Effect of Co on the martensitic transformation,crystal structure and magnetization of Ni<Subscript>52.5</Subscript>Mn<Subscript>23.5</Subscript>Ga<Subscript>24</Subscript> based ferromagnetic shape memory alloys

Five(Ni52.5Mn23.5Ga24)100-xCox(x = 0,2,4,6,8) alloys were prepared by arc melting,and the effects of Co addition on the martensitic phase transformation,crystal structure and magnetization were investigated.The phase transformation temperatures Ms,Mf,As and Af are proportional to the content of Co in the(Ni52.5Mn23.5Ga24)100-xCox alloys,which appears to be due to the variation in the valance electron concentration.The Curie temperature is sensitive to the composition of the alloy.As the amount of Co changes,both the Co-Mn exchange interaction and the distance between Mn atoms change.These,in turn,affect the Curie temperature and magnetization behavior of the alloy.The martensite phases in all the alloys are domained in three different orientations,the domain boundary was determined to belong to the family of {112} lattice planes.     

Improving the tensile properties of extruded Mg–Ga alloy by ageing treatment

A hot-extruded Mg-5Ga alloy was subjected to ageing treatment at 150 ?°C, 190 ?°C and 230 ?°C. The microstructures and mechanical properties of the extruded and aged alloy were examined in this study. Microstructure examinations suggested that particle-shaped and rod-shaped Mg₅Ga₂ were precipitated in the alloy after peak ageing treatment. The extruded alloy showed the yield strength, ultimate tensile strength and elongation to fracture of 157.6 ?MPa, 248.6 ?MPa and 17.5%, respectively. After peak ageing, the yield strength and ultimate tensile strength can be enhanced by as much as 15.7% and 8.6% reaching 182.3 ?MPa and 270 ?MPa, respectively. The improvement of the tensile strengths is mainly attributed to the enhanced precipitation strengthening by newly formed fine Mg₅Ga₂ precipitates. The ductility of the alloy was slightly increased by peak ageing at low temperatures (150 ?°C and 190 ?°C), but remarkably decreased by peak ageing at high temperature (230 ?°C) due to the formation of coarsened Mg₅Ga₂ particles which easily initiated the cracks during tensile deformation.     

Dendrite growth and micromechanical properties of rapidly solidified ternary Ni-Fe-Ti alloy

The rapid solidification of undercooled liquid Ni_(45)Fe_(40)Ti_(15)alloy was realized by glass fluxing technique.The microstructure of this alloy consists of primaryγ-(Fe,Ni)phase and a small amount of interdendritic pseudobinary eutectic.The primaryγ-(Fe,Ni)phase transferred from coarse dendrite to fragmented dendrite and the lamellar eutectic became fractured with the increase of undercooling.The growth velocity ofγ-(Fe,Ni)dendrite increased following a power relation with the rise of undercooling.The addition of solute Ti suppressed the rapid growth ofγ-(Fe,Ni)dendrite,as compared with the calculation results of Fe-Ni alloy based on LKT model.The microhardness values of the alloy and the primaryγ-(Fe,Ni)phase increased by 1.5 times owing to the microstructural refinement caused by the rapid dendrite growth.The difference was enlarged as undercooling increases,resulting from the enhanced hardening effects on the alloy from the increased grain boundaries and the second phase.     

Dependence of mechanical behavior on grain size of metastable Ti–Nb–O titanium alloy

Cold-rolled metastable β-type Ti–38Nb-0.2O alloy was subjected to annealing treatment to obtain different precipitates and grain sizes. The influence of annealing on microstructure and mechanical properties was investigated. The alloy annealed at 673 ?K or 773 ?K exhibited a single-stage yielding with high strength and low uniform elongation, due to the residual work hardening and the precipitation of ω or α phases. The alloy annealed at above 873 ?K exhibited an obvious double yielding behavior resulting from the stress-induced martensitic transformation. The grain growth kinetics of single β phase alloy is sensitive to temperature, and it is suggested that the existence of oxygen decreases the grain growth exponent and increases the required activation energy for grain growth. The critical stress for slip decreased monotonously with the increase of grain size, following the classic Hall-Petch relationship. However, the critical stress for martensitic transformation decreased to a minimum and then increased again, as the grain size increased. The results are worth for design of the heat-treatment parameters of the Ti–38Nb-0.2O alloy for engineering applications.     

Composition design of superhigh strength maraging stainless steels using a cluster model

The composition characteristics of maraging stainless steels were studied in the present work investigation using a cluster-plus-glue-atom model. The least solubility limit of high-temperature austenite to form martensite in basic Fe–Ni–Cr corresponds to the cluster formula [NiFe12]Cr3,where NiFe12is a cuboctahedron centered by Ni and surrounded by 12 Fe atoms in FCC structure and Cr serves as glue atoms. A cluster formula [NiFe12](Cr2Ni) with surplus Ni was then determined to ensure the second phase(Ni3M) precipitation,based on which new multicomponent alloys [(Ni,Cu)16Fe192](Cr32(Ni,Mo,Ti,Nb,Al,V)16) were designed. These alloys were prepared by copper mould suction casting method,then solid-solution treated at 1273 K for 1 h followed by water-quenching,and finally aged at 783 K for 3 h. The experimental results showed that the multi-element alloying results in Ni3M precipitation on the martensite,which enhances the strengths of alloys sharply after ageing treatment. Among them,the aged [(Cu4Ni12)Fe192](Cr32(Ni8.5Mo2Ti2Nb0.5Al1V1)) alloy(Fe74.91Ni8.82Cr11.62Mo1.34Ti0.67Nb0.32Al0.19V0.36Cu1.78wt%) has higher tensile strengths with YS?1456 MPa and UTS?1494 MPa. It also exhibits good corrosion-resistance in 3.5 wt% NaCl solution.     

Formation mechanism of interfacial microstructures and mechanical properties of Ti2AlNb/Ni-based superalloy joints brazed with NiCrFeSiB filler metal

Dissimilar brazing of Ti₂AlNb-based alloy and Ni-based wrought superalloy (GH536) was studied using NiCrFeSiB filler metal. The Ti₂AlNb/GH536 joints were analyzed by scanning electron microscope (SEM) equipped with an electron probe micro-analyzer (EPMA), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The formation mechanism of interfacial microstructure and mechanical properties of Ti₂AlNb/GH536 joints were studied. The results indicated that Ti₂Ni(Al,Nb), AlNi₂Ti and TiB₂ reaction layers were formed in the joint adjacent to Ti₂AlNb base metal. These layers resulted in high micro-hardness and the weak link of the joint. γ solid solution was formed through isothermal solidification and β₁-Ni₃Si phase precipitated in the γ solid solution during cooling process. Ni₃B, β₁-Ni₃Si and CrB phases appeared in the centre of the joint. Blocky and needle-like borides formed within the diffusion affected zone of GH536 base alloy. The maximum tensile strength of Ti₂AlNb/GH536 joints reached 425 ?MPa ?at room temperature and the strength value of 373 ?MPa was maintained at 923 ?K.     

Phase transition , structure and shape memory effect of Ni47Ti43Hf10 alloy

A Ni₄₇Ti₄₃Hf₁₀ high temperature shape memory alloy is fabricated. The martensitic transformation temperature (TT) is obtained by differential scanning calorimetry and four-probe electrical resistivity measurements. The effect of thermal cycling is investigated and it is found that the TT tends to be stable quickly, which is of benefit to practical applications. The martensite structure is determined to be B19' monoclinic by X-ray diffraction and transmission electron microscopy. One-way and two-way (which is seldom reported before) shape memory properties are studied by tensile and bending tests. The cycling number of two-way shape memory effect is tested for more than 20000 times.     

Effect of Ni addition on the glass-forming ability and soft-magnetic properties of FeNiBPNb metallic glasses

The effect of Ni addition on the glass-forming ability (GFA) and soft-magnetic properties of an (Fe1-xNix)75.5B14.5P7Nb3 (x=0-0.6) alloy system were investigated. We found that the addition of Ni was effective in allowing the alloy to approach a eutectic point as well as increasing the thermal stability of the supercooled liquid. By increasing the amount of Ni,the supercooled liquid region (ΔTx),the reduced glass transition temperature Trg (Tg/Tl) and the Y parameter [Tx/(Tg+Tl)] increased from 49 to 75 K,0...     

Preparation of Al<Subscript>72</Subscript>Ni<Subscript>8</Subscript>Ti<Subscript>8</Subscript>Zr<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Y<Subscript>3</Subscript> amorphous powders and bulk materials

Amorphous Al₇₂Ni₈Ti₈Zr₆Nb₃Y₃ powders were successfully fabricated by mechanical alloying. The microstructure, glass-forming ability, and crystallization behavior of amorphous Al₇₂Ni₈Ti₈Zr₆Nb₃Y₃ powders were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The isothermal crystallization kinetics was analyzed by the Johnson–Mehl–Avrami equation. In the results, the supercooled liquid region of the amorphous alloy is as high as 81 K, as determined by non-isothermal DSC curves. The activation energy for crystallization is as high as 312.6 kJ·mol?1 obtained by Kissinger and Ozawa analyses. The values of Avrami exponent (n) imply that the crystallization is dominated by interface-controlled three-dimensional growth in the early stage and the end stage and by diffusion-controlled two- or three-dimensional growth in the middle stage. In addition, the amorphous Al₇₂Ni₈Ti₈Zr₆Nb₃Y₃ powders were sintered under 2 GPa at temperatures of 673 K and 723 K. The results show that the Vickers hardness of the compacted powders is as high as Hv 1215.     

Towards understanding the influence of Mg content on phase transformations in the La3-xMgxNi9 alloys by in-situ neutron powder diffraction study

The present work is focused on the studies of the phase-structural transformations in the La_3-xMg_xNi₉ (x ​= ​1.0, 1.1 and 1.2) alloys as active materials of negative electrodes in the Nickel-Metal Hydride (Ni/MH) batteries. The phase equilibria and phase-structural transformations in the alloys were probed by in situ neutron powder diffraction (NPD) at the temperatures ranging from 300 ​K to 1273 ​K using the measurements of the equilibrated alloys at 8 setpoint temperatures of 300, 973, 1073, 1123, 1173, 1223, 1248 and 1273 ​K.Prepared by induction melting initial alloys were found to be multi-phase structured, containing up to 6 individual intermetallic compounds with different stoichiometric compositions. With the increase of the temperature and holding time, various transformations took place in the studied alloys. These included the formations and transformations of super-stacking intermetallics with variable ratios (La ​+ ​Mg)/Ni, 1:3, 2:7 and 5:19.With increasing temperatures, several systematic changes took place. (a) Abundances of (La,Mg)₂Ni₄ AB₂ and (La,Mg)Ni₃ AB₃ type intermetallics gradually decreased before they melted/decomposed above 1073 ​K; (b) The (La,Mg)₂Ni₇ A₂B₇ type intermetallics began to decrease in abundances above 1123 ​K; (c) The transformation in the (La,Mg)₅Ni₁₉ intermetallics from 3R to 2H proceeded above 1223 ​K.The increase of Mg content had no obvious influence on (La,Mg)₂Ni₄ and (La,Mg)₂Ni₇ phases, and corresponding reactions R1 and R3 took place at the same temperatures as in the La–Ni system. However, with increasing Mg content the melting point of (La,Mg)₅Ni₁₉ phase increased while the melting point of the (La,Mg)Ni₃ phase it decreased, leading to the variation of the reaction temperatures of the corresponding processes.The present study will assist in optimizing phase-structural composition of the alloys in the La–Mg–Ni system which contain Mg-modified layered structures by tailoring the high temperature annealing conditions.     

Effects of solidification parameters on microstructure and mechanical properties of continuous columnar-grained Cu–Al–Ni alloy

Effects of melt temperature and casting speed on microstructure and mechanical properties of Cu-14%Al-3.8%Ni(mass fraction) alloy wires fabricated by continuous unidirectional solidification technology were investigated.It was found that the average size of columnar grain in the alloy decreased and grain boundary turned clear and straight with increasing the casting speed at a given melt temperature.When the melt temperature was up to 1 280℃,theβ_1 phase gradually transformed into lozenged and lanciformγ...     

Effects of crystallization on corrosion behaviours of a Ni-based bulk metallic glass

The effects of microstructure change on the corrosion behaviours of Ni₅₅Nb₂₀Ti₁₀Zr₈Co₇ bulk glass-forming alloy were investigated in 1 mol/L HCl and 0.5 mol/L H₂SO₄ solutions. Different microstructures of the Ni-based alloy were achieved by annealing the bulk glassy rod prepared by copper mould casting. The microstructure, grain size, grain distribution, and phase composition were characterized. Electrochemical behaviours of the Ni-based alloy were revealed by static immersion and anodic potentiodynamic polarization tests. It is indicated that the corrosion behaviours of the Ni-based bulk glass-forming alloy are related to its microstructures, while the fully crystallized alloy exhibits a relatively lower corrosion resistance than those of the amorphous states.     

Ga对β基Co-Ni-Al铁磁性形状记忆合金马氏体和磁性转变的影响

借助光学金相、示差热分析、振动样品磁强计和x-射线能谱分析等分析方法，研究Ga含量对Co41Ni32-Al27-xGax合金马氏体相变和Curie点的影响。研究结果表明：合金马氏体相变温度与Ga含量成正比，在1573和1623K淬火时，X增加1，马氏体相变温度提高25K，但Ga含量对Curie点影响不大；在1573K淬火时，具有高有序度的马氏体相比B2结构相的Curie点高32K，说明结构有序度对Curie点影响较大；淬火温度升高会显著提高合金的马氏体相变温度和Curie点，当淬火温度从1573K升高到1623K时，马氏体相变温度升高43～69K，Curie点平均升高41K；随着Ga含量的增加，合金的熔点降低，在1623K淬火时Co41Ni32Al18Ga9合金发生部分熔化。     

Ni50.3Mn27.3Ga22.4磁性形状记忆合金薄膜的磁场增强马氏体相变应变研究

采用磁控溅射方法制备Ni50.3Mn27.3Ga22.4磁性形状记忆合金薄膜。研究薄膜的晶体结构、磁化行为以及磁场对马氏体相变应变的影响。试验结果表明,经823 K退火1 h的Ni50.3Mn27.3Ga22.4薄膜,室温下处于奥氏体态,呈较强的(110)织构特性,且室温饱和磁化强度约为40 emμ/g。试验还发现,当沿膜面方向施加0到0.8 T磁场时,Ni50.3Mn27.3Ga22.4薄膜的马氏体相变应变量随磁场强度的增大而增大,呈现出磁场增强马氏体相变应变效应。     

Morphology and quantitative analysis of O phase during heat treatment of hot-deformed Ti<Subscript>2</Subscript>AlNb-based alloy

A 1040℃-hot-deformed Ti₂AlNb-based alloy solution-treated at 950℃ and aged at different temperatures was quantitatively investigated. The microstructure, size of the phase, and microhardness of the deformed alloys were measured. The results indicated that the microstructure of the deformed Ti₂AlNb-based alloy specimens comprise coarse O lath, fine O lath, equiaxed O/α₂, and acicular O phase. More O phase was generated in the deformed alloy after heat treatment because the acicular O phase was more likely to nucleate and grow along the deformation-induced crystal defects such as dislocations and subgrain boundaries. After deformation and subsequent heat treatment, the acicular O phase of the resultant alloy became finer compared to that of the undeformed alloy, and the acicular O phase became coarser and longer with the elevated aging temperature, while the width of the O lath exhibited unobvious variations. The hot deformation facilitated the dissolution of the O lath but accelerated the precipitation of the acicular O phase. When the 950℃-solution-treated deformed Ti₂AlNb-based alloy was then aged at 750℃ for different periods, the phase content was nearly invariable, O and B2 phases eventually reached equilibrium, and the microstructure became stable and homogeneous.     

Abnormal thermal shock behavior in electrical conductivity of Ti2SnC

Some ternary carbide and nitride ceramics have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties. However, the influence of thermal shock on other properties is not clear. This work reports on the influence of thermal shock on electrical conductivity of Ti_2SnC as a representative member of ternary carbides. Abnormal change in electrical conductivity was first demonstrated during quenching Ti_2 SnC in water at 500-800 ℃. The residual electrical conductivity of the quenched Ti_2SnC gradually decreased with increasing temperature, but abnormally increased after quenching at 600 ℃. The microstructure of surface cracks was characterized. The main mechanism for the abnormal electrical conductivity recovery is that some narrow branching cracks are filled by metallic Sn precipitating from Ti_2SnC.     

Tailoring of microstructure and martensitic transformation of nanocrystalline Ti–Ni-Hf shape memory thin film

The amorphous Ti-Ni-Hf thin films with the specific compositions were prepared from single Ti-Ni-Hf alloy target by adjusting processing parameters of direct current magnetron sputtering deposition. Prior to the crystallization,a glass transition occurred in the present Ti-Ni-Hf thin films. The annealed Ti-Ni-Hf thin films were characterized by the nano-crystalline. With the annealing temperature increasing, the grain size firstly increased and then decreased owing to the presence of(Ti,Hf)_2Ni precipitate. Two endothermic and exothermic peaks corresponding to B19'■B2 martensitic transformation in heating and cooling curves were observed for the Ti-Ni-Hf thin films with the lower annealing temperature and shorter annealing time, which was closely related to the inhomogeneous composition. However, the Ti-Ni-Hf thin films annealed at higher annealing temperature and longer annealing time showed the single stage B19'■B2 martensitic transformation. In addition, the martensitic transformation temperatures firstly increased and then decreased with the annealing temperatures rising.     

A Ti–Zr–Cu–Ni–Co–Fe–Al–Sn amorphous filler metal for improving the strength of Ti–6Al–4V alloy brazing joint

Ti_(50)Zr_(27)Cu_8Ni_4Co_3Fe_2Al_3Sn_3(at%) amorphous filler metal with low Cu and Ni contents in a melt-spun ribbon form was developed for improving mechanical properties of Ti–6Al–4V alloy brazing joint through decreasing brittle intermetallics in the braze zone. Investigation on the crystallization behavior of the multicomponent Ti–Zr–Cu–Ni–Co–Fe–Al–Sn amorphous alloy indicates the high stability of the supercooled liquid against crystallization that favors the formation of amorphous structure. The Ti–6Al–4V joint brazed with this Ti-based amorphous filler metal with low total content of Cu and Ni at 1203K for 900s mainly consists of α-Ti, β-Ti,minor Ti–Zr-rich phase and only a small amount of Ti_3Cu intermetallics, leading to the high shear strength of the joint of about 460 MPa. Multicomponent composition design of amorphous alloys is an effective way of tailoring filler metals for improving the joint strength.     

Surface chemical structure of titania-silica nanocomposite powder

Titania-silica （TS） nanocomposite powder with three different composite structures, containing 10-30 mol% SiO2 in each structure, have been prepared by sol-gel processes. The surface characteristics of these titania-silica samples have been investigated by X-ray photo-emission spectroscopy （XPS） and high resolution transmission electron microscopy （HRTEM）. The study for all TS oxides annealed at 773 and 1173 K showed： an abnormal surface enrichment in Si increased with increasing annealing temperature; the Ti^3＋, Ti^2＋, Si^3＋ and Si^2＋ oxides coexisted with Ti^4＋ and Si^4＋ oxides, and the contents of these Ti/Si suboxides increased with increasing SiO2 content and annealing temperature; there was a layer rich in O on the topmost surface and the excess O could be attributed to the chem-adsorption of H2O; different composite structures could lead to different contents of Ti/Si suboxides. These results indicated that the surface of TS oxide powder derived by sol-gel process was a double layer with enriched O first and then SiOx/TiOy（x, y〈2）. Ti/Si suboxides could result from the thermal diffusion of Ti^4＋ and Si^4＋, which might be induced by the strong interaction between Ti^4＋ and Si^4＋.     

A FeNiMnC alloy with strain glass transition

Recent experimental and theoretical investigations suggested that doping sufficient point defects into a normal ferroelastic/martensitic alloy systems could lead to a frozen disordered state of local lattice strains(nanomartensite domains), thereby suppressing the long-range strain-ordering martensitic transition. In this study, we attempt to explore the possibility of developing novel ferrous Elinvar alloys by replacing nickel with carbon and manganese as dopant species. A nominal Fe_(89)Ni_5Mn_(4.6)C_(1.4) alloy was prepared by argon arc melting, and XRD, DSC, DMA and TEM techniques were employed to characterize the strain glass transition signatures, such as invariance in average structure, frequency dispersion in dynamic mechanical properties(storage modulus and internal friction) and the formation of nanosized strain domains. It is indicated that doping of Ni, Mn and C suppresses γ→α long-range strain-ordering martensitic transformation in Fe_(89)Ni_5Mn_(4.6)C_(1.4) alloy, generating randomly distributed nanosized domains by strain glass transition.