Homogenization of as-cast Ti-45AI-8.5Nb-0.5 （ W, B, Y） alloy

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。     

Microstructure,Microsegregation, and Mechanical Properties of Directional Solidified Mg–3.0Nd–1.5Gd Alloy

The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary a(Mg) phase and interdendritic a(Mg) ? Mg12(Nd, Gd) eutectic and Mg5 Gd phase. The primary dendrite arm spacing k1 and secondary dendrite arm spacing k2 were found to be depended on the cooling rate R in the form k1= 8.0415 9 10-6R-0.279 and k2= 6.8883 9 10-6R-0.205, respectively, under the constant temperature gradient of40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within a(Mg) matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved,while the corresponding elongation decreased with the increase of growth rate.     

Ti-45Al-8(Nb,Hf,Y)-0.2B合金高温抗氧化性研究

设计了铌当量约为8的三种成分合金：Ti-45Al-8Nb-0.2B,Ti-45Al-4Nb-0.5Hf-0.1Y-0.2B,Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B, (at.%),研究了这三种合金在900℃静止空气中的断续氧化行为。研究结果表明：Hf、Y联合微合金化的合金氧化膜与基体粘附性明显增强;低Nb/Hf比值的Ti-45Al-4Nb-0.4Hf-0.1Y-0.2B的合金氧化增重小、抗氧化性强,高Nb/Hf比值的Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B合金氧化增重大,抗氧化性差。对氧化膜的扫描电镜(SEM)、能谱分析(EDS)及X射线衍射（XRD）分析表明,Hf、Y的联合加入促进了Al2O3膜的须状生长形态,从而提高了氧化膜与基体粘附性,低Nb/Hf比值的合金中形成了较厚的连续致密的Al2O3膜,提高了合金的抗氧化性;高Nb/Hf比值的合金内存在明显的外氧化现象,导致了该合金抗氧化性下降。     

Directional solidification of Ti-45Al-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 d...     

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。     

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...     

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。     

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.     

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.     

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。