| 摘要 | Nb5Si3金属间化合物具有高熔点、低密度及较高的高温强度等特点，是有前途的新一代高温结构材料。然而，单相Nb5Si3在室温较低的塑性和韧性极大地限制了它的实际应用。为了解决这个关键问题，通过引入塑性Nbss相形成两相或多相Nb/Nb5Si3合金是非常有效的方法之一。但相对低强度Nbss的过量添加将显著降低其高温强度。通过合金化和定向凝固工艺可有效提高材料的性能。因此，本文采用电弧熔炼及定向凝固制备了不同成分的Nb/Nb5Si3原位复合材料，借助于X射线衍射术、扫描电子显微镜、透射电子显微镜及Gleeble1500压缩检测等分析测试手段，深入系统地研究了Nb/Nb5Si3原位复合材料的显微组织及力学性能。
铸态Nb-22Ti-16Si-xZr合金的组织是由Nbss，(Nb)3Si和γ(Nb)5Si3三相组成。Zr促进初生Nbss相的粗化及Nbss/γ(Nb)5Si3片层组织的形成。合金的室温屈服强度随Zr含量增加而增加，断裂应变随Zr含量增加，在Zr含量3 at.%时达到峰值。高温屈服强度随Zr含量增加先减小，随后在Zr含量3 at.%时达到最小值，然后再次增加。这些变化主要归因于Zr的加入而导致的合金组织和成分的改变。当无Zr时，初生Nbss粒子尺寸较小，在脆性相的约束下丧失其塑性变形的能力而呈现解理断裂。当加入Zr时，初生Nbss粒子尺寸的增大有助于界面剥离的发生，其断口呈现出穿晶解理和界面剥离混合断裂的特征。
Hf的添加显著地改变Nb-16Si合金的组织形貌。随Hf量的增加，组织显著细化，当Hf量增加到7at.%时，共晶团几乎消失。合金的断裂韧性随Hf量增加而增加，细小共晶组织的减小和初生Nbss相数量的增加都有利于合金断裂韧性的提高。Hf导致Nbss断裂模式从脆性解理过渡到塑性延伸。适量Hf的添加可以提高合金的高温强度，Hf含量为3at.%时合金的高温强度达到最大值。
1.5at.% Sn添加到Nb-20Ti-5Cr-3Al-18Si合金中，使组织没有发生明显的改变，Sn优先固溶到(Nb, Ti)ss而不是(Nb, Ti)5Si3中，Sn的添加降低了Cr在(Nb, Ti)ss中的固溶能力。Hf使细小的共晶组织消失，并且在(Nb, Ti)ss/β(Nb, Ti)5Si3两相界面形成一薄层富Hf的γ(Nb, Ti)5Si3相。Sn的添加使合金的强度降低，而塑性稍微增加，这归因于Cr在(Nb, Ti)ss中的固溶能力的下降。Hf的添加同时提高了合金的度和塑性，强度的增加是由于Hf的固溶强化作用；塑性增加是由于细小共晶组织的消失及富Hf的γ(Nb, Ti)5Si3相在相界的形成。因此，Hf对同时提高合金的强度和塑性是有益的。在室温，合金中β(Nb, Ti)5Si3相呈脆性断裂特征，而(Nb, Ti)ss出现明显的塑性变形，γ(Nb, Ti)5Si3呈现少量的塑性变形特征。
Ho添加到Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf合金中，细化了其微观组织，特别是减小了硅化物相粒子的尺寸，促进块状Ho2Hf2O7在(Nb, Ti)ss/(Nb, Ti)5Si3相界面处形成，抑制了(Ti, Nb)5Si3相的形成。适量Ho可改善合金的强度及塑性。在高应变速率下，微量Ho使合金的高温强度增加，而在较低的应变速率下，强化效应变弱。这种强化效应主要归因于Ho的添加导致的固溶强化和界面强化。含Ho合金的高温压缩行为可用温度补偿的幂规律方程表示为： ，动态回复和再结晶是其主要的软化机制。无Ho合金的断裂表面以解理断裂模式为主，而含Ho合金由于(Nb, Ti)ss局部塑性应变，断裂表面呈现混合断裂。
通过光悬浮区熔化炉成功制备了定向凝固的Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf
-0.1Ho合金。生长速率的增加导致合金组织的细化，这表现为共晶胞尺寸，胞间宽度及相间距的减小。然而，生长速率的增加也导致铌硅化物片层的不连续生长。相对于常规铸态合金，定向凝固合金由于慢的凝固速率，其组织比较粗大，而且(Ti, Nb)5Si3相呈几乎连续网状分布在胞间区。纵向组织为片层硅化物平行于生长方向排列并镶嵌在(Nb, Ti)ss基体内。定向凝固合金在生长方向显示了(Nb, Ti)ss的(110)和(Nb, Ti)5Si3的(310)的强烈方向性。而常规铸态合金没有显示出优先生长方向。选区电子衍射分析表明，在(Nb, Ti)5Si3和(Nb, Ti)ss或(Ti, Nb)5Si3和(Nb, Ti)ss相之间不存在确定的取向关系。界面是干净的，但不是光滑平直的，也没有中间相存在，两种硅化物相和(Nb, Ti)ss相结合的都很好。定向凝固可提高合金的力学性能。但生长速率过快可导致铌硅化物片层的不连续生长，降低合金的力学性能。定向凝固合金断裂呈现出混和断裂特征，断口表面粗糙，(Nb,Ti)ss相发生明显的塑性变形，而常规铸态合金的断口表面相对平坦，(Nb,Ti)ss相出现局部的塑性变形。 |
| 其他摘要 | As a promising candidate of high-temperature structural materials, intermetallic compound Nb5Si3 possesses high melting point, relatively low density and excellent high temperature strength. However, the low deformability and fracture toughness of single-phase Nb5Si3 at ambient temperature have limited seriously its practical applications. To circumvent this critical issue, the formation of two-phase or multiphase composites by incorporating a ductile Nb solid solution (Nbss) phase to Nb5Si3 phase has been proved to be one effective way. However, the over addition of low-strength Nbss will decrease significantly the high-temperature strength and this is not expected. Therefore, much effort has been devoted to get a better balance between room temperature fracture toughness and high temperature strength. It has been established that alloying and directional solidification are effective ways to improve mechanical properties of intermetallics. Therefore, the paper systematically studied the effects of alloying and directional solidification on microstructure and mechanical properties of cast Nb/Nb5Si3 in-situ composites by means of X-ray diffraction, scanning electron microscope and transmission electron microscope, etc.
The effect of Zr addition on the microstructures and mechanical properties of cast Nb-22Ti-16Si alloy was investigated. The results show that the Zr addition leads to coarseness of primary Nbss phase and promotes formation and coarseness of Nbss and γ(Nb)5Si3 lamellar eutectic structure. In addition, the Zr addition increases the hardness of Nbss and (Nb)3Si phases. The yield strength at room temperature of the alloys improves with increasing Zr content. The fracture strain reaches a peak value by 3 at.% Zr content, and then drops with the increase of Zr content. The high-temperature strength firstly decreases and reaches a minimal value by 3 at.% Zr content, and then increases again. These variations are attributed to the microstructure and composition changes of the alloys with increasing Zr content.
The effect of Hf addition on microstructure, room temperature fracture toughness and high-temperature strength of cast Nb-16Si alloy was investigated. The Hf addition changes significantly the microstructural morphology of Nb-16Si alloys, which includes microstructure refinement and disappearance of eutectic colonies. Fracture toughness of the alloys is improved, while high-temerature strength firstly increases, and then decreases, with increasing Hf content. The improvement in mechanical prpperties is mainly attributed to the microstructural change by Hf addition. The Hf addition leads to a transition of Nb solid solution fracture manner from brittle cleavage to plastic stretching.
The effects of Sn and Hf additions on microstructures and compressive properties of Nb-20Ti-5Cr-3Al-18Si alloy were investigated. The Sn addition has no significant effect on the microstructure of the alloy. Sn is preferentially partitioned in (Nb, Ti)ss rather than (Nb, Ti)5Si3. The addition of Sn decreases the solubility of Cr in (Nb, Ti)ss. The addition of Hf causes the coarsening of eutectic structure of (Nb, Ti)ss and (Nb, Ti)5Si3 and the formation of the Hf-rich γ(Nb, Ti)5Si3 along phase boundaries. With the addition of Sn the strength of the alloy decreases, while its ductility increases slightly. The Hf addition improves simultaneously strength and ductility of the alloy.
The addition of Ho causes remarkable microstructural refinement of Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf alloy. Ho addition inhibits the formation of the (Ti, Nb)5Si3 phase and the amount of (Ti, Nb)5Si3 phase decreases, even disappears, with increasing Ho content. Appropriate addition of Ho can improve the compressive ductility and yield strength at the temperature range of room temperature to 1300℃. The addition of minor Ho to the matrix alloy significantly increases its elevated temperature strength at a higher strain rate, while the strengthening effect becomes weak at a lower strain rate. The strengthening effect can mainly be attributed to solid solution strengthening and interface strengthening by Ho additions. The weakening of the strengthening effect is closely related to the disappearance of Ho segregation at the interface. The elevated temperature flow behavior could be generally described by the power-law equations. Dynamic recovery and recrystallization are the dominant softening mechanisms during high-temperature deformation for the alloy. In addition, the fracture surfaces of Ho-free alloys deformed at room temperature in compression tests exhibit cleavage for both the (Nb, Ti)ss and the (Nb, Ti)5Si3 intermetallic compound, while limited local plastic strain of the (Nb, Ti)ss is observed in Ho-doped alloys.
The directionally solidified (DS) Nb-22Ti-16Si-7Cr-3Al-3Ta-2Hf-0.1Ho alloy has been successfully fabricated by using an optical floating zone melting furnace. It is found that with the increase of growth rate the microstructure becomes finer, which can be characterized by the decreased average cell size, intercellular boundary width and interphase spacing. The microstructure in the DS alloy is coarser than that of the conventional cast sample due to the slower solidification rate. The (Ti, Nb)5Si3 phase in the transverse-section microstructure presents almost continuous network distribution at the intercellular regions. The typical microstructure of the longitudinal section shows that the lamellar or columnar silicide phases are aligned parallel to the growth direction and embedded within the (Nb, Ti)ss matrix. No consistent crystallographic orientation relationship either between (Ti, Nb)5Si3 and (Nb, Ti)ss or (Nb, Ti)5Si3 and (Nb, Ti)ss was found. The DS alloy shows strong orientations of (Nb, Ti)ss (110) and (Nb, Ti)5Si3 (310) along the growth direction. Compared to the conventional cast alloy, the DS alloy exhibits improved mechanical properties. However, due to the appearance of discontinuous Nb-silicide plates with the increase of growth rate, the improvement of yield stregth for the DS alloy from the refining microstructure is weakened. In addition, the refinement of Nbss plates with increasing growth rate is unfavorable to the enhancement of fracture toughness under the present DS conditions. |
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