Magnetic structure of single-domain antiferromagnetic FCT γ-Mn-Cu and γ-Mn-Fe alloys

Detailed neutron diffraction studies both by double axis and triple axis spectrometer are reported for the first time for fct single-domain single crystals of γ-Mn-Cu and γ-Mn-Fe. The main purpose of these studies was to elucidate further the nature of the diffuse (001) peak, forbidden by the AF1 magnetic structure, established earlier in this group of γ-Mn alloys. Line shapes and their temperature dependence were measured for (001), (100) and (010) reflections in Mn-Fe and in Mn—Cu, in the latter case both for homogeneous (quenched) and chemically decomposed (aged) samples. Only (001) reflection was found in Mn-Fe, while in Mn-Cu samples diffuse peaks were also observed at (100) and (010). Together with their temperature dependence and the results of energy analysis the data suggest a new model of the magnetic structure of these alloys. In Mn-Fe the spins deviate by a fixed angle from the c axis and this perturbation has an infinite correlation range and life time. In Mn-Cu a more complex set of perturbations is required to interpret the data.     

Scaling relationships in sharp conical indentation of shape memory alloys

Based on dimensional analysis and finite element calculations, several scaling relationships in the indentation of shape memory alloys with a sharp conical indenter were obtained. These scaling relationships illustrate the dependence of the indentation response on the material properties of shape memory alloys, such as phase transition and plastic deformation. It is shown that the yield stress and strain-hardening exponent of transformed martensite play important roles in the indentation response, in addition to the phase transition properties. Additionally, the general relationships between indentation hardness and phase transition stress, maximum transition strain, martensite yield stress and the strain-hardening exponent of shape memory alloys were obtained. The results show that the indentation hardness of shape memory alloys is not proportional to the phase transition stress or to the martensite yield stress, and cannot be used directly to measure the phase transition stress or the yield stress of shape memory alloys.     

Recent topics in microstructure control by magnetic field in Ni<Subscript>2</Subscript>MnGa and CoPt and new transformation behavior in Ti-Ni-Fe alloys

Three topics related to solid-solid phase transformations are presented. The first topic is related to ferromagnetic shape memory alloys. The general condition for rearrangement of martensite variants by magnetic field is discussed quantitatively. The second topic is related to microstructure control of CoPt (tetragonal L1₀-type structure) during ordering heat-treatment under a magnetic field. We show that a single variant state is realized by magnetic field, and magnetic field is especially effective at the early stage of ordering. The third topic is related to the so-called precursor phenomena in Ti-Ni-Fe shape memory alloys. In the topic we will show the existence of a commensurate phase, which inherits the microstructure of the incommensurate phase and is probably different from the R-phase.     

Baroelastic shape memory effects in titanium nickelide alloys subjected to plastic deformation under high pressure

The effects of a high pressure and torsional plastic deformation in Bridgman anvils on the structure and phase transformations in titanium nickelide-based shape memory alloys are studied by electron microscopy, neutron diffraction, and X-ray diffraction. The physical properties of the alloys are measured. It is found that the baroelastic effects related to the highly reversible B2 ? B19?? martensitic transformation can occur in metastable austenitic titanium nickelide alloys in both the standard polycrystalline and nanocrystalline states under high pressure.     

Resonant ultrasound spectroscopy – a tool to probe magneto-elastic properties of ferromagnetic shape memory alloys

Resonant ultrasound spectroscopy (RUS) was used to investigate the changes of elastic properties induced by magnetic field in magnetic shape memory alloys Ni-Mn-Ga and Co-Ni-Al. In contrast to large magneto-elastic response of Ni₂MnGa austenite, there is only very weak response of Co-Ni-Al. This indicates that the austenite phase of Ni-Mn-Ga can have a privileged position and this may be a reason for the existence of magnetic shape memory effect. In contrast to austenite, the magneto-elastic response in Ni-Mn-Ga martensite is very small with large damping due to existence of twin boundaries. The measurement showed that RUS can be a powerful method to probe magneto-elastic properties of shape memory alloys.     

Mn-Cu系高阻尼合金的凝固组织控制及阻尼特性的改善

Mn—Cu系阻尼合金兼有较高的阻尼特性和良好的力学性质，因而具有较大的实用前景。这类合金通常经过铸造，塑性加工和热处理来获得要求的性能和组织。加工后的Mn—Cu合金存在一个特定温度，在此温度以上合金的阻尼性能将会消失，因而影响了合金的在较高温度下的使用。该温度和合金的Mn含量有关，然而提高合金的Mn含量有会降低合金的加工性及力学性能。一般Mn-Cu合金的热处理都是利用400℃附近的时效来获得相分解后的局部高Mn组织。但是目前的时效处理后的Mn—Cu阻尼合金的最高使用温度只在80℃以下。为解决Mn—Cu阻尼合金使用温度的局限性，本研究选用凝固过程控制的方法在铸造组织中来获得较大幅度的Mn含量分布。从而在Mn-Cu合金得到较高的高阻尼特性温度。本工作利用铸型温度控制的方法，将M2052（Mn-20Cu-5Ni-2Fe）合金在250～0．1K／s冷却速度范围内控制凝固。随凝固冷却速度的降低在合金的铸态组织中观察到二次枝晶间距和晶粒尺寸的明显增大。同时还发现缓冷凝固的合金的成分比快冷凝固有较大的分布幅度。铸态下的合金阻尼性能评价也证实了凝固冷却速度对合金的凝固组织有很大的影响。尽管铸态组织的合金的高温阻尼性能并没有很大的改善，然而通过对铸态组织实施时效处理后发现缓冷凝固合金的高温阻尼性能有很大的改善。凝固冷却速度对时效处理后合金的阻尼性能有明显的影响。实验结果表明0．1K／s的冷却速度下缓慢凝固的合金在时效处理后高阻尼特性可持续高达120℃．     

Mn-Cu合金成分对去合金化法制备纳米多孔铜的影响

 采用电弧熔炼制备出前驱体Mn-Cu合金,在稀盐酸溶液中自由腐蚀去合金化,制备出具有双连续结构的纳米多孔铜。研究了前驱体合金的成分对纳米多孔铜微观结构及Mn的选择性腐蚀程度的影响。结果表明：前驱体Mn-Cu合金的Cu原子分数为43%时,其去合金化受到抑制,存在明显的未完全去合金化的岛状结构;前驱体Mn-Cu合金的Cu原子分数为32%～23%时,可完全去合金化,形成平均孔径尺寸为20～100 nm,平均系带尺寸为30～80 nm,具有双连续结构的纳米多孔铜;前驱体Mn-Cu合金的Cu原子分数低至20%时,去合金化后存在大量裂纹,形成纳米颗粒聚集体。纳米多孔铜中存在少量的残余Mn,残余Mn的原子分数随着前驱体合金Mn原子分数的增高而降低。实验表明腐蚀液浓度对纳米多孔铜形貌也存在影响。     

Martensitic transformation and related magnetic effects in Ni-Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni-Mn-based alloys are reviewed based on our recent research results.     

Theory of magnetic shape memory effect and pseudoelastic deformation in Ni-Mn-Ga alloys

The effect of the magnetic field on the deformation behavior of magnetic alloys of the Heusler type under different loading conditions is discussed in terms of the theory of diffuse martensitic transitions. The effects of magnetic shape memory, pseudoelastic deformation, and generation of reactive stresses in response to the magnetic field are considered. The theoretical relationships are compared with the experimental data available in the literature.

     

Magnetic phase diagram of ordered (Fe1-xMnx)Pt alloys

The magnetic structure and magnetic phase transitions of ordered (Fe_1-xMn_x)Pt alloys were investigated by neutron diffraction in the temperature range 4.2–900 K. A complete magnetic phase diagram is drawn up, where the regions for different magnetic states and the character of transitions between them are shown. The theoretical analysis of the magnetic phase stability is given.     

Martensitic transformations in some ferrous and non-ferrous alloys under magnetic field and hydrostatic pressure

Martensitic transformations are extensively influenced by external fields, such as temperature and uniaxial stress, in transformation temperatures, crystallography and amount and morphology of the product martensites. Therefore, to clarify the effect of external fields on martensitic transformations it is very important to understand the essential problems of the transformation, such as thermodynamics, kinetics and the origin of the transformation, whose information is naturally useful in technological applications using the transformation. Magnetic field and hydrostatic pressure are important in such external fields because there exist some significant differences in magnetic moment and atomic volume between the parent and martensitic states. In the present paper, therefore, we summarizz the effects of magnetic field and hydrostatic pressure on martensitic transfonnations in some ferrous and non-ferrous alloys by referring to past and recent works made by our group and many other researchers. The transformation start temperatures of all the ferrous alloys examined increase with increasing magnetic field, but those of non-ferrous alloys, such as Ti-Ni and Cu-Al-Ni shape memory alloys, are not affected. On the other hand, the transformation start temperature decreases with increasing hydrostatic pressure in some ferrous alloys, but increases in Cu-Al-Ni alloys. The magnetic field and hydrostatic pressure dependencies of the martensitic start temperature are in good agreement with those calculated by our proposed equations.

During investigations of ferrous Fe-Ni-Co-Ti shape memory alloy, we found that a magnetoelastic martensitic transformation appears and, in addition, several martensite plates grow nearly parallel to the direction of the applied magnetic field in a specimen of Fe-Ni alloy single crystal.

We further found that the isothermal process in Fe-Ni-Mn alloy changes to athermal under a magnetic field and the athermal process changes to isothermal under hydrostatic pressure. Based on these facts, a phenomenological theory has been constructed, which unifies the two transformation processes.     

Mechanism of the shape memory effect in martensitic alloys: an assessment

The (one-way) shape memory effect is a phenomenon that when a martensitic alloy is deformed in a martensitic state it recovers its original shape upon heating to the parent phase. This is a universal effect for certain martensitic alloys. We will assess the mechanism of the effect critically and select the essential factors which govern the effect. We try to understand it from a unified view, invoking the group–subgroup symmetry relation between the parent and martensite phase, along with analysis of reversible twinning modes in martensite. By such an assessment, we will show why typical shape memory alloys, such as Ti–Ni, Cu–Al–Ni etc., exhibit good shape memory characteristics, while others, such as ferrous alloys, do not. Thus, we will show that most of the shape memory characteristics of various martensitic alloys can be understood consistently from such an approach.     

Investigation of the spin-glass state in γ-Fe-Ni-Mn alloys

The composition region where there are no long-range ferro- and antiferromagnetic order has been determined in γ-Fe-Ni-Mn alloys. In these alloys strong small-angle magnetic neutron scattering has been obtained. The intensity has an unusual temperature dependence which is discussed in the framework of the critical neutron scattering thermodynamic theory. The ground state of these alloys is identified with the spin-glass state in which there are also regions of both short-range ferro- and antiferromagnetic order.     

Multi-functional properties of iron-based ferromagnetic shape memory ribbons

This paper presents measured multi-functional properties of Fe–Mn–Cr–Si–Tb–B ribbons developed by means of the melt-spinning technique in air. The alloys are multi-functional materials, which have both ferromagnetic and shape memory properties. If we can simultaneously improve the material properties, the applications of the shape memory alloys will be widened dramatically in the field of the electromagnetic sensors and actuators. The base shape memory material, Fe–Mn–Si alloy, is nonmagnetic due to its high manganese content (28–34 Mn, 4–6.5Si wt%). In order to improve ferromagnetic function of the Fe–Mn–Si alloy, we have investigated the addition of rare earth elements. Addition of about 0.7–1.0 wt% Tb was effective in increasing the saturation magnetization. However, ductility of the samples was not good and it was difficult to evaluate the shape memory properties with shape recovery strain measurements. The detailed magnetic and shape memory properties of the Fe–Mn–Cr–Si–Tb–B alloys are discussed in this paper.     

Development of an engineering model for ferromagnetic shape memory alloys

This paper presents a relationship among stress, temperature and magnetic properties of a ferromagnetic shape memory alloy. In order to derive an engineering model of ferromagnetic shape memory alloys, we have developed a measuring system of the relationship among stress, temperature and magnetic properties. The samples used in this measurement are Fe68–Ni10–Cr9–Mn7–Si6 wt% ferromagnetic shape memory alloy. They are thin ribbons made by rapid cooling in air. In the measurement, the ribbon sample is inserted into a sample holder winding consisting of the B-coil and compensation coils, and magnetized in an open solenoid coil. The ribbon is stressed with attachment weights and heated with a heating wire. The specific susceptibility was increased by applying tension, and slightly increased by heating below the Curie temperature.     

Long-range antiferromagnetic order in CeCu5.5Au0.5

The magnetic ordering previously discovered in CeCu_6–xAu_x heavy-fermion alloys is shown to be of long-range antiferromagnetic type by elastic neutron scattering performed on a polycrystalline sample. The data are compatible with an incommensurate wave vector (0.17, 0, 0.514). Although a definite assignment needs a single-crystal study, the magnetic structure is clearly different from the type of magnetic correlations in pure CeCu₆ found in inelastic scattering.     

Quantitative model of large magnetostrain effect in ferromagnetic shapell memory alloys

A quantitative model describing the large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported.The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar types of materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetization. A simple model of magnetization and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa. Received 29 September 1999     

Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys

A quantitative model describing large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported. The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar type materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetisation. A simple model of magnetisation and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa.     

定向凝固生长对Ni-Mn-Ga-RE (RE=TbSm)合金磁感生应变的影响

关键词：