【作者】 关锐峰；

【导师】 张新平；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2011， 硕士

【摘要】 本论文研究以制备近零膨胀的NiTi合金为目标,采用粉末烧结法成功制备出Ni/Ti原子比为43.8:56.2的多孔NiTi合金和NiTiCu合金,然后利用无压浸渗工艺制备出AZ91D/NiTi复合材料。采用物相分析（XRD）、热分析（DSC）以及压缩力学试验全面表征了材料的相组成、相变行为和循环压缩载荷下的力学性能,并对材料的热膨胀行为进行了全面研究,分析了影响材料负热膨胀性能的因素。研究结果表明,Ni/Ti的原子比显著影响多孔NiTi合金的热膨胀性能。当Ni原子百分含量小于46%时,多孔NiTi合金在100-150℃温度范围内具有一定的负热膨胀性能,其负热膨胀温度区间的大小随Ni含量的增加先增大后减小,其中具有Ni_43.8Ti_56.2成分的多孔合金具有最宽的负热膨胀温度区间。多孔Ni_43.8Ti_56.2合金中主要相为NiTi（B2和B19′）和NiTi₂相,在冷却和加热过程分别发生B2→B19′和B19′→B2相变。规则或球形的孔隙有利于降低负热膨胀行为出现的起始温度与结束温度。研究发现,孔隙率的增加使合金发生负热膨胀行为的起始温度和结束温度先增大后减小,负热膨胀温度区间变窄,压缩强度降低,强度下降率也增大。熔融镁合金可以在毛细附加压力作用下填充多孔NiTi合金中的开孔,制备的AZ91D/NiTi复合材料中尽管出现了MgO、MgO₂、Mg₁₇Al₁₂、MgAl)2O₄几种新相,但其相变行为与多孔NiTi合金相似。AZ91D合金对复合材料的性能影响很大,控制AZ91D含量在8.20-13.95 wt.%之间有利于获得近零膨胀材料;AZ91D可增强多孔预制体的压缩强度,但当含量超过15.66 wt.%时复合材料会在较低应变下提前失效。另外,复合材料的强度下降率随着多孔预制体孔隙率的增大而增大,其形状回复率在40%-70%之间。Cu原子可以很好地在NiTi合金中固溶,易于制备出NiTiCu合金,其相变过程只发生马氏体相变及其逆相变。Cu含量的增加时,合金的相变温度降低,压缩强度降低,强度下降率增大,负热膨胀行为的结束温度降低,负热膨胀系数趋近于零,负热膨胀温度区间也变窄;但当Cu的含量超过10 wt.%时,NiTiCu合金不具有负热膨胀性能。更多还原

【Abstract】 Aiming at development of near zero thermal expansion NiTi alloys, in this research the porous NiTi alloys and NiTiCu alloys as well as AZ91D/NiTi composites, all three materials have the same Ni/Ti atomic ratio of 43.8:56.2, have been successfully fabricated by using powder sintering and pressureless infiltration process. Phase analysis （XRD）, thermal analysis （DSC） and mechanical compression test were employed to characterize systematically the phase components, phase transformation behavior and mechanical properties under cycling compression loading. As the focus of this thesis study, the thermal expansion behavior of the fabricated materials was characterized systematically, and the influential factors on negative thermal expansion behavior of the materials were analyzed.The results show that the Ni/Ti atomic ratio has a significant influence on the thermal expansion properties of porous NiTi alloys. When the Ni/Ti atomic ratio is less than 0.46 the porous NiTi alloys exhibit negative thermal expansion property in the temperature range of 100-150℃, and the width of the temperature range increases initially and then decreases with increasing Ni content. The porous Ni_43.8Ti_56.2 alloy shows the widest negative thermal expansion temperature range.XRD analysis results show that the major phases in porous Ni_43.8Ti_56.2 alloy are NiTi （B2 and B19′） and NiTi₂, and B2→B19′or B19′→B2 phase transformation happens in the alloy during cooling or heating process. The regular pores could lower the starting temperature and end temperature of negative thermal expansion behavior. It has been proved that with increasing the porosity, both of the starting temperature and end temperature of negative thermal expansion behavior increase at first, then decrease; and the width of the temperature range of negative thermal expansion decreases, compressive strength of the alloy decreases and the strength of the alloy decreases greatly.During the fabrication of porous NiTi matrix composites, the molten AZ91D could infill successfully the open pores of porous NiTi alloys under the capillary force at the optimized processing temperature. AZ91D/NiTi composites exhibit the phase transformation behavior very similar to that of porous NiTi alloys, despite some new phases, such as MgO, MgO₂, Mg₁₇Al₁₂ and MgAl₂O₄, formed. It has been shown that AZ91D has a significant influence on AZ91D/NiTi composites’properties, the AZ91D content between 8.20 and 13.95 wt.% is beneficial for fabrication of near zero thermal expansion AZ91D/NiTi composites. Further, AZ91D could increase compressive strength of porous NiTi alloys, but if AZ91D content in the composit is too high, for example over 15.66 wt.%, early failure of the material may occur at a small strain. In addition, the strength of AZ91D/NiTi composites decreases greatly with increasing porosity of the preformed porous NiTi alloys, and the rates of shape recovery of the composites are in the range of 40%-70%.Moreover, as Cu atoms could form solid solution with NiTi, thus NiTiCu alloy can be fabricated easily. In NiTiCu alloys martensitic phase transformation and austenitic phase transformation occur during cooling and heating process. With increasing Cu content, the phase transformation temperatures decrease and the compressive strength of the alloy decreases strongly; in particular the end temperature of negative thermal expansion behavior decreases with the negative thermal expansion coefficient close to zero and the width of temperature range of negative thermal expansion decreasing. It is worth noticing that the NiTiCu alloys with the Cu content over 10 wt.% would not exhibit negative thermal expansion behavior.更多还原