Texture transition in Al–Mg alloys: effect of magnesium

ABSTRACT

The evolution of deformation texture and microstructure in commercially pure Al (cp-Al) and two Al–Mg alloys (Al–4Mg and Al–6Mg) during cold rolling to a very large strain (true strain ε_t? ≈?3.9) was investigated. The development of deformation texture in cp-Al, after rolling, can be considered as pure metal or Copper-type, which is characterised mainly by the presence of Cu {112}<111>, Bs {110}<112> and S {123}<634> components. The deformation microstructure clearly indicates that deformation mechanism in this case remains slip dominated throughout the deformation range. In the Al–4Mg alloy, the initial slip mode of deformation is finally taken over by mechanism involving both slip and Copper-type shear bands, at higher deformation levels. In contrast, in the Al–6Mg alloy, the slip and twin mode of deformation in the initial stage is replaced by slip and Brass-type shear bands at higher deformation levels. Although a Copper-type deformation texture forms in the two Al–Mg alloys at the initial stage of deformation, there is a significant increase in the intensity of the Bs component and a noticeable decrease in the intensity of the Cu component at higher levels of deformation, particularly in the Al–6Mg alloy. This phenomenon indicates the possibility of transition of the deformation texture from Cu-type to Bs-type, which is concurrent with the addition of Mg. Using visco-plastic self-consistent modelling, the evolution of deformation texture could be simulated for all three materials.     

Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

镍基单晶超合金Ni/Ni3Al晶界的分子动力学模拟

在镍基单晶超合金中，由于单晶Ni的晶格常数比单晶Ni₃Al的稍小，在Ni/Ni3Al晶界面 上必然要出现错配.采用分子动力学模拟了镍基单晶超合金的Ni/Ni₃Al晶界的结 构，考虑 了两个不同的初始模型，并进行了分子动力学弛豫.弛豫的结果均表明：由于晶格的差异形 成的错配能不是通过长程晶格错配的方式来释放，而是通过在局部区域形成位错的方式释放 的.由于Ni₃Al相周围Ni相环境的不同，形成的位错也有所不同. 关键词： 镍基单晶超合金 晶界 分子动力学模拟     

On Reasons for Anomalies of Properties in a Series of Mixed K2CoxNi1–x(SO4)2 · 6H2O Crystals

The microhardness and thermal stability of mixed K₂Co_xNi_1–x(SO₄)₂ · 6H₂O crystals have been studied for the {110} and {001} growth sectors. Maxima of the onset temperature of dehydration have been found at x ≈ 0.1 and x ≈ 0.9. The structure of the {110} and {001} growth sectors has been studied. The observed changes in the properties have been attributed to the degree of imperfection of crystals depending on their sectoral inhomogeneity.

     

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

The microstructure/texture evolution and strengthening of 316?L-type and 304?L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.     

Low core loss non-oriented silicon steels

Low core loss non-oriented silicon steels are produced with high (Si+Al) content to reduce eddy current losses. However, high alloy content has detrimental effect on mechanical properties, saturation polarization and thermal conductivity. A new generation of medium and, particularly, low core loss non-oriented silicon steels was developed, with lower alloy content than the conventional grades, based on improved purity and texture. The development allowed the production of new low loss grades, with maximum core loss (W_1.5/50) of 2.30 W/kg at 0.50 mm and 1.95 W/kg at 0.35 mm, with high permeability (J₅₀ of 1.7 and 1.72 T, respectively). Texture improvement was based on hot band structure control and higher boundary mobility. Large hot band grain size and low [1 1 0]∥RD fiber fraction in the hot band texture contribute to reduce the intensity of [1 1 1]∥ND and slightly increase the intensity of [0 0 1]∥RD in the final product. Higher grain boundary mobility and/or a two-stage cold rolling process, with an intermediate annealing, increase the fraction of [0 0 1]∥RD and reduce the fraction of [1 1 1]∥ND on recrystallization and lead to favorable texture evolution on grain growth.     

LiNbO3单晶体直拉法生长时组分过冷所形成的胞状界面和胞状组织

对于沿[0001]生长的LiNbO₃单晶体,组分过冷形成的胞状界面和胞状组织进行了形态学的研究(组分过冷是由于操作上的原因在熔体中掺入了Al₂O₃而引起的),确定了胞状界面和胞状组织的基本形态。胞状界面是由相毗邻的三角锥组成,三角锥的稜锥面为{0112}面组,毗邻的三角锥体间存在沟漕,其中的熔体中浓集溶质。在长成的晶体中的胞状组织或是由{1210}面构成的其母线平行于[0001]方向的稜柱体,或是由溶质浓集的三{1210}面相交而成的三叉。实验观测证实了白丝、白点、云层及溶质尾迹是胞状组织的某种表现,而温度起伏是形成间歇式胞状组织的原因。通过胞状界面的形态研究,确定了形态干扰的某些具体形式,并证实了破坏界面稳定性的形态干扰与界面邻近流体的动量边界层的稳定性有关。文章最后讨论了高熔点氧化物晶体与金属晶体的胞状组织在形态上的差异,认为是由于两类物质的不同熔化熵而产生的。 关键词：     

Recovery and recrystallization in nickel alloy–based tapes with various easy magnetization directions upon annealing in a high magnetic field

Ni_48.8Fe_51.2 and Ni₇₀Co₃₀ alloys are subjected to cold rolling at a reduction of 98.8–99.0% and are annealed in a dc magnetic field of up to 29 T and without a field at various temperatures. The effect of a magnetic field on the structure and the crystal texture is studied. The application of a magnetic field is found to retard recovery processes and the early stages of recrystallization. At the stage of grain growth, the number of orientations with an easy magnetization direction increases and the grain size increases provided that the easy magnetization direction coincides with the main texture of the material. If the easy magnetization direction does not coincide with the texture, the grain growth in a field is decelerated and the texture sharpness decreases.     

γ′晶体的蠕变及持久性能

本文报导了对以Ni₃Al为基的r′单晶体所做的蠕变及持久试验的结果。结果表明,r′晶体的稳态蠕变速ε_s遵从 ε_s=Cσ^mexp(-Q_c′/RT) 的关系。指数m仅和温度有关。m(850℃)=3.95,m(950℃)=3.62。对Ni₃Al,Q_c′=88kcal/g-at。合金元素Ti,Nb,W都使Q_c′升高,其中Nb的作用最大,然而W在提高蠕变强度上最有效。晶体取向对ε_s有显著影响。<111>取向的Ni₃Al晶体ε_s最低,持久寿命t_r最长。在1050℃,σ=5kg/mm²时,t_r可≥100小时,Ni₃Al单晶体也同样符合Monkman-Grant关系t_r(ε_s)^p=常数,其中p=0.82。r′晶体蠕变中的滑移系统是{111}<211>。     

Mag netic properties of $$\hbox{Ni}_{13- n}\hbox{Al}_ n$$ clusters with n = 0–13

We report the magnetic properties of small Ni_13-nAl_n\hbox{Ni}_{13-n}\hbox{Al}_n clusters with n = 0–13 calculated in the framework of density functional theory. The cluster magnetic moment decreases with the sequential substitution of Ni by Al atoms, which can be attributed to a greater degree of hybridization that forces the pairing of the electrons in the molecular orbitals of Ni and Al. For Ni₇Al₆, the complete quenching of the cluster magnetic moment appears to be due to the antiferromagnetic alignment of atomic spins as revealed by the spin density plots.     

A comparative study of the structure and crystallization of bulk metallic amorphous rod Pr60Ni30Al10 and melt-spun metallic amorphous ribbon Al87Ni10Pr3

Pr-based bulk metallic amorphous （BMA） rods （Pr60Ni30Al10） and Al-based amorphous ribbons （Al87Ni10Pr3） have been prepared by using copper mould casting and single roller melt-spun techniques, respectively. Thermal parameters deduced from differential scanning calorimeter （DSC） indicate that the glass-forming ability （GFA） of Pr60Ni30Al10 BMA rod is far higher than that of Al87Ni10Pr3 ribbon. A comparative study about the differences in structure between the two kinds of glass-forming alloys, superheated viscosity and crystallization are also made. Compared with the amorphous alloy Al87Ni10Pr3, the BMA alloy Pr60Ni30Al10 shows high thermal stability and large viscosity, small diffusivity at the same superheated temperatures. The results of x-Ray diffraction （XRD） and transmission electron microscope （TEM） show the pronounced difference in structure between the two amorphous alloys. Together with crystallization results, the main structure compositions of the amorphous samples are confirmed. It seems that the higher the GFA, the more topological type clusters in the Pr-Ni-Al amorphous alloys, the GFAs of the present glass-forming alloys are closely related to their structures.     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

Thermally driven solid-phase epitaxy of laser-ablated amorphous AlFe films on (0001)-oriented sapphire single crystals

Solid-phase epitaxy is demonstrated for the metallic binary alloy AlFe. Stoichiometric thin films are deposited at ambient temperature onto c-cut sapphire by pulsed laser deposition (PLD), resulting in smooth amorphous films as revealed by X-ray diffraction (XRD) and atomic force microscopy (AFM). By annealing at 600°C, still smooth epitaxial AlFe films are obtained exhibiting the B2 phase with the (110) direction parallel to the substrate normal and an in-plane orientation as given by AlFe[001]||Al₂O₃[11[`2]0]\mbox{AlFe}[001]\|\mbox{Al}_{2}\mbox{O}_{3}[11\bar{2}0]. While ferromagnetism is observed for the amorphous phase, the formation of the B2 structure is accompanied by paramagnetic behavior, confirming the high structural quality.     

Thin layers of aluminium: A molecular orbital study

Molecular beam epitaxy has been used to prepare thin layers (200 nm thickness) of aluminium grown either on aluminium or gallium arsenide substrates; their He(I) and He(II) photoelectron spectra have been recorded. The quasirelativistic CNDO/1 method has been applied to investigate the band structure of {Al}₁₇₂, {Al}₂₈₄ and {Al}₄₂₄ clusters obtained by a duplication of the unit cell: the DOS profiles and their projections were generated. These data were correlated with the periodic crystal orbitals of the EHT quality. The first excitation energy serves as a better estimate of the vanishing energy gap showing thus a metallic character of aluminium.     

Effect of gallium alloying on the structure,the phase composition,and the thermoelastic martensitic transformations in ternary Ni–Mn–Ga alloys

The effect of gallium alloying on the structure, the phase composition, and the properties of quasibinary Ni₅₀Mn_50–zGa_z (0 ? z ? 25 at %) alloys is studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures is analyzed. A general phase diagram of the magnetic and structural transformations in the alloys is plotted. The temperature–concentration boundaries of the B2 and L2₁ superstructures in the austenite field, the tetragonal L1₀ (2M) martensite, and the 10M and 14M martensite phases with complex multilayer crystal lattices are found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}_B2. Martensite crystals are twinned along one of the 24 \(24\left\{ {011} \right\}{\left\langle {01\bar 1} \right\rangle _{B2}}\) “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses.     

A study on the orientation inheritance in laminated NiAl produced by in situ reaction annealing

In order to promote the performance of B2 NiAl by texture control of orientation during in situ processing, phase transformation in laminated NiAl with bimodal grain size distribution manufactured by reaction annealing of Ni and Al foils has been studied. It turned out that there existed a Kurdjumov–Sachs orientation relationship (K–S OR) between parent Ni and product NiAl by crystallography analysis according to the electron backscatter diffraction (EBSD) results. The parent Ni did not transform to the product NiAl directly but via the formation of Ni₃Al firstly according to the transmission electron microscope (TEM) observation of the interface. This led to a new K–S OR between Ni₃Al and NiAl with a small atomic misfit, which made less residual stress generated through the formation of Ni₃Al than directly from the parent Ni.     

Shape memory behavior in Fe3Al-modeling and experiments

The Fe₃Al alloy with D0₃ structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe₃Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE – generalized stacking fault energy) for {1?1?0}〈1?1?1〉 and {1?1?2}〈1?1?1〉 slips in D0₃ Fe₃Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement.     

Grain-boundary faceting at a = 3, [110]/{112} grain boundary in a cubic zirconia bicrystal

The atomic structure of a = 3, [110]/{112} grain boundary in a yttria-stabilized cubic zirconia bicrystal has been investigated by high-resolution transmission electron microscopy (HRTEM). It was found that the grain boundary migrated to form periodic facets, although the bicrystal was initially joined so as to have the symmetric boundary plane of {112}. The faceted boundary planes were indexed as {111}/{115}. The structure of the {111}/{115} grain boundary was composed of an alternate array of two types of structure unit: {112}- and {111}-type structure units. HRTEM observations combined with lattice statics calculations verified that both crystals were relatively shifted by (α/4)[110] along the rotation axis to form a stable grain-boundary structure. A weak-beam dark-field image revealed that there was a periodic array of dislocations along the grain boundary. The grain-boundary dislocations were considered to be introduced by the slight misorientation from the perfect = 3 orientation. The fact that the periodicity of the facets corresponded to that of the grain-boundary dislocations must indicate that the introduction of the grain-boundary dislocations is closely related to the periodicity of the facets. An atomic flipping model has been proposed for the facet growth from the initial = 3, {112} grain boundary.     

The atomic structure of alloy surfaces: Ni3Al{001}

The atomic structure of the {001} surface of Ni₃Al has been determined by LEED (low-energy electron diffraction) intensity analysis to correspond to simple truncation of the bulk structure with the Ni-Al mixed layer on top rather than the pure Ni layer. The first interlayer spacing is essentially equal to the bulk interlayer spacing between {001} planes. First-principles calculations of the cohesive energies of slabs terminating in the two types of layers also indicate that the mixed layer termination is more stable.