Effects of deposition conditions of the Al film in Al/glass specimens and annealing conditions on internal stresses and hillock formations

In the present study, 25 kinds of specimen with five Al-film thicknesses were prepared to investigate the relation between the internal stress formed during the annealing process and the hillocks. In the preparation of specimens, the governing factors including deposition conditions, annealing temperature, and annealing time, were arranged following the orthogonal table of five-level and six-factorial (L₂₅(5⁶)) design. Stoney's formula is applied to describe the internal stresses before and after annealing (σ₀ and σ_f), respectively. The internal stress arising during the annealing process (σ_an) is evaluated using the model developed by Flinn et al. [1]. Then, the response surface methodology (RSM) is used to express the three stress parameters in terms of influential factors. The incipient σ_an value for hillocks appearing in the specimens was found to be between − 28.7 MPa and − 32 MPa in a compressive form. The annealing temperature, time, and Al-film thickness are the three major factors, affecting internal stress σ_an. An increase in the annealing time reduces the tensile stress or increases the compressive stress, or both. The tensile stress decreases and the compressive stress increases during the annealing process with increasing Al film thickness and annealing temperature. The number of hillocks formed in a unit of area is linearly proportional to both σ_an and (σ_f − σ_an).     

Effects of geometry of notched specimens on the local cleavage fracture stress σf of C-Mn steel

An elastic-plastic three-dimensional finite element method analysis is used to determine the stress and strain distributions ahead of notches of four-point bending (4PB) specimens with various sizes (W, B and a) and widths (B). By measuring the location of the cleavage initiation sites for a C-Mn steel, the local cleavage fracture stress σ_f is accurately determined. With increasing specimen sizes and widths the fracture load P_f increases considerably, but σ_f remains nearly constant. The reason that the σ_f of the specimen with minimum size is slightly larger than that of the other specimens is analyzed by an active zone model of cleavage fracture for notched specimens. The critical event for cleavage fracture is the propagation of a ferrite grain-sized crack into the neighboring matrix, and is independent of specimen sizes and widths. σ_f is mainly determined by the length of the critical microcrack, and the specimen sizes and widths have little effect on it.     

Effects of annealing and deforming temperature on microstructure and deformation characteristics of Ti–Ni–V shape memory alloy

Effects of annealing temperature T_an and deforming temperature T_d on microstructure and deformation characteristics of Ti–50.8Ni–0.5V (atomic fraction, %) shape memory alloy were investigated by means of optical microscopy and tensile test. With increasing T_an, the microstructure of Ti–50.8Ni–0.5V alloy wire changes from fiber style to equiaxed grain, and the recrystallization temperature of the alloy is about 580 °C; the critical stress for stress-induced martensite σ_M of the alloy decreases first and then increases, and the minimum value 382 MPa is got at T_an = 450 °C; the residual strain ?_R first increases, then decreases, and then increases, and its maximum value 2.5% is reached at T_an = 450 °C. With increasing T_d, a transformation from shape memory effect (SME) to superelasticity (SE) occurs in the alloy annealed at different temperatures, and the SME → SE transformation temperature was affected by T_an; the σ_M of the alloy increases linearly; the ?_R of the alloy annealed at 350–600 °C decreases first and then tends to constant, while that of the alloy annealed at 650 °C and 700 °C decreases first and then increases. To get an excellent SE at room temperature for Ti–50.8Ni–0.5V alloy, T_an should be 500–600 °C.     

Effects of loading rate on the local cleavage fracture stress σf in notched specimens

Four point bending (4PB) notched specimens with different notch sizes are tested at various loading rates at a temperature of −110 °C for a C-Mn steel. An elastic-plastic finite element method (FEM) is used to determine the stress and strain distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ_f is measured. The results show that the local cleavage fracture stress σ_f does not essentially change with loading rate V and notch size. The reason for this is that the cleavage micromechanism does not change in the different specimens at various loading rates. The cleavage micromechanism involves competition of two critical events of crack propagation and crack nucleation in the high stress and strain volume ahead of notch root. The large scatter of σ_f and notch toughness are mainly caused by the different critical events in different specimens.     

Effect of grain diameter on the work-hardening characteristics of Zn–1 wt.% Cu alloy during phase transformation

Stress–strain characteristics of slowly cooled Zn–1 wt.% Cu alloy specimens with different grain diameters have been studied in the temperature range from 453 to 573 K. The work-hardening parameters; coefficient of work-hardening, χ_p, yield stress, σ_y, and fracture stress, σ_f of the investigated specimens were found to decrease in a non-monotonic behaviour with increasing the deformation temperature. Two decreasing stages were observed before and after the transformation temperature 513 K. The rate of change of these parameters as well as the strain hardening exponent n (= d lnσ/d lnε) were investigated as function of deformation temperature. They showed two different behaviours before and after the transformation temperature. The effect of grain diameter on the above work-hardening parameters were also taken into consideration. The work-hardening parameters were found to increase with increasing grain diameter while the strain hardening exponent n is decreased. The obtained results were interpreted on the basis of the coarsening of ε-phase at the early stages of the first temperature range while the second stage referred to the thermal agitation of dislocation motion. The two observed temperature regions below and above 513 K may characterize a point of dislocation intersection mechanisms activated with ∼0.2 eV for the two regions. The microstructure of the samples under investigation was examined by optical microscopy (OM).     

Investigations on mechanical behaviour of brittle wear-resistant coatings II: Theory

From considerations concerning the loading conditions in thin elastic-brittle coatings on a steel substrate, fracture processes due to bending of the coated material are investigated. The most important features of this model are (1) statistical nucleation and growth of cracks, (2) crack interaction due to unloading and stress concentration and (3) features of crack growth and arrest due to coating-substrate interactions. Finally a functional relationship between the experimentally available number n(σ) of cracks per length and the strength distribution function F(σ) of coating elements is derived. This functional relationship may be used to identify the residual stress σⁱ, the strength parameters such as the lower limit σ_f⁰, mean value σ_f and coefficient β of variance as well as the structural dimensions from the experimental data given in Part I of this paper.     

Short crack threshold estimates to predict notch sensitivity factors in fatigue

The notch sensitivity factor q can be associated with the presence of non-propagating fatigue cracks at the notch root. Such cracks are present when the nominal stress range Δσ_n is between Δσ₀/K_t and Δσ₀/K_f, where Δσ₀ is the fatigue limit, K_t is the geometric and K_f is the fatigue stress concentration factors of the notch. Therefore, in principle it is possible to obtain expressions for q if the propagation behavior of small cracks emanating from notches is known. Several expressions have been proposed to model the dependency between the threshold value ΔK_th of the stress intensity range and the crack size a for very small cracks. Most of these expressions are based on length parameters, estimated from ΔK_th and Δσ₀, resulting in a modified stress intensity range able to reproduce most of the behavior shown in the Kitagawa–Takahashi plot. Peterson or Topper-like expressions are then calibrated to q based on these crack propagation estimates. However, such q calibration is found to be extremely sensitive to the choice of ΔK_th(a) estimate. In this work, a generalization version of El Haddad–Topper–Smith’s equation is used to evaluate the behavior of cracks emanating from circular holes and semi-elliptical notches. For several combinations of notch dimensions, the smallest stress range necessary to both initiate and propagate a crack is calculated, resulting in expressions for K_f and therefore for q. It is found that the q estimates obtained from this generalization, besides providing a sound physical basis for the notch sensitivity concept, better correlate with experimental data from the literature.     

Effect of fatigue loading on stiffness degradation,energy dissipation,and matrix cracking damage of CFRP [±45]3S composite laminate

In this study, a concept of using experimental matrix crack density, residual stiffness, and energy dissipation as a comparative life monitoring tool for any arbitrary stacking sequence is proposed. First, IMA/M21 [±45]_3S carbon fibre‐reinforced polymer (CFRP) multidirectional (MD) laminates were fabricated by autoclaving technique. The static tension and compression tests were conducted to determine the fatigue stress levels. Then, constant amplitude tension–tension fatigue tests were carried out at two stress ratios (R = σ_min/σ_max) of 0.1 and 0.5. Three stress levels on the basis of cycles to failure (N_f) were chosen, ie, lower stress run‐out (LSR), lower stress fractured (LSF), and higher stress fractured (HSF). The LSF and LSR specimens primarily degraded due to matrix cracking that caused higher stiffness degradation and lower energy parameter, whereas HSF specimens having the least possibility of matrix crack growth showed lower stiffness degradation and higher energy parameter in a fibre‐dominated failure.     

Microstructural influences on fatigue crack initiation in a model particulate-reinforced aluminium alloy MMC

Preliminary results are presented for fatigue crack initiation within a particulate-reinforced aluminium alloy metal-matrix composite. The study examines the response of a model system consisting of Si particles in an AlMgSi matrix age hardened to the peak strength T6 temper. Experimental observations indicate two sites for crack initiation for the high stress amplitudes (σ_max ⩾ 0.95 σ_ys) used in this study. In specimens containing smaller Si particles (≈ 4 μm), crack initiation occurs at ≈ 0.4 N_f within thin ligaments of the matrix which separate uncracked subsurface particles from the free specimen surface. In contrast, specimens containing larger Si particles (≈ 8 μm) are susceptible to crack initiation from particles which crack within 0.05 N_f. The potential contribution of thermally induced residual stresses is discussed qualitatively for the case of particles located at or near a free surface.     

Strain rate effects on viscoelastic crack bifurcation

The effects of applied strain rate on the viscoelastic crack bifurcation phenomenon in Polymethyl Methacrylate (PMMA) were investigated. It was still verified that the product $\text{σ}{}_{\mathrm{f}}\text{C}{}_{\mathrm{b}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ was constant, as was already observed by Congleton and Petch, and Anthony, Chubb and Congleton, for brittle elastic materials, for any strain rate, where σ_f = the gross fracture stress and C_b= the main crack length until the bifurcation starts. However, it was found that the higher strain rate increases the main crack length C_b resulting in the decrease in the gross fracture stress σ_f and vice versa. This might be interpreted that the higher stress concentration at the initiation crack tip, which is realized by becoming more brittle due to the higher strain rate owing to the predominance of the elastic element in the viscoelastic material, decreases the gross fracture stress leading to the longer main crack length.     

Fatigue strength of the Cu/Si interface in nano-components

The fracture behavior of the Cu/Si interface in a nano-cantilever specimen with a 200 nm-thick Cu film (Specimen-200), which possesses a nanometer-scale strain-concentrated region, is examined under a cyclic bending load. The fatigue strength is around GPa level owing to the high yield stress of the Cu nano-film and the deformation constraint associated with the neighboring hard materials. The S-N curve shows clear dependence of fatigue life on the applied stress in the high-stress range, Δσ. Specimens with a 20 nm-thick Cu film (Specimen-20) are also investigated for comparison. The stress range in the fatigue fracture of Specimen-20 is higher than that of Specimen-200 for the same fatigue life. However, there is good coincidence in the Δσ/σ_s (σ_s: strength in monotonic load) vs. N_f (number of cycles to fracture) at high Δσ. The S-N curves suggest the existence of a fatigue threshold (Δσ_w) at low Δσ. The ratio of fatigue limit to the fracture stress in a monotonic loading, Δσ_w/σ_s, is large compared with the magnitude of bulk metal, which suggests the brittle behavior of the interface. Moreover, the fatigue limits have good coincidence with their yield stresses.     

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (V_NS) were normalised with full size specimen (V_NF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σ_eq) and hydrostatic stress (σ_h) reaches ～2400 MPa (fracture stress, σ_f*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ～68 J of full size specimen used in the determination of nil ductility transition temperature.     

Effect of triaxial stress-state on creep fracture in Inconel alloy X-750

Smooth specimens and circumferentially notched bars with a “Bridgman” notch geometry were tested uniaxially at 700° C in air in the stress range of 340 to 700 MPa. The results indicated that the material was notch strengthened on the basis of net section stress,σ _a. However, when the fracture lifetimes were plotted as a function of the Bridgman effective stress,σ _e, all the data points fell approximately on one line. Cavity nucleation sites changed systematically from notch throat at the highest stress to notch root at the low stress. The notch rupture ductility in the notched specimens were found to have a lower value than in the smooth ones at all stresses.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.     

Tensile properties and fracture mechanisms of ZnO and ZnAl2O4-coated Al18B4O33 whisker reinforced aluminum composites

ZnO and ZnAl₂O₄-coated Al₁₈B₄O₃₃ whisker reinforced pure aluminum composites were fabricated by squeeze casting. The effects of the coating contents on the ultimate tensile strength (UTS), 0.2% proof stress (σ_0.2) and elongation to fracture (δ) of the composites were investigated. The results show that the UTS of ZnO-coated Al₁₈B₄O₃₃ whisker reinforced aluminum composites increases almost linearly as the coating contents increase. However, the UTS of ZnAl₂O₄-coated Al₁₈B₄O₃₃ whisker reinforced aluminum composites declines initially and increases later. The ZnAl₂O₄ coating of the whiskers is more favorable for an increase in the σ_0.2 of the composites. The δ of the composites obtains its maximum value with an appropriate coating content. Fracture mechanisms of the composites were also investigated.     

On the investigation of the biaxial fatigue behaviour of unidirectional composites

The paper illustrates the preliminary activity of an extensive research program oriented to investigate the multiaxial fatigue behaviour of unidirectional composite laminates, with particular attention to the analysis of the damage mechanisms and their correlation with the local multiaxial stress state to be used then as the basis for the development of multiaxial fatigue criterion. The definition of an effective experimental procedure for multiaxial fatigue testing is carefully discussed in terms of specimen geometry, specimen manufacturing and local stress state. Once identified in the thin-walled tubular specimens under tension–torsion loading the best test configuration for the aims of the research, the results of comparative fatigue tests investigating the influence of the tubes geometry (wall thickness to diameter ratio) on the transverse fatigue response are presented. In the final part of the paper the effects of an increasing shear stress component (σ₆) on the transverse (σ₂) fatigue strength and damage evolution in UD glass–epoxy tubes are illustrated.     

Strain-induced martensite formation in austenitic stainless steel

In situ measurements of the strain-induced martensitic transformation (SMTs) of SUS304 stainless steel that takes place during tensile loading at room temperature were performed around the notch of a dumbbell-shaped specimen where high stress concentration occurs. Even in the low plastic strain regime, with loading to 0.2 % proof stress (σ _0.2), some SMTs occurred. However, the area fraction of the Fe-α′-martensite phase did not increase significantly even when the sample was loaded to the ultimate tensile strength (σ _UTS). After the σ _UTS point, the total fraction of the Fe-α′ phase increased dramatically to the fracture point (σ _f). The phase textures of Fe-α′ and Fe-γ were almost equal at (σ _UTS ? σ _f)/2, and the Fe-α′ phase was observed over almost the entire measurement area around the notch at the σ _f point. However, the area fraction of the Fe-α′ phase at the σ _f point decreased far away from the fracture surface, to an extent that the total fraction of the Fe-α′ phase was almost the same as that of the Fe-γ phase in an area about 1.7 mm from the fracture face. Different martensite characteristics were detected in the stainless steel, depending on the applied load level. This was attributed to the severity of deformation. In particular, deformation twinning, created around σ _UTS, and severe plastic deformation before fracture make a strong Fe-α′ phase. Details of this phenomenon are interpreted using various approaches, including electron backscatter diffraction analysis and finite element analysis.     

Plastic flow and fracture of titanium at low temperatures

The following features of the mechanical behaviour of titanium at low temperatures are discussed: operative slip systems, twinning, serrations in the stress-strain curves near 4.2 K, grain size hardening, strain hardening, interstitial solid solution strengthening, deformation kinetics, and fracture and ductility. Regarding plastic flow, interstitial solutes mainly influence the thermal component of the flow stress, the rate controlling mechanism being the thermally activated overcoming of interstitial solute obstacles by dislocations gliding on the first order prism planes. Strain hardening is given by the commonly observed linear relationship between the flow stress σ and the square root of the dislocation density ρ,

$\text{σ= σ}{}_{\mathrm{<mtext>f</mtext>}}\text{(T,}\text{ε}\text{?}\text{, C}{}_{\mathrm{<mtext>i</mtext>}}\text{)+ λEb[p(?, d)]}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$

where σ_f, the stress due to all obstacles other than dislocations, increases with increase in strain rate ε and interstitial content C_i and with decrease in temperature T. The constant α is of the order of 0.7 and E is Young's modulus. Both σ_f and α are relatively independent of grain size, d, supporting the work hardening model for the effect of grain size on the flow stress.The fracture stress increases with decrease in grain size and with increase in interstitial content. As a general trend, the total elongation tends to increase as the temperature is lowered to about 77 K, below which a decrease elongation occurs. The improvement in ductility with decreasing temperature to 77 K is attributed to the occurrence of twinning. Therefore, the effects of grain size, temperature, and interstitial content on ductility at low temperatures reflects in part the influence these variables have on twinning.     

Serrated yielding in Nb–V dual phase steel

Abstract

The characteristics of serrated yielding (the Portevin–Le Chatelier effect) in a Nb–V dual phase steel have been studied in the temperature range 85–210°C at strain rates between 1·2 × 10^?5 and 1·2 × 10^?2 s^?1. Serrated yielding was found to initiate only after a critical strain ?_c was reached. The strain between two successive serrations ??_s increases almost linearly with strain, while the stress drop ?σ_c increases with strain up to ?σ_max, then decreases. The exponent β in the mobile dislocation density–plastic strain relationship (ρ_m= ?^β) is 1·09 in the temperature range 85–140°C and 1·34 in the temperature range 140–210°C. The results also indicate that in the same temperature ranges there are two values of activation energy for type A serrations, i.e. 79 and 119 kJ mol^?1 respectively. The results are discussed in terms of substitutional–interstitial solute atom interaction and changes of concentration of interstitial atoms.

MST/934