【作者】 王俊；

【导师】 马广义； 王宣平；

【作者基本信息】 大连理工大学 ， 机械工程， 2025， 硕士

【摘要】 NiTi形状记忆合金（SMA）具有形状记忆效应、超弹性及弹热效应,以及优异的耐腐蚀性、耐磨性及生物相容性,被广泛应用于生物医学、航空航天、汽车工业、土木交通等领域。使用传统方法（如锻造、粉末冶金等）制备的NiTi合金很难成形形状复杂构件。增材制造是一项革命性技术,在复杂构件一体成形、梯度结构成形,个性化定制等方面优势明显。但是,增材制造具有较快的冷却速率、周期性热历程,易使成形构件产生元素偏析、残余应力等现象,影响其相变及形状记忆性能。虽然通过激光功率、扫描参数等制造工艺参数的调整可以避免一些缺陷,但是仍然需要后续的热处理方法进一步提升质量和性能。本文聚焦研究时效热处理对激光定向能量沉积（LDED）NiTi SMA微观组织、相变行为及形状记忆效应的影响,通过讨论不同时效温度（400℃、500℃、600℃）和时间（2 h、4 h、6 h）的热处理工艺,分析了相成分及分布、相变行为、晶粒形态及织构、显微硬度和形状记忆效应等演变过程。研究表明,时效热处理一方面通过影响第二相的析出和分布影响相变过程,另一方面通过减少大角度晶界降低相变阻力,最终提升形状记忆性能。本文的主要工作与结论如下:（1）揭示了时效热处理后LDED制备的NiTi SMA微观组织演变规律。LDED制备的NiTi形状记忆合金晶粒形貌以柱状晶为主,主要相有NiTi B2、NiTi B19’、NiTi₂、Ni₄Ti₃及Ni₃Ti相,时效热处理不会改变相种类和晶粒形态。400℃时效处理对微观组织影响不大;500℃时效处理会促进非均匀分布的Ni₄Ti₃相的析出,并大幅减少大角度晶界和增加织构强度;600℃时效处理会促进Ni₄Ti₃相的均匀分布,同时也会大幅减少大角度晶界,对织构强度影响不大,时效时间延长会导致Ni₄Ti₃相的尺寸增大。（2）探究了时效热处理对LDED制备的NiTi形状记忆合金相变行为的影响。时效热处理会引起相变温度升高,并且相变焓也会增加,最大能提升900%以上。时效时间延长,相变温度略有升高,但不会改变相变历程。非均匀分布的Ni₄Ti₃相会造成降温时出现两阶段相变,少量的或均匀分布的Ni₄Ti₃相不会对相变历程造成影响。使用分子动力学模拟的方法探究了晶界含量对相变过程的影响,发现晶界原子含量增加会增加相变阻力,明确了500℃和600℃时效热处理后相变焓大幅提升的主要是与大角度晶界大幅减少紧密相关。（3）揭示了时效处理工艺-微观组织-力学性能关联机制。对不同时效处理工艺的NiTi SMA样件进行了纳米压痕测试和形状记忆效应测试。经过时效处理后试样的内应力和位错密度降低,因此显微硬度随时效温度升高和时效时间延长下降。试样的弹性模量对时效热处理工艺不敏感。所有热处理试样在4%变形条件下均能达到95%以上的回复率;500℃热处理后试样在6%变形条件下最高能达到97%左右的回复率,600℃热处理后回复率也均能达到90%以上。形状记忆效应的提升一方面得益于更低的晶界对马氏体相变的阻力,一方面得益于Ni₄Ti₃相促进了马氏体形核。更多还原

【Abstract】 NiTi shape memory alloys（SMAs）are widely used in biomedical,aerospace,automotive,civil engineering and transportation fields due to their shape memory effect,superelasticity and elasto-thermal effect,as well as excellent corrosion resistance,wear resistance and biocompatibility.NiTi alloys prepared by traditional methods（e.g.,forging,powder metallurgy,etc.）are difficult to form complex shapes.Additive manufacturing is a revolutionary technology,in the complex components of one-piece forming,gradient structure forming,personalized customization and other advantages.However,additive manufacturing has a fast cooling rate,cyclic thermal history,and is prone to elemental segregation and residual stress,which affects the phase transformation and shape memory properties of the formed components.Although some defects can be avoided by adjusting the manufacturing process parameters such as laser power and scanning parameters,subsequent heat treatment methods are still needed to further improve the quality and performance.In this paper,we focus on the effect of aging heat treatment on the microstructure,phase transformation behavior and shape memory effect of laser directed energy deposition（LDED）NiTi SMA.By discussing the heat treatment process with different aging temperatures（400℃,500℃,600℃）and times（2 h,4 h,6 h）,we analyze the evolution of phase composition and distribution,phase transformation behavior,grain morphology and weave,microhardness and shape memory effect.shape memory effect and other evolutionary processes.The study shows that the aging heat treatment affects the phase transformation process by influencing the precipitation and distribution of the second phase on the one hand,and reduces the resistance to phase transformation by reducing the large-angle grain boundaries on the other hand,which ultimately improves the shape memory properties.The main work and conclusions of this paper are as follows:（1）The microstructure evolution of NiTi SMA prepared by LDED after aging heat treatment is revealed.The grain morphology of NiTi shape memory alloy prepared by LDED is dominated by columnar crystals,and the main phases are NiTi B2,NiTi B19’,NiTi₂,Ni₄Ti₃,and Ni₃Ti.The aging heat treatment does not change the phase types and grain morphology.400℃aging heat treatment has little effect on the microstructure of the sample;500℃aging heat treatment will promote the precipitation of non-uniformly distributed Ni₄Ti₃ phase,and significantly reduce the large angular grain boundaries and increase the textile strength;600℃aging heat treatment will promote the uniform distribution of Ni₄Ti₃ phase,and also significantly reduce the large angular grain boundaries,but does not significantly affect the strength of the textile strength.The extension of aging time will lead to the increase in the size of Ni₄Ti₃ phase.（2）The effect of aging heat treatment on the phase transition behavior of NiTi shape memory alloys prepared by LDED was investigated.Ageing heat treatment causes an increase in the phase transition temperature,and the enthalpy of phase transition increases,up to a maximum of more than 900%.Prolonged aging time slightly increases the phase transition temperature,but does not change the course of the phase transition.The non-uniformly distributed Ni₄Ti₃ phase causes a two-stage phase transition at lower temperatures,while little or uniformly distributed Ni₄Ti₃ phase has no effect on the phase transition history.The effect of grain boundary content on the phase transformation process was investigated using molecular dynamics simulations.It was found that an increase in the atomic content of grain boundaries increases the resistance to phase transformation,and it was clarified that the substantial increase in the enthalpy of phase transformation after aging heat treatment at 500℃and 600℃was mainly closely related to the substantial reduction of HAGBs.（3）The mechanism of aging treatment process-microstructure-mechanical properties correlation was revealed.Nanoindentation tests and shape memory effect tests were conducted on NiTi SMA samples with different aging treatment processes.The internal stresses and dislocation densities of the samples were reduced after aging treatment,and thus the microhardness decreased with the increase of aging temperature and aging time.The elastic modulus of the specimens was insensitive to the aging heat treatment process.All the heat-treated specimens can reach more than 95%recovery rate at 4%deformation;the highest recovery rate of the specimens can reach about 97%at 6%deformation after heat treatment at500℃,and the recovery rate at 6%deformation can reach more than 90%after heat treatment at 600℃.The enhancement of shape memory effect is partly attributed to the resistance of lower grain boundaries to martensitic phase transformation,and partly attributed to the promotion of martensitic nucleation by Ni₄Ti₃ phase.更多还原