EBSD Study of Delamination Fracture in Al–Li Alloy 2090

Aluminum–lithium (Al–Li) alloys offer attractive combinations of high strength and low density for aerospace structural applications. However, a tendency for delamination fracture has limited their use. Identification of the metallurgical mechanisms controlling delamination may suggest processing modifications to minimize the occurrence of this mode of fracture. In the current study of Al–Li alloy 2090 plate, high quality electron backscattered diffraction (EBSD) information has been used to evaluate grain boundary types exhibiting delamination fracture and characterize microtexture variations between surrounding grains. Delamination was frequently observed to occur between variants of the brass texture component, along near-Σ3, incoherent twin boundaries. EBSD analyses indicated a tendency for intense deformation along one side of the fractured boundary. A through-thickness plot of grain-specific Taylor factors showed that delaminations occurred along boundaries with the greatest difference in Taylor factors. Together, these suggest a lack of slip accommodation across the boundary, which promotes significantly higher deformation in one grain, and stress concentrations that result in delamination fracture.     

Bifurcation Analysis of Parametrically Excited Rayleigh–Liénard Oscillators

A parametrically excited Rayleigh–Liénard oscillator is investigatedby an asymptotic perturbation method based on Fourier expansion and timerescaling. Two coupled equations for the amplitude and the phase ofsolutions are derived and the stability of steady-state periodic solutionsas well as parametric excitation-response and frequency-response curvesare determined. Comparison with the parametrically excited Liénardoscillator is performed and analytic approximate solutions are checkedusing numerical integration. Dulac's criterion, thePoincaré–Bendixson theorem, and energy considerations are used in order to study the existence and characteristics of limit cycles of the twocoupled equations. A limit cycle corresponds to a modulated motion forthe Rayleigh–Liénard oscillator. Modulated motion can be also obtainedfor very low values of the parametric excitation, and in this case, anapproximate analytic solution is easily constructed. If the parametricexcitation is increased, an infinite-period bifurcation is observed because the modulation period lengthens and becomes infinite, while themodulation amplitude remains finite and suddenly the attractor settlesdown into a periodic motion. Floquet's theory is used to evaluatethe stability of the periodic solutions, and in certain cases,symmetry-breaking bifurcations are predicted. Numerical simulationsconfirm this scenario and detect chaos and unbounded motions in theinstability regions of the periodic solutions.     

Deformation mechanism in a coarse-grained Mg–Al–Zn alloy at elevated temperatures

Deformation behavior of a coarse-grained AZ31 magnesium alloy was investigated at elevated temperatures using commercial rolled sheet. The as-received material had equiaxed grains with an average grain size of 130 μm. The tensile tests revealed that the material exhibited high ductility of 196% at 648 K and 3×10⁻⁵ s⁻¹. Stress exponent, grain size exponent and activation energy were characterized to clarify the deformation mechanism. It was suggested from the data analysis that the high ductility was attributed to the deformation mechanism of glide-controlled dislocation creep. In addition, constitutive equation was developed for the present alloy.     

The Response of a Rayleigh–Liénard Oscillator to a Fundamental Resonance

A Rayleigh–Liénard oscillator excited by a fundamentalresonance is investigated by using an asymptotic perturbation method based on Fourier expansion and time rescaling. Two first-order nonlinear ordinarydifferential equations governing the modulation of the amplitude andthe phase of solutions are derived. These equations are used todetermine steady-state responses and their stability. Excitationamplitude-response and frequency-response curves are shown and checkedby numerical integration. Dulac's criterion, the Poincaré–Bendixsontheorem, and energy considerations are used in order to study the existenceand characteristics of limit cycles of the two modulation equations. Alimit cycle corresponds to a modulated motion for the Rayleigh–Liénardoscillator. For small excitation amplitude, the analytical results arein excellent agreement with the numerical solutions. In certain caseswhen the excitation amplitude is very low, an approximate analyticsolution corresponding to a modulated motion can be obtained andnumerically checked. Moreover, if the excitation amplitude is increased,an infinite-period bifurcation occurs because the modulation periodlengthens and becomes infinite, while the modulation amplitude remainsfinite and suddenly the attractor settles down into a periodic motion.     

Influence of nanoprecipitates on the creep strength and ductility of a Fe–Ni–Al alloy

Creep studies of a duplex Fe–Ni–Al intermetallic alloy, in two microstructural states, have been carried out at temperatures between 725 and 800 °C (about 0.6 T_m). In the as-cast state, the alloy contains a large volume fraction of nanoprecipitates (50–100 nm) which confer a very high creep strength with a stress exponent of 3 and an activation energy of 280 kJ/mol. The different microstructure obtained in the second state of the alloy, obtained after annealing at 1000 °C for 24 h, leads to a much lower creep strength with a higher stress exponent as well as a large value of the apparent activation energy. While volume diffusion appears to control creep in the as-cast state, both thermal and athermal processes seem to contribute to the different creep rate of material in the annealed state. The latter also exhibits a much larger ductility (12%) relative to that observed in the as-cast material (3%), due to the presence of large numbers of interfaces between the two phases present where strain incompatibilities can be accommodated.     

Whisker alignment of Ti–6Al–4V/TiB composites during deformation by transformation superplasticity

We examine the effect of 10 vol.% TiB whisker reinforcement on transformation superplasticity of Ti–6Al–4V, induced by thermal cycling about the α/β phase transformation range of the matrix under a uniaxial tensile stress. During superplastic deformation, the whiskers gradually align along the external loading axis, as measured by electron back-scattering diffraction of deformed specimens. The composites exhibit a gradual decrease in effective deformation rate during superplastic elongation. This effect is attributed to increased load transfer from matrix to whisker upon whisker alignment, and is explained with elastic calculations of stress partitioning.     

Adiabatic shear banding in high speed machining of Ti–6Al–4V: experiments and modeling

An experimental analysis of orthogonal cutting of a Ti–6Al–4V alloy is proposed. Cutting speeds are explored in a range from 0.01 to 73 m/s by using an universal high-speed testing machine and a ballistic set-up. The evolution of the cutting force in terms of the cutting speed and the development of adiabatic shear banding are analyzed. The shear band width and the distance between bands have been determined by micrographic observations. Their dependence upon the cutting velocity is analyzed. A modeling is proposed which restitutes well this velocity dependence.     

Crystal plasticity modeling of slip activity in Ti–6Al–4V under high cycle fatigue loading

Deformation micromechanisms of a Ti–6Al–4V alloy under fatigue loading at room temperature are studied using a three-dimensional crystal plasticity constitutive model. The model employs a minimum set of fitting parameters based on experimental data for Ti–6Al–4V. Single slip is strongly favored through a softening law that affects mainly the driving force for slip on the first activated slip system. Cyclic deformation behavior at the macroscopic scale and at the local scale of grains is analyzed through the simulation of 20 cycles of fatigue on a polycrystalline structure of 900 randomly oriented grains. The progressive activation of slip (basal, prismatic, and pyramidal) is analyzed and compared to experimental observations.     

Preparation of nano-sized Al2O3–2SiO2 powder by sol–gel plus azeotropic distillation method

Nano-sized amorphous Al₂O₃–2SiO₂ powder was prepared by a sol–gel method coupled with azeotropic distillation. The structure of the powder was investigated by DTS, BET, TEM, FT-IR, TG-DTA and XRD, showing that n-butanol azeotropic distillation could effectively remove water from the aluminosilicate gels and prevent the formation of hard agglomerates in the drying process. The average particle diameter of the powder was about 70 nm. The largest BET specific surface area of the powder was 669 m²/g. To examine the alkali-activation reactivity of the powder, alkali-activation tests were performed with the powder reacting with sodium silicate solution. The synthetic powder was found to be highly reactive.     

Adiabatic shear banding and scaling laws in chip formation with application to cutting of Ti–6Al–4V

The phenomenon of adiabatic shear banding is analyzed theoretically in the context of metal cutting. The mechanisms of material weakening that are accounted for are (i) thermal softening and (ii) material failure related to a critical value of the accumulated plastic strain. Orthogonal cutting is viewed as a unique configuration where adiabatic shear bands can be experimentally produced under well controlled loading conditions by individually tuning the cutting speed, the feed (uncut chip thickness) and the tool geometry. The role of cutting conditions on adiabatic shear banding and chip serration is investigated by combining finite element calculations and analytical modeling. This leads to the characterization and classification of different regimes of shear banding and the determination of scaling laws which involve dimensionless parameters representative of thermal and inertia effects. The analysis gives new insights into the physical aspects of plastic flow instability in chip formation. The originality with respect to classical works on adiabatic shear banding stems from the various facets of cutting conditions that influence shear banding and from the specific role exercised by convective flow on the evolution of shear bands. Shear bands are generated at the tool tip and propagate towards the chip free surface. They grow within the chip formation region while being convected away by chip flow. It is shown that important changes in the mechanism of shear banding take place when the characteristic time of shear band propagation becomes equal to a characteristic convection time. Application to Ti–6Al–4V titanium are considered and theoretical predictions are compared to available experimental data in a wide range of cutting speeds and feeds. The fundamental knowledge developed in this work is thought to be useful not only for the understanding of metal cutting processes but also, by analogy, to similar problems where convective flow is also interfering with adiabatic shear banding as in impact mechanics and perforation processes. In that perspective, cutting speeds higher than those usually encountered in machining operations have been also explored.     

An experimental investigation of a superplastic constitutive equation in Al–Mg–Si alloy composites reinforced with Si3N4 whiskers

Superplastic properties at 818 K were investigated by tensile tests for Al–Mg–Si alloy composites reinforced with Si₃N₄ whiskers whose volume fractions were 0–30%. The 20 and 30 vol.% composites exhibited large elongation of 615 and 285% at a high strain rate of 2×10⁻¹ s⁻¹, respectively. High strain rate superplasticity of the composites is attributed to the very small grain size of less than 3 μm. The stress–strain rate relation for the composites was almost the same as that of the alloy, taking into consideration the influences of threshold stress and grain size, and the relation was independent of the volume fraction of whisker. This is probably because grain boundary sliding was not hampered by the whiskers due to the presence of liquid phase for the composites.     

Strain Mapping of Al–Mg Alloy with Multi-scale Grain Structure using Digital Image Correlation Method

Al–Mg alloy powder was mechanically milled in liquid N₂ (cryomilling) to produce thermally stable powder with nanocrystalline (NC) microstructure for the manufacture of high-strength alloys. A multi-scale microstructure was achieved by blending unmilled coarse-grained (CG) powder with cryomilled powder and subsequently consolidating. The final bulk alloy was comprised of ultra-fine grained (UFG) regions and discrete CG bands. Dynamic observations of tensile deformation of the alloy were recorded using a micro-straining module attached to a light microscope, and the displacements were measured by digital image correlation (DIC). Strain inhomogeneity between UFG regions and ductile CG bands was observed in the micro-strain (strain order of 10⁻⁴–10⁻⁶) range, and the strain behavior was interpreted in terms of dislocation plasticity. Special emphasis was given to the distinct displacements between adjoining regions during deformation.     

Diffusion–reaction of aluminum and oxygen in thermally grown Al2O3 oxide layers

The diffusion–reaction of aluminum (Al) and oxygen (O), to form thermally grown oxide (TGO) layers in thermal barrier coatings (TBCs), is studied through an analytical model. A nonsymmetrical radial basis function approach is used to numerically solve the mass balance equations that predict the TGO growth. Correct boundary conditions for the Al and O reactions are laid out using scaling arguments. The Damköhler number shows that the O–Al reaction is several orders of magnitude faster than diffusion. In addition, a comparison between aluminum and oxygen diffusivities indicates that TGO growth is governed by aluminum diffusion. The results are compared with experimental measurements on air plasma spray-deposited TBCs treated at 1,373 K with exposure times ranging from 1 to 1700 hours. We found that, for several time decades, the thickness of the thermally grown layer has power law dependence of time with an exponent of ½, following the diffusion control mechanism. At later times, however, the presence of other oxides and additional kinetics modify the diffusive exponent.     

Evolution of subsequent yield surfaces and elastic constants with finite plastic deformation. Part III: Yield surface in tension–tension stress space (Al 6061–T 6511 and annealed 1100 Al)

Subsequent yield surfaces for aluminum alloys are determined for three proportional loading paths (i.e., axial, hoop, and combined hoop and axial stress) using 10 με deviation from linearity as the definition of yield. This paper is in continuation with Parts I and II of the author’s previous papers on subsequent yield surfaces under tension–torsion (σ11–√3σ₁₂) stress space using similar small offset definition of yield. In the current paper comprehensive experimental results on subsequent yield surfaces under tension–tension (σ₁₁–σ₂₂) stress space are presented. Comparison of subsequent yield surfaces under (σ₁₁–√3σ₁₂) stress space, investigated in the earlier papers, clearly indicated distinctive hardening behavior under various loading paths. However, subsequent yield surfaces for Al 6061–T 6511 (a low work hardening alloy) showed contraction and negative cross-effect with finite deformation as compared to the annealed 1100 Al (a high work hardening alloy) where expansion and positive cross-effect was observed.     

Alkali-activation reactivity of chemosynthetic Al2O3–2SiO2 powders and their 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectra

Pure Al₂O₃–2SiO₂ powders were prepared by sol–gel and coprecipitation methods, and their alkali-activation reactivities were compared. The alkali-activation reactivity of the powder prepared by the sol–gel method was higher than that of the powder prepared by the coprecipitation method. The powders were investigated by ²⁷Al and ²⁹Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to understand the relationship between their structure and alkali-activation reactivity. The ²⁷Al MAS NMR data showed that the five-coordinate Al content of the powder prepared by the sol–gel method was higher than that of the powder prepared by coprecipitation. The higher content of five-coordinate Al corresponded to higher alkali-activation reactivity. The ²⁹Si MAS NMR data showed that for the powder prepared by the sol–gel method, silicon was replaced by aluminum at secondary coordination sites of the central Si atoms during calcination. However, for the powder prepared by single-batch coprecipitation, the main change was from a low degree of polycondensation to a high degree of polycondensation.     

One parameter model for creep in a whisker reinforced anisotropic rotating disc of Al–SiCw composite

Steady state creep in a rotating disc of anisotropic aluminum silicon carbide whisker composite has been studied in the present study. The creep behavior is described by Norton's power law. Stress and strain rate distributions for anisotropic discs have been calculated and compared with those obtained for isotropic disc. It is concluded that the radial strain rate which always remained compressive for the isotropic composite ($\alpha =1.0$) and anisotropic disc ($\alpha =1.3$), becomes tensile in the middle region of the disc when the anisotropy parameter $\alpha =0.7$. Also if α is reduced from 1.3 to 0.7 the variation of tensile strain rate in the tangential direction remains similar but the magnitude reduces by five orders of magnitude. The study revealed that anisotropy introduces significant change in the strain rates although its effect on the resulting stress distribution may be relatively small.