Critical driving force for martensitic transformation fcc (γ)→hcp(ε) in Fe-Mn-Si shape memory alloys

By the application of Chou's new geometry model and the available data from binary Fe-Mn, Fe-Si and Mn-Si systems, as well as SGTE DATA for lattice stability parameters of three elements from Dinsdale, the Gibbs free energy as a function of temperature of the fcc(γ) and hep(ε) phases in the Fe-Mn-Si system is reevaluated. The relationship between the Neel temperature of the γ phase and concentration of constituents in mole fraction, is fitted and verified by the experimental results. The critical driving force for the martensitic transformation fcc (γ)→ hep (ε), △ G_C~(γ→ε), defined as the free energy difference between γ and ε phases at M_s of various alloys can also be obtained with a known M_s. It is found that the driving force varies with the composition of alloys, e. g. △ G_C~(γ→ε) = - 100.99 J/mol in Fe-27.0Mn-6.0Si and △ G_C~(γ→ε) = - 122.11 J/mol in Fe-26.9Mn-3.37Si. The compositional dependence of critical driving force accorded with the expression formulated by Hsu of the     

Thermodynamic prediction of M_s in Fe-Mn-Si shape memory alloys associated with fcc(γ)→hcp(ε) martensitic transformation

By means of X-ray diffraction profile analysis of three different composition Fe-Mn-Si alloys, the relationship between stacking fault probability P_(sf) with the concentrations of constituents in alloys, 1/P_(sf)=540.05 23.70×Mn wt％-138.74×Si wt％, was determined. According to the nucleation mechanism by stacking fault in this alloy,the equation between critical driving force ΔG_c and P_(sf), ΔG_c=67.487 0.1775/P_(sf)(J/mol), was made. Therefore,the relationship between critical driving force and compositions was established. Associated with the thermodynamic calculation, the M_s of fcc(γ)→hcp(ε) martensitic transformation in any suitable composition Fe-Mn-Si shape memory alloys can be predicted and results seem reasonable as compared with some experimental data.     

Nucleation barrier of fcc(γ)→hcp(ε) martensitic transformation in Fe-based alloys

Based on the dislocation theory and Olson's stacking fault model, a model describing the nucleation of an hcp(ε) martensite embryo at low-angle grain boundary is proposed with the influence of external stress field taken into account. The dependences of temperature (T), shear stress (τ) and dislocation density at grain boundary on the martensite nucleation in FeMnSi based alloy, as an example, are numerically simulated. It has been shown that there exist the subcritical and critical embryos during the course of ε-phase nucleation. The free energy difference between them is just the energy barrier of embryo growth. Depending on T and τ, the characteristic embryo sizes may vary in wide ranges and decrease with increasing a and decreasing T. The energy condition of martensitic transformation at Ms and critical shear stress (τc) is discussed from the viewpoint of kinetics and thus the TEM observed result that stacking fault energy is not zero at Ms temperature is reasonably explained. Besides, it is pr     

Dendritic morphology evolution and microhardness enhancement of rapidly solidified Ni-based superalloys

The mechanisms of microstructure evolution and dendritic growth of Inconel alloys in a drop tube were investigated in this paper.The Vickers microhardness of the solidified alloy droplets was also measured to explore the effects of grain size and precipitated phase on the mechanical properties. From XRD results, the rapidly solidified Inconel 600, 617, 625, and 718 alloys are characterized by γ phase solid solution with fcc structure. Further analyses based on EDS and SEM reveal that Laves phase precipitates at the grain boundaries of γ phase in Inconel 718 alloy. With the decrease of droplet diameter(D), the dendritic morphology experiences a transformation of "orthogonal long dendrites→irregular stubby dendrites→equiaxed grains" in Inconel 600 alloy, "orthogonal long dendrites → stubby irregular dendrites" in Inconel 625 alloy and "orthogonal long dendrites→equiaxed grains" in Inconel 617 and 718 alloys. Although the dendrites become coarse locally, the dendritic grain size obviously reduces with the decrease of droplet diameter, leading to the increase of Vickers microhardness. For Inconel 718 alloy,the Vickers microhardness firstly decreases, when the droplet diameter decreases from 1000 to 900 μm, and then increases linearly with the decrease of droplet diameter. It is found that the Vickers microhardness of Inconel 625 alloy is the largest among the four alloys.     

Microstructure,martensitic transformation and shape memory effect of polycrystalline Cu-Al-Mn-Fe alloys

In this study, two Cu-Al-Mn-Fe polycrystalline alloys were prepared, and their microstructure, reversible martensitic transformation, mechanical properties and shape memory effects were investigated. The results show that the reversible martensitic transformation temperatures of the studied alloys are between room temperature and 373 K, which are suitable for practical applications. Two typed martensites of 18R and 2H coexist both in two alloys. The bcc β(FeAl) nanoparticles are Fe-rich, Mnrich and Cu-poor, whereas the martensite is Cu-rich, Fe-poor and Mn-poor. The size of nanoparticles ranges from tens to hundreds of nanometers. Full shape recovery property is displayed in Cu-12.9Al-4.5Mn-2.6Fe alloy all the time while applying different deformation from 5% to 8%. The maximum recoverable strain is up to 4.4% with a recovery rate of 100%.     

The influence of temperature on stacking fault energy in Fe-based alloys

Temperature has great influence on the stacking fault energy (SFE). Both SFE and dr0/ d T for Fe-based alloys containing substitutional or interstitial atoms increase with increasing tempera-ture. Based on the thermodynamic model of SFE, the equation dr0/dT=drcn/dT+drseg/dT+drMG/dT and thoseexpressions for three items involved are established. The calculated dr0/dT is generally consistent with the experimental. The influence of chemical free energy on the temperature dependence of SFE is almost constant, and is obviously stronger than that of magnetic and segregation contributions. The magnetic transition and the segregation of alloying elements at stacking faults cause a decrease in SFE of the alloys when temperature increases; that is, drMG/dT<0 and dyseg/dT<0. Meanwhile, such an influence decreases with increasing temperature, except for the drseg/d 7 of Fe-Mn-Si alloys. With these results, the experimenal phenomena that the SFE of Fe-based alloys is not zero at the thermo-dynamically equilibrated tem     

Influences of 2.5wt% Mn addition on the microstructure and mechanical properties of Cu-Al-Ni shape memory alloys

The influences of 2.5wt%Mn addition on the microstructure and mechanical properties of the Cu-11.9wt%Al-3.8wt%Ni shape memory alloy(SMA) were studied by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),and differential scanning calorimeter(DSC).The experimental results show that Mn addition influences considerably the austenite-martensite transformation temperatures and the kind of martensite in the Cu-Al-Ni alloy.The martensitic transformation changes from a mixedβ_1→β'_1...     

Application of the C-Me segregating theory in solid alloys to ceramics

Because of its excellent properties, zirconia ceramics has already been widely applied. Its phase transformation affects its properties. The research on the mechanism of its phase transformation is very important to control the phase transformation as well as its properties. The valence electron structure of cubic zirconia, tetragonal zirconia and monoclinic zirconia are calculated with the em-pirical electron theory in solids and molecules in this paper. The results show that the total numbers of the covalent electron pairs which form their strong bond framework are 3.19184, 3.45528 and 3.79625, respectively. According to the view-point of the C-Me segregating theory in solid alloys, it can be deduced that the phase transformation order of zirconia should be liquid phase→cubic phase→tetragonal phase→monoclinic phase. The deduction from valence electron struc-ture is completely in accordance with the experimental results, so the electron theory of phase transformation in alloys can be expanded to ceramics materials.     

Theoretical analysis and simulation of thermoelastic deformation of bimorph microbeams

In this paper, a purely mechanical model for the thermoelastic behavior of a bimorph microbeam is presented. The thermoelastic coupling problem of the microbeam is converted to a mechanical problem by simply replacing the thermal stress in the beam with a bulk force and a surface force. Thermoelastic deformation of the bimorph microbeams with constraints frequently used in micro-electro-mechanical systems (MEMS) devices has been derived based on this model and is characterized by FEA simulation. Coincidence of the results from theory and simulation demonstrates the validity of the model. The analysis shows that a bimorph microbeam with a soft constraint and a uniform temperature field has a larger thermoelastic deformation than that with a hard constraint and a linear temperature field. In addition to the adoption of materials with large CTE mismatch,thickness ratio and length ratio of the two layers need to be optimized to get a large thermoelastic deformation.     

Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu (wt%, signed as 3C), fabricated by traditional cast (TC) and spray forming (SF) processes, were investigated by dif-ferential scanning calorimetry (DSC) and scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that Al7Cu2Fe phase can be formed and transformed in TC- and SF-3C alloys between 802-813 K. and 800-815 K, respectively. The transformation fromβ-Al5FeSi toδ-Al4FeSi2 phase via peritectic reaction can occur at around 858-870 K. and 876-890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature ofδ-Al4FeSi2 phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation ofδ-Al4FeSi2→β-Al5FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.     

Phase composition, transition and structure stability of functionally graded cemented carbide with dual phase structure

The phase composition, phase transition and phase structure transformation of the wire-cut section of functionally graded WC-Co cemented carbide with dual phase structure were investigated by XRD phase analysis. It is shown that the composition of η phase in the core zone is Co3W3C （M6C type）. The structure of cobalt based solid solution binder phase is fcc type. At the cooling stage of the sintering process, the phase transition of η phase, i.e. M6C→M12C and the martensitic phase transition of the cobalt based solid solution binder phase, i.e. fcc→hcp are suppressed, which facilitates the strengthening of the alloy. Because the instantaneous temperature of the discharge channel is as high as 10 000 ℃ during the wire cutting process, the processed surface is oxidized. Nevertheless, the oxide layer thickness is in micro grade. In the oxide film, η phase is decomposed into W2C and CoO, and cobalt based solid solution binder is selectively oxidized, while WC remains stable due to the existence of carbon containing liquid organic cutting medium.     

Phase transformation behaviors and shape memory effects of TiNiFeAl shape memory alloys

Measurements of electrical resistivity, X-ray diffraction, and tensile test at room temperature and ?196°C were performed to investigate the effects of Al addition substituting Ni on the phase transformation behaviors, the mechanical properties, and the shape memory effects of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys. It is found that 1at% Al addition dramatically decreases the martensitic start transformation temperature and expands the transformation temperature range of R-phase for TiNiFeAl alloys. The results of tensile test indicate that 1at% Al improves the yield strength of Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys by 40% and 64%, but de- creases the plasticity to 11% and 12% from 26% and 27% respectively. Moreover, excellent shape memory effect of 6.6% and 7.5% were found in Ti50Ni47Fe2Al1 and Ti50Ni46.5Fe2.5Al1 alloys, which results from the stress-induced martensite transformation from the R-phase.     

Magnetic properties and XPS of Dy-Y and Dy-Nd alloys

The magnetic properties and X-ray photoelectron spectroscopy (XPS) spectra in the Dy100-xYx and Dy100-zNdz rare-earth alloys are studied.The magnetism measurements show that the dhcp crystal-field influence in the Dy-Nd alloys,which tends to destroy the orbital moment,does not vanish even though a field of 6T is applied.With increasing Y (or Nd) content the temperature region corresponding to helical phase becomes wider (or narrower).The XPS records suggest that the property of the valence fluctuation in neodymium tends to increase the Fermi wave vector kF of the Dy-Nd alloys.This testifies that the abrupt dropping of the Neel temperature,as Nd is introduced,is not only due to a dilution effect of Nd.The introduction of light rare earths into heavy ones is favorable for suppressing the helical phase in the alloys and overcoming the negative magnetocaloric effect.This result will play a guiding role in designing composite magnetic refrigeration working substances.     

Microstructural developments and precipitate transformation during transient liquid-phase bonding of a duplex stainless steel

Effect of holding time on microstructural developments and transformation of precipitates formed at the interface during transient liquid-phase bonding of a duplex stainless steel using a Ni-based amorphous insert alloy was studied. The experimental results reveal that the microstructure of the adjacent base metal varies clearly as a function of holding time. The migration of Cr and Ni elements and the → transformation seem to play relevant roles in this microstructure evolution. The scanning electron microscopy (SEM) and electron prob X-ray microanalysis (EPMA) results indicate the transformation of BN→BN and (N, Mo) boride→BN at the interface with the holding time of 60-1 800 s. N content changes with holding time increasing at locations at the interface might be a controlling factor contributing to this transformation.     

Cooling rate effects on the structure and transformation behavior of Cu-Zn-Al shape memory alloys

Different fragments of a hot-rolled and homogenized Cu–Zn–Al shape memory alloy（SMA） were subjected to thermal cycling by means of a differential scanning calorimetric（DSC） device. During thermal cycling, heating was performed at the same constant rate of increasing temperature while cooling was carried out at different rates of decreasing temperature. For each cooling rate, the temperature decreased in the same thermal interval. During each cooling stage, an exothermic peak（maximum） was observed on the DSC thermogram. This peak was associated with forward martensitic transformation. The DSC thermograms were analyzed with PROTEUS software： the critical martensitic transformation start（Ms） and finish（Mf） temperatures were determined by means of integral and tangent methods, and the dissipated heat was evaluated by the area between the corresponding maximum plot and a sigmoid baseline. The effects of the increase in cooling rate, assessed from a calorimetric viewpoint, consisted in the augmentation of the exothermic peak and the delay of direct martensitic transformation. The latter had the tendency to move to lower critical transformation temperatures. The martensite plates changed in morphology by becoming more oriented and by an augmenting in surface relief, which corresponded with the increase in cooling rate as observed by scanning electron microscopy（SEM） and atomic force microscopy（AFM）.     

Atomistic simulations of martensitic transformation processes for metastable FeMnCoCr high-entropy alloy

Non-equiatomic Fe Mn Co Cr high-entropy alloy(HEA), which exhibits a great potential to break the strength-ductility trade-off relationship, has drawn abundant attention from researchers in experiments. However, atomic simulations of such excellent alloys are limited due to the lack of proper interatomic potentials. In this work, the complete martensitic transformation of nonequiatomic HEA is reproduced via atomic simulations with a novel interatomic potential under EAM framework. The physical p...     

Novel spin glasses by mechanical milling

Novel spin-glass alloys were synthesized by milling intermetallic compounds and also by milling mixtures of crystalline elemental powder in a high-energy ball mill.Spin glass behaviour was found in amorphous Co2Ge,which was amorphised by milling in mechanically disordered crystalline GdAl2 in ball-milled crystalline and amorphous CoZr,and in mechanically alloyed Co-Cu,which formed a supersaturated f.c.c.solid solution.All these materials are binary alloys and tlie concentration of the magnetic element is high,which makes them novel types of spin glasses.It is shown that ball milling may not only lead to structural metallic glasses,but can also generate the magnetic pendant of a structural glass,namely the spin glass.     

Martensitic nucleation mechanism

A sort of special dislocation configuration was deformation-induced in an Fe-Ni-V-C alloy by in-situ elongation tests of TEM. The cooling in-situ observations, as well as the SADPs from the region of the special dislocation configurations, proved that they are martensitic nuclei. In martensitic transformation, a nucleus changed into a small martensitic sub-plate, and a group of parallel sub-plates that formed from a group of parallel nuclei made up a big martensitic plate Martensitic transformation involved opposite shear between adjacent martensitic nuclei. By using the reduced-cell method, the crystallographic structure of observed martensitic nuclei was indexed as a face-centered orthogonal (FCO) lattice, which was explained by the nucleation mechanism proposed by the present authors. The crystallographic analysis confirmed that the defect faulting involved in martensitic nucleation took place among three close pakked planes, instead of between two adjacent planes as an ordinary stacking fault.     

Three-Dimensional Kinematics Simulation of Robot Fighting Platform in Virtual Environment

A method of 3D kinematics simulation of robot fighting platform (RFP) in virtual environment is proposed with the aim of enhancing vision telepresence. Based on the theory of space coordinate transformation, kinematics equations of RFP are formulated; followed by applying a method of modeling using 3DMAX software to build an RFPs 3D geometric model before a 3D kinematics simulation system of RFP is completed based on virtual reality technology and Open Inventor VC++. Test results have indicated that this system can perform RFPs kinematics simulation in virtual environment. It can also imitate RFPs motion states and environmental features well. Moreover, not only can better real-time performances and interactions be achieved but also operators vision telepresence be enhanced, therefore this approach may help lay the foundation for the realization of RFPs teleoperation with vision telepresence.     

Bionic Attitude Transformation Combined with Closed Motion for a Free Floating Space Robot

In order to realize the small error attitude transformation of a free floating space robot,a new method of three degrees of freedom(DOF) attitude transformation was proposed for the space robot using a bionic joint.A general kinematic model of the space robot was established based on the law of linear and angular momentum conservation.A combinational joint model was established combined with bionic joint and closed motion.The attitude transformation of planar,two DOF and three DOF is analyzed and simulated by the model,and it is verified that the feasibility of attitude transformation in three DOF space.Finally,the specific scheme of disturbance elimination in attitude transformation is presented and simulation results are obtained.Therefore,the range of application field of the bionic joint model has been expanded.