Reaction synthesis of Ni/Ni3Al multilayer composites using Ni and Al foils: High-temperature tensile properties and deformation behaviour

The effects of temperature on the tensile properties and deformation behaviour of Ni/Ni₃Al multilayer composites have been systematically investigated. With increasing tensile test temperature from room temperature to 600 °C, the ultimate tensile strength decreased. The ‘abnormal’ strengthening of Ni₃Al gave rise to a reduction in the capability for cooperative deformation between the Ni and the Ni₃Al layers at elevated temperatures. As a result, the ultimate tensile strength of the multilayer composites decreased at elevated temperatures. A mixture of transgranular cleavage and intergranular failure at relatively low temperatures, and an intergranular failure at elevated temperatures were observed in the fracture surfaces of the Ni₃Al layers in the multilayer composites. The splitting of the coarse precipitates along the {0 0 1} planes at 800 °C resulted from the differences in solubility of Al in Ni between room temperature and 800 °C.     

Notch sensitivity of heavily cold-rolled Ni3Al foils

The notch sensitivity of 95% cold-rolled Ni₃Al foils was examined using double U-notched tensile specimens. The foils were found to be essentially insensitive to the presence of the notch. Plastic deformation occurred locally at the notch root to relieve the stress concentration, resulting in the notch insensitivity.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix; a striking contrast in behavior from normal γ/γ′ alloys

Coarsening of γ precipitates in a γ′ (Ni₃Al) matrix depends strongly on volume fraction, in dramatic contrast with coarsening of γ′ precipitates. Coalescence of γ precipitates is easy, unlike the resistance to coalescence of γ′ precipitates. We regard this as indirect proof that anti-phase relationships among γ′ precipitates inhibit coalescence.     

极薄带微轧制中的系列新发现

在对极薄带微轧制的研究中,陆续发现一系列用传统轧制理论不能解释的新现象,如工业纯铜的超延展现象、脆性材料的非典型塑性现象、异种金属多层复合中的变形诱导反应扩散现象等。这些新现象与微成形中的尺寸效应有关,对其进行深入研究,一方面,能够有助于揭示微成形的深层规律,加深对塑性变形本质的认识;另一方面,有助于在微成形领域产生新思路,开发微成形新工艺,促进微成形产品开发,推动微成形技术的进步。     

The kinetics of two-stage formation of TiAl3 in multilayered Ti/Al foils prepared by accumulative roll bonding

The solid-state reactions of Ti/Al multilayered samples produced by Accumulative Roll Bonding (ARB) have been investigated using differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. The kinetics of the formation of the intermetallic compound TiAl₃ was highlighted. Experimental evidence and analysis of the data shows that, there was a two-stage process in the formation of TiAl₃ in the ARB Ti/Al reactive multilayered samples. Calorimetric and microstructural analyses also suggest that the interdiffusion of Al and Ti which led to solid solutions preceded the formation of intermetallic compounds. Despite the apparent chaos in the thickness of the ARB multilayered samples, the distribution of layer spacing did not become broad enough to lose the main features of the double exothermal behaviour. Isothermal DSC shows a larger Avarami constant in ARB Ti/Al multilayered structures than was found in Ti/Al thin films. A modified model based on thin films was set up to describe the kinetic characteristics of the formation of the intermetallic compound TiAl₃ in ARB samples.     

剪切变形方向特征对高纯铝箔轧制织构的影响

运用取向分布函数(ODF)研究了每个轧制道次的剪切变形特征对高纯铝箔轧制织构的影响.提出用中性角的相对大小来量化轧制剪切变形作用方向改变的位置,运用Taylor晶体塑性变形理论模拟计算了中性角的相对大小对轧制织构演变的影响.实测与模拟的轧制织构特征表明:中性角靠近轧制变形区的中心位置有利于形成{001}<110>剪切织构,从而证实了除剪切变形的程度外,剪切变形方向改变的位置也是影响高纯铝箔轧制织构的重要因素.     

Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

On the peritectoid formation of Ni5Al3

The peritectoid formation of Ni₅Al₃ from the two phases NiAl and Ni₃Al was studied in a Ni---Al alloy containing 66 at% Ni by means of transmission electron microscopy. The product phase does not form as a uniform layer between the two initial phases as expected and already observed in a few systems. In the system studied here there are only very few nucleation sites located at the NiAl/Ni₃Al interface. The further growth of Ni₅Al₃ takes place only into one of the initial phases which is NiAl. A strict orientation relationship between NiAl and Ni₅Al₃ was observed; the growth direction was [221]. The transformation is presumably diffusion controlled; it is very sluggish and it can be described by a nucleation and growth process. From the study presented here we conclude that the formation of Ni₅Al₃ proceeds by a micromechanism which differs from that normally assumed for peritectoid reactions.     

The effect of chemical coating with Ni on the electrode properties of Mg2Ni alloy

Mg₂Ni alloy prepared by powder sintering was chemically coated with 10% nickel by weight. The effects of the nickel coating on the surface appearance, the structure of the alloy and the electrode characteristics were investigated. The discharge capacity and cycle life of the nickel-coated alloy electrode were greatly increased in comparison with values for the bare alloy because of the changed phase structure and surface properties.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

Influence of metastable Al9Ni2 phase on the sequence of phase transformations initiated by heating of Al/Ni multilayer foils produced by EBPVD method

Al/Ni multilayer foils (MF) undergo a cascade of phase transformations at heating, initiated by diffusion interaction of Al and Ni layers. It is found that phase transformations sequence at initial stages depends on the method of producing MF: at sputtering or ion-beam deposition of elements, metastable Al₉Ni₂ phase forms at phase transformations initial stages, and in the case of MF produced by electron-beam physical vapour deposition (EBPVD) method or cold rolling of laminates, this is Al₃Ni phase. Such a difference in phase transformations sequence is associated with the influence of the method of MF production on the possibility of intermixed zone (IZ) formation on layer interfaces. In the study it was suggested that such anomalously high diffusion mobility of atoms can be achieved in the presence of excess vacancies in MF structure. With this purpose, MF structure was produced by high-rate (up to 30 nm/s) layer-by-layer deposition of elements by EBPVD method. Phase transformations and MF were studied by the method of X-ray diffraction (XRD) and differential-thermal analysis (DTA). It is shown that irrespective of MF composition and modulation period, at their heating phase transformations start with formation of metastable Al₉Ni₂ phase. At further MF heating, a stable Al₃Ni phase forms alongside Al₉Ni₂ phase. Later on, Al₉Ni₂ and Al₃Ni phases turn into stable intermetallics characteristic of MF chemical composition.     

Ab initio electronic structure calculations and optical properties of ordered and disordered Ni3Al

The electronic structure and optical properties of the Ni₃Al intermetallic alloy are studied by the first-principles orthogonalized linear combination of atomic orbitals method. Disordered models at different temperatures were constructed using molecular dynamics and the Vienna ab initio simulation package. The average charge transfer from Al to Ni increases steadily with temperature until the liquid phase is reached. The localization index shows the presence of relatively localized states even above the Fermi level in the disordered models. The calculated optical conductivity of the ordered phase is rich in structures and in reasonable agreement with the experimental data. The spectra of the disordered Ni₃Al models show a single broadened peak at 4.96 eV in the 0 K model which shifts towards 6.62 eV at 1400 K and then down to 5.83 eV in the liquid phase. Other results on the band structure and density of states are also discussed.     

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H₂ (reactive mechanical grinding) with oxides Cr₂O₃, Al₂O₃ and CeO₂. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH₂ during reactive mechanical grinding. The Mg+10wt.%Cr₂O₃ powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. The Mg+10wt.%Cr₂O₃ powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. Mg+10wt.%Cr₂O₃ absorbs 5.87wt.% H at 573 K, 11 bar H₂ during 60 min at the first cycle. The Mg+10wt.%Cr₂O₃ powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H₂ during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr₂O₃ in the Mg+10wt.%Cr₂O₃ powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr₂O₃ after cycling.     

Effect of Al concentration on pseudoelasticity in Fe3Al single crystals

Pseudoelasticity in Fe₃Al single crystals with different Al contents was investigated focusing on the dislocation configuration and the ordered domain structure. Giant pseudoelasticity only appeared in the D0₃-ordered Fe₃Al single crystals, while the B2-ordered crystals and those with the disordered phase exhibited small strain recovery. The amount of shape recovery in the D0₃-ordered crystals showed a maximum near 23.0at.%Al and decreased with increasing deviation from this Al concentration. In the D0₃ phase at 22.0–25.0at.%Al, 1/4[1 1 1] superpartial dislocations moved individually dragging the nearest-neighbour antiphase boundaries (NNAPB), while couplets of the superpartials were observed to bow out, dragging the next-nearest-neighbour antiphase boundaries (NNNAPB) in Fe–28.0at.%Al. In Fe–22.0–25.0at.%Al single crystals, the NNAPB pulled back the superpartials to decrease its energy during unloading, resulting in the giant pseudoelasticity. In contrast, the surface tension of the NNNAPB was lower than that of the NNAPB, leading to the small strain recovery in Fe–28.0at.%Al.     

Sulphidation/oxidation behaviour of TiAlCr and Al2Au coated Ti45Al8Nb alloy at 750 °C

In this paper, we present the sulphidation/oxidation behaviour of a Ti45Al8Nb (at%) alloy coated with different protective surface films. Two intermetallic coatings are considered; TiAlCr and Al₂Au deposited by physical vapour deposition. The coated alloy was subjected to a H₂/H₂S/H₂O yielding pS₂ - 10⁻¹ Pa and pO₂ - 10⁻¹⁸ Pa potentials at 750 °C for up to 1000 h. The corrosion kinetics were determined by means of discontinuous gravimetry and the as-received and exposed samples were characterised using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD). The materials showed the development of a multilayered structure. In the case of the TiAlCr coated Ti45Al8Nb - base alloy, Al₂O₃, TiO₂ and Cr₂S₃ developed. For the Al₂Au coated Ti45Al8Nb samples an Al₂O₃ scale containing TiO₂ nodules was observed at the surface.     

Shear-induced chemical disordering in Ni3Al at different temperatures

Molecular dynamics simulations have been used to study shear-induced chemical disordering in Ni₃Al lattices at different temperatures and strain rates. Shearing determines the formation of an amorphous layer, the thickness of which increases linearly with the square root of time. The rate at which the amorphous layer grows is both shearing rate- and temperature-dependent. A linear correlation between the amorphous layer growth rate and the shear modulus is found. This suggests that mechanical properties could play a central role in shear-induced disordering processes.     

CVD deposition and characterization of coloured Al2O3/ZrO2 multilayers

Coloured Al₂O₃/ZrO₂ multilayers have been deposited onto WC-Co based inserts by a CVD process. Through physical as well as optical analysis of such multilayers, colour is believed to originate from interference. The coatings are obtained with good process reproducibility. It was found that the ZrO₂ process used in the multilayer, with ZrCl₄ as the only metal chloride precursor, results in a mixture of tetragonal and monoclinic ZrO₂ phases. However by adding a relatively small amount of AlCl₃ during such a process results in ZrO₂ layers being composed of predominantly tetragonal ZrO₂ phase. Corresponding multilayers seem to have a more fine grained and smoother morphology whereas multilayers containing monoclinic ZrO₂ phase seem to be less perfect with existence of larger grains of ZrO₂ which are believed to scatter light and alter the reflectance of such a multilayer. In addition to this, such multilayers were found to be free of or with greatly reduced amount of thermal cracks, normally present in pure CVD grown Al₂O₃ layers.It is believed that, in the studied Al₂O₃/ZrO₂ multilayers, the observed tetragonal ZrO₂ phase is the result of a size effect, where small enough ZrO₂ crystallites energetically favor the tetragonal phase. However as the ZrO₂ crystallite size distribution is shifted to larger sizes it is believed that a mixture of crystallites with both stable and metastable tetragonal phases as well as a stable monoclinic phase is obtained. The proposed metastable tetragonal ZrO₂ phase may in fact explain the absence of thermal cracks in such multilayers through a transformation toughening mechanism, well known in ZrO₂ based ceramics.     

Effects of aluminum concentration and compact thickness on reaction synthesis of Ni3Al---NiAl alloys

The effects of aluminum concentration and green compact thickness on the reaction synthesis of powder compacts containing 25 to 40 at. %Al were studied under a uniaxial compressive stress of 10 MPa. The results indicate that both reaction product and material density are sensitive to these two parameters. For all powder compacts with a thickness of 5 mm, no temperature increase was detected during reaction synthesis, indicating no formation of transient liquid phases and thus no self-sustaining combustion reaction. For 6- and 7-mm-thick compacts containing 40% Al, a full synthesis reaction took place, resulting in production of the NiAl alloy with a high material density (>98%) and a fine grain size (14 μm). Other compacts containing 25 and 30% Al showed a coexistence of partially and fully reacted zones, with the reaction zone size strongly dependent on aluminum concentration and compact thickness. All the results are discussed in terms of local heat accumulation and heat transfer as well as thermodynamic analyses of exothermic reactions during each reaction step.     

Effect of α-Al/Al3Ni microstructure on the corrosion behaviour of Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing

The effect of microstructure on corrosion behaviour of an Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing (ECAP) was investigated by means of potentiodynamic polarization test. The Al-5.4 wt% Ni alloy samples were severely deformed by ECAP with two strain introduction methods of route A and route B_C and the ECAP process was repetitively carried out up to 6 passes (strain 6). The anodic polarization measurements indicated that pitting potential of the ECAPed Al-Ni alloy samples in chloride containing neutral buffer solution increased with ECAP passes. The pitting corrosion resistance of Al-Ni alloy after ECAP by route B_C was better than that by route A. It is attributable to that the size of α-Al crystal region was reduced with ECAP passes and route B_C was able to obtain more homogeneous α-Al/Al₃Ni structure than route A. On the other hand, pitting corrosion resistance of pure Al samples showed an obvious declining with increasing ECAP passes. It was indicated that more homogeneous and finer α-Al/Al₃Ni structure obtained by ECAP played a vital role on improving the corrosion resistance of Al-5.4 wt% Ni alloy.     

Corrosion behaviour of Al/Al3Ti and Al/Al3Zr functionally graded materials produced by centrifugal solid-particle method: Influence of the intermetallics volume fraction

Intermetallic particles, Al₃Ti and Al₃Zr were formed in Al–5mass%Ti and Al–5mass%Zr alloys, respectively, by centrifugal casting, in order to create functionally graded materials (FGMs). At present, no information is available on the influence of the amount of intermetallics on the electrochemical properties of these alloys.In this paper, the corrosion resistance of Al/Al₃Ti and Al/Al₃Zr FGMs was investigated by open-circuit measurements, potentiodynamic polarization and electrochemical impedance spectroscopy. Results suggests that the corrosion resistance of the FGMs is affected by galvanic effects between the intermetallic particles and the metallic matrix. Lower centrifugal forces resulted in an improvement of the electrochemical properties.