磁性金属Fe纳米线的制备及其性能

本文尝试采用纳米Au颗粒作为催化剂,利用化学气相沉积法合成铁纳米线。并着重研究了不同沉积温度对纳米线生长过程的影响。研究结果表明,过低沉积温度无法分解二茂铁,而过高沉积温度则会导致二茂铁高温分解副反应发生,生成大量碳颗粒,从而阻碍铁纳米线的生长。作者在600℃沉积温度下,以二茂铁为反应前驱体和蓝宝石作为基板,通过纳米Au颗粒的催化作用首次成功获得了大批量的铁单晶纳米线,并进一步揭示了纳米线的气-固生长机制。此外,本文还对合成的单晶Fe纳米线的磁各向异性进行了探讨。     

Internal friction behaviour of Ni–Mn–Ga

The internal friction (IF) behaviour of shape memory alloys (SMA) is characterised by an IF peak and a minimum of the elastic modulus during the martensitic transformation (MT), and a higher IF value in the martensitic state than in parent phase. The IF peak is considered to be built of three contributions, the most important of them being the so-called “transient” one, existing only at non-zero temperature rate. On the other hand, the ferromagnetic Ni–Mn–Ga system alloys undergoes a MT from the L2₁ ordered parent phase to martensite, the characteristics of the transformation depending largely on the e/a ratio of the alloys. Indeed, a variety of transformation sequences, including intermediate phases between parent and martensite and intermartensitic transformations, have been observed for a wide set of studied alloys. Furthermore, the IF and modulus behaviour during cooling and heating these alloys show specific characteristics, such as modulus anomalies, strong temperature dependence of the elastic modulus, temperature dependent internal friction in martensite, and, as a general trend, a low transient contribution to the IF. In the present work, the IF and modulus behaviour of several Ni–Mn–Ga alloys will be reviewed and compared to that observed for “classical” systems like Cu- or NiTi-based shape memory alloys.     

Structure and Microscopic Magnetism of Epitaxial Ni–Mn–Ga Films

We report on the structural and magnetic properties of epitaxial thin films of the ferromagnetic shape memory material Ni–Mn–Ga prepared by DC magnetron sputter deposition. Different substrate materials, i.e., MgO(100) and Al₂O₃(11?20) allow for a tailored epitaxial growth. Using a sacrificial chromium buffer layer freestanding epitaxial films are obtained. In combination with photolithography partially freestanding structures such as microbridges are fabricated. The complex martensite crystal structure in substrate‐constrained and freestanding films is studied by means of X‐ray diffraction. The identified asymmetric twin variant configuration is associated with a macroscopic surface pattern observed by optical microscopy. The absence of magnetic‐field induced strain in the (100) oriented samples is explained on basis of the detected twin variant configuration using a simplified model. Taking advantage of the thin film geometry spectroscopic methods are applied to the samples. The measurements provide the first experimental test for changes in the electronic structure of the involved 3d metals during a martensitic transition. Exploiting the X‐ray magnetic circular dichroism quantitative information on the element‐specific spin and orbital magnetic moments are accessed. In addition, angular‐dependent experiments allow us to trace the microscopic origin of the magnetic anisotropy in Ni₂MnGa improving the fundamental understanding of this material.     

Ni–Mn–Ga shape memory alloys development in China

Study on Ni–Mn–Ga ferromagnetic shape memory alloys recently keeps active. Intermartensitic transformation was found. Magnetic field enhanced phase transformation strain was discovered, and achieved up to 4%. Fifteen percent super high strain induced by variant reorientation under stress was obtained in non-modulated martensite. Six percent large magnetic field induced strain was achieved, and the temperature dependence was investigated in 5-layered martensite single-variant Ni–Mn–Ga alloys. Several other systems of magnetic shape memory alloys and high temperature shape memory alloy Ni–Mn–Ga are also reviewed.     

Modulated structures of Fe–10Mn–2Cr–1.5C alloy

In Fe–10Mn–2Cr–1.5C alloy the superlattice diffraction spots and satellite reflections have been observed by transmission electron microscopy, these results show that the ordering structure and modulated structure have taken place in this alloy. X-ray diffraction proved that austenitic steel in this alloy is more stable than in traditional austenitic manganese steel. Based on this investigation, we consider that the C–Mn ordering clusters were existing in austenitic manganese steel and the chromium could strengthen this effect by linking the weaker C–Mn couples together. These structures may play an important role in the work hardening of austenitic manganese steel.     

Design and deformation behavior of high strength Fe–Mn–Al–Cr–C duplex steel

The influence of cold rolling reduction on microstructures and mechanical properties at room temperature of the duplex Fe–28Mn–7Al–5Cr–0.3C steel was investigated. In the Fe–28Mn–7Al alloy system, the duplex microstructure was obtained by lowering the carbon content to about 0.3 wt.%. The steel was austenito-ferritic with a low to moderate stacking fault energy. Two thermomechanical cycles were performed, which included cold rolling/annealing at 1100 °C, and cold rolling/annealing at 1100 °C/cold rolling/annealing at 1000 °C.The effects produced by cold rolling on the duplex steel were grain refinement and different strain-induced marks within the ferrite and austenite phases. They were easily observed within the austenite phase at a relatively smaller reduction than within the ferrite phase. Mechanical twinning plays a dominant role within the austenite phase during deformation at room temperature, resulting in extreme mechanical properties. No edge or longitudinal cracks were observed during cold rolling of the duplex steel.     

Magnetic Interactions in Ni‐Mn‐Based Magnetic Shape‐Memory Heusler Alloys

Ni‐Mn‐based Heusler alloys exhibit a variety of features related to martensitic transformations and are materials that are sought to be employed in actuation applications. To be able to exploit their properties, it is necessary to understand the rich variety and subtle magnetic coupling mechanisms occurring in these alloys. We review complementary neutron polarization analysis and ferromagnetic resonance experiments and give an account on the complex magnetism of these alloys in the austenite and martensite states.     

Synthesis of Al–Mn–Ce alloy by the spark plasma sintering

The bulk Al₉₀Mn₈Ce₂ alloy is sintered by spark plasma sintering (SPS) method. The microstructures and the hardness and wear resistance of the Al₉₀Mn₈Ce₂ samples are investigated. The results show that bulk Al₉₀Mn₈Ce₂ alloy with less than 2% porosity has been obtained at 673 K. At 723 K the Rockwell hardness of the alloy reaches 97 HRB and the wear resistance of the alloy is three times as high as that of the conventional A390 aluminum alloy. The high wear resistance of the Al₉₀Mn₈Ce₂ alloy is attributed to the existence of the large amount of the intermetallic compounds.     

Microstructure characterization of the non-modulated martensite in Ni–Mn–Ga alloy

The microstructure of the non-modulated martensite in a Ni–Mn–Ga alloy has been characterized in detail by conventional transmission electron microscopy. Bright field images show that the martensite exhibits an internal substructure consisting of a high density of narrow twins. Using electron diffraction, it is found that the martensite has a tetragonal crystal structure. The lattice correspondence between the parent phase and the non-modulated martensite is investigated. Furthermore, the four twinning elements describing the microtwinning have been graphically and quantitatively determined. The results indicate that the microtwinning within the non-modulated martensite belongs to the compound type.     

Martensitic transformation and shape memory effect in Fe–Mn–Si based alloys

The research status of the Fe–Mn–Si based alloys is reviewed with emphasis on the recent progress in the martensitic transformation and the associated shape memory effect (SME). Particular interest is given to the fcc(γ)–hcp(ε) transformation mechanism in the alloys featured by low stacking fault energy and the approaches aiming to the enhancement of SME through alloy design including microalloying and microstructure control by introducing texture and precipitates into the parent γ matrix. Potential topics of oncoming focus are briefly highlighted.     

Fatigue, monotonic and fracture toughness properties of a Cr–Mn–N steel

The low-cycle fatigue, monotonic and fracture toughness behaviour of E3949, a Cr–Mn–N austenitic stainless steel, used for drillcollar connections was studied. Low-cycle fatigue tests were carried out at room temperature under total strain control in the range of 0.40 to 1.50% using Companion Specimens Test (CST) and Incremental Step Test (IST) methods. Cyclic softening without saturation was observed in all tests. Massing cyclic stress–strain behaviour was observed only with the IST method. The fatigue life behaviour obeyed Basquin and Coffin–Manson relationships and the high value obtained for ′_f imparts a significant improvement in fatigue resistance of this alloy compared to AISI 304LN. The J–R curves and J_IC values were obtained at room temperature and at 150°C by using single specimens and the elastic compliance technique for crack length measurement. The observed decrease in crack initiation fracture toughness at 150°C is proposed to be due to a dynamic strain ageing effect, which impairs ductility.     

Characterization of phase development in non-isothermally annealed mould-cast and heat-treated Al–Mn–Sc–Zr alloys

The effect of Mn addition on microstructure and mechanical properties during isochronal annealing in the temperature range of 20 °C–570 °C of the mould-cast and heat-treated Al–Sc–Zr alloys with a various content of Mn and Zr was studied. The electrical resistometry together with the microhardness (HV0.3) measurements were compared to microstructure development. The microstructure development was examined by scanning electron microscopy, transmission electron microscopy and electron diffraction. Relative resistivity changes and the microhardness of the mould-cast and heat-treated Al–Mn–Sc–Zr alloys exhibit similar dependence on annealing temperature. Precipitation of the Al₃Sc particles is responsible for the peak microhardness in all these alloys. The microhardness decrease is slightly delayed during the isochronal annealing and during the high temperature heat treatment in the mould-cast alloy with the higher Zr-content due to a higher oversaturation of Zr. The decomposition sequence of the oversaturated solid solution of the studied Al–Mn–Sc–Zr alloys is compatible with the recently published decomposition sequence of the Al–Sc–Zr system and also with the formation of Mn,Fe-containing particles. It seems very probable that the addition of Mn does not influence the decomposition of solid solution of the ternary Al–Sc–Zr system.     

Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel

The transformations during continuous cooling and isothermal processes, the effects of hot deformation and the morphology of the final microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel designed for acicular ferrite microstructure were investigated. The results show that there are three independent “C” curves for isothermal phase transformation, i.e., TTT diagram, of low-carbon microalloyed steel, namely, polygonal ferrite–pearlite transformation “C” curve, the massive ferrite transformation “C” curve and the bainitic transformation “C” curve, respectively. Hot deformation accelerates acicular ferrite transformation and refines the steel's matrix. The microstructure of acicular ferrite for pipeline steels was discussed.