Directional solidification of Ti-45AI-8Nb-（W,B,Y） alloy

Using a Bridgman vertical vacuum furnace, Ti-45Al-8Nb-(W,B,Y) (at.%) bars, which were prepared from a plasma arc melting (PAM) ingot, were directionally solidified at growth rates of 10, 15, and 20 μm/s. Polysynthetic twinned (PST) crystal with an aligned lamellar microstructure was obtained at the growth rate of 15 μm/s because of high Nb addition. The principle of PST crystal growth and the effect of Nb element were discussed. The results of investigations on microstructure and micromechanical properties of the directionally solidified (DS) bars of Ti-45Al-8Nb-(W,B,Y) alloy are briefly summarized.     

热处理对定向凝固Ti-45Al-8Nb-（W,B,Y）合金组织的影响

研究两种热处理制度对Bridgeman法制备的典型定向凝固Ti-45Al-8Nb-（W,B,Y）（摩尔分数,%）合金组织的影响。两种典型的定向凝固显微组织分别为胞状生长形貌全片层组织和枝晶状生长形貌块状组织,对其热处理发现：热处理1250℃,24h＋900℃,30min＋AC能有效消除其定向凝固合金中的B2相,并将快速定向凝固时产生的块状组织转变为片层组织,在胞状间及枝晶臂上分别获得了宽度为150-200μm和50-100μm的柱状晶。热处理1400℃,2h＋900℃,30min＋AC能同时消除B2相、块状组织及凝固偏析,但造成了晶粒的严重长大。热处理能有效改进定向凝固合金的显微组织。     

铸造Ti-45Al-8.5Nb-0.5(W,B,Y)合金组织均匀化处理

采用水冷铜坩埚感应熔炼制备了Ti-45Al-8.5Nb-0.5(W,B,Y)(at%)合金铸锭,研究了铸造合金组织特点及均匀化处理温度对显微组织的影响规律。结果表明,铸锭组织呈全层片,有大量β相网,富Al相等成分偏析;调整热处理温度使合金偏析区域处于α单相区,充分保温后缓慢冷却可消除偏析形成全层片组织;经过1420℃×12 h炉冷,β相与富Al相偏析均得以消除,获得平均尺寸约为320μm,层片间距约为0.8μm的均匀全层片组织。     

热处理工艺对 Ti-45Al-7Nb-0.15B-0.7W合金微观组织的影响

用电子探针对Ti-45Al-7Nb-0．15B-0．7W的铸态和经过热处理之后的微观组织进行了观察,研究热处理工艺对该合金微观组织的影响。结果表明：该合金的铸态组织经过热等静压和均匀化处理后,其晶团尺寸减小,片层组织更为完整;Ti-45Al-7Nb-0．15B-0．7W合金的α相转变温度在1280—1295℃之间。该合金在超过1295℃的温度进行热处理时,其微观组织由近片层组织转变为全片层组织。     

定向凝固NiAl-9Mo共晶合金的凝固组织特性

采用液态金属冷却法在恒定温度梯度GL=334 K/cm,大生长速率范围内（2~300μm/s）对Ni-45.5Al-9Mo （摩尔分数,%）共晶合金进行定向凝固制备。研究生长速率（v）对纤维间距（λ）、纤维直径（d）和纤维体积分数的影响。在实验中发现平界面和胞界面两类共晶生长界面。在平界面和胞界面组织中,生长速率（v）与纤维间距（λ）和纤维直径（d）的关系经回归分析分别为：λv1/2=5.90μm·μm1/2·s1/2和 dv1/2=2.18μm·μm1/2·s1/2。Mo纤维的体积分数可在一定的范围内随生长速率进行调整,这是由生长过程中界面前沿过冷度的增加及共晶组织中各组成相的生长特性引起的。     

Hot deformation behavior and microstructural evolution of as-forged Ti-44Al-8Nb-(W,B, Y) alloy with nearly lamellar microstructure

The hot deformation behavior and microstructural evolution of as-forged Ti-44Al-8Nb-(W, B, Y) alloy with nearly lamellar structure were investigated by means of uniaxial hot compression. Its stress exponent and activation energy are 3.81 and 494 KJ/mol, respectively. The efficiencies of power dissipation and instability parameters are evaluated, and processing maps at strains of 0.12, 0.25, and 0.5 are developed. It is demonstrated that the microstructural evolution is dependent on the temperature and strain rate. Moreover, the recovery and recrystallization of γ phases as well as the spheroidization of α phases play important roles in refining the microstructure. Reasonable parameters for secondary hot working are above 1150 °C with a strain rate of less than 0.25 s⁻¹ at a strain of 0.5. Additionally, the hot working window can be expanded to the region with lower temperature and higher strain rate at a strain of 0.12. Finally, crack-free TiAl sheets were successfully prepared by hot pack rolling. The as-rolled alloy is characterized by duplex microstructure with a mean grain size of 10 μm, exhibiting a failure strength of 1021 MPa with 0.78% ductility at room temperature. At 800 °C, the failure strength remains high: above 650 MPa.     

定向凝固Ti-46Al-8Nb合金的微观组织演变与微观偏析(英文)

在3～70μm/s的生长速度范围内对Ti-46Al-8Nb(摩尔分数,%)合金进行定向凝固实验,研究其微观组织演变和微观偏析形式。在该生长速度范围内,固-液界面表现为规则的枝晶生长,一次枝晶间距随着生长速度的加快而逐渐减小。在定向凝固过程中观察到典型的L+β→α包晶反应,最终得到具有α2/γ层片和B2相的微观组织。在各个晶粒中,层片与β相枝晶初始生长方向呈0°或45°。包晶反应导致严重的成分偏析,在凝固过程中,铝富集在枝晶间,铌富集在枝晶心部。随着α相的形核和生长,铌的偏析程度逐渐增大,从而促进B2相的析出,而富集在枝晶间的铝在包晶反应发生后逐渐变得均匀、一致。     

TiAl金属间化合物的定向凝固和晶向控制

在航空航天及能源领域有重要应用价值的TiAl基合金，由于其γ α2片层结构的特点，采用定向凝固技术控制片层方向可以更好地发挥其潜能。从相与组织选择及晶体择优生长特性角度，讨论了具有包晶反应的高温γ-TiAl合金的定向凝固与晶向控制，分析了晶向控制中引晶、合金化、调节成分与凝固参量及它们的综合作用所得到的结果与存在的问题，并提出了此领域今后应注意的方面。     

生长速度与试样直径对定向凝固Ti-46Al-8Nb合金显微组织和硬度的影响

对Ti-46Al-8Nb（摩尔分数,%）合金进行布里奇曼型定向凝固实验,考察生长速度和试样直径对合金显微组织、相变路径和硬度的影响。结果表明,随着生长速度的增加和试样直径的减小,凝固过程由完全β相凝固转变为具有包晶反应的凝固过程,其最终显微组织为α2/γ层片及有α2/γ层片和B2相组成的多相组织。以上结果是由扩散和对流的减弱造成的溶质富集而引起的。包晶反应的发生往往导致严重的溶质偏析,其中溶质Al和Nb的偏析使得层片组织较为粗大。因此,在较高生长速度下发生包晶反应时,合金硬度值急剧下降,硬度曲线随着生长速度的增加呈不连续变化。严重溶质偏析导致的粗大层片组织也使得该成分合金的硬度低于其他Ti Al基合金的硬度。     

Microstructure characteristics of Ni-43Ti-4Al-2Nb-2Hf alloy prepared by conventional casting and directional solidification

To further investigate the microstructure characteristic and solidification mechanism,so as to provide knowledge for the microstructure control of a NiTi-Al based high-temperature structural material,t...     

Effect of V and Nb additions on microstructure,properties,and deformability of Ti-45Al-9 (V,Nb,Y) alloy

Ti-45Al-9(V, Nb, Y) alloys with four different x=V/Nb (atomic ratio x = 1, 1.5, 2 and 3.5) have been prepared, and the microstructures, properties and hot deformation behaviors were investigated. SEM, XRD and TEM results showed that Ti-45Al-9(V, Nb, Y) alloys were mainly composed of γ, α 2 , and β phase, and the volume fraction of β phase increased with the increase of the atomic ratio of V/Nb. The alloys were featured with lamellar microstructure with β and γ phases locating at the colony boundaries, and some β precipitates appearing at γ/γ interfaces. It was found that the colony size decreased with the increase of x. The alloys exhibited moderate mechanical properties at room temperature, with a yield strength of over 600 MPa, and fractures showed mainly translamellar character. The alloy with x=3.5 exhibited the best deformability at elevated temperature and that with x=1 had superior oxidation resistance at 800 ℃.     

生长速度对Ti-45%Al包晶合金定向凝固组织的影响

通过电磁感应加热和液态金属冷却相结合的实验方法制备了Ti-45%Al(at%)包晶合金的定向凝固试样,并观察在不同生长速度下的微观组织和界面形态。结果发现,抽拉速度为10μm/s时,界面以胞晶形式生长,最终组织的α2 γ片层与生长方向垂直;抽拉速度为50μm/s时,观察到明显的枝晶生长界面,α2 γ片层与生长方向成45°夹角。低速下的胞状间距明显大于高速下的枝晶间距,说明快速凝固有利于减小枝晶间距。     

Effect of carbon on microstructures of Ti-45Al-3Fe-2Mo-xC alloy

The effect of carbon addition on the microstructures of TiAl-based alloy (Ti-45Al-3Fe-2Mo) was studied. The proportion of β/B2 phase reduces with the content of carbon increasing, while the colony size increases. With increasing the carbon content, the lamellar spacing first decreases from 267 nm (Ti-45Al-3Fe-2Mo) to 237 nm (Ti-45Al-3Fe-2Mo-0.3C) and 155 nm (Ti-45Al-3Fe-2Mo-0.5C), but then increases to 230 nm (Ti-45Al-3Fe-2Mo-1.0C) with further increase in C level, which is affected by the inhibition of carbon atom and precipitation of carbides at the lamellar interface. Precipitation of carbides shows a response to aging time at 800 °C. P-type carbides grow up at the boundaries and near the dislocation areas with the prolonging of aging time. And these carbides are projected different morphology in different beam directions (BD). The effects of these microstructural modifications were examined and the observations were discussed.     

Lamellar boundary alignment of DS-processed TiAl−W alloys by a solidification procedure

In this study, a β solidification procedure was used to align the lamellae in a Ti-47Al-2W (at.%) alloy parallel to the growth direction. The Bridgman technique and the floating zone process were used for directional solidification. The mechanical properties of the directionally solidified alloy were evaluated in tension at room temperature and at 800°C. At a growth rate of 30 mm/h (with the floating zone approach), the lamellae were well aligned parallel to the growth direction. The aligned lamellae yielded excellent room temperature tensile ductility. The tensile yield strength at 800°C was similar to that at room temperature. The orientation of the γ lamellar laths in the directionally solidified ingots, which were manufactured by means of a floating zone process, was identified with the aid of electron backscattered diffraction analysis. On the basis of this analysis, the preferred growth direction of the bcc-β dendrites that formed at high temperatures close to the melting point was inferred to be [001]_β at a growth rate of 30 mm/h and [111]_β at a growth rate of 90 mm/h.     

High temperature tensile properties of directionally solidified Ni-43Ti-4Al-2Nb-2Hf alloy

By liquid metal cooling(LMC)process,the Ni-43Ti-4A1-2Nb-2Hf(％,atomic fraction)alloy was directionally solidified(DS).The microstructure and tensile properties at room and elevated temperature were investigated.It was found that the DS process significantly improves the room temperature tensile strength,increasing by 70％ compared with the as-cast alloy.After appropriate heat treatment(HT),the average tensile strength reaches above 1900 MPa,nearly twice of the as-cast one.At 800 and 900 ℃,the tensile strengths are about 308 and 169 MPa,respectively.     

Microstructures,micro-segregation and solidiifcation path of directionally solidiifed Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb (at.%) aloy,Bridgman-type directional solidiifcation and thermodynamics calculations were performed on the aloy. The microstructures, micro-segregation and solidiifcation path were investigated. The results show that the β phase is the primary phase of the aloy at growth rates of 5-20 μm?s-1 under the temperature gradients of 15-20 K?mm-1, and the primary phase is transformed into an α phase at relatively higher growth rates (V >20 μm?s-1). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb aloy at a growth rate of 10 μm?s-1 under a temperature gradient of 15 K?mm-1. The increase of temperature gradient to 20 K?mm-1 can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the aloy and induce the solidiifcation path to become complicated. The equilibrium solidiifcation path of Ti-45Al-5Nb aloy can be described as , in which βR and γR mean the residual β andγ.     

Ti-47Al-6Nb-2Cr合金的制备与表征

采用非自耗电弧熔炼铸造方法,制备了Ti-47Al-6Nb-2Cr合金。采用金相观察、扫描电镜及透射电镜对其显微组织进行表征。结果表明,Ti-47Al-6Nb-2Cr合金在双相区近γ处理加循环快速热处理,可有效地将粗大的柱状组织细化成50μm~100μm的层状组织,这一细化过程主要是利用了块型转变的逆反应γ→α转变。     

Study of microstructure and mechanical properties of Ti-45Al-(Fe,Nb) (at. %) alloys

In the present work, the microstructure and compression properties of two novel γ(TiAl) based alloys, Ti-45Al-5Fe and Ti-45Al-5Fe-5Nb, have been investigated. Both alloys had a relatively fine as-cast structure generally consisting of the γ(TiAl) and τ₂(Al₂FeTi) phases with a minor amount of the α₂(Ti₃Al) and β(B2) phase. The compression properties of the novel alloys were measured at room and elevated temperatures. The Ti-45Al-5Fe-5Nb alloy showed higher room temperature ductility and similar strength at room and elevated temperatures as well as improved workability at elevated temperatures as compared to β-solidifying γ(TiAl) alloys of last generation (TNM alloys).     

Lamellar orientation control in directionally solidified TiAl intermetallics

TiAl-based alloys are potentially used as high-temperature structural materials with a high specific strength in the range of~900°C.However,the mechanical properties of TiAl-based alloys are extremely anisotropic with respect to the lamellar orientation of the microstructures.A balance combination of room-temperature ductility and strength can be achieved when the lamellar orientation are aligned parallel to the tensile stress direction.Lamellar orientation control of TiAl-based alloys by directional solidification technique has been widely studied in recent years.Two different directional solidification processes can be used to modify the lamellar orientation.One is a seeding technique and the other is adjusting the solidification path.This paper reviews the principles of the two methods and their progress.The influence of alloy composition and solidification parameters on lamellar orientation control is also discussed.     

TiAl合金的热加工、组织和性能

采用水冷铜坩埚感应熔炼技术制备了高质量的Ti-43Al-9V-0．3Y合金铸锭,该合金铸态组织为近层片组织结构,层片团簇的体积分数为85％左右,大小约为80μm,块状卢和γ相位于层片团簇边界。层片结构中除了γ和α2相外,还存在少量的口相析出物。Ti-43Al-9V-0．3Y合金具有良好的热加工性能,通过包套锻造和包套轧制技术,成功制备了大尺寸TiAl合金锻饼和国内最大尺寸TiAl合金板材,其尺寸分别为犯60mm×24mm和500mm×300mm。经热变形后,Ti-43Al-9V-0．3Y合金的显微组织明显细化,力学性能得到了显著提高。