Grain refinement and texture evolution during high precision machining of a Ni-based superalloy

Abstract

The grain refinement and texture evolution in the surface gradient microstructure of a Ni-based superalloy induced by high speed machining was studied in this research. The direct evidence of grain refinement induced by dislocation–twin interaction was revealed and the detailed grain refinement process was summarised as deformation twinning, dislocation-twin reaction, localied thinning of nanotwin lamellae and final fracture. The underlying dislocation–twin interaction mechanism was elucidated from the crystallographic perspective. Using electron backscatter diffraction and precession electron diffraction techniques, a multiscale texture analysis covering undeformed coarse grain region, ultrafine grain region and nanograin region was carried out. The texture evolution with decreasing depth to the machined surface was identified as cube in the bulk interior and a mixture of rotated cube {0?0?1}<1?1?0>, cube {1?0?0}<0?0?1>, copper {1?1?2}<1?1?1 > and Goss {1?1?0}<0?0?1> textures in the topmost 1.3-μm-thick nanograin layer. The intrinsic thermomechanical effects of high precision machining are responsible for crystallographic texture transformation.     

Selective role of bainitic lath boundary in influencing slip systems and consequent deformation mechanisms and delamination in high-strength low-alloy steel

We elucidate here the deformation behaviour and delamination phenomenon in a high-strength low-alloy bainitic steel, in terms of microstructure, texture and stress evolution during deformation via in situ electron back-scattered diffraction and electron microscopy. Furthermore, the selective role of bainitic lath boundary on slip systems was studied in terms of dislocation pile-up and grain boundary energy models. During tensile deformation, the texture evolution was concentrated at {1 1 0}<1 1 1> and the laths were turn parallel to loading direction. The determining role of lath on the deformation behaviour is governed by length/thickness (l/t) ratio. When l/t > 28, the strain accommodates along the bainite lath rather than along the normal direction. The delamination crack initiated normal to (0 1 1) plane, and become inclined to (0 1 1) plane with continued strain along (0 1 1) plane and lath plane. This indicated that the delamination is not brittle process but plastic process. The lack of dimples at the delaminated surface is because of lack of strain normal to the direction of lath. The delaminated (0 1 1) planes were associated with cleavage along the (1 0 0) plane.     

Depth-Sensing Indentation on REBa2Cu3O7?δ Single Crystals Obtained from Xenotime Mineral

A natural mixture of heavy rare-earth oxides extracted from xenotime mineral have been used to prepare large single crystals of the high-temperature REBa₂Cu₃O_7??? superconductor, grown using the CuO?CBaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic moduli were measured by the Oliver and Pharr method, which yielded 7.4?±?0.2?GPa and the range 135?C175?GPa at small depths, respectively. Increased loads promote the nucleation of lateral cracks, which reduce hardness and measured elastic modulus, as indicated by instrumented indentation at higher loads. The indentation fracture toughness, estimated by measuring the radial crack length from cube corner indentations at various loads, was found to be 0.8?±?0.2?MPa m^1/2. The observed slip systems of REBa₂Cu₃O_7??? single crystals were [100](001) and [010](001), the same as for YBa₂Cu₃O_7??? single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus are not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. In conclusion, the mechanical properties of REBa₂Cu₃O_7??? single crystals prepared from xenotime are equivalent to those of YBa₂Cu₃O_7??? single crystals from conventional rare-earth oxides.     

Indentation studies on alkaline earth fluoride crystals at elevated temperatures

Indentations have been made on the (111) planes of CaF₂, SrF₂ and BaF₂ at various temperatures up to 700°C. By etching the crystals, the indentation dislocation rosettes (IDRs) have been recorded and the relative hardness is estimated from the dimensions of the IDRs. The hardness of all these crystals falls with temperature according to the equationH=A exp(−BT). Dramatic changes are observed in the shape of the IDR at elevated temperatures indicating activation of new slip systems.     

Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites

Electron backscattered diffraction has been used to determine the orientation of WC crystals in a WC-Co composite and atomic force microscopy has been used to measure the shapes of planar sections of the same crystals. A stereological analysis has been used to determine that {101¯0} prism facets and the {0001} basal planes are the WC surfaces that are most frequently in contact with Co. Further, the WC habit is an approximately equiaxed trigonal prism bound by three prism facets and two basal facets. An analysis of 15,600 grain boundaries shows that certain interfaces occur with a frequency that is much higher than would be expected in a random distribution and that the grain boundary habit planes also have {101¯0} and {0001} orientations. Eleven percent of all the observed WC-WC interfaces are 90° twist boundaries about [101¯0]. Two types of boundaries with a 30° rotation about [0001], a twist and an asymmetric tilt, comprise 3% of the population.     

The nature of dislocation sources in proton bombarded GaAs

The technique of transmission electron microscopy (TEM) has been applied to study the nature of damage rafts, which act as dislocation sources, in proton bombarded (5×10¹⁵, 10¹⁶ protons cm²) GaAs. The results show that the rafts consist of a planar array of voids lying on the {110} planes. The dislocations generated at these rafts are glissile, of the (a/2) <110> type and glide on the {111} planes intersecting the rafts.     

L p -Boundedness of the Wave Operator for the One Dimensional Schrödinger Operator

Given a one dimensional perturbed Schrödinger operator H =  ? d ²/dx ² + V(x), we consider the associated wave operators W  _± , defined as the strong L ² limits $\lim_{s\to\pm\infty}e^{isH}e^{-isH_{0}}Given a one dimensional perturbed Schr?dinger operator H = − d ²/dx ² + V(x), we consider the associated wave operators W _± , defined as the strong L ² limits . We prove that W _± are bounded operators on L ^p for all 1 < p < ∞, provided , or else and 0 is not a resonance. For p = ∞ we obtain an estimate in terms of the Hilbert transform. Some applications to dispersive estimates for equations with variable rough coefficients are given.     

The Feynman Propagator on Perturbations of Minkowski Space

The singular values squared of the random matrix product \({Y = {G_{r} G_{r-1}} \ldots G_{1} (G_{0} + A)}\), where each \({G_{j}}\) is a rectangular standard complex Gaussian matrix while A is non-random, are shown to be a determinantal point process with the correlation kernel given by a double contour integral. When all but finitely many eigenvalues of A*A are equal to bN, the kernel is shown to admit a well-defined hard edge scaling, in which case a critical value is established and a phase transition phenomenon is observed. More specifically, the limiting kernel in the subcritical regime of \({0 < b < 1}\) is independent of b, and is in fact the same as that known for the case b =  0 due to Kuijlaars and Zhang. The critical regime of b =  1 allows for a double scaling limit by choosing \({{b = (1 - \tau/\sqrt{N})^{-1}}}\), and for this the critical kernel and outlier phenomenon are established. In the simplest case r =  0, which is closely related to non-intersecting squared Bessel paths, a distribution corresponding to the finite shifted mean LUE is proven to be the scaling limit in the supercritical regime of \({b > 1}\) with two distinct scaling rates. Similar results also hold true for the random matrix product \({T_{r} T_{r-1} \ldots T_{1} (G_{0} + A)}\), with each \({T_{j}}\) being a truncated unitary matrix.     

Dislocations in Fe-3% Si alloy single crystals deformed at a higher rate

The method of etching dislocations is used to study the distribution of dislocations and twins in Fe-3% Si alloy single crystals prepared from the melt after plastic deformation with higher speed. The crystals are deformed by twinning in the 〈111〉 directions along the {112} planes and by slip in the 〈111〉 directions along the {110} planes. The results prove that the dislocations causing plastic deformation move in the {110} planes during both fast and slow deformation. The difference in the slip surfaces during fast and slow deformation is explained by the different number of cross slips per unit dislocation path.     

FCC Rolling Textures Reviewed in the Light of Quantitative Comparisons between Simulated and Experimental Textures

The crystallographic texture of metallic materials has a very strong effect on the properties of the materials. In the present article, we look at the rolling textures of fcc metals and alloys, where the classical problem is the existence of two different types of texture, the “copper-type texture” and the “brass-type texture.” The type of texture developed is determined by the stacking fault energy of the material, the rolling temperature and the strain rate of the rolling process. Recent texture simulations by the present authors provide the basis for a renewed discussion of the whole field of fcc rolling texture. We simulate the texture development with a model which allows us to vary the strength of the interaction between the grains and to vary the scheme for the calculation of the lattice rotation in the individual grains (type CL/MA or PR/PSA). For the deformation pattern we focus on {111}<110> slip without or with deformation twinning, but we also consider slip on other slip planes and slip by partial dislocations. We consistently make quantitative comparison of the simulation results and the experimental textures by means of a scalar correlation factor. We find that the development of the copper-type texture is best simulated with {111}<110> slip combined with type CL/PR lattice rotation and relatively strong interaction between the grains — but not with the full-constraint Taylor model and neither with the classical relaxed-constraint models. The development of the brass-type texture is best simulated with {111}<110> slip combined with PR/PSA lattice rotation and weak interaction between the grains. The possible volume effect of deformation twins on the formation of the brass-type texture is a controversial question which we discuss on the basis of our simulations as seen together with other investigations.     

Optical properties of ZnO nanocrystals embedded in PMMA

We report in this work the preparation of thin films of ZnO nanocrystals synthesized and dispersed in polymethylmethacrylate using a easy route and deposited in class substrate by spin coating technique. Their structural and optical properties were investigated by X-ray, absorption and photoluminescence spectroscopy. The XRD patterns exhibit sharp peaks at $2\uptheta $ corresponding to the hexagonal (wurtzite) phase diffraction planes. The optically characterization, exhibit a wide absorption band in the range of the study and a large emission band with three peaks at 481.5, 531.09 and at 671.28 nm.The crystallites radius (R) was estimated by applying the effective mass approximation model and was about 1.8 nm. From measurements of second order susceptibilities using harmonic generation technique at $\lambda = 1,064\,\text{ nm }$ in picoseconds regime we deduced $\lambda _\mathrm{eff}^{<2>}$ equal to $5.95\times 10^{-10}$  m/V. Obtained $\lambda _\mathrm{eff}^{<2>}$ was four order of magnitude larger compared with ZnO bulk material (2.5 pm/V).     

A nanoindentation study of the mechanical properties of ZnO thin films on (0 0 0 1) sapphire

The mechanical properties of epitaxial ZnO thin films grown on (0 0 0 1) sapphire substrate were investigated by nanoindentation with a Berkovich tip and compared with that of bulk ZnO single crystal. In all indents on ZnO film a single discontinuity (‘pop-in’) in the load versus indentation depth data was observed at a specific depth of between 13 and 16 nm. In bulk ZnO, however only 65% of indents showed pop-in event at a specific depth of between 12 and 20 nm. The mechanism responsible for the ‘pop-in’ event in the epitaxial ZnO thin films as well as in bulk ZnO was attributed to the sudden propagation of dislocations, which had been pinned down by pre-existing defects, along the pyramidal and basal {0 0 0 1} planes (cross slip). The elastic modulus and hardness of the epitaxial ZnO thin films were determined to be 154 ± 5 and 8.7 ± 0.2 GPa, respectively, at an indentation depth of 30 nm.     

On slip in AgCl crystals at room temperature

At moderately wavy and branched slip bands on the surfaces of sheets of AgCl crystals the distribution of slip lines has been observed by means of the electron microscope. From the results it can be deduced that the cross slip of screw dislocations takes place. The divergence of the ends of slip bands has also been observed, which can be as well explained by the cross slip. From the active slip direction and the directions of straight slip lines in the cross-slip regions the microscopic slip planes have been determined. They lie in the region roughly limited by the {113} and {331} planes and the problem is discussed whether these planes are low-index crystallographic planes.     

Two modes of slip systems in iron silicon crystals

Single crystals of iron 11·6 wt.% silicon were deformed in compression and slip bands were observed by optical microscope in specimens of different orientations. The deformation took place mostly by crystallographic slip along {110} planes, however, non-crystallographic slip in specimens of particular orientations was observed, too. On the basis of the present and previous results a simple qualitative model is proposed which explains which slip planes should be observed in crystals of iron with different silicon content deformed under various conditions. The main idea of the model is the eixstence of two different modes of slip systems in these crystals. The model predicts the conditions at which one slip mode prevails.The author would like to thank Dr. F. Kroupa, Dr. A. Gemperle, Dr. B. esták and Dr. J. Blahovec for useful discussions.     

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.     

集中力作用下硅中位错结构

本文用化学侵蚀法研究了硅单晶样品在800—1000℃印压得到的位错“花结”。实验结果说明:印压产生的位错分布在{111}滑移面上;位错线的取向大部分是<110>或<112>方向。分析并观察到在压印下有两种位错环,一种是柏格斯矢量沿<110>方向并平行于(111)印压面;一种是柏格斯矢量沿<110>方向并与印压面相交。对位错环的结构进行了分析。     

Segregations at antiphase boundaries in hexagonal close-packed superstructures. II

As was shown in the first part of this study [1], slip occurs in polycrystalline ordered Mg₃Cd, which has a DO₁₉ superstructure, along the {0001} basis planes, {1010} prismatic planes, and {1011} and {1012} pyramidal planes [2,3]. The formation of segregations at antiphase boundaries (APB's) in the {0001} basis and {1011} pyramidal planes was also examined there [1]. Segregations at APB's in planes are treated in this second part of the study. A zero-energy APB may form in the prismatic plane [4]. Segregation of atoms, on the other hand, is not allowed at such an APB according to the Gorskii-Bragg-Williams theory if correlation and interaction in the second and higher coordination spheres are not taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol. 12, No. 5, pp. 62–70, May, 1969.The authors thank N. S. Golosov and N. V. Kozhemyakin for discussion of this study and valuable advice.     

First-principles characterisation of the pressure-dependent elastic anisotropy of SnO2 polymorphs

Using DFT calculations, this study investigates the pressure-dependent variations of elastic anisotropy in the following SnO₂ phases: rutile-type (tetragonal; P4₂/mnm), CaCl₂-type (orthorhombic; Pnnm)-, α-PbO₂-type (orthorhombic; Pbcn)- and fluorite-type (cubic; Fm-3m). Experimentally, these polymorphs undergo sequential structural transitions from rutile-type → CaCl₂-type → α-PbO₂-type → fluorite-type with increasing pressure at 11.35, 14.69 and 58.22 GPa, respectively. We estimate the shear anisotropy (A₁ and A₃) on {1?0?0} and {0?0?1} crystallographic planes of the tetragonal phase and (A₁, A₂ and A₃) on {1?0?0}, {0?1?0} and {0?0?1} crystallographic planes of the orthorhombic phases. The rutile-type phase shows strongest shear anisotropy on the {0?0?1} planes (A₂ > 4.8), and the degree of anisotropy increases nonlinearly with pressure. In contrast, the anisotropy is almost absent on the {1?0?0} planes (ie A₁ ~ 1) irrespective of the pressure. The CaCl₂-type phase exhibits similar shear anisotropy behaviour preferentially on {0?0?1} (A₃ > 5), while A₁ and A₂ remain close to 1. The α-PbO₂-type phase shows strikingly different elastic anisotropy characterised by a reversal in anisotropy (A₃ > 1 to < 1) with increasing pressure at a threshold value of 38 GPa. We provide electronic density of states and atomic configuration to account for this pressure-dependent reversal in shear anisotropy. Our study also analyses the directional Young’s moduli for the tetragonal and orthorhombic phases as a function of pressure. Finally, we estimate the band gaps of these four SnO₂ phases as a function of pressure which are in agreement with the previous results.