On-chip laboratory suite for testing of free-standing metal film mechanical properties,Part II – Experiments

In this paper, we demonstrate the fabrication of electrostatically loaded, free-standing Al–0.5 wt.%Cu thin-film samples, realizing a near-zero compliance support post. We measure Young’s modulus E = 74 GPa using cantilevers, in good agreement with grain texture measurements. We measure residual stress σ_R ranging from 30 to 60 MPa using fixed–fixed beams and find that processing induces significant plastic straining, which leads to residual stress values significantly less than the as-deposited value. Strength of this alloy is at least 172 MPa if the film is not severely strained, and the material exhibits no room-temperature fatigue up to 1 billion cycles at this stress level. Notched devices that have been subjected to process-induced plastic straining of ∼4% are weaker and fatigue logarithmically with the number of cycles. We compare deformation processes on the samples using ex situ TEM. The mechanism for the high strength value is attributed to the grain size and the thin surface oxide which constrain dislocation glide, while fatigue of the highly strained material is associated with the appearance of persistent slip bands.     

The fatigue behavior of I-phase containing as-cast Mg–Zn–Y–Zr alloy

The fatigue behavior of as-cast Mg–12%Zn–1.2%Y–0.4%Zr alloy has been investigated. The S–N curve showed that the fatigue strength at 10⁷ cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 × 10⁷ cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 × 10⁵ cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 × 10⁵ cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m^1/2, respectively. When the maximum stress intensity factor K_max was lower than 4.2 MPa m^1/2, in the steady crack propagation region, due to the retarding effect of I-phase/α-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/α-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m^1/2, in the sudden failure region, the rigid bonding of I-phase/α-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/α-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation.     

Simple solvothermal routes to synthesize nanocrystalline Bi2MoO6 photocatalysts with different morphologies

Nanocrystalline Bi₂MoO₆ photocatalysts were successfully synthesized by conventional solvothermal and microwave–solvothermal routes, respectively. The prepared samples were characterized by X-ray diffraction, BET surface area analysis, UV–vis diffuse reflectance spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The samples have high surface areas in the range of 10–32 m² g⁻¹. Their average crystallite sizes are in the range of 16–35 nm. The absorption edge of the samples is at ∼491 nm, corresponding to a band gap energy of about 2.53 eV. Different morphologies with nanosheets and nanorods were also observed. The photocatalytic activities of Bi₂MoO₆ photocatalysts were evaluated by the decomposition of Rhodamine B under visible-light irradiation (λ > 420 nm). Nanocrystalline Bi₂MoO₆ samples obtained via different conditions exhibited different photocatalytic performances. The effects of the crystallinity, specific surface area and morphology of the samples on the photocatalytic activities are also discussed.     

Enhanced efficiency of polymer:fullerene bulk-heterojunction solar cells with the insertion of thin CdS layer near the Al electrode

The insertion layer of cadmium sulfide (CdS) between polymer–fullerene blend and Al electrode is used to enhance the short-circuit current (I_sc) and the power conversion efficiency (PCE). The solar cells based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C₆₀ with the function layer of CdS (∼10 nm) shows the open-circuit voltage (V_oc) of ∼0.7 V, short-circuit current (I_sc) of ∼4.6 mA/cm², filling factor (FF) of ∼0.28, and the power conversion efficiency (PCE) of ∼5.3% under monochromatic light (532 nm) photoexcitation of about 16.7 mW/cm². Compared to cells without the CdS layer, the power conversion efficiency increases about an order of magnitude. The thickness of CdS layer was varied from 10 to 40 nm using e-beam deposition, and we obtained optimum current density–voltage characteristics for 10 nm thick CdS layer.     

Fatigue testing and microstructural characterization of tungsten heavy alloy Densimet 185

A rotating target consisting of helium-cooled tungsten has been chosen for the European Spallation Source (ESS) facility to be built in Lund. Thermo-mechanical cycling due to the incidence of the proton beam every 2 s on any given tungsten slab in the rotating wheel could lead to crack formation and failure over the lifetime of the target. This work reports tensile and fatigue data obtained at room temperature for the Densimet 185 alloy in the non-irradiated condition. Methods for extracting relevant parameters from fatigue curves with small sets of data are discussed. Fatigue results show a large spread of data for which the application of such methods is challenging.Stress controlled fatigue testing was carried out in this study with mean stress approaching zero and amplitudes in the range 250 to 450 MPa, with 50 MPa increments. A frequency of 25Hz was employed and the fatigue tests lasted until failure was registered or until the upper limit of 2 × 10⁶ cycles was reached. No failure due to fatigue occurred in specimens subjected to stress amplitudes below 300 MPa. Microstructural and fractographic studies on the fatigue samples using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) showed that the samples had low porosity, large and nearly spherical tungsten grains, and with a fairly uniform distribution of the ductile phase rich in nickel and iron. However, bonding between tungsten grains in some areas was found to be inadequate. Intergranular fracture was predominant in the specimens at room temperature. Data for the D185 alloy are compared to those for IT180 and D176 alloys obtained in a previous study and strategies for improving the fatigue strength are discussed.     

On the effect of environment on spontaneous growth of lead whiskers from commercial brasses at room temperature

In this paper we report on a detailed statistics-based study of the effect of various environments – air, argon, vacuum, oil and water – on the room temperature spontaneous nucleation and growth of lead whiskers from hacksaw-cut surfaces of three leaded commercial brasses. The samples were initially held in the various environments for 166 h, before being stored in ambient air for a total of 1126 h. The environment was found to have a strong effect. The highest whisker density, ∼30,000 cm⁻², was observed for the sample initially kept under a mechanical (∼1 Pa) vacuum; the lowest, ∼3000 cm⁻², was observed for the sample initially immersed in water; the densities of the others were in between. The samples held in water only grew whiskers when removed from the water. Once exposed to air, a few whiskers grew at an average rate that, at 0.9 nm s⁻¹, was the highest measured. When the samples were exposed to air, after the initial storage in the different environments, both nucleation and growth of the whiskers were accelerated, before ultimately ceasing to grow roughly 400 h after creating the surfaces from which the whiskers grew. These results are consistent with a scenario in which oxygen and/or moisture diffusion down the Pb/brass interfaces results in a volume expansion that provides the driving force for whisker growth. The results also indicate that the oxygen levels needed for whisker growth can be as low as a few parts per million.     

Temperature dependence of sound attenuation and shear modulus of ultra fine grained copper produced by equal channel angular pressing

The influence of temperature on shear modulus and internal friction in ultrafine-grained copper processed by equal channel angular pressing (ECAP) was investigated in the temperature range from 150 to 520 K. Acoustic measurements were performed on the inverted torsion pendulum at the frequencies of ∼18 and ∼45 Hz. An irreversible shear modulus increase and a concurrent decrease in sound attenuation were observed in the temperature region from ∼350 to 450 K on the first heating of specimens. The activation energy E ≈ 1.05 eV and the attempt frequency ν₀ ≈ 10¹⁰ s⁻¹ of the irreversible relaxation process were determined using the measurements at different heating rates. The overall decrease in the shear modulus in ECAP-processed copper was shown to be made up by two components: a temperature-independent and a temperature-dependent ones. The latter is accompanied with an additional internal friction of the relaxation type, which is reversible up to ∼350 K. An estimate of the activation energy for this reversible relaxation process was obtained. Possible mechanisms responsible for the anomalous behavior of the shear modulus and the sound attenuation in ultrafine-grained copper are discussed.     

Mechanical properties and rolling behaviors of nano-grained copper with embedded nano-twin bundles

By means of dynamic plastic deformation (DPD) at liquid nitrogen temperature (LNT), bulk nano-grained copper samples with embedded nano-twin bundles were prepared. Subsequent cold rolling (CR) of the LNT-DPD Cu led to a reduction in quantity of nano-twin bundles and a slight grain coarsening, accompanied by a decrease in grain boundary (GB) energy from 0.34 to 0.22 J m⁻². An increasing CR strain leads to a saturation grain size of ∼110 nm, which is less than half of that in the severely deformed Cu from the coarse-grained form. Decreased strength and enhanced ductility were induced by CR in the LNT-DPD sample. The saturation yield strength in the LNT-DPD Cu during CR was ∼105 MPa higher than that in conventional severely deformed Cu, which originates from the finer grains as well as the nano-scale twins in the LNT-DPD sample. The enhanced ductility is primarily attributed to CR induced GB relaxation.     

Deformation behavior of a two-phase Mo–Si–B alloy

Mo–Si–B alloys are being considered as possible candidates for high-temperature applications beyond the capabilities of Ni-based superalloys. In this paper, the high-temperature (1000–1400 °C) compression response over a range of quasi-static strain rates, as well as the monotonic and cyclic crack growth behaviors (as a function of temperature from 20 °C to 1400 °C) of a two-phase Mo–Si–B alloy containing a Mo solid solution matrix (Mo(Si,B)) with ∼38 vol% of the T2 phase (Mo₅SiB₂) is discussed. Analysis of the compression results confirmed that deformation in the temperature–strain-rate space evaluated is matrix-dominated, yielding an activation energy of ∼415–445 kJ/mol. Fracture toughness of the Mo–Si–B alloy varies from ∼8 MPa√m at room temperature to ∼25 MPa√m at 1400 °C, the increase in toughness with temperature being steepest between 1200 °C and 1400 °C. S–N response at room temperature is shallow whereas at 1200 °C, a definitive fatigue response is observed. Fatigue crack growth studies using R = 0.1 confirm the Paris slope for the two alloys to be high at room temperature (∼20–30) but decreases with increasing temperature to ∼3 at 1400 °C. The crack growth rate (da/dN) for a fixed value of ΔK in the Paris regime in the 900–1400 °C range, increases with increasing temperature.     

Investigation of the low-cycle fatigue mechanism for micron-scale monocrystalline silicon films

This study investigated the cyclic and static fatigue properties of 10 μm thick, deep reactive ion etched, monocrystalline silicon films. Stress–life fatigue curves and fatigue degradation rates vs. stress curves were generated at both 4 and 40 kHz, at 30 °C, 50% relative humidity (RH). A significant frequency effect was observed, with shorter fatigue lives and faster damage accumulation rates at 4 kHz. Static fatigue was also observed with shorter static lifetimes at 80 °C, 90% RH than at 30 °C, 50% RH. Fracture surface evaluation did not reveal any major difference between cyclically and statically fatigued devices. These experimental results confirm that the fatigue of micron-scale silicon is not purely mechanical. The study also proposes a fatigue scenario based on time-dependent subcritical crack growth to account for the low-cycle fatigue regime.     

Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10⁵–10⁹ cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.     

Oxidation-induced embrittlement of TiAl alloys

The ductility of oxidised TiAl-based alloys is reduced even when the oxygen-rich region is of the order 100 nm thickness; this loss in ductility is smallest in lamellar samples. Removal of this oxidised region restores ductility. Acoustic events are observed during tensile tests at stresses above 300 MPa and cracks at about 250 MPa. In-situ tensile tests on samples, with part of the oxygen-enriched region removed, have shown that cracks are formed only in regions where the oxygen-rich region is present. X-ray diffraction measurements have shown that the oxygen-rich surface generates a tensile stress in the top 1 or 2 μm of the alloy of about 250 MPa corresponding to a compressive stress in the oxygen-rich layer of 2000 MPa. It is concluded that embrittlement is caused by (i) the tensile stress induced by the oxygen-rich region and (ii) the corresponding ease of crack nucleation in this region. Subsequent propagation is controlled by the fracture toughness.     

Preparation and electrical properties of highly (1 1 1) oriented antiferroelectric PLZST films by radio frequency magnetron sputtering

Highly (1 1 1) oriented lead lanthanum zirconate stannate titanate (PLZST) films were synthesized on Pt/Ti/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering. The microstructure and electrical properties of the films were investigated as a function of post-annealing temperature. Smooth and crack-free films obtained by post-annealing at 700 °C for 30 min, and exhibit a dense columnar microstructure with a grain size of ∼0.85 μm. The sputtered PLZST films of nominal composition Pb_0.97La_0.02 (Zr_0.60Sn_0.30Ti_0.10)O₃ display a high saturation polarization of ∼70 μC cm⁻², a low antiferroelectric-to-ferroelectric switching field (<100 kV cm⁻¹), a reasonable dielectric constant and a low loss tangent. This combination of properties makes them attractive for microdevice applications.     

Effects of stress ratio on fatigue crack propagation properties of submicron-thick free-standing copper films

The dominant mechanics and mechanisms of fatigue crack propagation in ca. 500 nm thick free-standing copper films were evaluated at the submicron level using fatigue crack propagation experiments at three stress ratios, R = 0.1, 0.5 and 0.8. Fatigue cracking initiated at the notch root and propagated stably under cyclic loading. The fatigue crack propagation rate (da/dN) vs. stress intensity factor range (ΔK) relation was dependent on the stress ratio R;da/dN, increases with increasing R. Plots of da/dN vs. the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max ? 4.5 MPa m^1/2) irrespective of R, indicating that K_max is the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode irrespective of the R value. The region ahead of the fatigue crack tip is plastically stretched by tensile deformation, causing necking deformation in the thickness direction and consequent chisel-point fracture. In contrast, in the low-K_max region (K_max < 4.5 MPa m^1/2), the da/dN vs. K_max function assumes higher values with decreasing R; in this region, the fracture mechanism depends on R. At the higher R value (R = 0.8), the fatigue crack propagates in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appears: within several grains, intrusions/extrusions form ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagates preferentially through the intrusions/extrusions.     

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Electric fatigue tests have been conducted on pure and manganese-modified Pb(In_0.5Nb_0.5)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) single crystals along different crystallographic directions. Polarization degradation was observed to suddenly occur above 50–100 bipolar cycles in 〈1 1 0〉 oriented samples, while 〈0 0 1〉 oriented samples exhibited almost fatigue free characteristics. The fatigue behavior was investigated as a function of orientation, magnitude of the electric field and manganese dopant. It was found that 〈0 0 1〉 oriented PIN–PMN–PT crystals were fatigue free, due to its small domain size, being on the order of 1 μm. The 〈1 1 0〉 direction exhibited a strong electrical fatigue behavior due to mechanical degradation. Micro/macro cracks developed in fatigued 〈1 1 0〉 oriented single crystals. Fatigue and cracks were the result of strong anisotropic piezoelectric stress and non-180° domain switching, which completely locked the non-180° domains. Furthermore, manganese-modified PIN–PMN–PT crystals were found to show improved fatigue behavior due to an enhanced coercive field.     

Fatigue crack growth behavior of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass-forming alloy

Fatigue crack growth behavior was studied for a Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass in ambient air, demonstrating a fatigue threshold of ΔK_TH = 1.4 MPa√m and a Paris law exponent of 1.7. A nearly stress intensity-independent crack growth regime occurred at 2.5 × 10^?8 m cycle^–1, suggesting an environmental influence of ambient air on the fatigue crack growth, as has been observed for Zr–Ti–Ni–Cu–Be bulk metallic glasses. However, this environmental fatigue effect was shifted to 25× higher growth rates due to the different chemistry.     

Fatigue behavior of Al0.5CoCrCuFeNi high entropy alloys

Research was performed on an Al_0.5CoCrCuFeNi high entropy alloy (HEA) in an attempt to study the fatigue behavior. The present fatigue investigation shows encouraging fatigue resistance characteristics due to the prolonged fatigue lives of various samples at relatively high stresses. The current results indicate that the fatigue behavior of HEAs compares favorably with many conventional alloys, such as steels, titanium alloys, and advanced bulk metallic glasses with a fatigue endurance limit of between 540 and 945 MPa and a fatigue endurance limit to ultimate tensile strength ratio of between 0.402 and 0.703. Some unpredictability in the fatigue life of the samples was observed as scattering in the stress vs. lifetime plot. Weibull models were applied to predict the fatigue data and to characterize the variability seen in the HEAs. A Weibull mixture predictive model was used to separate the data into two, strong and weak, groups. This model predicts that at stresses above 858 MPa the median time to failure of specimens in the strong group will be greater than 10⁷ cycles. It was shown that microstructural defects, such as aluminum oxide inclusions and microcracks, may have a significant effect on the fatigue behavior of HEAs. It is believed that a reduction in the number of these defects may result in a fatigue behavior which exceeds that of conventional alloys.     

Size distribution of precipitated Ni clusters on the surface of an alkaline-treated LaNi5-based alloy

The size distributions of precipitated Ni clusters on the surface of a LaNi₅-based alloy immersed in alkaline solution (alkaline treatment) at 383 K for 0–110 min were precisely determined by combining superparamagnetic analysis and transmission electron microscopy (TEM) observations. The superparamagnetic analysis indicated that the diameters of the Ni clusters were smaller than ∼25 nm in all samples, while their average values increased approximately from 5 to 9 nm with increasing alkaline treatment time. The spatial distribution of the Ni clusters was successively observed by TEM, which agreed fairly well with the estimated size distribution by superparamagnetic analysis. Therefore, estimation of the actual size distribution of Ni clusters by superparamagnetic analysis was proved to be feasible. Based on the above results, a precipitation process for Ni clusters by alkaline treatment is proposed.     

Pulse current plasma assisted electrolytic cleaning of AISI 4340 steel

An electrolytic plasma process (EPP) for cleaning AISI 4340 steel was performed in a 10% solution of sodium bicarbonate operated at 70 °C. The effects of the pulse frequency (f) and duty cycle (δ) on the surface morphology, microstructure, mechanical and corrosion properties were investigated. Compared to the conventional DC process, the pulsed EPP cleaning resulted in reduced surface roughness and compressive residual stress at the surface. Minimal reduction in hardness and no reduction in toughness due to hydrogen embrittlement (ASTM F519) were found. At the same time, rotating bending beam fatigue tests indicated a noticeable reduction in fatigue life, which could be offset by a shot peening treatment prior to EPP cleaning at 10 kHz and δ = 0.8. Glow discharge optical emission spectroscopy indicated minimal changes in the surface composition and potentiodynamic corrosion studies revealed a slight ennoblement of the surface attributable to an increased rate of cathodic processes. Optimal process parameters were identified for δ = 0.8 and f = 100–10,000 Hz.     

Size-independent stresses in Al thin films thermally strained down to −100 °C

Size and temperature dependencies of thermal strains of {1 1 1} textured Al thin films were determined by in situ X-ray diffraction (XRD) in the temperature range of −100 to 350 °C. The experiments were performed on 50–2000 nm thick Al films sputter-deposited on oxidized silicon (1 0 0) substrates. The in-plane stresses were assessed by measuring the {3 3 1} lattice plane spacing at each temperature in steps of 25 °C during thermal cycling. At high temperatures, the films could only sustain small compressive stresses. The obtained stress–temperature evolutions show the well-known increase of flow stresses with decreasing film thickness for films thicker than 400 nm. However, for thinner films, the measured stress on cooling is independent of the film thickness. This lack of size effect is caused by the flow stresses in the thinnest films exceeding the maximum stress that can be applied to these samples using thermomechanical loading down to −100 °C. Thus, the measured stresses of ∼770 MPa in the thinnest film represent a lower limit for the actual flow stresses. The observed stresses are also discussed taking microstructural information and possible constraints on dislocation processes into account.