Effect of Thermomagnetic Treatment on Structure and Properties of Cu–Al–Mn Alloy
© The Author(s). 2017
Received: 1 January 2017
Accepted: 6 April 2017
Published: 20 April 2017
The paper studies the influence of magnetic field on magnetic and mechanical properties of Cu–Mn–Al alloy under annealing. The comparative analysis of the magnetic field orientation impact on solid solution decomposition processes in a fixed annealing procedure is held using the methods of low-field magnetic susceptibility, specific magnetization, and microhardness test. The paper highlights changes in the magnetic and mechanical properties of Cu–Al–Mn alloy as the result of change in a critical size of forming precipitated ferromagnetic phase and determines correlation in the behavior of magnetic and mechanical properties of the alloy, depending on a critical nucleus size of forming precipitated ferromagnetic phase.
KeywordsNanoparticles Magnetization Magnetic susceptibility Magnetic field Microhardness
In the market of new materials, the functional materials having unusual properties are in great demand, among which the ferromagnetic shape memory alloys are predominant. The control over such properties is exercised using force, thermal, and magnetic fields. An important aspect of improvement in the material properties is to create a nanostructured state, which has significant advantages in magnetic and mechanical characteristics in contrast to the bulk materials in crystalline or amorphous state. Magnetization and magnetic anisotropy in case of nanoparticles can be significantly greater than that of a bulk sample, and a difference between the Curie temperature (T c) reaches hundreds of degrees [1–3]. Magnetic nanomaterials have a number of unusual properties, in particular, giant magnetoresistance, abnormally large magnetocaloric effect, and others .
Cu–Al–Mn alloys are one of the most interesting ferromagnetics with shape memory (SM). They demonstrate an unusual magnetic behavior, in particular, superparamagnetism  and giant magnetoresistance , and specific mechanical properties such as SM effect, thermoelasticity, superelasticity, and plasticity of transformation [7, 8]. Moreover, the Cu–Al–Mn alloys exhibit a superelastic strain of about 7%, which is comparable to that of Ti–Ni alloys [9, 10].
It was reported [9, 11, 12] that polycrystalline Cu–Al–Mn shape memory alloys with a low Al content between 16 and 18 at.% have a good balance between workability and SM properties. These alloys demonstrate excellent ductility and exhibit SM properties such as superelasticity and the one- or two-way memory effect based on martensitic transformation. The β (L21) phase significantly extends to the low Al content region by the addition of Mn . The degree of order in the phase is lowered by decreasing the Al content. Doping of binary Cu–Al alloys with Mn slows the decomposition of solid solution under aging and alters the physical and mechanical characteristics of the material. Thus, Cu-based alloys with 14 wt% Al and 3 wt% Mn and with 13 wt% Al and 7.5 wt% Mn have the same start temperature of martensitic transformation (Ms ≈ −80 °C); however, the hardness of the matrix differs by 50% (300 and 200 HV) .
To get optimal properties, these alloys usually undergo an additional thermal, mechanical, or magnetic treatment. Thus, aging of Cu–Al–Mn alloys leads to the formation of a system of nanoscale particles of ferromagnetic Cu2MnAl phase in a paramagnetic Cu3Al matrix , and annealing in magnetic field increases the T c of Cu–Al–Mn alloys . At the same time, the heat treatment allows to control number and size of particles in the alloy and also the martensitic transformation temperature and hysteresis, which depend on characteristics of precipitated particles [15, 16]. Thus, the clarification of the possibility to control the magnetic and mechanical characteristics of Cu–Al–Mn alloys under annealing in magnetic field is of particular interest.
The purpose of this paper was to study the effect of preliminary thermomagnetic treatment (TMT) on structure, magnetic, and mechanical properties of Cu–Al–Mn alloy under the formation of the system of nanoscale magnetic particles.
The object of investigation was the alloy of the following composition: 84.7Cu-11.4Al-3.9Mn (% by weight), smelted in an induction furnace in an argon atmosphere. Samples were prepared as rods of the length of 15 mm and the cross section of 3.5×3.5 mm. After the homogenizing annealing at 1123 K for 10 h, the alloy samples were quenched in water and then annealed in air at 473 K for 3 h in magnetic field with intensity of 1.5 kOe having different orientation (perpendicular or parallel to the main axis of sample) or without field. Magnetization was measured with a ballistic magnetometer, electrical resistance was determined using a four-point method, and low-field magnetic susceptibility was studied using an induction method. The phase composition of samples was examined using an X-ray diffractometer Rigaku Ultima IV in Kα-monochromatic radiation of Cu anode. The chemical composition of the alloy was defined according to the data of energy dispersive X-ray fluorescence analysis accurately to within ±0.5%. The microstructure of alloy was studied using a scanning electron microscope (SEM). A size of precipitated nanoparticles was estimated by the two-pass method of atomic force microscopy (AFM) using a scanning probe microscope (SPM) Solver PRO-M with a magnetic cobalt probe NSG01/Co with the size of 130 × 35 × 2 μm and the distance from the probe to the surface for the second pass of 100 nm.
Results and Discussion
The weak intensity of reflections can indicate that the particles of precipitated phase are high-dispersed or nanosized. In its turn, the change in orientation of samples relative to the magnetic field direction (perpendicular or parallel) affects quantity and size of precipitated particles, which is manifested in the redistribution of the intensities of diffraction peaks.
It is assumed that during the annealing of aging alloys, the magnetic field lines up the precipitated anisotropic ferromagnetic (β3-Cu2AlMn) particles elongated in the field direction in regular-oriented chains. Unfortunately, since the aging time was short and, as a consequence, a low volume fraction of particles has been precipitated during the initial stages of the solid solution decay, such kind of particle orientation along the field was not observed when using X-ray and SEM analyses. However, there are some changes in the magnetic parameters of the alloy after TMT, contrary to the alloy annealed without field.
To confirm the presence of nanoparticles on the sample surface of the Cu–Al–Mn alloy, the two-pass method of AFM was used. The first pass was applied for the pre-measurement of a sample surface profile, and the second pass was performed in the phase image mode to measure phase shift distribution on the sample surface, which reflects distribution of the material characteristics.
According to (2), the shift of T B on the χ/χ max (T) curves towards lower temperatures under effect of TMT can be caused by a decrease in volume of the precipitated ferromagnetic particle. At the same time, the shift of T B towards high temperatures for the same TMT regimes under applied magnetic field of different intensity (100 ÷ 700 Oe) can be caused by the strong field dependence of K V , although the magnitude of magnetic field has no effect on the transition temperature (T B ).
As it is shown on σ(H) curves (Fig. 5), the alloy magnetization at higher temperatures (at 40 °C) exhibits a characteristic superparamagnetic behavior. A theory of superparamagnetism  provides for the relationship μ ef H s ≈ k B T. The magnetization σ(H) for the samples aged without field is of a larger magnitude as compared to the sample aged in magnetic field (Fig. 5).
A size of the nanoparticles can be estimated by the slope of the linear section of σ(H) dependence at H → 0, which in the case of temperature T = 40 °C has a smaller slope and a smaller magnetization magnitude that also points to reduction of a size of the particles when the aging occurs in magnetic field.
Microhardness of alloys after different TMT
Treatment (annealing at 200 °C for 3 h)
Average microhardness H μ(ave) (GPa)
Without magnetic field
In magnetic field perpendicular to the sample axis
In magnetic field parallel to the sample axis
No anomalies were observed on the temperature curves of electrical resistivity of the investigated samples.
This paper studies the effect of thermomagnetic treatment of 84.7Cu-11.4Al-3.9Mn (wt.%) alloy on the structure, magnetic, and mechanical properties. The correlation between the magnetic and mechanical behavior of the investigated alloy in terms of the morphology of nanoparticle formation of ferromagnetic precipitated phase at 200 °C for 3 h both in magnetic field having different orientation, and without it, was found, and the magnetic field dependence on a critical size of the precipitated phase was shown. The treatment in parallel magnetic field causes the reduction in a size of nanoparticles that appears in the decrease of magnetic characteristics of the alloy and in the increase of the alloy microhardness.
Atomic force microscopy
Scanning electron microscope
Scanning probe microscope
The authors thank I.V. Sharai from the Institute of Magnetism of the NAS and MES of Ukraine for the assistance in the AFM experiments.
This work was carried out within the frame of the state budget project of National Academy of Sciences of Ukraine #0115U003536 “Development and research of magnetic and magnetoelectric elements of devices based on multilayer ferrite-piezoelectric structures” in Institute of Magnetism of the NAS and MES of Ukraine.
The idea of the study was conceived by AT. AT, LD, AP, and OG performed the experiments. LD carried out the X-ray phase analysis and the microstructure investigations using SEM. AT measured the electrical properties and carried out the AFM investigations. AP measured the magnetic properties. OG performed the microhardness test. AT and LD interpreted the experiments and wrote this manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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