【作者】 张喆；

【导师】 张学习；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2015， 硕士

【摘要】 本文采用真空感应熔炼法制备3种不同成分的Ni50Mn25Ga25-x Cux合金(x=2.2、3.0、3.8)。利用扫描电子显微镜(SEM)和金相显微镜(OM)观察Ni-Mn-Ga-Cu合金的微观组织结构,利用X射线衍射仪(XRD)、差热扫描量热仪(DSC)分析Ni-Mn-Ga-Cu合金的物相和马氏体相变过程,发现Cu元素含量低于6at%的掺杂不会出现第二相;对x=3.8的Ni-Mn-Ga-Cu合金采用熔体纺丝法制备直径尺寸为20μm到60μm的纤维,利用扫描电子显微镜(SEM)分析热处理前后合金纤维的物相成分,并使用综合物性测量系统(PPMS)测量热处理态纤维的磁热性能;使用动态机械分析仪(Q800 DMA)研究了Ni50Mn25Ga25-x Cux(x=3.8)合金纤维的力学性能,结果表明Cu元素的掺杂明显改善了Ni-Mn-Ga合金本身的脆性。选择Cu元素作为第四组元对Ni-Mn-Ga合金进行掺杂,合金元素成分配比为Ni50Mn25Ga25-x Cux(x=2.2、3.0、3.8)时微观组织内部几乎没有第二相的析出,通过X射线衍射分析发现室温下合金为单相马氏体。使用DSC分析合金的相变温度和居里温度,发现合金相变温度随Cu含量的增加而显著提高。随着Cu含量由2.2增加到3.8,合金马氏体相变开始温度由19.5℃增加到了100.2℃。随Cu元素含量的提高,价电子浓度变化,相变温度随之升高,然而合金居里温度为70℃受Cu元素含量变化影响较小。考虑到熔体纺丝法制备Ni50Mn25Ga25-x Cux纤维过程以及后续热处理过程中的成分损失,选择Cu含量为3.8时的合金铸锭制备纤维。对制备态纤维进行化学有序化热处理和去应力退火,采用PPMS测量其相变温度发现其马氏体相变点与居里点发生了耦合,有利于产生较高磁热效应。纤维在356K温度附近下具有最大的磁熵变,以356K为中心设计测量纤维的等温磁化曲线,计算纤维在5T、359K下条件的最大磁熵变为-5.3J/kg·K,与Ni-Mn-Ga合金不同制备状态相比,磁热效益明显提高。利用DMA分别对标距为2mm、5mm和10mm的纤维进行室温拉伸,发现标距为2mm的纤维由于其具有较高的断裂强度和延伸率,本文选择其作为研究Ni-Mn-Ga-Cu合金纤维室温力学性能的标准。对直径分别为30μm、40μm、50μm的Ni-Mn-Ga-Cu合金纤维进行室温拉伸,发现随着纤维直径的增加,纤维的断裂强度从713MPa降低到447MPa、439MPa,弹性应变基本没有发生变化,分别为1.52%、1.62%、1.68%。30μm直径纤维断裂后最大塑性变形为为3.89%,表现出较大的拉伸塑性。40μm、50μm直径纤维变形过程过程中拉伸塑性较小,断裂后最大塑性变形分别为0.57%、0.62%。纤维力学性能的变化与纤维内部微观结构、纤维成形过程中增加的缺陷有关。对比制备态与热处理态的纤维室温拉伸性能,发现经过热处理后由于晶粒长大以及界面成分损失等因素,纤维的断裂强度和拉伸塑性发生了明显的降低。对比Cu元素掺杂前后纤维的力学性能,可以发现Cu元素可以显著降低Ni-Mn-Ga合金的本征脆性。此外对Ni-Mn-Ga-Cu合金纤维进行了断裂可靠性评价。制备态Ni-Mn-Ga-Cu合金纤维具有较高的Weibull模数m和门槛应力值σu,相对于未掺杂Cu元素的Ni-Mn-Ga合金纤维,具有更高的断裂可靠性和使用安全性。更多还原

【Abstract】 In this paper, 3 kinds of Ni50Mn25Ga25-x Cux alloys(x=2.2, 3.0, 3.8) with different compositions were prepared by vacuum induction melting method. The microstructures of Ni-Mn-Ga-Cu alloy were observed by using scanning electron microscopy(SEM) and optical microscope(OM). X-ray diffraction(XRD) and differential scanning calorimetry(DSC) were utilized to analyze the phase transformation of Ni-Mn-Ga-Cu alloys, no second phase was found. Ni50Mn25Ga25-x Cux microwires(x=3.8) with diameter of 20 to 60μm were prepared by melt-extraction method. Chemical ordering annealing and internal stress release treatment were applied. Magnetocaloric effect of the microwires after annealing was analyzed on the physical property measurement system(PPMS). Finally, room temperature mechanical properties of the as-extracted and annealed microwires were studied by using a dynamic mechanical analyzer(Q800 DMA) under the tension mode, the results show that Cu doping Ni-Mn-Ga significantly reduced the brittleness of Ni-Mn-Ga alloy.Cu element was choose to take the place of Ga element and act as the fourth element in Ni50Mn25Ga25-x Cux with different composition of x=2.2, 3.0 and 3.8. No secondary phase was found in all alloys after polishing and etching using OM and SEM. XRD results showed that all three alloys are single martensite at room temperature. DSC analysis indicated that the phase transformation temperature increased with increasing Cu content. Actually, the valence electron concentration would be changed with the increase of Cu content, thus, the transformation temperature changed. The Curie temperature of the alloys maintained 70℃and less affected by the Cu element content.Higher Cu content of 3.8 was chosen to prepare the microwires due to the composition loss during melt-extraction and the later heat treatment process. Chemical ordering annealing and internal stress release treatment was applied. PPMS was used to measure the martensitic transformation of the annealed microwires and coupled martensitic transformation and Cuire temperature was found which was considered to have better magnetocaloric effect.At 356 K the microwires has a maximum magnetic entropy change, so designed the isothermal magnetization curves to calculate the microwires’ maximum magnetic entropy change is-5.3J/kg·K at 5T, 359 K.Different gauge lengths of 2mm, 5mm and 10 mm were chosen to test the mechanical properties of the microwires at room temperature. The 2mm gauge length sample exhibited the best fracture strength and elongation and was chosen to perform all the mechanical properties of the microwires in this work. Tensile tests were performed at microwires with different diameter of 30μm, 40μm, 50μm. The tensile strength decreased from 713 MPa to 447 MPa with increasing diameter while the elastic deformation remained unchanged of 1.52%、1.62%、1.68%, respectively. The maximum plastic strain of 3.89% was found in microwires with diameter of 30 μm, indicating excellent ductility. However, 0.57% and 0.62% plastic strain were found in microwires with diameter of 40 and 50 μm, indicating a decreasing ductility with increasing diameter. The microstructure and the defects formed during fabrication were considered to be the key factors affecting the mechanical properties of the microwires By comparing the mechanical properties of alloy with and without Cu, the mechanical properties of Ni-Mn-Ga alloy can be improved significantly. In addition, the fracture reliability evaluation of Ni-Mn-Ga-Cu alloy fiber was carried out. As-extracted Ni-Mn-Ga-Cu alloy microwires has higher Weibull modulus m and threshold stress value sigma u, indicating higher fracture reliability and safety in comparison to the ternary Ni-Mn-Ga microwires.更多还原