Ni2MnGa单晶马氏体相变过程摩擦耗能的热动力学计算

根据相界面摩擦原理 ,在推导出计算Ni2 MnGa系统热动力学参量的一般表示式的基础上 ,结合马氏体相变温度分别在室温以下、室温附近、室温以上三种非正配分比Ni2 MnGa单晶自发相变应变和交流磁化率随温度变化的测量结果 ,计算了三种样品马氏体相变过程中界面摩擦所消耗的能量 .结果进一步表明正是相变过程中的界面摩擦导致了相变的热滞后 ,而三种样品马氏体相变过程的摩擦耗能和相变热滞后存在较大差别的原因在于三种样品马氏体相变生成物具有不同的结构     

铁磁形状记忆合金Ni52.2Mn23.8Ga24的马氏体相变及其物理表征

对Ni52.2Mn23.8Ga24的单晶样品在马氏体相变过程中的相变潜热、磁性、电阻以及应变等物理序参量进行了测量.测量结果表明:不同的物理机制表征的相变温度有所不同.利用马氏体相变的GT关系予以分析,解释了不同测量方法获得的相变温度差别的原因.研究指出,Heusler合金Ni2MnGa的相变是分布晶格畸变类型,磁结构的变化发生在第二步晶格的非均匀切变,但相变应变与GT模型有区别. 关键词： 马氏体相变 Ni52.2Mn23.8Ga24     

铁磁形状记忆合金Ni52.5Mn23.5Ga24马氏体相变热滞后的研究

用相界面摩擦原理计算了Ni525Mn235Ga24单晶样品在马氏体相变过程中由于相界面摩擦所消耗的能量.计算结果表明,克服相界面摩擦所需要的能量为1314Jmol,仅占相变潜热的一小部分.另外,精细的交流磁化率测量样品的转变循环回线结果表明,相变热滞后的大小和马氏体的转变百分数成正比,从而进一步证明了热弹性马氏体相变的热滞后来源于相界面推移过程中的摩擦 关键词： 马氏体相变 Ni2MnGa     

Ni52Mn24Ga24单晶中取向内应力的热动力学计算

测量了Ni52Mn24Ga24单晶样品在磁场加载和未加载情况下马氏体相变时的相变应变.分析结果表明:用提拉法生长单晶时在晶体内部引入了单一取向的内应力,该取向内应力可诱导马氏体变体择优取向,从而导致马氏体相变时产生大的相变应变.从理论上计算了该内应力的大小.另外,对样品在马氏体态单纯磁诱导应变的热动力学研究,表明取向内应力在马氏体态依然存在.     

单晶Ni52Mn24Ga24中马氏体变体择优取向的物理表征

利用各种实验手段对M52Mn24Ga24单晶中马氏体变体的择优取向进行了表征.针对金相观察、磁场干预的相变应变、磁感生应变等实验结果,分析了马氏体相变过程变体自发择优取向和磁诱导择优取向的机理.根据不同方向磁场干预相变应变的结果,计算了Ni52Mn24Ga24单晶中等效取向内应力的大小约为2.45 MPa.从变体择优取向造成的有效弹性和磁畴分布两个方面,对单晶样品在[001]和[010]两个等价的晶体学方向上磁感生应变特性的差别,包括最大应变值、饱和场、滞后效应和起始磁场数值的参数,进行了分析和讨论.     

Si掺杂的铁磁形状记忆合金Co50Ni21Ga29Six的物性研究

成功生长了Co50Ni21Ga29:Si(x=1,2)单晶样品,对其磁性,马氏体相变及其相关性质进行了细致的测量.发现掺si成分的单晶具有非常迅速的马氏体相变行为、2.5%的大相变应变、大于100 ppm的磁感生应变和4.5%的相变电阻.进一步研究指出,CoNiGa合金中掺入适量Si元素,能够降低材料的马氏体相变温度,减小相变热滞后,提高材料的居里温度,并使得磁性原子的磁矩有所降低.尤其重要的是Si元素的添加能够增大材料马氏体的磁晶各向异性能,改善马氏体变体的迁移特性,从而获得更大的磁感生应变.     

Ni52Mn24Ga24单晶中取向内应力的热动力学计算

测量了Ni₅₂Mn₂₄Ga₂₄单晶样品在磁场加载和未加载情 况下马氏体相变时的相变应变.分析结果表明：用提拉法生长单晶时在晶体内部引入了单一取向的内应力，该取向内应力可诱导马氏体变体择优取向，从而导致马氏体相变时产生大的相变应变.从理论上计算了该内应力的大小.另外，对样品在马氏体态单纯磁诱导应变的热动力学研究，表明取向内应力在马氏体态依然存在. 关键词： 马氏体相变 磁感生应变 内应力     

热磁预处理对Ni-Mn-Ga单晶磁学和力学性能的影响

研究了预先热磁处理对Ni_503Mn_287Ga₂₁单晶的磁学和力学性能的影响.首先将样品加热到居里温度之上让其冷却，冷却方式分为两种：一种是施加一定大小的磁场从高于居里温度冷却至室温，另一种是在样品经历顺磁-铁磁相变后但还未发生奥氏体-马氏体相变前施加相同大小和方向的磁场并冷却至室温.室温时的拉伸-压缩实验结果表明单晶样品在经历前一种处理后，其可逆应变、磁化强度的变化 （ΔM）比后一种处理的相应值要小很多.在后一种热磁处理的样品中，顺磁-铁磁相变发生后形成了自发磁畴，但这种磁畴不具有择优取向.在顺磁-铁磁相变结束后施加磁场，容易导致择优的马氏体准单畴出现，从而表现出大的可逆应变和ΔM.但对于前者，我们认为样品从居里温度降到室温过程中，其中的磁畴在相同的磁场作用下获得择优生长，形成大磁畴，导致磁诱导的强各向异性.这种择优取向的大磁畴在随后马氏体相变期间影响着马氏体的自发排列方式，不利于马氏体准单畴的出现，结果导致较小的可逆应变和ΔM. 关键词： 磁和力学锻炼 Ni-Mn-Ga单晶 铁磁和马氏体相变     

Ni4Ti3沉淀相对NiTi形状记忆合金相变行为的影响

采用第二近邻修正型嵌入原子势的分子动力学方法，建立了共格沉淀相与半共格沉淀相块状/柱状模型，模拟了温度诱发相变和应力诱发相变，分析了Ni₄Ti₃沉淀相对Ni Ti形状记忆合金相变行为的影响．结果表明，Ni₄Ti₃沉淀相本征应变诱发的弹性应力场对相变中马氏体变体类型、形核位置、分布等有重要影响.在温度诱发相变时，共格沉淀相促进部分马氏体变体的形核生长，能显著提高Ni Ti超弹性形状记忆合金的马氏体相变开始温度;在应力诱发相变时，Ni₄Ti₃沉淀相使马氏体早于无沉淀相区域形核，导致了相变应力降低、抑制了马氏体解孪，减小了应力-应变曲线的滞回环.     

哈斯勒合金Ni-Mn-Ga的马氏体相变和磁增强双向形状记忆效应

通过研究铁磁性金属间化合物Ni_2+xMn_1-xGa(x=-0.1,0,0.08,0.13,0.18,0.2)和Ni_2-xMn_1+x/2Ga_1+x/2(x=-0.1,0,0.04,0.06,0.1)两个系列多晶样品的交流磁化率随温度的变化行为,得到了化合物在不同组分下的马氏体相变温度TM和居里温度T_C.发现随着Ni成分的增加,前者的马氏体相变温度T_m增加,而居里温度T_C降低,后者的马氏体相变温度T_m和居里温度T_C均是先增大后减小.报道了T_m在室温附近的单晶样品Ni₅₂Mn₂₄Ga₂₄的磁场增强双向形状记忆效应.发现伴随着马氏体相变,样品在[001]方向可产生1.2%的收缩.如果在该方向施加1.2T的偏磁场可以使该应变值增大到4.0%.而垂直于[001]方向施加1.2T的偏磁场时,在[001]方向产生1.6%的膨胀.阐明了产生大应变的原因并非相界移动,而是单晶的杂散内应力小和外加磁场通过孪晶界移动使马氏体变体重组的共同结果. 关键词： 形状记忆效应 马氏体相变 2MnGa')" href="#">Ni₂MnGa     

掺Mn对NiFeGa磁性形状记忆材料单晶特性的影响

在单晶Ni₅₄Fe₁₉Ga₂₇中掺入少量Mn，对增强磁交换相互作用，提高居里温度和稳定了B2相并增强晶格的刚度，起到了非常明显的作用.Mn的存在使样品中取向内应力得以保持，提高了变体择优取向的水平，大大提高了单晶的相变应变和磁场增强作用.发现内应力对超弹性性质的影响也很明显. 关键词： 磁性形状记忆合金 NiFeGa 单晶 超弹性     

Strain characteristics and superelastic response of single crystals

The intermartensitic transformation, in a two-step complete thermoelastic martensitic transformation in Ni_53.2Mn_22.6Ga_24.2 single crystals, provides a much larger strain than that of the martensitic transformation. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress–strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. Moreover, a well-defined character of the twin-boundary motion, similar to the soliton motion, has been observed upon loading or unloading.     

Method of phase-dispersion introscopy

A method is described for studying the internal structure of noncrystalline objects which are weak x-ray absorbers. The method uses the conventional arrangement of plane-wave topography. The object to be investigated is immersed in a monochromatic pseudoplane wave formed by a highly asymmetric reflection off a perfect crystal or several crystals and introduces phase disruptions at the wave front. A perfect single crystal mounted behind the object in a Laue arrangement reveals these disturbances, forming a contrast image of the boundaries of the structural components of the object. The possibilities of the method are illustrated by photographs of objects of various natures. The features of the image contrasts are discussed and confirmed by experiments on test objects. Zh. Tekh. Fiz. 67, 68–77 (January 1997)     

单晶Ni52Mn24Ga24中马氏体变体择优取向的物理表征

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Characterization of preferential orientation of martensitic variants in a single crystal of NiMnGa

We report the detailed observation of martensitic variants in NiMnGa single crystals. The variants that are twinned with each other in different ways can be clearly identified in our single crystals by optical observation. We also investigated the preferential orientation of the martensitic variants in NiMnGa single crystals. We observed the motion of the variant boundary in response to application of a magnetic field. This observation can be used to explain phenomenologically the magnetic-field-induced strain. In the single crystal with composition Ni₅₂Mn₂₄Ga₂₄, martensite with seven modulated layers (7M) shows preferentially oriented variants. A completely recoverable two-way shape-memory behavior was also observed by measuring the free sample in three different directions during a complete temperature cycle. It was found that the largest strains in the [001] and [010] directions occur in different temperature ranges.     

The influence of a magnetic field on the mosaic spread and growth rate of small Rochelle salt crystals

The growth rates of small (length <1 mm) crystals of Rochelle salt grown with and without a magnetic field have been measured at constant supersaturation (4%). It has been shown that some crystals grown in the presence of a magnetic field exhibit a decrease in the linear growth rate in the [010] direction relative to that shown under normal conditions. It was further noticed that a few crystals of the total number showed slight increases in growth rate. The fact that a majority of crystals showed a decrease in growth rate has given rise to speculations that this should be caused by an increase of strain (mosaic spread), resulting from a change of the mode of incorporation of the growth units into the crystal surface brought about by the applied magnetic field. The results show that most of crystals grown in the magnetic field have a higher mosaic spread and lower growth rate than observed during growth without an applied field. An increase in growth rate in the presence of the magnetic field, observed for a few crystals, might be explained by the relaxation of this strain by formation of dislocations, which in turn enhance the growth rate.     

Magnetomechanical effect in silicon (Cz-Si) surface layers

The mechanical properties of near-surface layers of Czochralski-grown silicon crystals Cz-n-Si(111) have been found to undergo changes in response to an external constant magnetic field (B ?? 0.1 T). A magnetically induced variation in the microhardness, Young??s modulus, and coefficient of plasticity of silicon crystals correlates with the change in the lattice parameter and internal stresses of the sample. The growth of an oxide film under exposure to a magnetic field plays the principal role in the magnetomechanical effect due to a decrease in the concentration of oxygen complexes in the near-surface layers of the sample. In microstructured silicon, where the surface is considerably more developed, the magnetic field induces more profound changes in the internal stresses as compared to single crystals.     

Three-dimensional structure of polystyrene colloidal crystal by synchrotron radiation X-ray phase-contrast computed tomography

Colloidal crystal with long-range ordered structure has attracted great attention for their applications in various fields. Although perfect colloidal crystals have been achieved by some fabrications for utilization, little is known about their exact structures and internal defects. In this study, we use synchrotron radiation (SR) phase-contrast computed tomography (CT) to noninvasively access the internal structure of polystyrene (PS) colloidal crystals in three dimensions (3D). The phase-attenuation duality Paganin algorithm phase retrieval was employed to achieve a satisfactory contrast and outline of the spheres. After CT reconstruction, the positions of individual PS particles and structural defects are identified in three dimensions, and the local crystal structure is revealed. Further quantitative analysis of the void system in colloidal crystal illustrates that single voids can be mostly attributed to tetrahedron void of sphere close packing, but the interconnected voids with large volume induce a sphere volume fraction of 59.39 % that reflects a metastable glass behavior of colloidal crystal arrangement. The void orientation result reveals that the 3D close-packing difficulty mainly lies in the stacking of interlayer.     

Stress–strain–temperature behaviour of [001] single crystals of Co49Ni21Ga30 ferromagnetic shape memory alloy under compression

The conventional shape memory effect (SME) and pseudoelasticity (PE) in as-grown [100] single crystals of Co₄₉Ni₂₁Ga₃₀ alloy under compression are reported. The parent single crystals exhibit about 5% transformation strain at compressive stress levels as low as 4?MPa, and a pseudoelastic strain of 4.5%. Complete PE was observed in the temperature range from 35 to 285°C, along with increasing stress hysteresis with temperature. The latter is attributed to increasing number of variants and the corresponding variant–variant interactions. We demonstrate that the current material can be utilized in applications that demand high strength at elevated temperatures. Moreover, the current results also indicate the potential of this material to exhibit magnetic shape memory effect, which could broaden the scope of utility of this material upon further research.