Processes of atomic ordering in microcrystalline ternary alloys based on Ni3Mn

Stable and metastable phases are formed in the production of microcrystalline alloys By quenching from the melt, The atomic ordering of these phases has not been completely studied until recently. In ternary systems with two ordered phases having different Kurnakov temperatures TK (the temperature of the order-to-disorder transition), for example, an intetmetallic compound and an ordinary superstructure, preferential formation of a superstructure with a highT _K is expected and the alloying atoms occupy vacancies in the ordered lattice of the intermetallic compound. The present work concerns neutron diffraction analysis of atomic ordering in the ternary systems Ni₃Mn-Ni₃Al, Ni₃Mn-Ni₃Si, and Ni₃Mn-Ni₃Ti.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 2–4, August. 1995.     

Phase relation and microstructure in multi-phase intermetallic alloys based on Ni3Al–Ni3Nb–Ni3V pseudo-ternary alloy system

The phase relation and microstructure of alloys based on the Ni₃Al–Ni₃Nb–Ni₃V pseudo-ternary alloy system at 1273 and 1373 K were investigated by optical microscopy, X-ray diffraction, and scanning electron microscopy (attached with a wavelength dispersive spectroscope). There was no other intermediate phase besides L1₂ (Ni₃Al), D0_a (Ni₃Nb) and D0₂₂ (Ni₃V). The phase fields of L1₂ (Ni₃Al), D0_a (Ni₃Nb) and D0₂₂ (Ni₃V) have been shown to primarily depend upon the free electron concentration (e/a) of the constituent atoms. Among the three constituent intermetallic phases, one kind of three-phase relation (i.e. L1₂–D0_a–D0₂₂) and three kinds of two-phase relations (i.e. L1₂–D0_a, D0_a–D0₂₂ and L1₂–D0₂₂) were found to exist at 1273 K. The prepared alloys exhibited widely different microstructures, depending on alloy composition and temperature. In the low Nb concentration region, dual multi-phase microstructures composed of L1₂+D0_a+A1(→L1₂+D0₂₂) were formed by a eutectic reaction.     

Atomic ordering in nano-layered FePt

Monte Carlo simulation of chemical ordering kinetics in nano-layered L1₀ AB binary intermetallics was performed. The study addressed FePt thin layers considered as a material for ultra-high-density magnetic storage media and revealed metastability of the L1₀ c-variant superstructure with monoatomic planes parallel to the surface and off-plane easy magnetization. The layers, originally perfectly ordered in a c-variant of the L1₀ superstructure, showed homogeneous disordering running in parallel with a spontaneous re-orientation of the monoatomic planes leading to a mosaic microstructure composed of a- and b-L1₀-variant domains. The domains nucleated heterogeneously on the surface of the layer and grew discontinuously inwards its volume. Finally, the domains relaxed towards an equilibrium microstructure of the system. Two “atomistic-scale” processes: (i) homogeneous disordering and (ii) nucleation of the a- and b-L1₀-variant domains showed characteristic time scales. The same was observed for the domain microstructure relaxation. The discontinuous domain growth showed no definite driving force and proceeded due to thermal fluctuations. The above complex structural evolution has recently been observed experimentally in epitaxially deposited thin films of FePt.     

Long-range ordering kinetics and ordering energy in Ni3Al-based γ′ alloys

Chemical long-range order kinetics were studied for the Ll₂-ordered γ′-phase domains of the quasi-binary systems Ni₇₅AL_25−xM_x (M = Fe, Mn, Cr, 0 ≤ x ≤ 25), where the γ′-phase stability (ordering energy) rapidly decreases with increasing M-component concentration x.The sample γ′ homogeneity was tested by means of transmission electron microscopy. The ‘order-order’ relaxations measured by means of in-situ resistometry indicated that an increase in x caused a systematic decrease in the activation energy, E_A, of homogeneous long-range order kinetics.Discussing a possible influence of various parameters, it was concluded that the observed decrease in E_A was predominantly due to an effect of the ordering energy on the bulk vacancy diffusion in these Ll₂-ordered compounds.     

Coherency strain-induced ordering in substitutional alloys

It has been well known that one of the driving forces for ordering is the relaxation of elastic strain energy due to difference in atomic size, a clear evidence being the change in lattice parameter between ordered and disordered states. Because of mathematical complexity, however, only a few strain models have been presented for substitutional alloys. This work reports coherency-induced ordering in a simple model with a two-dimensional square lattice. At an equi-atom composition, a square lattice displays two elementary, ordered structures, Sl₀ and S1₁, similar to Ll₁(CuAuI) and L1₁ (CuPt) in fcc. The stability of two superlattices at the ground state depends strongly on elastic anisotropy. For a homogeneous system, Zener’s anisotropy ratio is sufficient to measure the stability of one structure against the other. If the elastic constants are different, however, the stability depends not only on the elastic moduli of both elements but also on the solvent-solute bond length. The order-to-disorder transition is studied via the Discrete Atom Method, which is predicated upon statistical mechanics and linear elasticity. When both elastic and chemical interactions join together for ordering, the transition temperature is raised to a value greater than the sum of the two individual cases-an indication of a coupling between the two driving forces. This article based on a presentation made in the symposium “The 1999 KSEA Materials Symposium in Honor of Professor Sang Joo Kim’s 70th Anniversary”, held at UCLA, Los Angeles, CA, USA, August 12~14, 1999 under the auspices of Korea—US American Scientists and Engineers Association.     

Cu基微晶钎不料高频钎焊OCr18Ni9Nb/YG6分析

应用Cu-xSn(x=7,13.5,质量分数%)微晶钎料对OCr18Ni9Nb不锈钢与YG6硬质合金进行了高频钎焊,分析了钎焊接头的相组成、组织形态和界面元素分布特征,探索了钎料成分和原子互扩散与接头组织之间的相关规律.结果表明,在感应钎焊过程中,Cu基微晶钎料表现出了良好的润湿性和铺展性,液态钎料与母材之间发生了一定程度的原子互扩散,实现了OCr18Ni9Nb/YG6的焊接.钎接接头组织致密均匀,无夹渣和气孔缺陷产生.Ocr18Ni9Nb/Cu-xSn/YG6钎缝组织由β-Cu固溶体和少量金属间化合物组成,钎料中锡含量的增加使钎缝化合物数量有所增加.近界面母材组织无明显粗化现象.     

Effect of atomic ordering on the phase transformations in Ni–Mn–Ga shape memory alloys

The influence of the L2₁ order degree in single crystalline Ni–Mn–Ga alloys is analysed in the present work. The alloys studied display a sequence of martensitic (MT) and intermartensitic (IMT) transformations on cooling, the reverse transformation to austenite, on heating, taking place in a single step. Quenching from different temperatures produces distinct effects on the MT and IMT: while the MT temperatures are not practically affected by the performed heat treatments, the IMT shifts towards lower temperatures after quenching from increasing temperatures. Such evolution can be related to changes in the L2₁ order degree, in the sense that ordering favours the occurrence of the IMT while it hardly affects the MT temperatures. The closeness of the free energies of the different martensitic structures and the differences between the MT and IMT entropy changes allow this behaviour to be explained. In turn, the Curie temperature increases with the L2₁ ordering for an alloy undergoing magnetic transition in martensite phase, no changes being detected if the Curie temperature is above the martensitic transformation.     

Phase diagram of Ni3V-Co3V pseudobinary system

A phase diagram of Ni₃V-Co₃V pseudobinary system is proposed based on x-ray diffraction and differential thermal analysis. Four invariant reactions are located in the phase diagram—two eutectoid and two peritectoid reactions. Six kinds of geometrically close-packed phases are encountered. The stability of these close-packed phases correlate well with the electron concentration.     

Hydrogen diffusivity in B-doped and B-free ordered Ni3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni₃Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni₃Fe alloy. Comparing with the B-free ordered Ni₃Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni₃Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni₃Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

Phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys after different heat treatments简

The effects of heat treatments on the phase transformation behavior of Ti49 Ni49.5 Fe1 V0.5 and Ti48 Ni48.5 Fe1 V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a twostage B2→R→B190martensitic transformation on cooling, but a B190→B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.     

Properties of Co−Fe−V−Ni alloys alloyed with titanium and aluminum

I. P. Bardin Central Research Institute of Ferrous Metallurgy. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 29–32, April, 1992.     

Oxidation resistance of nanocrystalline vis-à-vis microcrystalline Fe-Cr alloys

This paper investigates the hypothesis that nanocrystalline structure may confer considerable oxidation resistance to iron-chromium alloys at much lower chromium contents. Discs of nanocrystalline Fe-10%Cr alloy were produced by ball milling of Fe and Cr powders and compaction of the powder without considerable grain growth (by processing within a suitable time-temperature window). Corrosion resistance of discs of nanocrystalline and microcrystalline alloys was compared by subjecting them to oxidation in air and post-oxidation characterisation of the oxide scales by secondary ion mass spectroscopy (SIMS). Nanocrystalline Fe-10%Cr alloy showed considerably greater oxidation resistance than the microcrystalline alloy of the same composition. Chromium content of the inner scale formed over the nanocrystalline alloy was detected to be nearly five times greater than that in the case of microcrystalline alloy, facilitating formation of the passive layer and providing an explanation for the greater oxidation of the former.     

Grain boundary segregation,chemical ordering and stability of nanocrystalline alloys: Atomistic computer simulations in the Ni–W system

Atomistic computer simulations are used to investigate the equilibrium solute distribution and alloying energetics in nanocrystalline Ni–W. Composition and grain size-dependent trends in grain boundary segregation and chemical ordering behavior are evaluated and we find the equilibrium state to be significantly influenced by the nanostructure. The energetics of alloying are assessed through computation of the segregation, formation, and grain boundary energy, and these quantities are linked to previous thermodynamic models of nanostructure stability. With comparison to experiments, we conclude that nanocrystalline Ni–W alloys are synthesized in a metastable state. These findings have important consequences for theories of nanostructure control in general and particularly for the thermal stability of nanocrystalline Ni–W.     

Diffusion pressure welding of NiAl and Ni3Al-based alloys with St3S steel

Alloys containing phases on intermetallic structures, known as intermetallic compounds, belong to the new generation of metallic materials which possess properties lying between those of metals and ceramics. They are alloys of two or more metals which have crystalline structures that are different from those of the crystalline structures of individual component metals. For a number of years, intermetallic compounds were regarded as useful when they appeared as dispersed phases in conventional alloys, but in many situations, for example in the bonding of ceramics with metals, such phases are undesirable because of their brittleness. However, as the knowledge of their unique properties, especially of their behaviour at elevated temperatures and in hostile environments increases, the idea of using this group of materials for constructional purposes has been born. Systems containing intermetallic compounds had been investigated and it was shown that a new and unique group of alloys can be created by combining the matrices of nickel, iron and titanium with that of aluminium.     

Structural ordering and magnetic properties of arc-melted FeGa alloys

X-ray diffraction, Mössbauer spectrometry and magnetic properties have been performed on FeGa arc-melted binary alloys in order to study the compositional dependence of the structural ordering and magnetic properties of these alloys. The average magnetic hyperfine field at 300 K decreases with increasing Ga content as does the average magnetic moment per Fe atom, and a linear dependence between both quantities is found for Ga content up to 20 at%. The substitution of Fe by Ga atoms increases the lattice parameter and causes a change in the order of the crystal structure that has been modelled using a binomial distribution method. The effect of Ga atoms as near neighbours of Fe on the average magnetic hyperfine field is evaluated.