外磁场对Ni-Mn-Ga磁性形状记忆合金相变应变及显微组织的影响

研究了外磁场对Ni52Mn24.6Ga23.4(%,原子分数)单晶马氏体相变及其相变应变的影响,并对磁场增强相变应变的微观机制进行了探讨。研究结果表明无外加磁场时,NiMnGa合金发生马氏体相变时可产生约0.3%的收缩形变,沿单晶[100]方向施加外磁场,其相变应变随磁场的增加而呈近线性增加。当外磁场强度为6.37×105A/m时,应变量达到最大值(3.5%)。磁场作用下冷却形成的马氏体虽然孪晶亚结构不变,但自协作组态消失,并伴随有孪晶板条的增厚。磁场对马氏体相变应变的增强效应来自于磁场作用下的马氏体变体的择优取向。     

Ni1.95Mn1.36Ga0.69磁控形状记忆合金的相变与结构有序化

利用差示扫描量热计(DSC)研究了非化学计量Ni1.95Mn1.36Ga0.69合金的马氏体相变及结构有序化处理条件.根据DSC峰型特征的变化,可确定在800℃进行结构有序化处理是合适的.在800℃又进行了不同保温时间的处理,并测定了马氏体相变的特征温度.马氏体相变特征温度随保温时间的增加呈规律变化,相变热滞随保温时间的增加呈下降趋势.经50h的有序化处理后,再继续延长保温时间,马氏体相变特征温度和热滞均不再有明显变化.本文研究结果表明,对于Ni1.95Mn1.36Ga0.69合金,应选用800℃×50h的结构有序化处理工艺.     

Ni55.30Fe17.60Ga27.10磁致形状记忆合金马氏体相变

研究了热处理对Ni55.30Fe17.60Ga27.10磁致形状记忆合金组织和马氏体相变的影响.电弧态合金组织中马氏体板条模糊.在氩气保护,800℃保温4h缓慢冷却条件下热处理后,马氏体板条规整平直.DSC结果表明Ni55.30Fe17.60Ga27.10合金在加热/冷却时发生热弹性马氏体相变.热处理后Ms=43.24℃,Mf=36.72℃,As=46.84℃,Af=51.83℃.相比电弧态,热处理后合金Ms和Mf提高,As和Af降低,相变滞后减小.XRD结果证明热致马氏体为14M马氏体.     

高温形状记忆合金Ni54Mn25Ga21相变及其形状记忆效应的研究

主要研究了马氏体相变温度Ms高于居里温度Tc的Ni54Mn25Ga21合金的相变及其单晶的形状记忆效应.采用真空电弧炉熔炼,然后用磁悬浮区熔晶体生长炉进行Ni54Mn25Ga21合金的单晶生长,成功制备了Ni54Mn25Ga21单晶.对多晶粉末样品进行了原位X射线衍射变温分析,结果表明Ni54Mn25Ga21合金具有可恢复的热弹性马氏体相变性能.对Ni54Mn25Ga21单晶进行的形状记忆效应实验结果表明,当总预应变不超过6%时,压缩变形后残留的应变可在随后的加热过程中完全回复.     

NbRu合金室温马氏体的相变和显微组织

利用示差扫描量热分析、X射线衍射分析、透射电子显微分析和高分辨电子显微分析研究了等原子比NbRu合金室温马氏体的相变和显微组织．确定了其在一个冷热循环中发生两步相变时不同的相变温度，研究了其室温热马氏体组织的微观组织及其亚结构，实验结果表明：该合金的马氏体为热弹性马氏体；马氏体变体间界面呈现典型的自协作组态；变体内部存在大量的(101)I型孪晶亚结构，孪晶亚结构板条间界面平直、清晰。     

Fe85Ga15多晶在模拟海水中的应力腐蚀行为研究

研究了新型磁致伸缩材料Fe-Ga合金的应力腐蚀性能。采用恒载荷和慢应变速率拉伸(SSRT)试验,结合电化学测试技术,研究了铸态Fe85Ga15多晶在模拟海水中的应力腐蚀。结果表明,开路电位下,模拟海水中薄板光滑试样恒载荷拉伸能够发生低于材料抗拉强度的断裂,且断裂时间明显依赖于外加应力的大小,外加应力愈大,断裂时间愈短。这表明铸态Fe85Ga15合金在模拟海水中能够发生应力腐蚀开裂(SCC),且SCC归一化门槛应力为σscc/σb=0.34。慢应变速率拉伸显示,SCC敏感性在应变速率为5×10-7/s时最大,用强度损失表示为Iσ=(1-σc/σb)×100%=35%。阴极极化升高而阳极极化降低恒载荷SCC断裂时间。这些结果初步表明铸态Fe85Ga15合金在模拟海水中的应力腐蚀为阳极溶解型。     

Co-Ni磁性形状记忆合金的显微组织和力学行为

采用光学显微分析、X射线衍射分析及拉伸试验研究了合金成分对Co-Ni磁性形状记忆合金的显微组织和力学行为的影响.结果表明,Co-Ni合金均匀化退火后其热致马氏体为密排六方结构,点阵参数为a=0.251nm,c=0.407 nm,热轧后马氏体组织呈透镜状.母相为面心立方结构,其点阵参数为a=0.355 nm.试验合金在弹性阶段过后,呈现连续的加工硬化,没有明显的屈服平台,随Ni含量的增加,σ02、E值下降,δ增加.     

人工时效对激光选区熔化AlMg4.5Sc0.55Mn0.5Zr0.2合金显微组织和力学性能的影响

采用激光选区熔化制备AlMg_4.5Sc_0.55Mn_0.5Zr_0.2合金,研究人工时效工艺参数对合金维氏硬度的影响规律,分析沉积态和优选时效态合金的室温拉伸性能和显微组织。结果表明：人工时效使该合金的维氏硬度由102HV提升至140HV以上。随着时效温度升高（305~335℃）或时效时间延长（1.5~48 h）,维氏硬度呈现先增加、再降低、最后逐渐趋于稳定的规律。在315℃时效3 h或12 h后,合金的室温拉伸性能基本相当,无明显的各向异性;抗拉强度和屈服强度分别达到470 MPa和410 MPa,断后伸长率保持在15.0%。力学性能的提升得益于人工时效过程中弥散析出且与基体共格的纳米增强颗粒Al₃（Sc,Zr）。     

Ni52 Mn23Ga24.5 Sm0.5合金的马氏体相变和磁致伸缩性能

研究了多晶Ni52 Mn23 Ga24．合金添加微量的稀土元素Sm后，对合金的马氏体相变和磁致伸缩性能的影响。结果发现，微量稀土元素Sm的掺入，降低了合金的马氏体相变温度和居里温度，但并未改变合金的晶体结构，同时由于晶粒细化作用，使合金室温时的磁致应变性能下降。     

添加元素Al对Tb0.3Dy0.7Fe1.95合金组织结构及性能的影响

研究了Tb0.3Dy0.7Fe1.95合金中添加少量Al替代Fe时,对合金晶体结构、显微组织、磁致伸缩系数影响.结果发现,添加Al没有改变合金的晶体结构,随着Al添加量的增加,晶体显微组织中的析出物增加;在一定的磁场强度下,磁致伸缩系数随Al添加量而变化,当磁场强度低于40kA/m时,磁致伸缩系数随Al添加量的变化曲线出现一峰值,当磁场强度高于40kA/m时,磁致伸缩系数随Al添加量的增加而降低.     

Martensitic transformation and shape memory effect in Ni54.75Mn13.25Fe7Ga25 ferromagnetic shape memory alloy

The martensitic transformation and shape memory effect of Ni_54.75Mn_13.25Fe₇Ga₂₅ (at.%) alloy are studied in the present paper. It is shown that tetragonal martensite with parallel bands substructure transforms to parent phase heated by electron beam. It can be clearly observed that the martensite band becomes smaller and smaller, then transforms to parent phase completely in the end. A large reversible transformation strain, about 1.5%, is obtained in this undeformed polycrystalline alloy due to martensitic transformation and its reverse transformation. This transformation strain is also increased to 1.8% by the external magnetic field. It is believed that the effect of the magnetic field on the preferential orientation of martensitic variants increases the transformation strain.     

Structural transition and atomic ordering of Ni49.8Mn28.5Ga21.7 ferromagnetic shape memory alloy powders prepared by ball milling

The Ni_49.8Mn_28.5Ga_21.7 powders of micro-scale irregular equiaxial particles are prepared by ball milling method, and characterized by XRD, DSC and SEM techniques. The powders are found to contain disordered fct structure. Upon heating to high temperatures, the crystal structure of the as-milled powder is found to evolve from disordered fct to disordered bcc and then to a Heusler-type structure sequentially. The critical temperature for the transition from the bcc phase to the Heusler phase is 360 °C. This phase transition process is also a disorder–order transition. An atomic ordering model similar to the grain nucleation and growth is established to explain the annealing temperature dependence of the phase transformation temperature.     

Phase transformation,thermoelastic and magnetic properties of Ni50Mn30Ga20−xCux ferromagnetic shape memory alloys

Effect of addition of Cu on phase transformation temperatures, enthalpy and entropy changes, Curie temperature, magnetization saturation were investigated in Ni–Mn–Ga ferromagnetic shape memory alloy. The results show that the Ni₅₀Mn₃₀Ga_20−xCu_x alloys exhibit thermoelastic martensitic transformation. The martensitic and reverse martensitic transformation temperatures, enthalpy and entropy changes and thermal hysteresis increase with increase of Cu content. Martensite structure changes from 7 M with 0–0.5 at.% Cu to non-modulated T martensite when the content of Cu is more than 0.5 at.%. In addition, the Curie temperature almost remains unchanged at low-Cu content and subsequently decreases obviously. Magnetization saturation of alloys decrease with increasing Cu content since it is sensitive to ordered atomic arrangement.     

Microstructural, mechanical and magnetic properties of shape memory alloy Ni55Mn23Ga22 thin films deposited by radio-frequency magnetron sputtering

The near-stoichiometric Ni₂MnGa ferromagnetic alloys are one of the smart materials, that are of a great interest when they are deposited as a thin film by r.f. sputtering. These thin films of shape memory alloys are prospective materials for micro and nanosystem applications. However, the properties of the shape memory polycrystalline thin films depend strongly on their structure and internal stress, which develop during the sputtering process as well as during the post-deposition annealing treatment. In this study, about 1 μm Ni₅₅Mn₂₃Ga₂₂ thin films were deposited in the range 0,45 to 1,2 Pa of Ar pressure and P = 40 to 120 W. Their composition, crystallographic structure, internal stress and stress gradient, indentation modulus, hardness, deflection induced by magnetic field and magnetic properties were systematically studied as a function of the temperature of the silicon substrate ranging from 298 to 873 K and the vacuum annealing treatment at 873 K for 21,6 ks and 36 ks. A silicon wafer having a native amorphous thin SiO_x buffer layer was used as a substrate. This substrate influences the microstructure of the films and blocks the diffusion process during the heat treatment.The crystal structure of the martensitic phase in each film was changed systematically from bct or 10 M or 14 M. In addition, the evolution of the mechanical properties such as mean stress, stress gradient, roughness, hardness and indentation modulus with the temperature (of substrate or of heat treatment) were measured and correlated to crystal structure and morphology changes.Moreover, it has been shown that it is necessary to associate a high temperature (873 K) annealing during a long time (21 ks and 36 ks) to obtain good ferromagnetic properties. Thus, for the well annealed films (36 ks at 873 K) the magnetostrain is about - 170 ppm for a magnetic field of 1 MA m^- 1 applied along the beams.As a conclusion, the response of free-standing magnetic shape memory films to a magnetic field of 0,2 MA m^- 1 depends strongly on the martensitic structure, internal mechanical stress (mean and gradient) and magnetic properties. The free-standing annealed film at 873 K for 36 ks points out a considerable magnetic actuation associated with bct or 10 M or 14 M martensitic structures.     

Effect of pre-strain on martensitic transformation of Ni43Mn43Co7Sn7 high- temperature shape memory alloy

In the present work, the effect of pre-strain on martensitic transformation of Ni₄₃Mn₄₃Co₇Sn₇ (at.%) alloy was investigated. The results show that Ni₄₃Mn₄₃Co₇Sn₇ alloy undergoes a martensitic transformation at 288 °C upon cooling. The thermal cycling does not affect the transformation behavior of the alloy, indicating the good thermal stability. The reverse transformation of the deformed martensite is pre-strain dependent. When the pre-strain is higher than 7.5%, the reverse transformation occurs in two-stage manner upon first heating due to the nonuniform martensite deformation. In contrast, during the first martensitic and second reverse transformation, the pre-strain shows little effect on the transformation temperatures.     

Effects of aging on martensitic transformation of Ti50Ni25Cu25 shape memory alloy

Martensitic transformation has been studied in Ti₅₀Ni₂₅Cu₂₅ shape memory alloy by internal friction (IF) measurement and X-ray diffraction. It shows that the martensitic transformation proceeds from B2 to B19 for the solution-treated Ti₅₀Ni₂₅Cu₂₅ alloy. B2 phase is stabilized, and aging the alloy at 723–923 K decreases internal friction values. Part of the remaining B2 parent phase transform to B19′ monoclinic martensite at much lower temperatures.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.