新型轻质超高强NiTi-Al结构材料力学行为及强韧化机制

NiTi - Al intermetallic compound,as a new ultra-high strength structural materials, is expected to meet the requirement of the medium temperature range of advanced aeroengine. Recently, an ultra-high strength B2 structural NiTi-Al intermetallic compound, with tensile strength more than 2000 MPa and fracture toughness more than 35 MPaom^1/2, were obtained by directional solidification and micro alloying method. The mechanical behavior is is characterized by ultra linear elasticity and ductile fracture, which is different from the brittle fracture behavior of the same B2 structure NiAl and FeAl, and also different from the pseudo elasticity behavior caused by martensitic transformation in NiTi shape memory alloy. TEM results show that the ultra-high strength and ductile fracture are associated with the deformation of twins, the interaction between twins and dislocations, precipitation and micro-crack initiation in the non-martensitic transformation type NiTi-Al intermetallic compound. The project will take directional solidified NiTi-Al intermetallic compound as the research object, aiming to study the macro-deformation behavior, to find the key factors determining high strength and high plasticity, and to discover the twin deformation mechanism, clarifying the interaction between twins and dislocations, precipitates, as well as between micro crack initiation and propagation. It is from atomic scale to reveal the mechanical behavior, strengthening and toughening mechanism of NiTi-Al intermetallic compound, for the development of a new light-weight and high-temperature structure material.

NiTi-Al金属间化合物是有望满足先进航空发动机中温区使用的超高强新型结构材料。申请者前期采用定向凝固和微合金化法获得了拉伸强度超过2000MPa，断裂韧性为35 MPaom^1/2以上的B2型NiTi-Al金属间化合物，其力学特性表现为超线弹性变形及韧性断裂特征，既不同于B2结构NiAl和FeAl的脆性断裂行为，也不同于NiTi形状记忆合金马氏体相变引起的伪弹性行为。TEM表明，NiTi-Al超高强度和韧性断裂与其形变孪晶与位错、析出相及微裂纹萌生均发生了相互作用有关。本项目拟以定向凝固NiTi-Al结构材料为研究对象，阐明其宏观力学行为，确定高强度和高韧性的关键可控因素，明确孪晶形成机理，孪晶与位错、共格析出相、以及微裂纹萌生与扩展之间相互作用机制，建立宏观力学行为和微观形变机制内在联系，揭示NiTi-Al金属间化合物力学行为和强韧化机理，为发展新型轻质高性能超高强结构材料奠定基础。

项目完成了超高强定向凝固NiTi-Al基金属间化合物的制备及取向优化控制，并揭示其微观力学行为和强韧化机制。[001]取向合金具有高强度和超线弹性，弹性极限最高可达4%，但其断口组织处未见位错和孪晶等塑性变形特征。[102]取向合金应力应变曲线展现出塑性，弹性极限下降为2%左右，有明显屈服现象，断口处存在{112}<111> 变形孪晶。SEM原位拉伸结果发现[102]取向合金裂纹源首先在脆性强化相Ti2Ni相或Nb /Ti2Ni共生组织中萌生，裂纹扩展缓慢，当应力积累到1500N时，裂纹扩展开裂。断口表面基体组织中发现较为密集的变形迹线，对变形迹线进一步分析表明，它们包括滑移带和孪晶，主要是（-211）<111>位错滑移和（12-1）<-111>变形孪晶。TEM原位拉伸结果进一步证实，在裂纹扩展前未发现位错和孪晶，裂纹扩展同时在裂尖前方应力诱发形变孪晶，释放应力，裂纹沿孪晶界呈锯齿状继续扩展，引发裂尖位错形成，应力不断集中与释放的过程，使裂尖形态不断重复由尖锐变钝过程。TEM高分辨发现在部分变形孪晶界上发现了ω相析出，与基体的取向关系为[-113]B2//[2-1-1-3]ω, (110)B2//(1-101)ω和 (2-11)B2//(0-110)ω，ω相晶格常数与bcc基体关系如下：a_ω=√2 a_bcc，c_ω=√3⁄2 a_bcc，c_ω⁄a_ω ≈0.612，空间群为D_6h^1 (P6⁄mmm)，其惯习面位于bcc基体相的{112}面。高温拉伸时富Nb相和Ti2Ni相动态球化，大量纳米级β-Nb相析出，800℃拉伸时纳米相对位错有一定的钉扎作用。随着拉伸温度增加至900℃，有再结晶现象发生，纳米级β-Nb粗化，Nb的固溶强化作用和对位错钉扎作用减弱，高温强度取决于纳米Nb析出相与位错交互强化作用，Nb析出后引起固溶强化效果弱化以及高温再结晶软化几方面共同作用。在NiTi-Al基合金中发现了一种含20%Nb的新强化相，具有孪晶或非孪晶两种形态，晶体结构确定为底心单斜结构：a = 0.987 nm, b = 0.504 nm, c = 1.172 nm, α = γ = 90°, β = 130.18°，空间群为C2/c。NiTi-Al金属间化合物通过取向优化和成分优化可获得塑性、高强度、高弹性极限的良好匹配。完成了项目的研究内容，并达到研究目标。