sintering of a boron and tungsten containing alloy (Ti_49,92Al₄₈W₂B_0,08, called IRIS) as a function of the sintering temperature. Microstructures of sintered alloys are analyzed by scanning and transmission electron microscopies. Investigations mainly focus on a fine near-lamellar microstructure. Attention is paid to both characteristic dimensions of this microstructure and orientation relationships between various phases.The fine near-lamellar microstructure is formed by lamellar grains surrounded by extended γ zones containing β₀ precipitates. The size of lamellar grains ranges from 35 to 45 μm while the width of the borders remains between 5 and 10 μm. Effects of both boron addition and sintering temperature are studied. Orientation relationships between γ lamellae and γ grains in the borders, as well as between the γ matrix and the β₀ precipitates are investigated. As a result of these investigations, a formation mechanism of this microstructure is proposed and discussed. The origin of the grain growth limitation during the SPS processing is particularly analyzed.     

不同SPS烧结温度Ti-45Al-5.5(Cr,Nb,B,Ta)合金的显微组织及力学性能(英文)

采用高能球磨和放电等离子烧结(SPS)技术,制备成分为Ti-45Al-5.5(Cr,Nb,B,Ta)的TiAl合金块体,随后对TiAl合金进行热处理。研究在不同SPS烧结温度下制备的TiAl合金经过热处理后的显微组织和力学性能。结果表明:高能球磨后的合金粉末形状不规则,粉末颗粒尺寸大约为几十微米。XRD分析表明,机械球磨后的粉末由TiAl和Ti3Al两相组成;烧结后的Ti-45Al-5.5(Cr,Nb,B,Ta)合金块体主要是TiAl相,以及少量的Ti3Al和TiB2相。当烧结温度为900°C和1000°C时,合金的显微组织为双相结构,并伴随有一些细小的等轴γ晶粒和细小的针状TiB2相。当烧结温度从900°C上升到1000°C时,Ti-45Al-5.5(Cr,Nb,B,Ta)合金的显微硬度变化不大,抗压强度从1812MPa提高到2275MPa,压缩率从22.66%增加到25.59%,合金的断裂方式为穿晶断裂。     

烧结温度对快速凝固TiAl合金组织及力学性能的影响(英文)

将快速凝固Ti-46Al-2Cr-4Nb-0.3Y(摩尔分数,%)合金薄带破碎成粉末,然后通过等离子烧结(SPS)制备成块体。研究烧结温度对烧结块体的组织和力学性能的影响。在1200℃烧结时得到完全致密的块体。进一步升高烧结温度对合金密度的影响不大,但是对烧结块体的显微组织及相结构有显著影响。烧结块体主要由γ和α2相组成,随着烧结温度的升高,α2相的体积分数降低,块体合金由近γ组织演变为近层片组织,且组织逐渐粗化,但是长大不明显。1260℃烧结得到的块体组织细小、均匀,没有明显微偏析,具有良好的综合力学性能,室温压缩断裂强度和压缩率分别为2984MPa和41.5%,高温(800℃)拉伸断裂强度和伸长率分别为527.5MPa和5.9%。     

Microstructure and mechanical properties of duplex tiAl alloys containing Mn

A study has been made of the effect of Mn on the structure and compressive mechanical properties of duplex TiAl alloys. In order to clarify the separate effect of Mn and Al contents, the effect of Ti/Al ratio was also studied by analyzing the behavior of alloys with different Ti/Al ratios at the fixed Mn content. Addition of 3 at.% Mn to the binary Ti₅₃Al₄₇ alloy decreases the tetragonality and the unit cell volume of γ structure. It also promotes the formation of twin-related structure, the refinement of interlamellar spacing and grain size, and the increase in the volume fraction of lamellar grains. Increase in Ti/Al ratio at the fixed Mn content results in the further decrease in the tetragonality and unit cell volume and the refinement of grain size, while it decreases the volume fraction of lamellar grains. The modification of microstructure directly influences the compressive properties and deformation mode of duplex TiAl alloys. It has been found that Mn addition and increase in Ti/Al ratio enhance the plasticity of duplex alloys. Generation of mobile dislocations at the twin intersections has been found to occur in Mn containing alloys. Such dislocation generation at the twin intersections as well as the promotion of deformation twins in Mn containing alloys are all beneficial for improving the ductility of duplex TiAl alloys.     

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α₂ lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).     

全片层高Nb含量TiAl合金长期热循环后的显微组织不稳定性

对全片层Ti-45Al-8.5Nb-（W,B,Y）合金在900℃下进行长期热循环（500次和1000次）实验,采用扫描电镜（SEM）及透射电镜（TEM）研究该合金长期热循环后的显微组织不稳定性。结果表明：合金经热循环后主要产生两种类型的组织不稳定性：1）长期热循环特别是1000次热循环后,在Al偏析处易产生因晶界迁移引起的不连续粗化,随着循环次数的增加,元素扩散致使Al偏析逐渐减少;2）1000次热循环后,α2片层变细且发生断裂,这是由α2→γ相变导致的α2片层溶解所致。同时,γ晶粒在α2片层或（α2＋γ）片层内部以任意方向形核。