钛合金冷坩埚电磁约束铸造工艺研究

针对液态下化学性质相对活泼的钛合金,采用冷坩埚技术对电磁约束铸造工艺过程进行了试验研究,考察了工艺条件对成形坯件表面质量和内部组织的影响.结果表明:试样抽拉速度变化对表面质量和宏观组织的影响明显,加热功率虽然对组织变化影响不大,但对表面质量影响较为显著.通过优化工艺,可以获得既有较光滑铸造表面又具有一次定向凝固组织的钛合金铸件.讨论了铸造过程裂纹和组织缺陷的形成机理,获得到了表面光滑无裂纹,宏观组织为柱状晶的钛合金大尺寸圆坯.     

Nb-Si基超高温合金及其定向凝固工艺的研究进展

具有低密度和高熔点特性的Nb-Si基超高温合金是下一代航空发动机热端部件的重要候选材料之一.但Nb-Si基超高温合金的低室温断裂韧性限制了其工业化应用,合金化和定向凝固是改善室温断裂韧性的有效方法,本文综述了这2个方面的研究进展.合金化方面重点介绍了合金元素通过位错增韧和相改变增韧实现铌基固溶体(Nbss)相的韧化,通过固溶强化和相变提高硅化物相的高温性能,促进硅化物以近"Y"型生长,改善两相的界面等影响,分析发现Ti、Hf、Zr、B和Mg等元素均可改善室温断裂韧性.定向凝固方面综述了Nb-Si基超高温合金的定向凝固方法及特点,不同定向凝固工艺对Nb-Si基合金的组成相、组织形貌、室温断裂韧性以及高温强度的影响,定向凝固过程的组织演变规律及强化机理,分析发现调控工艺可获得Nbss/Nb5Si3良好单向生长的组织.在保证Nbss/Nb5Si3共晶耦合单向生长的情况下,减小Nbss相的厚度,提升其连续性是提高室温断裂韧性的有效方法.还展望了Nb-Si合金化与定向凝固的未来发展趋势.     

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.     

Effects of Rapid Cooling Rate on Microstructure Formation and Microhardness of Binary Ti-44Al Alloy

快速冷却速率对Ti-44Al合金组织和显微硬度的影响

陈晓宇^1,2,方虹泽¹,王琪¹,陈瑞润¹,丁宏升¹,苏彦庆¹,郭景杰¹

（1. 哈尔滨工业大学 材料科学与工程学院,哈尔滨 150001;

2. 青岛大学 机电工程学院,山东 青岛 266071）

中文说明：

为细化TiAl合金的显微组织与晶粒,提高其力学性能,采用甩带快速凝固方法在不同冷却速率条件下制备了Ti-44Al（at.%）合金,研究了铸态与快速凝固合金的显微组织和显微硬度。XRD测试结果表明,合金中α2相含量随着冷却速率增加而增加,这是由于高的凝冷却率导致α相直接转变为α2相。随着冷却速率的增加,晶粒形态首先由树枝晶转变为等轴晶和树枝晶混合的组织,而后转变为等轴晶。经快速凝固制备后,合金晶粒尺寸由铸态的200-600 μm降低到快速凝固的18 μm。片层厚度由铸态的4.5 μm降低到快速凝固的1.1 μm。随着冷却速率的增加,α2相含量逐渐增加,γ相含量逐渐减少,并且快速凝固可以降低基体中元素偏析。快速凝固的合金晶粒细化,片层厚度降低,并且α2相含量增加,这导致显微硬度由铸态的247 HV提高到快速凝固的556 HV。

关键词：Ti-44Al合金,显微组织,显微硬度,甩带快速凝固,凝固路径

     

高熵形状记忆合金的研究进展

高熵形状记忆合金是在等原子比NiTi合金的基础上,结合高熵合金的概念,逐渐发展起来的一种新型高温形状记忆合金。近年来,已开发出了综合性能优异的(TiZrHf)50(NiCoCu)50系和(TiZrHf)50(NiCuPd)50系高熵形状记忆合金,引起了广泛的关注和研究兴趣。本文从物相组成、微观组织、马氏体相变行为、形状记忆效应和超弹性等角度出发,综述了高熵形状记忆合金的研究进展,并对高熵形状记忆合金未来的研究重点进行了展望。     

Efficient Melt Stirring Induced by the Coupled Effects of Alternating Magnetic Field and Configuration of Cold Crucible

In the present study, the fluid flow in cold crucible and its effects on solute transport were numerically and experimentally investigated. It was found that the coupled effects of alternating magnetic fields and the configuration of cold crucible induce a vigorous three-dimensional melt flow. Further, solute transport results indicate that the three-dimensional flow contributes to more efficient melt stirring, and the stirring efficiency for obtaining uniform melt is improved by 77.5 pct compared with only meridional flow.     

Comparison of a Directionally Solidified TiAl Alloy by Φ15 mm Cylindrical and 29 × 6 mm Plate Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Molds

To begin a preliminary exploration on the effect of shape, the effect of a novel shaped 29 × 6 mm plate mold on the microstructure and mechanical properties of directionally solidified (DS) Ti-45Al-2Cr-2Nb was investigated. For comparison, Φ15 mm cylindrical molds with an equivalent cross-sectional area were used. Experimental results show that the typical microstructures of the DS region in all samples consist of regular well aligned α ₂/γ lamellar structure, B2 phase and Y₂O₃ particles. Due to the lower strength of flow generated by the shape factor of the mold, the plate sample has coarser lamellae and more B2 phase, but fewer Y₂O₃ particles. Additionally, the room temperature (RT) fracture toughness and tensile properties of the plate sample are improved.     

TiAl基合金方坯冷坩埚电磁约束铸造工艺研究

利用冷坩埚电磁约束铸造工艺制备了Ti-46A1-0.5W-0.5Si的定向凝固试样,并集中考察了冷坩埚法定向凝固条件下方坯试样的表面质量、宏观组织及微观组织.研究表明,电磁场的干扰对试样表面质量的影响比较大;随抽拉速度的增加,晶粒尺寸逐渐减小,晶粒数目增多;并且发现Ti-46A1-0.5W-0.5Si合金定向凝固是一个非平衡态的过程,凝固过程中初生相既有β相又有α相.