TEM study and magnetic measurements of precipitates formed in Cu-Fe-Ni alloys

The precipitation behavior of Cu-Fe alloys with Ni addition on isothermal annealing at 878 K was investigated by means of transmission electron microscopy (TEM), electron dispersive X-ray spectroscopy (EDS), and field-emission scanning electron microscopy (FE-SEM). Magnetic element atoms were segregated from the solid solution in supersaturated state, and nano-scale magnetic particles were randomly formed in the copper matrix at the initial stage of annealing at 873 K. With increasing the isothermal annealing time, however, the striking feature that two or more nano-scale magnetic particles with a cubic shape aligned linearly along (100) directions were observed upon the isothermal annealing at 873 K. To investigate the relationship between micro-structures and magnetic properties of the heterogeneous Cu-Fe-Ni alloys, magnetic measurements such as M-H measurements were also carried out, using a superconducting quantum interference device (SQUID) magnetometer. In this study, it was revealed that the magnetic properties of the specimen presented the ferromagnetic behavior, during the precipitation process in a Cu-Fe-Ni alloy.     

Nano-scale precipitates formed in Cu-Co based alloys and their magnetic properties

The microstructural evolutions of nano-scale magnetic Co particles formed in Cu-Co base alloys have been investigated on isothermal annealing at 973 K, using transmission electron microscopy (TEM). After the solution treatment and short annealing, nano-scale magnetic particles appeared randomly in the Cu-rich matrix. With increasing the isothermal annealing time, however, pairs and sometimes more than two of Co precipitates were linearly arranged along <100> directions in Cu-Co alloys. On the other hand, such linear arrangements of precipitates were extended in Cu-Ni-Co alloys. Co precipitates were cubic in the coherent stage and octahedral in incoherent stage of precipitation in binary alloys, while the precipitates rendered rectangular shapes even in the incoherent stage in Cu-Ni-Co. The magnetic properties of the specimens have also been measured at the similar conditions, with the superconducting quantum interference device (SQUID) magnetometer. The present study revealed that coersive forces of the specimens were correlated with the microstructural evolution occurring during the isothermal annealing.     

Discontinuous reactions in melt-spun Cu–10 at. %Co alloys and their effect on magnetic anisotropy

The occurrence of discontinuous reactions under isothermal annealing of melt-spun Cu–10 at. % Co alloys, consisting of ribbons (20 µm thick) with columnar grains in the as-solidified state, has been investigated. The microstructure of the ribbons for different annealing temperatures (723–923 K) and annealing times (5–60 min) was determined by transmission electron microscopy, including analysis by energy-dispersive X-ray spectroscopy. Magnetic properties at room temperature were measured by means of hysteresis curve measurements in a vibrating sample magnetometer. Different types of microstructure were observed within grains and at grain boundaries. The spinodal decomposition microstructure was observed during the early stages of annealing for all annealing temperatures. Spherical precipitates grew from the modulated structure at a later stage, forming homogenous distributions throughout the grains. Heterogeneous distributions of incoherent precipitates formed at T > 873 K. As result of discontinuous precipitation, all grain boundaries exhibited arrays of rod-like Co precipitates with diameters and inter-rod spacing of few nanometers. The coarsening of discontinuous precipitates is attributed to a grain boundary-controlled phenomenon, called discontinuous coarsening (DC). The columnar morphology of the grains in the as-solidified alloy was connected with Co rods that were primarily oriented along the ribbon plane. This structure is connected with magnetic anisotropy, which is later weakened by DC. These results elucidate the unusual magnetic behavior of melt-spun Cu–Co alloys and provide a key to understanding their higher magnetoresistance in comparison with other heterogeneous systems.     

Degeneration and rejuvenation of shape memory effect associated with the precipitation of coherent nano-particles in a Co-Ni-Si shape memory alloy

In a solution treated Co-20Ni-6Si shape memory alloy,coherent nano-particles were precipitated after annealing at 873 K for 1 min,but the shape memory effect almost vanished.It is attributed to that the coherent nano-particles not only suppressed the stress-induced face-centered cubic to close-packed hexagonal martensite transformation but also damaged the crystallographic reversibility of reverse martensite transformation.After further annealing at 1073 K for 1 min,the shape memory effect was reju-venated owing to the dissolution of nano-particles.Besides,the recovery strain significantly increased to 5.1％ from the solution treatment of 3.1％ after annealing at 1073 K for 1 min.     

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%.     

Shape Memory Effect of an Aged Fe-Mn-Si-Co-Mo Alloy

The effect of ageing on the microstructure, mechanical properties and shape memory effect (SME) in a newly developed Fe-24Mn-5Si-8Co-4Mo shape memory alloy has been studied. It was found that Fe2Mo particles precipitate during ageing and thereby increase hardness and strength of the alloy The SME of the alloy can be remarkably improved by ageing and a maximum SME can be obtained when aged at 873 K. When the ageing temperature is over 873 K, the SME decreases with increasing ageing temperature. The reason for the improvement of SME by ageing in the Fe-Mn-Si-Co-Mo alloy is discussed     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Ageing behaviour in a dental low-gold alloy with high copper content

Phase transformation behaviour in a dental low-gold alloy with high copper content during continuous heating was investigated by hardness tests, electrical resistivity measurements, X-ray diffraction, scanning and transmission electron microscopies. Two kinds of solution treatment conditions (at 873 K and 1073 K) followed by iced-brine quench, represented different ageing behaviours. Although subsequent anisothermal annealing produced same phase separation of face-centred cubic disordered and ordered (Cu3Au) phases in both specimens, the specimen quenched from 1073 K had already been hardened by a spinodal decomposition. © 1998 Chapman & Hall     

Microstructures and mechanical property of AlCoNiCrFe alloy annealed in high magnetic field

A rapidly solidified high entropy alloy AlCoNiCrFe was annealed at different temperatures with high magnetic field applied up to 4 T. Both precipitation and coarsening of the precipitates were promoted during annealing in a high magnetic field, and nanosized arrayed particles as well as boundary oriented secondary phases were formed with effects of magnetic field. The microstructural features were obtained owing to enhancement of atomic diffusion by applying high magnetic field. It was found that both hardness and yield strength were not strongly dependent on the magnetic field, but the ultimate compression strength is reduced as higher magnetic field is applied due to formation and coarsening of the precipitates on grain boundaries.     

Mechanical behavior and shape memory effect of an aged NiAl–Fe alloy

The mechanical behavior and shape memory effect of an aged NiAl–Fe alloy has been investigated. It was found that the first yielding stress of NiAl–Fe alloy in a compression test was decreased with the precipitation of a Ni₅Al₃ phase after aging at 473–673 K, and increased as the aging temperature increased higher. The one-way shape recovery of NiAl–Fe alloy increased as the aging temperature increased from 473 to 673 K, and decreased as the aging temperature increased higher than 673 K. The morphology of precipitates in the NiAl–Fe alloy aged at 473–873 K was investigated, and the mechanism of its effect on the mechanical properties is discussed.     

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.     

流变铸造亚共晶白口铸铁组织结构的演变

利用电磁搅拌方法研究了亚共晶白口铸铁半固态浆料凝固组织的演变。结果表明,在等温搅拌过程中,球形颗粒半固态组织的形成是一种渐变过程,经历了一次枝晶和二次枝晶的分离、枝晶的弯曲和粒化阶段,搅拌温度、激磁电流和搅拌时间三者恰当的配合会导致为理想的球形颗粒组织,由温度降低造成的搅拌力或搅拌速度的突然降低导致颗粒簇聚的现象。在连续冷却过程中,流变铸造组织出现了枝晶断裂迹象,而在等温过程中则较少见。     

Annealing Influence on the Microstructure and Magnetic Properties of Ni–Mn–In Alloys Ribbons

We report on the structure, microstructure and inverse magnetocaloric effect associated with the first-order martensitic phase transition, in Heusler Ni_50.0Mn_35.5In_14.5 alloy ribbons. We have studied the short-time vacuum annealing influence at 1048?K, 1073?K, 1098?K, and 1123?K in these properties. At room temperature, an increase in the degree of structural order for ribbons annealed up to 1098 K was observed, corresponding to cubic L2₁ austenite phase. Meanwhile, for the sample annealed at 1123?K a monoclinic 10M martensitic phase was detected. A comparison of magnetic entropy change as a function of the applied field, after using zero-field-cooling thermomagnetic and isothermal magnetization measurements, has been made for the sample annealed at 1073?K.     

Effect of a high magnetic field on the shape of the γ′ precipitates in cast nickel-based superalloy K52

The shape change of the γ' precipitates of cast Ni-based superalloy K52 after aging treatment under a high magnetic field was investigated. The results show that duplex γ' precipitates are present in the γ matrix after aging treatment with or without the magnetic field. One is the coarse particles with average size of 500 nm; the other is fine spherical γ' precipitates with average of 100 nm in diameter. The application of a 10T magnetic field only results in the shape of the coarse γ' particles changing from spherical to cuboidal when the alloys subjected to the same heat treatments. This shape change was mainly discussed based on the strain energy increase caused by the difference in magnetostriction between the γ matrix and γ' precipitates. The fine γ' particles still keep spherical. Further TEM observations shows that a number of γ particles in nano-scale precipitate in the coarse γ' particles in the specimens treated without the magnetic field. In addition, it was found that the magnetic field caused the decrease of the hardness in the alloy, and the hardness was associated with the field direction.     

Small angles X-ray diffraction and Mössbauer characterization of annealed Tb/Fe multilayer

The effect of thermal annealing on the structure and magnetic properties of crystalline Tb/Fe multilayers has been studied using conversion electron M?ssbauer spectrometry and small-angle X-ray diffraction. The growth of Tb–Fe amorphous alloy from the interface is observed with increasing annealing temperature. After annealing at 873 K, a clear total mixing of the multilayers by interdiffusion has been evidenced. The results are compared with the effect of ion irradiation in the same system.     

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep of a Fe₃Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873–1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms of the threshold stress concept. A rapid fall of the stress exponent and of the threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results of long-term creep tests in tension performed recently on the same alloy. __________ Translated from Problemy Prochnosti, No. 1, pp. 117–120, January–February, 2008.     

Decomposition of supersaturated solid solution in splat-cooled Sn- 10 at% Sb alloy

A supersaturated solid solution was obtained in an Sn-10 at. %Sb alloy rapidly quenched from the liquid state. Decomposition of this solution, taking place during isothermal annealing at 373 K, was investigated by transmission electron microscopy. In the first stage of decomposition, clustering of Sb atoms in definite planes of the tetragonal lattice of tin was observed. Subsequently, needle-like precipitates of the equilibrium compound SnSb were formed. The sequence of decomposition of the supersaturated solid solution Sn(Sb) and a model for the formation of the compound within the tetragonal lattice of tin are proposed.     

Nanoscale Grain Growth Behaviour of CoAl Intermetallic Synthesized by Mechanical Alloying

Grain growth behaviour of the nanocrystalline CoAl intermetallic compound synthesized by mechanical alloying has been studied by isothermal annealing at different temperatures and durations. X-ray diffraction method was employed to investigate structural evolutions during mechanical alloying and annealing processes. The disordered CoAl phase with the grain size of about 6 nm was formed via a gradual reaction during mechanical alloying. The results of isothermal annealing showed that the grain growth behaviour can be explained by the parabolic grain growth law. The grains were at nanometric scale after isothermal annealing up to 0·7 T _m. The grain growth exponent remained constant above 873 K indicating that grain growth mechanism does not change at high temperatures. The calculated activation energy indicated that the grain growth mechanism in the disordered CoAl phase at high temperatures was diffusing Co and Al atoms in two separate sublattices. Furthermore, an equation has been suggested to describe the grain growth kinetics of nanocrystalline CoAl under isothermal annealing at temperatures above 873 K (T/T _m ≥ 0·5).     

变形程度对强变形Al-4%Cu合金退火组织性能的影响

为了研究变形程度对强变形Al-4%Cu合金退火行为的影响,通过透射电镜观察和拉伸试验,研究了人工时效Al-4%Cu合金经过不同变形量的多向压缩变形(MAC),退火(120℃/60 min)后的显微组织和力学性能.研究表明:试样中的第二相在MAC过程中破碎溶回基体后,会在后续退火过程中再次析出,且析出相回溶的程度对退火组织性能的影响很大.含θ″相试样和含θ’相试样经MAC变形后,析出相完全回溶于基体,在退火过程中有新的第二相析出,试样强度升高;同时试样的的塑性也得到了提高,这与再析出粒子对超细晶粒长大的阻碍作用有关.析出相未完全回溶的含θ相试样,退火后强度低于退火前,析出相回溶和再析出交替进行.析出相基本回溶态强韧化效果最佳.     

等温退火对Alnico8合金微观结构的影响

利用透射电镜(TEM)和X射线小角衍射技术(SAXS)研究了磁场等温退火工艺参数对Alnico8合金的微观组织的影响,样本的晶格结构清晰可见。结果表明,在等温退火阶段Alnico8合金通过调幅分解形成两相结构,棒状的铁磁相(1α相)粒子高度有序地弥散在弱磁性相(2α相)当中。其中,1α相的平均长度由SAXS测得,并与TEM照片数据进行了验证比对。本文着重研究了1α相粒子平均长度随等温退火处理的温度及时间参数的改变规律,结果显示样本的磁性能与其微观结构有一定相关性。