Cyclic plasticity of single crystal nickel oriented for single slip

The cyclic deformation behavior of single crystal nickel was investigated by performing uniaxial fully reversed constant plastic strain amplitude fatigue experiments at plastic shear strain amplitudes ranging from 1.1 × 10⁻⁴ to 8.8 × 10⁻³. Digitally acquired stress–strain hysteresis loops were used to calculate friction and back stresses and to relate the shapes of the loops to the evolving dislocation structures and magnetomechanical effects. The results indicate that the cyclic stress–strain curve exhibits a plateau of 50 MPa between plastic strain amplitudes of 1 × 10⁻⁴ and 7 × 10⁻³. Saturation friction stress, calculated using the Cottrell method, is reasonably constant over the entire range of plastic strain amplitudes at a value of 15 MPa. The back stress is 35 MPa within the plateau and increases to 48 MPa at the highest strain amplitude. When cycled at low plastic strain amplitude, magnetomechanical effects account for a significant portion of the measured inelastic strain.     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

Cyclic deformation of tungsten monofilament-reinforced multicrystalline copper composites

In an attempt to understand the cyclic deformation behavior of continuous fiber reinforced metal matrix composites, plastic strain controlled tests have been performed on tungsten monofilament-reinforced, multicrystalline, copper composites. The cyclic hardening response of the composites greatly depends on the fatigue dislocation structures corresponding to the strain amplitude. For example, at high strain amplitude, i.e. 1×10⁻³, secondary slip stimulated by the self-stresses of the primary dislocations becomes more active, and secondary hardening even occurs during saturation. At low strains, loop patches form and are associated with fine slip. At intermediate strains, persistent slip bands occur, but their distribution is altered by the presence of the fiber. The paper introduces a simple model to link the cyclic stress–strain response of the multicrystalline composites to those of monolithic single crystals and fibers. This model not only represents the fiber reinforcement by the rule of mixtures, but also adopts the Sachs model for the single crystal–polycrystal conversion factor. The results calculated by the model show very good agreement with the experimental data in all strain amplitudes at which the composites were fatigued. This encouraging outcome suggests that the new model could be applied to high-cycle fatigue of commercial continuous-fiber-reinforced polycrystalline metal matrix composites.     

THE ELECTRON CHANNELLING CONTRAST TECHNIQUE APPLIED TO THE CHARACTERISATION OF DISLOCATION STRUCTURES IN THE VICINITY OF A FATIGUE CRACK

Abstract— In this investigation the Electron Channelling Contrast (ECC) technique in scanning electron microscopy (SEM) was applied to reveal the dislocation structures in the vicinity of surface fatigue cracks in comparison to those of cyclically-deformed recrystallized polycrystalline copper. The plastic zone around a fatigue crack was found to consist of an innermost region containing cells, followed by a region containing dense veins and PSBs, surrounded by a structure of loose veins, bundles and loop patches typical of the cyclically deformed matrix. A relation between plastic strain amplitude values deduced from cyclic stress-strain investigations and the dislocation structures near fatigue cracks are given. Typical regions of damage accumulation were identified and plastic strain contours for surface fatigue cracks established. The essentially non-destructive ECC technique is particularly suited to identify the changes in mesoscopic dislocation structures from surface layers to the interior of specimens over large specimen areas.     

Fatigue crack growth retardation under constant amplitude and variable mean stress

Fatigue crack growth retardation under stress spectra with constant amplitude and variable mean stress, respectively, was studied. Flat specimens with a central through crack were tested under tension-tension load. The specimens were made with low alloy steel 4 mm thick, with yield strength of 625 Mpa and ultimate tensile strength of 784 MPa. Overload affected crack length increments Δa¹ were studied. The best correlation was obtained between monotonic plane stress plastic zone size 2r_y and Δa¹. The cyclic plastic zone size 2r_pc correlated with crack length increment of minimum crack growth rate after overload. Forman's equation and Willenborg's model of fatigue crack growth retardation were used for theoretical prediction of fatigue crack propagation life. The best agreement between theoretical and experimental results was also obtained using monotonic plastic zone size instead of monotonic plastic zone radius or cyclic plastic zone size. The agreement is reasonably good, even though in the case of one spectrum, cracks were arrested for several thousand cycles.     

Microstructure and superplasticity of Mg–Al–Ca electromagnetic casting alloys after hot extrusion

Microstructure and superplastic properties of the plates extruded from the Ca containing Mg alloy (1 wt.% Ca–AZ31) billets fabricated by electromagnetic casting (EMC) without and with electromagnetic stirring (EMS) were examined. The linear intercept grain sizes of the extruded materials were 3.7 μm and 2.1 μm, respectively. The material extruded from the EMC + EMS billet exhibited good superplasticity at low temperatures as well as at high strain rates, including the tensile elongations of 370% at 1 × 10⁻³ s⁻¹, −523 K and 550% at 1 × 10⁻² s⁻¹, −673 K. These values largely exceeded those of the AZ31 alloys with the similar grain sizes. The superior superplasticity of the extruded EMC + EMS billet could be attributed to fine grains and high grain stability at elevated temperatures by the presence of finely dispersed particles of thermally stable (Al,Mg)₂Ca phase. The constitutive equations were developed for describing the high-temperature deformation behavior of the fine-grained 1 wt.% Ca–AZ31 alloys with different grain sizes in wide range of temperature and strain rate.     

Internal stress behavior of the short ceramic fiber reinforced aluminum alloy under tensile deformation

The in situ measurement of phase stress under tensile deformation on an A6061 alloy reinforced with SiC whiskers (Al/SiCw MMC: Metal Matrix Composite) was performed using the X-ray diffraction technique. In order to raise a preciseness of measurements, we applied a profile fitting technique to separate the nearby located diffraction peak. Tensile deformation on elastic to plastic range was applied by four points bending device and discussed internal stress behavior in the short ceramic fiber reinforced MMC. Phase stress in Al matrix was increased linearly up to 2800×10⁻⁶ in strain and then saturated immediately. On the other hand phase stress in SiC whiskers shows an unstable stress behavior. It was decreased at first because of the Poisson's effect from Al matrix but reversed over 500×10⁻⁶ applied strain. The measured phase stress behavior in elastic region agreed with the calculations using micromechanics based on Eshelby/Mori–Tanaka model except for this unstable internal stress region. The macro stress behavior in plastic region was extremely small than that of the tensile test results. It supposed that the mechanism of strength is not so much the fiber reinforcing as the dispersion strengthening like the Orowan mechanism. Regarding the fatigue property, ΔK_th of the Al/SiC MMC, this was lower than that of the A6061 alloy. On the Al/SiCw MMC specimen, many micro void formations were observed around the fatigue crack tip even under the ΔK_th of A6061. It was considered that these were caused by the high gradient of residual stress on composite process and the unstable stress behavior in low ΔK region.     

Plastic deformation behavior in dual-phase Ni–31Al intermetallics at elevated temperature

Plastic deformation behavior of dual-phase Ni–31Al intermetallics at elevated temperature was examined. It was found that the alloy exhibited good plasticity under an initial strain rate of 1.25 × 10⁻⁴ s⁻¹ to 8 × 10⁻³ s⁻¹ in a temperature range of 950–1075 °C. A maximum elongation of 281.3% was obtained under an initial strain rate of 5 × 10⁻⁴ s⁻¹ at 1000 °C. The strain rate sensitivity, m value was correlated with temperature and initial strain rate, being in the range of 0.241–0.346. During plastic deformation, both the two phases Ni₃Al and NiAl in dual-phase Ni–31Al could co-deform without any void formation or debonding, the initial coarse microstructure became much finer after plastic deformation. Dislocation played an important role during the plastic deformation in dual-phase Ni–31Al alloy, the deformation mechanism in dual-phase Ni–31Al could be explained by continuous dynamic recovery and recrystallization.     

Microstructural evolution and superplasticity of rolled Mg-9Al-1Zn

Microstructural evolution and superplasticity of a Mg-9Al-1Zn alloy rolled at 673 K were investigated at 573 K and 1.5×10⁻³ s⁻¹. The grain size of the as-rolled Mg alloy was 39.5 μm. However, the grain size of the specimen deformed to a true strain of 0.6 was 9.1 μm. The grain refinement was attributed to dynamically continuous recrystallization during an initial stage of tensile test. Stabilization of subgrain boundaries by fine particles and stimulation of continuous recrystallization by prior warm-deformation were not needed to attain dynamically continuous recrystallization in the Mg alloy. As a result of the grain refinement, the rolled Mg alloy exhibited superplastic behavior.     

Superplastic behavior of a fine-grained ZK60 magnesium alloy processed by high-ratio differential speed rolling

Grain size of the ZK60 alloy was effectively reduced to 12 μm through high-ratio differential speed rolling (HRDSR) for a thickness reduction of 70% in a single pass. Due to the strengthening effects of grain boundaries and particles, the HRDSR processed ZK60 exhibited a high tensile strength of 340 MPa. Low temperature superplasticity was attained at 473–493 K at low strain rates (5 × 10⁻⁴ s⁻¹) and high strain rate superplasticity was attained at 523–553 K at high strain rates (10⁻² s⁻¹). The optimum superplastic temperature was found to be 553 K where a maximum tensile elongation of 1000% was obtained at 1 × 10⁻³ s⁻¹. The deformation behavior of the HRDSR processed ZK60 at elevated temperatures could be depicted by considering contribution of grain boundary sliding and slip creep to total plastic flow. Difference in superplastic deformation behavior between the HRDSR processed and equal channel angular press processed ZK60 alloys was examined and discussed.     

NUCLEATION AND SHORT CRACK GROWTH IN FATIGUED POLYCRYSTALLINE COPPER

Surface evolution in polycrystalline copper specimens with a shallow notch has been studied in interrupted constant strain amplitude cyclic loading. The inhomogeneous strain distribution close to stress amplitude saturation leads to the formation of extrusions and intrusions along persistent slip bands within the grain and also in suitably oriented grain boundaries. Numerous primary cracks within a grain or at a grain boundary are nucleated. Some cracks can grow further either by linking with existing cracks or by nucleation of new elementary cracks ahead of the crack tip. Crack growth rates of individual cracks fluctuate considerably but for each strain amplitude, which results in a saturated plastic strain amplitude, a crack growth rate of an equivalent crack can be established. This crack growth rate was found to depend strongly on the plastic strain amplitude in agreement with the Manson-Coffin law.     

Dynamic strain aging effect on the fatigue resistance of type 316L stainless steel

Mechanism of dynamic strain aging (DSA) and its effect on the high-temperature low-cycle fatigue resistance in type 316L stainless steel were investigated by carrying out low-cycle fatigue tests in a wide temperature range from 20 to 650 °C with strain rates of 3.2×10⁻⁵–1×10⁻²/s. The regime of DSA was evaluated using the anomalous features of material behavior associated with DSA. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It is reasonably concluded that the pipe diffusion of solute atoms along the dislocation core is responsible for DSA. Dynamic strain aging reduced the fatigue resistance by ways of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening.     

Solid–liquid interfacial energy of camphene

The Gibbs–Thomson coefficient and the solid–liquid interfacial energy for camphene have been measured to be (8.58±0.96)×10⁻⁸ K m and (4.43±0.49)×10⁻³ J m⁻², respectively, by a direct method. The grain boundary energy of camphene has also been calculated to be (8.36±0.92)×10⁻³ J m⁻² from the observed grain boundary groove shapes.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.     

Mechanical fatigue of Sn-rich Pb-free solder alloys

Recent fatigue studies of Sn-rich Pb-free solder alloys are reviewed to provide an overview of the current understanding of cyclic deformation, cyclic softening, fatigue crack initiation, fatigue crack growth, and fatigue life behavior in these alloys. Because of their low melting temperatures, these alloys demonstrated extensive cyclic creep deformation at room temperature. Limited amount of data have shown that the cyclic creep rate is strongly dependent on stress amplitude, peak stress, stress ratio and cyclic frequency. At constant cyclic strain amplitudes, most Sn-rich alloys exhibit cycle-dependent and cyclic softening. The softening is more pronounced at larger strain amplitudes and higher temperatures, and in fine grain structures. Characteristic of these alloys, fatigue cracks tend to initiate at grain and phase boundaries very early in the fatigue life, involving considerable amount of grain boundary cavitation and sliding. The growth of fatigue cracks in these alloys may follow both transgranular and intergranular paths, depending on the stress ratio and frequency of the cyclic loading. At low stress ratios and high frequencies, fatigue crack growth rate correlates well with the range of stress intensities or J-integrals but the time-dependent C* integral provides a better correlation with the crack velocity at high stress ratios and low frequencies. The fatigue life of the alloys is a strong function of the strain amplitude, cyclic frequency, temperature, and microstructure. While a few sets of fatigue life data are available, these data, when analyzed in terms of the Coffin–Mason equation, showed large variations, with the fatigue ductility exponent ranging from −0.43 to −1.14 and the fatigue ductility from 0.04 to 20.9. Several approaches have been suggested to explain the differences in the fatigue life behavior, including revision of the Coffin–Mason analysis and use of alternative fatigue life models.     

Effect of γ′ size on room temperature low cycle fatigue behaviour of a nickel base superalloy

Abstract

The results of a study on the effect of γ′ particle size on the room temperature (23°C) low cycle fatigue (LCF) behaviour of a Ni base superalloy, Nimonic 90, is reported. The γ′ particle sizes were estimated from transmission electron micrographs. Ranges of particle sizes corresponding to underaged, peak aged, and overaged conditions were identified by examining the age hardening response curve. The solutionised samples had longer LCF lives compared with the aged alloys. Coffin-Manson and cyclic stress–strain plots showed bilinearity at a plastic strain amplitude of around 0.4% in the solutionised, underaged, and peak aged conditions. The observed bilinearity could be attributed to a change in the deformation mode from single slip to multiple slip. The cyclic stress response showed relatively stable behaviour for the peak aged and the overaged specimens compared with the underaged and the solution treated specimens.     

Deformation of PC/ABS alloys at elevated temperatures and high strain rates

The objective of this paper is to experimentally study the deformation behavior of the alloys of polycarbonate (PC) and acrylonitrile–butadiene–styrene (ABS) at elevated temperatures and high strain rates. Four kinds of PC/ABS alloys with the ratio of PC to ABS being 80:20, 60:40, 50:50 and 40:60 and three different strain rates 8.0 × 10² s⁻¹, 2.7 × 10³ s⁻¹ and 1.0 × 10⁴ s⁻¹ are considered. The Split Hopkinson Pressure Bar (SHPB) experiments are carried out at 293 K and 343 K, respectively. The curves of engineering stress and engineering strain and true stress and true strain are obtained for the PC/ABS alloys at different temperatures and different strain rates, respectively. The effects of temperature, strain rate and the fraction of ABS on the deformation behavior of PC/ABS alloys are discussed in details, and then a temperature and strain rate-dependent phenomenological constitutive model for PC/ABS alloys is developed.     

Effect of load ratio and hydrogen concentration on the crack growth rate in Zr–2.5Nb tubes

Crack growth rates (CGR's) were determined under sustained and cyclic loads using 17 mm compact tension and cantilever beam specimens taken from Zr–2.5Nb tubes charged to 6–100 ppm H. The cyclic load effect on the CGR was investigated at 250 °C where load ratios, R were varied from 0.13 to 1 with a constant K_max. Under sustained loads, the CGR of the Zr–2.5Nb tube increased with supersaturation of hydrogen, ΔC and leveled off above 20–35 ppm H of the ΔC. Under cyclic loads with 1 cycle/min, the CGR at 250 °C decreased with decreasing R: 3.2 × 10⁻⁸ m/s at R = 1 and 4.8 × 10⁻⁹ m/s at R = 0.13. The striation spacing, corresponding to the critical hydride length, decreased with decreasing R, indicating easier cracking of the hydrides under cyclic loads. The decreased CGR under cyclic loads and its dependence on the ΔC are discussed using Kim's delayed hydride cracking model.     

Cyclic surface deformation behaviour of ZrH2-purified iron: Effects of solute carbon,strain amplitude,strain rate and environment

The surface deformation behaviour in ZrH₂-purified interstitial-free iron was studied during fatigue in the push-pull plastic strain control mode under various combinations of plastic strain amplitude (5×10^–4, 5×10^–3), strain rate (5×10^–4, 3× 10^–2s^–1) and environment (ultrahigh vacuum, oxygen). Comparative tests of vacuum-melted commercially pure iron (CPI) containing 170 p.p.m. C were also conducted to investigate the effect of interstitials. At a plastic strain amplitude of 5×10^–4, the environmental effect is clearly exhibited regardless of the strain rate and the presence of interstitials. Fatigue in ultrahigh vacuum produces very fine slip lines not only in interstitial-free iron but also in vacuum-melted CPI. In the presence of oxygen, fatigue produces prominent slip lines, but those developed in CPI are more intense and coarser than those developed in interstitial-free iron. At the higher plastic strain amplitude of 5×10^–3, the gaseous environmental effect on the cyclic surface deformation is insignificant. The surface deformation behaviour is discussed in terms of the environmental effect and the basic mechanisms that govern the cyclic plasticity of iron.     

Crack and slip band formation in iron during ultrasonic fatigue at various temperatures

Armco iron specimens with notches were cyclic loaded at stress amplitudes that produced fatigue lives greater than 4 × 10⁷ cycles at temperatures of 23, 60 and 100°C and a frequency of 23 kHz. Simultaneously crack initiation and propagation as well as slip bands formation were observed by optical microscopy. The main fatigue cracks initiates crystallographically from a surface layer at the grain boundaries and in regions with a low density of slip bands or non-crystallographically in regions without slip bands. The crack path is surrounded by a thin plastic region formed by slip bands which tend to be in a shear direction or may be in other directions. The thickness of the plastic region and orientation of cracks nuclei are a function of both the grain orientation and temperature during loading. The ultrasonic plastic deformation at higher temperature is found in greater number of grains in contrast to observations at room temperature. At higher temperatures the concentration of external stress in the notch root is less significant than that room temperature also at high frequency loading.