Low Plastic Strain Amplitudes Research Articles

Cyclic deformation of al-single crystals at low constant plastic strain amplitudes

Cyclic deformation of Al-single crystals with the crystal axis parallel to (1̄23) has been performed at room temperature and at 77 K at plastic shear strain amplitudes in the range 2.4·10 −4−1.2·10 −2. At room temperature, mechanical saturation was not obtained at any strain level, with a possible exception at the very lowest value. Instead, a hardening-softening-secondary hardening sequence was generally found. The hysteresis loops did not change shape during the cycling. At 77 K, mechanical saturation was reached, and the cyclic stress-strain curve had a plateau region in the plastic shear strain amplitude range 4.2·10 −3−1.2·10 −2. The hysteresis loop became more rectangular when saturation was approached, similar to what is found in Cu single crystals. The slip line pattern consisted of macrobands, which were made up by small segments of PSBs on the primary slip plane. Slip activity was also found on other slip systems. The macrobands are explained by considering the nucleation and propagation of PSBs. The dislocation structure formed at 77 K was found to be rather similar to the structure developed in Cu single crystals oriented for single slip. A high density of dislocation loops was found. The ladder structure was not observed.

Cyclic stress response and deformation behaviour of precipitation-hardened aluminium-lithium alloys

The cyclic stress response of two lithium-containing aluminium alloys aged to contain ordered precipitates was studied in different environments over a range of plastic strains. The specimens were cycled using tension-compression loading under total strain control. The peak-aged AlLiMn alloy cyclically hardened to failure, whereas the peak-aged AlLiCu alloy displayed softening for most of the fatigue life. The presence of shearable softening for most of the fatigue life. The presence of shearable precipitates in the two alloys results in a local decrease in resistance to dislocation movement, leading to a progressive loss of ordering contributions to hardening and slip concentration. This, coupled with the presence of precipitate free zones, promotes strain localization in intense slip bands and results in early crack nucleation. Transmission electron microscopy observations revealed homogeneous deformation in specimens cycled at high plastic strain amplitudes. However, at lower plastic strain amplitudes, deformation was inhomogeneous in the two alloy systems with the formation of intense planar slip bands. Results of this study reveal that the initial hardening observed is due to dislocation-dislocation and dislocation-precipitate interaction and that the softening observed in the AlLiCu alloy is a mechanical and not an environmental effect.

Cyclic deformation of molybdenum single crystals II. Dislocation substructures and cyclic hardening in crystals deformed at 400 K

Abstract The study of the cyclic deformation behaviour in tension-compression of molybdenum single crystals described in Part I has been completed with the observation by transmission electron microscopy (TEM) of the dislocation substructures developed at 400 K. The dislocation microstructures' morphologies were determined, the dislocation densities measured and the dislocation Burgers vectors evaluated. The different regions of the c.s.s. curve were correlated with characteristic dislocation substructures. At low plastic-strain amplitudes the dislocation substructure consists of poorly developed tangles and, as the plastic-strain amplitude increases, dislocation bundles, walls and cells are formed and developed. Probably the most important feature is the presence of secondary dislocations, mainly with conjugate Burgers vectors, which increase very strongly with increasing plastic-strain amplitude. It is the activity of secondary sources which governs the cyclic hardening behaviour of the crystals. This a...

Fatigue microcrack initiation in polycrystalline alpha-iron with polished and oxidized surfaces

Plastic-strain-controlled fatigue crack initiation experiments were conducted on unoxidized and oxidized, vacuum-melted iron. In the unoxidized, as-polished condition at low plastic strain amplitudes, e.g., 1 or 5 X 10−4, microcracks initiated along well defined slip bands and in the troughs of surface rumples. Such microcracks tended to stop short of grain boundaries. On increasing the plastic strain amplitude, initiation of fatigue cracks along grain boundaries became important. When the specimens were surface oxidized, intergranular microcrack initiation was the dominant mode even at the plastic strain amplitude of 5 x 10−4, where transgranular microcracks formed only very infrequently and then only considerably later in the fatigue lifetime. At 1 x 10−3 amplitude, transgranular microcracks initiated very early in the cycling compared to the polished condition, but such cracks did not grow or increase in number. Intergranular cracks formed later and led to failure. Surface oxidation led to approximately a 20 pct reduction in lifetime to final failure.

The cyclic stress-strain behavior of polycrystalline Al-5wt.%Mg

Push-pull fatigue tests have been carried out on binary Al-5wt.%Mg specimens in the annealed and cold-worked conditions at constant plastic strain amplitudes between 4 × 10 −5 and 2 × 10 −2 to characterize the cyclic stress-strain response and the strain-fatigue life behaviour. The annealed material hardens substantially at all strain levels and, at saturation, exhibits a constant plateau stress of 270 MPa for plastic strain amplitudes between approximately 10 −4 and 2 × 10 −3. The high degree of cyclic hardening is related to the formation of a high density of fine dislocation loops. Intense slip bands are also observed to penetrate the bulk of the specimen. The strain-fatigue life plots of the annealed and cold-worked material reveal a two-slope behaviour. The high fatigue life exponent at low plastic strain amplitudes is thought to be due to rapid crack initiation in isolated surface slip bands.

Secondary cyclic hardening in fatigued copper monocrystals and polycrystals

Copper single crystals oriented for single slip and copper polycrystals were deformed cyclically at constant plastic strain amplitudes (in the range about 10 −3–10 −2) up to large cumulative plastic strains. It was noted for monocrystals and polycrystals that, during prolonged cycling in the so-called cyclic saturation regime, slow but significant microstructural changes continued to occur in the bulk, giving rise to “secondary” cyclic hardening. The observed microstructural changes result from a slow but persistent enhancement of secondary (multiple) slip which leads ultimately to the formation of misoriented dislocation cells. In single crystals the enhancement of secondary slip begins in the persistent slip bands (PSBs) at the PSB-matrix interfaces, causing a gradual transformation of the PSB-wall structure into misoriented cells. In parallel to the microstructural changes, the hysteresis loops become more pointed and, in some cases, a renewed increase in the peak stress occurs. The described secondary cyclic hardening was found to be a general bulk phenomenon whose occurrence can be described by an empirical law. It occurs the earlier the larger the plastic strain amplitude is. In polycrystals, secondary cyclic hardening is observable at lower plastic strain amplitudes and at an earlier stage than in single crystals oriented for single slip.

The cyclic stress-strain curve of polycrystals

The internal stresses implied by the Sachs model are estimated for individual PSBs at low plastic strain amplitudes and for homogeneously sheared grains at higher plastic strain amplitudes. The analysis shows that the Sachs model can account semi-quantitatively for experimentally measured cyclic stress-strain curves for copper. A similar approximative analysis of the Taylor model cannot account for the data. An interesting feature of the Sachs model is that, although it is assumed that the flow condition is entirely controlled by the PSBs. the predicted cyclic stress-strain curve displays an upward slope at high plastic strain amplitudes.

Cyclic deformation behavior of a low carbon steel in the temperature range between room temperature and 850 K

Stress-controlled uniaxial push-pull fatigue tests were carried out with cylindrical specimens of low carbon steel with a carbon content of 0.1 percent. Resistent heated grips provided testing temperatures up to 550°C. During all fatigue tests the time-dependent strain was measured. The strain at zero load, the plastic strain amplitude plotted against the number of cycles is known as the cyclic deformation curve. At room temperature, this curve depends on the applied stress amplitude. The shape of the cyclic deformation curve at the same stress amplitude is strongly temperature-dependent. In the temperature region of blue brittleness the endurance shows a maximum connected with a very low plastic strain amplitude. Also the cyclic stress strain curves have a different appearance within varied temperature ranges.

Fatigue of copper polycrystals at low plastic strain amplitudes

Single crystals and polycrystals of pure copper were fatigued in tension-compression at constant low amplitudes of plastic strain and low cycling frequencies at room temperature in air. Surface patterns of persistent slip bands were quantitatively examined by optical microscopy. Bulk dislocation microstructures were examined by transmission electron microscopy. The microstructure and mechanical behaviour observed for the single crystals are in close quantitative agreement with comparable existing fatigue data. The cyclic stress-strain curve of the polycrystals shows a plateau in a linear plot of the saturation stress versus the plastic strain amplitude. The area fraction of PSB's on the polycrystals increases roughly linearly with the plastic strain amplitude. The dislocation microstructure in bulk grains consists of regular wall structures embedded in a matrix of less regular structures. A Sachs type model in which the small plastic strain incompatibility caused by bulk persistent slip is elastically accommodated is consistent with the experimental observations.

Fatigue of copper single crystals at low constant plastic strain amplitudes

Single crystals of copper with single slip orientation were cycled at constant plastic strain amplitudes until fatigue failure. The strain amplitudes were selected to vary the volumetric fraction of persistentslip bands in the gauge of the specimen. The cyclic response has been measured by monitoring the changes in hysteresis loop shape, energy absorption and peak, average and saturation stresses. The hysteresis loop shape is quantitatively expressed through a parameter which turns out to be particularly sensitive to changes in deformation characteristics. The results show that the plastic strain amplitude affects the initial hardening rate, the cumulative strain value at which the maximum peak stress is reached, the shape variation of the hysteresis loop, the energy input per unit amount of deformation and the fatigue life in general as measured by the number of cycles to failure or by the energy absorption per cubic millimetre to failure. The fatigue results are explained by assuming that the crack propagation rates in Stage 1 are dependent on the plastic strain amplitude even when only the volumetric fraction of persistent slip bands is varied in the gauge of the specimen.

The effect of grain refinement on the low cycle fatigue behavior of an AluminumZincMagnesium(Zirconium) alloy

The microstructure and low cycle fatigue (LCF) behavior of Al 3Zr grain-refined AlZnMg alloy were investigated and compared with those of a large-grained AlZnMg alloy. Improvements in LCF resistance were obtained in the AlZnMgZr by an improved homogeneity of deformation which delayed crack initiation and failure. An overaged microstructure of large η′ and η particles, obtained by aging for 24 h at 150 °C, exhibited the best LCF resistance observed in this study. The improved LCF life of this aging treatment was attributed to the increased homogeneity of slip resulting from the occurrence of dislocation looping as the primary deformation mechanism. The Coffin-Manson parameters vary with the microstructure and deformation behavior of the alloys. Failure of the empirical Coffin-Manson relationship for samples aged for 24 h at 150 °C was related to a change in the capacity of the microstructure to absorb energy prior to crack initiation at high- and low plastic strain amplitudes.

Dislocation Microstructures in Fatigued Copper

A characteristic feature of fatigue in many materials is the concentration of deformation into bands of intense slip. In single crystals of copper fatigued at low constant plastic strain amplitudes these "persistent slip bands" (PSB's) are first observed as the stress amplitude reaches saturation.(1,2).Their formation in the immediately succeeding cycles is rapid but after perhaps twice the number of cycles required to reach saturation the specimen enters a more stable state which is maintained until large cracks are formed near the end of the test. In this state, although PSB formation may continue slowly its rate is very much reduced - few new bands form and the existing ones do not show much increase in width. The fatigue deformation is carried in a quasi-reversible manner by the PSB's whilst the remainder of the specimen (known as the matrix) becomes inactive. At the specimen surface the PSB's are marked by intense rumpling which is known to play a part in crack formation and fatigue failure.

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65a and a small amount of incoherent η (MgZn2). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35a. Aging the 7050 alloy to produce particles larger than 50a gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δep/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.

Weak-beam study of dislocation structures in fatigued copper

Abstract Single crystals of pure copper oriented for single slip were fatigued to saturation at low constant plastic strain amplitude. A weak-beam study of the dislocation microstructures revealed a very high density of dipoles in both the ‘wall’ and ‘matrix’ structures. Secondary dislocations were not observed

A model for the fatigue of copper at low plastic strain amplitudes

Abstract Single crystals of pure copper oriented for single slip were fatigued in air at room temperature at low constant plastic strain amplitudes in the range 0·1 % ≤ e p≤ 0·6%. After a few thousand cycles the specimens entered what appeared to be an equilibrium state which contained two phases: a hard and almost inactive ‘matrix’ and the softer ‘persistent slip bands’ (PSBs) into which the deformation tended to concentrate. In the equilibrium state the stress amplitude, the shape of the stress-strain loop and the volume fraction of the specimen occupied by the PSBs were invariant. Furthermore, the volume fraction occupied by the PSBs was found to be linearly related to the plastic strain amplitude. Thus, no matter what the average strain amplitude in the crystal (so long as both phases were present) the PSBs suffered a fixed amplitude which was measured as 0·90%; the specimen adapted itself to the applied amplitude by adjusting the relative amounts of the two phases. A number of predictions of this mod...

Etching studies of dislocation microstructures in crystals of copper fatigued at low constant plastic strain amplitude

Abstract An etching technique was used to study the dislocation microstructure in single crystals of copper fatigued to saturation in push-pull at constant plastic strain amplitude. In agreement with earlier electron microscope studies a structure of plane, regularly spaced walls of dislocations perpendicular to the primary slip direction was found to be associated with the persistent slip bands. The wall spacing (1·38 ± 0·05 yum) was measured and found to be virtually independent of fatiguing frequency (between 0·01 and 25 Hz) at room temperature. The internal consistency of the results was used to justify the etching technique.