Acta Metallurgica Sinica, 2017, 53(5): 583-591
doi: 10.11900/0412.1961.2016.00502
直流电流对Ti-48Al-2Cr-2Nb合金组织和性能的影响

Effects of Direct Current on Microstructure and Properties of Ti-48Al-2Cr-2Nb Alloy
陈占兴, 丁宏升, 刘石球, 陈瑞润, 郭景杰, 傅恒志

摘要:

将直流电流作用于定向凝固过程中的Ti-48Al-2Cr-2Nb合金,利用OM、XRD、SEM和TEM分析了合金的凝固组织、相组成和片层组织,测试了合金的显微硬度及800 ℃压缩力学性能。结果表明,电流在一定程度上促进了合金凝固组织的细化及成分的均匀性,减少或消除了片层间偏析。随着电流密度的增大,平均晶粒尺寸和片层厚度呈现先减小后增大的趋势,α2相相对含量先增大而后减小,合金的显微硬度、压缩断裂与屈服强度也呈现先增大后减小的趋势。平均晶粒尺寸最小约0.46 mm,片层间距最小为0.19 μm,分别比未加载电流时降低70%和29%,α2相相对含量从18.5%增至39.4%。片层间距或晶粒尺寸越小,合金的强度越高,变形能力越均匀,塑性也越好。合金的最大显微硬度达542 HV,合金的压缩屈服强度与断裂强度分别达到1200和1365 MPa,与未施加电流时相比均有所提高。加载直流电流引起固-液界面相前沿过冷度减小,可认为是TiAl二元相图中的L→β+L→α+β的包晶反应成分向富Al侧微小偏移,此时初生β相增多,从而造成了TiAl合金室温相组织α2相的相对含量增加。

关键词: TiAl合金 ; 直流电流 ; 凝固 ; 微观组织 ; 显微硬度 ; 高温压缩

Abstract:

TiAl based alloys have been widely used as promising aerospace structural materials, which benefit from their unique combination of mechanical properties. However, they yield poor plasticity and low process ability, thus restricting the wide application. In this work, an efficient way was proposed by which direct current (DC) was imposed on the solidification process of TiAl-based alloy. Influences of DC on the microstructure and properties of directionally solidified Ti-48Al-2Cr-2Nb alloy using water cold crucible directional solidification equipment has been investigated. The changes of solidification microstructure, phase structure and composition of the alloy and γ/α2 interlamellar structures were characterized by OM, XRD, SEM and TEM. The effect of DC on the size of eutectoid colony, interlamellar spacing and relative content of α2 phase had been studied by Image Pro Plus. Furthermore, the mechanical properties of the directionally solidified Ti-48Al-2Cr-2Nb alloy at 800 ℃ were performed. The results revealed that the DC can evidently promote the homogeneity of the solidification component and refiner the structure, and the segregation in lamellar colonies can be efficiently reduced or eliminated to a certain extent. With the increasing of the current density, the grain size and lamellar spacing decreased first and then increased, however, the α2 phase content showed a totally different trend. Moreover, the microhardness, compression yield strength and the fracture strength of the alloy also revealed a trend of decrease after the first increase too. With the current density increasing, the average grain size and interlamellar spacing declined to the lowest of 0.46 mm and 0.19 μm, respectively, and the content of α2 phase increased from 18.5% to 39.4%. The microhardness of sample reached 542 HV, the compression yield strength and the fracture strength were remarkably improved, and the maximum values reached 1200 and 1365 MPa, respectively. DC can cause a reduction of the supercooling in front of the liquid phase during the solidification process. The results can be seen as the peritectic reaction L→β+L→α+β moving a tiny drift to the direction of the Al-rich side in TiAl binary phase diagram, consequently, the primary β-phase increased, and the content of α2 phase, microstructure under room temperature, increased evidently.

Key words: TiAl alloy ; direct current ; solidification ; microstructure ; microhardness ; high temperature compression

TiAl合金密度较小、比强度和比模量高、高温抗蠕变及高温抗氧化性能好,具有良好的力学性能、物理性能及特殊的机械性能,是一种最具潜力的轻质高温结构材料,广泛应用于航空、航天、军事等领域,是当今金属间化合物研究的热点之一[1~3]。然而TiAl金属间化合物的室温塑性与断裂韧性不足,这成为制约TiAl合金继续发展和扩大应用的关键问题[4,5]。而对材料组织和性能的不断需求,推动了在传统材料制备和处理技术的基础上新型凝固过程控制方法及工艺的发展。

材料的电场处理是将电场应用于材料的制备、加工及处理过程,从而实现对材料加工过程的控制及改善组织性能的方法,具有污染少、能量密度高、制备效率高、工艺参数可控精度高等优点,有良好的应用前景[6,7]。科研人员将电流作用于低熔点的纯Al、Al-Cu、Al-Si合金、Pb-Sn、Pb-Sb-Sn合金和较高熔点的铸铁、高温合金、难混熔合金等金属的凝固及控制过程,并取得了一定的研究成果[8~17]。纯金属或合金在一定的电流参数下晶粒细化,凝固组织中柱状晶转化为等轴晶,屈服强度和抗拉强度等性能发生较大变化[8,10~12,14~17]。然而,适合改善某种金属组织与性能的参数范围并不普适于其它金属。Zhou[18]通过计算指出,随金属熔点的升高,纯金属获得相同晶粒尺寸所需的电流密度也逐渐提高,在凝固过程中不同金属的晶粒尺寸与电流密度存在一定的对应关系。目前,利用电流作用于高熔点的TiAl合金的凝固控制过程及其相关工艺参数的匹配、电流对高熔点金属凝固组织及性能的影响等问题的研究相对不足。

本工作将直流电流作用于Ti-48Al-2Cr-2Nb的凝固过程,以避免高活性的TiAl合金熔体添加细化剂引入细化剂杂质污染,研究了直流电流作用下该合金凝固组织的变化及其对合金显微硬度、高温压缩性能的影响,并初步探讨直流电流对TiAl合金凝固过程的影响。

1 实验方法

实验材料选用名义成分为Ti-48Al-2Cr-2Nb (原子分数,%)的合金,加工成直径14 mm、长90 mm棒材,然后放入内壁涂有Y2O3的Al2O3陶瓷管中。主要设备为:电源频率50 kHz、感应加热功率0~100 kW连续可调的多功能冷坩埚定向凝固设备及工作电压60 V、输出电流150 A的外加直流稳压电源设备。利用多功能冷坩埚定向凝固设备,将0~15 A的直流电流通过直径1 mm的Nb丝插入到TiAl棒上部熔池,负极与TiAl棒下端相连接,电流从熔池端向未熔化端流经定向凝固过程中的TiAl料棒而形成闭合回路。在定向凝固实验进行过程中,正电极持续送入合金熔体中的速率与TiAl棒下抽拉速率保持相同。实验过程中通过热电偶装置测温,在加热功率为36 kW时熔池温度达1650 ℃,保温5 min;经计算在此加热功率下该定向凝固设备工作过程中的温度梯度约为15 K/mm[19,20]。本实验的下抽拉速率为0.6 mm/min,开始抽拉后接通直流电流,电流密度范围为0~96 mA/mm2

沿料棒轴向方向距底部35 mm处切取一组横截面进行组织观察及XRD分析。利用GX71型金相显微镜(OM)对试样的横截面组织进行观察;利用X'Pert Pro MPD型X射线衍射仪(XRD)对经直流电流作用的定向凝固试样截面进行物相扫描分析,扫描角度20°~90°;利用Quanta 200 FEG扫描电子显微镜(SEM)对试样的微观组织及偏析情况进行观察,利用其自带的能谱仪(EDS)对样品成分进行分析;利用Tecnai G2 F30 型透射电子显微镜(TEM)观察试样稳定凝固区微观结构;利用Image-Pro Plus软件测定γ/α2片层厚度、α2相含量及横截面晶粒尺寸。利用Micro-586型显微硬度仪测定热影响区、初始凝固区、定向凝固区及等轴晶区的显微硬度,每个区域在若干不同点进行测定并取其平均值,载荷500 g,加载时间10 s。沿料棒轴向方向距底部40 mm处切取直径3 mm、长度4.5 mm的压缩试样,在Gleeble-1500D动态热力模拟试验机上进行高温压缩性能实验,压缩过程中变形温度800 ℃,变形速率0.1 s-1,应变量0.4,加热速率10 ℃/s,保温时间2 min。

2 实验结果
2.1 组织分析

图1所示为加载直流电流的定向凝固Ti-48Al-2Cr-2Nb合金横、纵截面的宏观凝固组织。为直观地观察加载电流前后定向凝固组织演化及晶粒长大过程,分别从距开始加载电流相同位置处切取横截面,并测量平均晶粒尺寸。宏观上纵截面凝固组织可分为:铸态组织区、热影响区、初始凝固区、定向凝固稳态生长区及等轴晶区,如图1所示。实验过程中,浸于Ga-In冷却剂中的TiAl棒下端受到的感应加热影响较小而保持原始的铸态组织(A区);冷却剂以上未熔化的部分是热影响区(B区),经历了一个回复、再结晶及晶粒长大的过程[21];定向凝固过渡区(C区)由于定向凝固过程中单向传导热流及晶粒形成条件的影响不同,从而长大成为接近柱状晶倾向的粗大等轴晶[22,23];在未加载电流和加载电流密度达到最大(96 mA/mm2)时出现柱状晶区(D区),而加载电流密度较小时(32~64 mA/mm2),等轴晶区(E区)取代柱状晶区。初始凝固阶段,由于在热影响区粗大等轴晶基础上生成的柱状晶不稳定生长,且在生长过程中互相之间竞争激烈,其取向偏离度较大,具有择优生长方向的晶粒取代或吞并劣势生长方向的晶粒,并最终被保留下来[24,25],此区域组织由粗大等轴晶向柱状晶转变。结晶过程的继续进行一方面依赖于在上一稳态条件下所形成晶体的继续长大,另一方面是在新的平衡条件下重新生核长大的过程。本实验条件下,未加载电流时的定向凝固稳定生长区由稳定生长的粗大柱状晶构成,生长方向并不严格平行于轴线方向,这是由冷坩埚加热过程中熔体中温度场、流场分布及侧向散热不平衡造成的。Al2O3管内的TiAl棒受感应加热并产生侧向散热,由于冷坩埚内部沿轴向不同位置处磁感应强度不同,导致试棒受热不均匀,因此试棒的表面及径向温度分布亦不均匀[19,20,24,25]。从图1b和c可以看出,电流明显改变了Ti-48Al-2Cr-2Nb合金定向凝固组织,在电流密度较小时(32~64 mA/mm2),由于电流的Joule热效应造成二次枝晶熔断,熔断的二次枝晶作为非均匀形核的基底,并同时向各个方向生长,晶核的增多是造成等轴晶区形成的必要条件。从图1d可以看出,当电流密度增加到96 mA/mm2时,在定向凝固稳定生长区的柱状晶沿着过渡区的一次枝晶方向继续生长。电流密度的增大减小了固液界面前沿过冷度[9,12,13],这一过冷度不足以生成新的晶核,但利于某些择优取向晶粒的继续长大。由于凝固过程中温度梯度的存在而继续长大并形成定向生长的柱状晶组织。

图1 直流电流作用下定向凝固Ti-48Al-2Cr-2Nb合金的宏观组织

Fig.1 Macrostructures of directionally solidified Ti-48Al-2Cr-2Nb alloy without direct current (DC) (a) and with the DC densities of 32 mA/mm2 (b), 64 mA/mm2 (c) and 96 mA/mm2 (d) (Zone A—original as-cast zone, zone B—heat affected zone, zone C—transition zone, zone D—columnar crystal zone, zone E—equiaxed crystal zone)

图2a~d分别为从图1直线1处(D区及E区)即加载电流相同距离处切取的一组横截面组织的OM像。可以看出,Ti-48Al-2Cr-2Nb合金凝固过程中横截面晶粒由不同方向的层片组织所组成;随电流密度在0~96 mA/mm2间变化,平均晶粒尺寸呈现先减小后增大的变化趋势。

图2 直流电流作用下Ti-48Al-2Cr-2Nb组织的OM像

Fig.2 OM images of microstructures of Ti-48Al-2Cr-2Nb alloys solidified without DC (a) and with the DC densities of 32 mA/mm2 (b), 64 mA/mm2 (c) and 96 mA/mm2 (d)

图3所示为直流电流作用下Ti-48Al-2Cr-2Nb合金组织的SEM像。EDS分析表明,未加载电流时,由于凝固过程中扩散不均匀导致偏析出现B2相及γ相,其中Cr、Nb在偏析相中大量富集甚至局部区域含量更高。加载电流密度在32~64 mA/mm2时,晶界间偏析逐渐减少,溶质元素扩散且成分分布均匀化,片层间距变小;电流密度增大至96 mA/mm2时,溶质合金元素进一步均匀化分布,偏析消失。这是金属熔体凝固过程中加载电流促进溶质扩散,溶质分配系数发生变化的结果[9,17]

图3 直流电流作用下Ti-48Al-2Cr-2Nb合金组织的SEM像

Fig.3 SEM images of microstructures of Ti-48Al-2Cr-2Nb alloy solidified without DC (a) and with the DC densities of 32 mA/mm2 (b), 64 mA/mm2 (c) and 96 mA/mm2 (d)

2.2 物相分析

根据“Al当量”将Ti-48Al-2Cr-2Nb合金中的Cr、Nb含量折算成二元TiAl合金的Al含量:C'Al=48.8<49.4,该合金在平衡凝固条件下的凝固路径为:L→[β]+L→[β+α]+L→[β+α]+γ→[α]+γα2+γ→[α2+γ]+γ[24~26],β相是初生相,室温组织为α2/γ全片层结构(FL)。图4为电流作用下Ti-48Al-2Cr-2Nb合金凝固试样的XRD谱。可以看出,该合金室温组织主要由γ相、α2相及少许Y2O3杂质相组成,未加载电流时还有少量B2相出现。定向凝固过程中,随电流密度的增加,样品表面α2相的XRD峰呈先减弱后增强的趋势。一般来说,XRD峰对应原子分布与晶面取向,衍射峰相对强度取决于该物质的组成与结构。衍射强度与对应相的体积分数成正比,峰值越高则构成该晶面的原子阵列在该晶体材料中的存在越多,衍射峰宽化表明晶粒尺寸变小。由此初步定性分析,未加载电流时的Ti-48Al-2Cr-2Nb合金晶粒尺寸较大,在相转变过程中,晶界处残余的少量β相在室温下以B2相的形式存在;随电流密度不断增大,晶粒尺寸、α2γ相的相对含量发生变化,B2相基本消失。

图4 电流作用下Ti-48Al-2Cr-2Nb的XRD谱

Fig.4 XRD spectra of Ti-48Al-2Cr-2Nb alloy solidified with and without DC

图5为不同电流作用下Ti-48Al-2Cr-2Nb合金的片层凝固组织的TEM像。可以看出,未加载电流时凝固组织的α2/γ片层间距较大且组织不均匀,相界有析出相;随着电流密度增大,片层间距细化并且组织不断趋于均匀化;在电流密度达到最大时虽然平均片层间距有所粗化,但其组织均匀性增大。进一步确定其组成相主要为深色衬度的α2相和浅色衬度的γ相,片层组织中的α2γ相之间存在着 0001 α 2 111 γ < 11 2 ̅ 0 > α 2 < 1 1 ̅ 0 > γ 的位相关系。

图5 直流电流作用下Ti-48Al-2Cr-2Nb合金的片层组织TEM像

Fig.5 TEM images of lamella structures of Ti-48Al-2Cr-2Nb alloy solidified without DC (a) and with the DC densities of 32 mA/mm2 (b), 64 mA/mm2 (c) and 96 mA/mm2 (d)

在本实验条件下,加热功率、生长速率等定向凝固参数相同的情况下,加载电流密度成为影响定向凝固Ti-48Al-2Cr-2Nb合金微观组织的的主要因素,主要体现在所含各相的相对含量、晶粒尺寸及片层间距变化方面。

根据图5中γ相与α2相的对比度的区别,并根据XRD定性分析α2相的变化得到的信息(图4),利用Image-Pro Plus测量出的面积近似代表体积,定量统计分析合金中γ相、α2相的相对含量变化。图6给出了Ti-48Al-2Cr-2Nb合金在定向凝固中随电流密度增大时α2相的相对含量变化情况。可以看出,在由γα2两相构成的组织中,α2相相对含量随直流电流密度的增加而增大,在电流密度达到64 mA /mm2后持续增加时,α2相相对含量有所减小。未加载直流电流时α2相占18.5%,外加直流电流密度为64 mA/mm2α2相最高可达39.4%,但加载电流密度达到96 mA /mm2α2相减少至35%。

图6 直流电流作用下Ti-48Al-2Cr-2Nb合金中的α2相含量变化

Fig.6 Volume fraction of α2 phase in Ti-48Al-2Cr-2Nb alloy solidified with and without DC

图7为根据金相及TEM分析并辅助Image-Pro Plus软件测定的Ti-48Al-2Cr-2Nb合金平均晶粒直径及α2/γ片层间距随电流密度的变化。横截面平均晶粒直径变化呈先减小后增大的变化趋势,片层间距随电流密度增大时的变化与晶粒尺寸的变化趋势基本相同。未加载直流电流时的平均晶粒直径约为2.5 mm;加载电流密度为64 mA/mm2时平均晶粒直径约为0.46 mm,达到最小值,与未施加直流电流时相比降低70%;电流密度持续增大到96 mA/mm2时,平均晶粒直径增大到1.5 mm。未施加直流电流时的平均片层间距为0.65 μm;电流密度为32 mA/mm2时,片间距最小为0.19 μm,是未加载电流时的29%。

图7 直流电流作用下Ti-48Al-2Cr-2Nb合金的晶粒尺寸及片层间距

Fig.7 Grain size and lamella width of Ti-48Al-2Cr-2Nb alloy with and without DC

2.3 显微硬度

Ti-48Al-2Cr-2Nb合金是由α2/γ两相构成的全片层组织,凝固过程中的组织变化对合金力学性能有很大的影响[1,2,4]。直流电流影响了TiAl合金的定向凝固过程,导致了微观组织结构如析出相、晶粒尺寸及片层间距的差异。图8为不同密度直流电流作用下定向凝固Ti-48Al-2Cr-2Nb合金不同区域的显微硬度,图8中(B、C和D/E区)显微硬度分别对应于图1宏观组织各区域。可以看出,在热影响区(B区),由于回复再结晶导致此区域位错减少,内应力消除[21],故显微硬度较低;由于在定向凝固过渡区(C区)晶粒不稳定生长、晶粒取向差异及应力,显微硬度较热影响区有所升高;由于等轴晶区晶粒稳定生长及晶粒细化等因素的存在,显微硬度进一步增大。对于位于合金的稳定生长区域(D/E区),直流电流密度为64 mA/mm2时的显微硬度最大为542 HV,与未加载电流时的显微硬度相比提高了31.5%,这是由于晶粒尺寸与片层间距尺寸都较小且α2含量最高。当电流密度为32 mA/mm2时,层片间距较小但晶粒尺寸相对较大,因此显微硬度比电流密度为64 mA /mm2时有所减小。未加电流时的此区域显微硬度偏小是由凝固偏析及成分分布不均所导致。

图8 直流电流作用下定向凝固Ti-48Al-2Cr-2Nb合金各区域的显微硬度

Fig.8 Microhardness in various zones of directionally solidified Ti-48Al-2Cr-2Nb alloy with and without DC current

根据图8电流作用下Ti-48Al-2Cr-2Nb合金凝固稳定生长区(D/E区)的显微硬度,综合考察不同电流密度下α2相含量、片层间距及晶粒尺寸的差异(图6和7),可以得出α2相含量及晶粒尺寸相较于片层间距,在显微硬度中起主要作用。这是由于显微组织的长度结构参数对TiAl基合金的力学性能有着重要影响,根据Hall-Petch公式[27~30]

σ = σ 0 + k y D - 1 / 2 (1)

式中,ky是材料的常数; σ 0 是恒定应力部分,通常与其它类型的滑移障碍物有关;D是显微组织的长度结构参数,一般为多晶体中各晶粒的平均直径或片层间距。从式(1)可以看出,片层间距或晶粒尺寸越小,TiAl合金的强度越高,并且其变形能力愈均匀,变形能力增加则塑性也越好。

2.4 高温压缩性能

图9为直流电流作用下定向凝固Ti-48Al-2Cr-2Nb合金在应变量0.4、应变速率0.1 s-1、变形温度800 ℃时的高温压缩真应力-应变曲线。可以看出,未经加载直流电流处理的凝固试样屈服强度最低,约为720 MPa,压缩断裂强度为1198 MPa。当试样凝固过程中电流密度在32~64 mA/mm2时,该合金的屈服强度、断裂强度和塑性性能呈明显上升趋势,并在64 mA/mm2时压缩屈服强度及断裂强度达到最高,分别为1200和1365 MPa,比未经外加直流电流处理时的材料分别提高了67%和14%。当电流密度继续增大到96 mA/mm2时,该合金的屈服强度与断裂强度又有所下降。可见,直流电流在一定的电流参数下,有效提高了Ti-48Al-2Cr-2Nb合金的屈服强度与断裂强度。

图9 直流电流作用下Ti-48Al-2Cr-2Nb合金的高温压缩真应力-应变曲线

Fig.9 True stress-true strain curves of Ti-48Al-2Cr-2Nb alloy solidified with and without DC

3 分析讨论

在金属的凝固过程中加载电流,导电粒子在电流作用下产生电迁移现象[31,32],同时液态金属内部产生Joule热和Lorentz力及浓度梯度的作用,驱使金属熔体中不同性质离子产生运动。随电流密度变化,固液界面前端熔体流动及液态金属溶质分配系数随之发生变化。电流作用下液态金属凝固过程中的有效界面分配系数KE[32]为:

K E = K 0 ( 1 + UE R ) K 0 + ( 1 + UE R - K 0 ) exp [ - ( 1 + UE R ) D ] (2)

式中,K0为平衡溶质分配系数;U为熔体中原子相对迁移速率,单位电场中溶质、溶剂原子运动速率差;D为液态金属中的溶质扩散系数;E为熔体两端的电势差;R为平面固液界面的移动速率,即晶体的长大速率;δ为固液界面前沿液相侧溶质富集层厚度,δ越小越不利于发生成分过冷。式(2)表明,电流驱动熔体中带电离子向两极移动并促进了熔体中溶质元素Al的扩散,从而偏析减小,溶质的有效分配系数KE (K0<KE<1)增大。根据Wagner作出的电传输在二元合金熔体界面稳定性影响的成分过冷判据[31]

G R m C s ( 1 - K E - V R ) D K E (3)

式中,G为固液界面前沿液相温度梯度, V 为溶质的电传输速率,m为液相线斜率,CS为界面上固相一侧溶质浓度。由式(3)可以看出,KE (KE<1)增大不利于成分过冷。直流电流作用下过冷度减小[9,12,13],加之电流起伏效应的存在,临界形核功和形核半径都随之减小,结果是向易于形成晶核的方向发展[8],形核率是决定晶粒尺寸的重要因素。由于在较小电流密度(32~64 mA/mm2)时Joule热熔断二次枝晶,非均匀形核质点增加,这也造成了电流密度在32~64 mA/mm2时形成细化的等轴晶组织。当电流密度继续增大到96 mA/mm2时,虽然固液界面前沿过冷度有所减小,但这一过冷度不足以生成新的晶核,并且在定向凝固过程中温度梯度存在的条件下择优生长为柱状晶粒。因此随着加载电流密度的变化,随之出现不同晶态凝固组织的转变。

直流电流会引起定向凝固过程中液相前沿过冷度的减小,根据TiAl二元相图,在C'Al=48.8时的非平衡结晶条件下促进了固液转变过程中β相的转变析出,导致TiAl合金的L→β+L→α+β的包晶反应成分向富Al侧微小偏移(如图10虚线所示),导致初生β相增多,β完全参与包晶反应过程,根据杠杆定律可知,导致室温下TiAl片层中α2相的含量增多。

图10 直流电流作用下TiAl二元合金非平衡转变示意图

Fig.10 Schematic of equivalent binary phase diagram of TiAl system with direct current

4 结论

(1) 加载直流电流在一定程度上促进了定向凝固的Ti-48Al-2Cr-2Nb合金组织的细化及成分的均匀化,合金偏析减小或消失,在较小电流密度时(32~64 mA/mm2)柱状晶向等轴晶转变。横截面平均晶粒尺寸和片层厚度总体上均呈现先减小后增大的变化趋势,最小尺寸分别约0.46 mm和0.19 μm,与未外加直流电流时相比分别减小了70%和29%;随电流密度的增大,室温下α2相相对含量提高,比未加载电流时高出113%。

(2) 片层间距或晶粒尺寸越小,则合金的强度越高并且变形能力愈均匀,变形能力越强,塑性也越好。加载直流电流64 mA/mm2凝固的Ti-48Al-2Cr-2Nb合金的最大显微硬度是542 HV,与未加载电流时相比提高了31.5%;压缩屈服强度及断裂强度分别达到1200和1365 MPa,与未加载电流时相比分别提高了67%和14%。

The authors have declared that no competing interests exist.

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ABSTRACT Directionally solidified TiAl microstructures were investigated and it was found that there is competitive growth between the stable phase (β) and metastable phase (α) near the peritectic reaction L+β→α in Ti–Al binary system. The phase selection phenomena of Ti–Al system containing (44–50) at.% Al were theoretically studied based on the criterion of the highest interface temperature and with solidification interface response function model of single-phase alloys. Firstly, according to a thermodynamic model of Ti–Al binary system, a part of the phase diagram with the Al content of (44–50) at.% and above 1740 K was calculated. The peritectic reaction temperature is 1763 K, and the peritectic composition is Ti–47.3 at.% Al. The solidus and liquidus of α and β were described as polynomials. Using these polynomials, Co, Tm, me and ke were determined. Suppose that the solidification of α and β phases conforms to the solidification theory of single phase, the interface temperatures were calculated. For Ti–47 at.% Al, when temperature gradient (G) is 1064000 K/m, the critical growth rate of α phase from planar to columnar is about 3.6×10616 mm/s. The critical growth rate linearly increases with increasing temperature gradient. With the same computational program, the interface temperatures of α and β phases with Al content from 44 to 50 at.% were calculated. Comparing the interface temperatures of α and β phases, and assuming the phase with the higher interface temperature to grow preferentially from the melt, the phase-selection map with the reference frame of the melt composition and the ratio of temperature gradient to growth velocity (G/V) was constructed. The theoretical results are in good agreement with experimental results.
DOI:10.1016/j.intermet.2004.07.010      URL     [本文引用:2]
[26] Li X, Fautrelle Y, Ren Z M.Influence of thermoelectric effects on the solid-liquid interface shape and cellular morphology in the mushy zone during the directional solidification of Al-Cu alloys
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[27] Hansen N.Hall-Petch relation and boundary strengthening[J]. Scr. Mater., 2004, 51: 801
Abstract The Hall–Petch relation is discussed separately for the yield stress of undeformed polycrystalline metals and for the flow stress of deformed metals. Key structural parameters are the boundary spacing, between grain boundaries in the former case and between dislocation boundaries and high angle boundaries in the latter. An analysis of experimental data supports the Hall–Petch relation for undeformed metals over a grain size range from about 20 nm to hundreds of micrometers. For deformed metals, boundary strengthening is not a constant and the Hall–Petch relation must be modified.
DOI:10.1016/j.scriptamat.2004.06.002      URL     [本文引用:1]
[28] Yamamoto Y, Takeyama M.Physical metallurgy of single crystal gamma titanium aluminide alloys: orientation control and thermal stability of lamellar microstructure[J]. Intermetallics, 2005, 13: 965
This paper summarizes our recent work on the development of cast single crystal gamma TiAl alloys (PST) focusing on both lamellar orientation control of PST crystal in process and the thermal stability of the lamellar microstructure in use at elevated temperatures. PST crystals can easily be produced by unidirectional solidification regardless of the kinds of primary solidification phases of bcc β-Ti or hcp α-Ti solid solutions. The lamellar orientation control can be achieved by controlling the orientation of α single crystal existing just underneath the liquid/solid interface. Seeding with Ti-rich PST crystal does not work because of random nucleation of new grains due to the reverse phase transformation of γ to α during heating. However, an Al-rich γ single crystal with no solid/solid phase transformation up to the melting point is a promising seed when the average composition of the seed and alloys to grow is in the region of α solidification. During exposure at elevated temperatures interface reaction of energetically unstable variant interface takes place, resulting in the coarsening of γ plates and eventually leading to the collapse of the lamellar microstructure unless α 2 plates exist. Thus, thermodynamically stable α 2 plates play an important role in pinning the coarsening of γ plates across the lamellae, which is responsible for the high thermal stability of the lamellar microstructure.
DOI:10.1016/j.intermet.2004.12.011      URL     [本文引用:0]
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[30] Sato Y S, Urata M, Kokawa H, et al.Hall-Petch relationship in friction stir welds of equal channel angular-pressed aluminium alloys[J]. Mater. Sci. Eng., 2003, A354: 298
The effect of grain size on hardness in the stir zones of friction stir (FS) welds of equal channel angular (ECA)-pressed Al alloys 1050 and 5083 was examined. The hardness was found to be essentially related to grain size through the Hall–Petch relationship in the stir zone of Al alloy 1050. The k H slope of the Hall–Petch equation for the stir zone of Al alloy 1050 was different from the previously reported ones, which was attributed to dynamic recrystallisation during friction stir welding (FSW). On the other hand, the relationship between hardness and grain size in the stir zone of Al alloy 5083 was expressed by the Hall–Petch equation with a change in slope. The change in slope was attributed to the homogeneous distribution of many fine particles.
DOI:10.1016/S0921-5093(03)00008-X      URL     [本文引用:1]
[31] Pfann W G, Wagner R S.Principles of field freezing[J]. Trans. Metall. Soc. AIME, 1962, 224: 1139
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[32] Prodhan A, Sivaramakrishnan C S, Chakrabarti A K.Solidification of aluminum in electric field[J]. Metall. Mater. Trans., 2001, 32B: 372
Abstract The casting properties of molten aluminum during solidification in the presence of electric field were discussed. The effects of varying current on cooling behavior, microstructural refinement and the extant of pinhole porosity were investigated in aluminum ingots. Cooling curves were obtained by recording thermocouple output data in a computer. Hydrogen analysis and tensile tests were also performed on samples.
DOI:10.1007/s11663-001-0060-4      URL     [本文引用:2]
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关键词(key words)
TiAl合金
直流电流
凝固
微观组织
显微硬度
高温压缩

TiAl alloy
direct current
solidification
microstructure
microhardness
high temperature compres...

作者
陈占兴
丁宏升
刘石球
陈瑞润
郭景杰
傅恒志

CHEN Zhanxing
DING Hongsheng
LIU Shiqiu
CHEN Ruirun
GUO Jingjie
FU Hengzhi