Evaluation of Ca-doped BaZrO3 as the crucible refractory for melting TiAl alloys

In this study, a new Ca-doped BaZrO₃ refractory was designed by using thermodynamics approaches and tested for its applicability for vacuum induction melting (VIM) of TiAl alloys. The influence of CaO on the BaZrO₃ phase constitution and microstructure, as well as the key features of the TiAl melt interaction with the Ca-doped BaZrO₃ crucibles were investigated by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM). Results revealed that the Ca-doped BaZrO₃ refractory consisted of Ba_1-xCa_xZrO₃ and CaO phases. An obvious interaction occurred during the melting of the TiAl alloy in the Ca-doped BaZrO₃ crucible along with the generation of BaAl₂O₄ as a reaction product, with formation of a reaction layer up to 5?µm thick. Dissolution of Ca-doped BaZrO₃ refractory in the TiAl melt was the main reason for the alloy-crucible reaction. Moreover, the Ca-doped BaZrO₃ crucible was found to substantially reduce the contamination of the TiAl alloy, with lower oxygen concentration as compared with other conventional oxide crucibles. Overall results confirmed that vacuum induction melting using the Ca-doped BaZrO₃ refractory can be considered as an appropriate method for the fabrication of TiAl alloys.     

Influence of thermodynamic activities of different masteralloys in pack powder mixtures to produce low activity aluminide coatings on TiAl alloys

Titanium aluminides are interesting high temperature materials, but show insufficient oxidation resistance as well as embrittlement at higher temperatures (>750 °C). Al-enriched coatings can be manufactured by pack cementation on many high temperature alloys to promote the formation of a protective alumina layer at high temperatures, which not only protects the alloy from oxidation but is also expected to impede embrittlement of TiAl at high temperatures. One drawback of such coatings is that Al-rich phases are very brittle. Therefore the major intermetallic aluminide phase in the coating plays a critical role for the protection behavior. Based on thermodynamic calculations different masteralloys were chosen to control the pack cementation process. Particular attention is given to the gradient between the aluminum activity of the different masteralloy powders and the aluminum activity of the substrate surface (alloy TNM^®-B1) in order to control the deposited phase at the surface. It is revealed that powder pack with Al as masteralloy provides a high Al activity and produces thick multi-layered coatings consisting of brittle TiAl₃ and TiAl₂ phase and aluminum-rich TiAl. By using different chromium aluminides as masteralloys, thinner, low-activity coatings could be produced, consisting of a bi-layer of brittle TiAl₂ phase and aluminum-rich TiAl or just the targeted pure aluminum-rich TiAl, which is known to have much better mechanical properties.     

Producing fully-lamellar microstructure for wrought beta-gamma TiAl alloys without single α-phase field

Fine-grained fully-lamellar (FL) microstructure is desired for TiAl components to serve as compressor/turbine blades and turbocharger turbine wheels. This study deals with the process and phase transformation to produce FL microstructure for Mo stabilized beta-gamma TiAl alloys without single α-phase field. Unlike the α + γ two-phased TiAl or beta-gamma TiAl with single α-phase field, the wrought multi-phase TiAl–4/6Nb–2Mo–B/Y alloys exhibit special annealing process to obtain FL microstructure. Short-term annealing at temperatures slightly above β-transus is recommended to produce the desired FL microstructure. The related mechanism is to guarantee the sufficient diffusion homogenization of β stabilizers during single β-phase annealing, and further avoid α decomposition by α → γ + β when cooling through α + β + γ phase field. The colony boundary β phase contributes to fine-grained nearly FL microstructure, by retarding the coarsening of the α phase grains.     

双态组织高Nb-TiAl合金的缺口敏感性

通过对圆棒状缺口试样和无缺口光滑试样进行单向拉伸试验,研究了双态组织高Nb-TiAl合金的缺口敏感性。研究表明,缺口根半径R≥0. 5 mm的U型试样和缺口角度60°、R≥1 mm的V型试样的抗拉强度RmN都大于光滑试样的抗拉强度Rm,缺口敏感度NSR均大于1,合金对缺口不敏感。当缺口类型相同时,缺口敏感性随缺口根半径R的减小而增大; V型缺口敏感性大于U型缺口;对于V型缺口试样,当R=0. 5 mm时,其NSR在0. 98～1. 03之间,RmN在Rm附近波动,易出现缺口敏感。     

锻态Ti-44Al-4Nb-4V-0.3Mo-Y合金等轴及片层组织高温拉伸性能及组织演变

等轴γ晶粒和α2/γ片层是beta-gamma TiAl合金的2种主要变形组织形态。研究了锻态Ti-44Al-4Nb-4V-0.3Mo-Y合金等轴组织及片层组织的高温拉伸性能及组织演变。结果表明:拉伸温度对Ti-44Al-4Nb-4V-0.3Mo-Y合金的力学性能和显微组织有显著的影响。在相同温度下,Ti-44Al-4Nb-4V-0.3Mo-Y合金等轴组织的抗拉强度和屈服强度略高于片层组织,而延伸率相差不大。随拉伸温度的升高,合金的抗拉强度和屈服强度逐渐减小,而延伸率迅速增大。对于等轴组织,提高温度,等轴γ晶粒被拉长,发生完全的动态再结晶,从而细化合金的显微组织。对于片层组织,α2/γ片层的分解和γ板条的再结晶程度随拉伸温度的升高而增大。Ti-44Al-4Nb-4V-0.3Mo-Y合金的韧脆转变温度在750～800℃之间。     

硼化物原位自生增强TiAl基复合材料的组织演化、热变形行为及加工工艺设计（英文）

硼元素添加造成的相转变和硼化物析出等因素会对原位TiAl基复合材料显微组织演化及热变形行为产生影响。利用等温压缩实验、扫描电子显微技术以及透射电子显微技术等研究材料的动态再结晶和动态回复机制,并计算出其表现变形激活能为691.506 k J/mol。在1100～1200℃温度区间,再结晶γ和α晶粒的形核长大分别主导α₂→α相转变温度上、下的热变形行为。α相的动态回复主导材料在1250℃低应变速率下的热变形行为;同时,硼元素会提高α相含量,降低γ→α和α₂→α相转变温度,进而促进加载过程中回复α相晶粒的形核长大。根据新建的本构模型,对TiAl基复合材料的变形机制和加工工艺进行详细阐述.     

TiAl合金与异种材料的摩擦焊接

TiAl合金与42CrMo直接摩擦焊接性较差,为此分别引入高温合金GH3039、K418、N80A和纯镍N6作为中间材料,对TiAl-GH3039/K418/N80A/N6-42CrMo异种材料的摩擦焊接工艺进行了研究。采用硬度计、扫描电镜和电子万能试验机对焊后接头区域的硬度、组织和焊合区成分变化以及接头力学性能进行了分析。研究表明,TiAl合金与异种材料焊后接头中形成了复杂的多层状金属间化合物;TiAl合金与GH3039、N80A的摩擦焊接性较好,与K418、N6的摩擦焊接性较差;根据不同材料线膨胀系数随温度的变化规律、与TiAl合金摩擦焊接后接头的性能及其与42CrMo的摩擦焊接性,最终选择GH3039作为中间材料。通过引入中间材料,摩擦焊制备了TiAl合金涡轮-42CrMo转轴的异种材料整体转子,使得TiAl合金在涡轮增压器领域的应用成为可能。     

TiAl金属间化合物纳米粉末的相转变

对采用自悬浮定向流技术制备的TiAl金属间化合物纳米粉末进行真空退火,考察退火工艺参数对其相转变过程的影响。X射线衍射等分析表明:随着退火温度的升高以及退火时间的延长,纯a2-Ti3Al相纳米粉末向a-TiAl相转化的程度逐渐增加,其晶粒尺寸也逐渐变大。将其纳米粉末真空压制成块体也有助于上述相转变。对复相的纳米块体进行真空退火,可以得到纯a-TiAl相的纳米晶合金。