Seeded growth of Ti-46Al-8Nb polysynthetically twinned crystals with an ultra-high elongation

Large size polysynthetically twinned crystals of Ti-46 Al-8 Nb alloy with a parallel lamellar microstructure were successfully prepared using a Ti-43 Al-3 Si seed by our new operation.A large amount of columnar B2 phase paralleling to the growth direction was found in the final lamellar microstructure.Higher growth rate(>30 mm/h)led to the failure of seeding process.Based on these results,a new mechanism is proposed to describe the seeding process of the hypo-peritectic Ti Al alloys.The peritecticαphase is suggested to directly nucleate from the melt,and then act as nucleus for transformedαphase in the subsequentβtoαtransformation.At the higher growth rate,the appearance ofβphase secondary dendrites and homogeneous nucleation lead to the failure of seeding process.High Nb addition leads to a large amount of residualβphase,and theseβdendrites finally evolve into B2 phase.The room temperature tensile elongation was measured to be 11.9-18.5%for Ti-46 Al-8 Nb PST crystals,which is the highest ever reported value for Ti Al based alloys.

Microstructure control and mechanical properties of directionally solidified TiAl-Nb alloys

The double directional solidification（DS） technique was developed to control the lamellar microstructures in primary β TiAl-Nb alloys.Polysynthetically twinned（PST） crystals with lamellar boundaries parallel to or inclined 45o to the growth direction were achieved due to the complete peritectic transformation during directional solidification of the alloys with the dendritic solid/liquid interface.The PST crystals with aligned lamellar boundaries only parallel to the growth direction were produced when lamellar grains with lamellar boundaries in the same orientation were seeded by themselves under appropriate growth conditions.Low boron addition is harmful to align the lamellar orientation because of the growth of non-peritectic α phase.Due to the larger yttria particles and boride ribbons in the directionally solidified TiAl-Nb alloys,the tensile plastic elongations of the alloys are only close to 2%.

层状TiAl原位透射电镜拉伸揭示界面主导的塑性各向异性

基于功能基元和序构范式将功能基元序构成特定取向组织,可以最大限度的利用层状结构材料的力学性能各向异性.然而,其各向异性变形行为的微观起源尚不清楚.本文以全片层的γ-TiAl/α_(2)-Ti_(3)Al双相单晶为模型,通过沿特征取向的原位透射电镜拉伸试验和多尺度结构表征,揭示了界面功能基元主导的塑性各向异性及相应的变形机制.相比于片层内部本征的位错行为,跨γ/α2界面取向序构主导的滑移系连续性和界面强度在塑性各向异性中起决定性作用.因此,当平行或垂直于片层方向拉伸时,层状材料发生穿片层的应力传递或界面分层,进而导致强度和延展性的各向异性.这些机理对理解层状结构材料的塑性各向异性具有普遍意义,有利于高性能合金的结构设计.