Acta Metallurgica Sinica, 2017, 53(5): 583-591
doi: 10.11900/0412.1961.2016.00502

Effects of Direct Current on Microstructure and Properties of Ti-48Al-2Cr-2Nb Alloy

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。

1 实验方法

2 实验结果
2.1 组织分析

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)

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)

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

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

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)

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

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。

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

$σ = σ 0 + k y D - 1 / 2$ (1)

2.4 高温压缩性能

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

3 分析讨论

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

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

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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