Low-temperature fatigue behaviour and development of dislocation structure in aluminium single crystals with single-slip orientation

Al single crystals oriented for single slip were cyclically deformed under constant plastic strain amplitudes between 1?×?10^?3 and 5?×?10^?2 at 77?K. Al single crystals showed hardening to saturation at all applied shear stress amplitudes. The resultant cyclic stress–strain curve (CSSC) showed a stress plateau in a range of plastic strain amplitude from 2?×?10^?3 to 2?×?10^?2. Surface observation revealed that multiple slip systems were active even at the strain amplitude in the plateau region. At plastic strain amplitudes corresponding to the plateau of the CSSC, persistent slip bands (PSBs) were formed parallel to the primary slip plane. In the PSBs, well-developed dislocation walls parallel to the {100} planes were observed. The microstructure in the PSBs was explained by the fact of multiple activation of the primary and critical slip systems. The above results indicate that the high stacking fault energy of Al is an important factor affecting the fatigue behaviour even at 77?K.     

Strain bursts during cyclic deformation and coarse slip

Summary Single crystals of 99·98% copper were cyclically deformed in push-pull at 50 cps by an electrodynamical vibrator designed to make the stress amplitude independent of changes in plastic compliance of the specimen. Over a period ofN cycles (280 < < N < 10⁶) the stress amplitude was increased linearly with time up to 3·2 kp/mm² giving a fatigue life of 10⁶. During this monotonic increase of stress amplitude withN > 5000 the plastic strain amplitude did not rise monotonically but showed about 20 regularly spaced, strong maxima. These slip bursts lasted typically about 50 cycles and originated from slip distributed throughout the specimen. The next burst occurred when the stress amplitude had increased by about 12%. Observations of the influence of the rate of stress amplitude increase, unidirectional prestrain, and orientation are reported. If one stops increasing the stress amplitude between two strain bursts at a stress amplitude of only about 1·5 kp/mm² and then pulls the specimen unidirectionally, a yield point with a lower yield stress is observed just at the stress where the next strain burst would have occurred had the increase of cycling stress been continued. During the subsequent yield elongation in a unidirectional test there form very strong (0·3 m) and widely spaced (5 m) slip lines. This coarse slip cannot be explained in terms of point defect clusters at the very beginning of a fatigue test of a pure metal. All of these observations can, however, be interpreted in terms of the formation and dissociation of dislocation dipoles formed by mutual trapping of dislocations of opposite sign on different slip planes. It seems reasonable that the main part of cyclic hardening is due to this mutual trapping even in tests performed so that no bursts occur. This is in agreement with the ample evidence supplied by electron microscopy that almost nothing but dipoles are observed after cyclic deformation.Published in Z. f. Metallkunde59 (1968), 927.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 295 K

The dislocation density in iron single crystals deformed at 295 K has been studied by measuring the coercive field, the initial susceptibility, the Rayleigh constant, and the reversible susceptibility in the approach to ferromagnetic saturation as functions of the resolved shear stress. The influence of different dislocation types on the saturation susceptibility has been calculated. In this way it is possible to distinguish dislocation structures composed of screw or edge dislocations and to reveal long-range internal stresses, which govern the work-hardening in the deformation stage II/III. The dislocation density increases in stage I linearly and in stage II/III quadraticaly with the resolved shear stress. In stage O mainly isolated screw dislocations are created.     

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.     

Work-hardening of Fe,Fe-0·5 wt% Si and Fe-0·9 wt% Si single crystals

Work-hardening curves of single crystals of iron and its alloys with 0·5 and 0·9 wt.% Si and paths of the tensile axis of specimens during deformation were investigated. Single crystals oriented for single glide were deformed in tension at temperatures 113, 201, 295 and 403 K at a nominal strain rate of 5·5×10^?5 sec^?1. It appears that with increasing silicon content (a) the transition between the high-temperature regime of plastic deformation (characterized by three-stage work-hardening) and the low-temperature regime is shifted to lower temperatures, (b) stage I-hardening is enhanced and (c) the flow stress level increases. These observations are explained by strong solution hardening of iron by silicon atoms which suppresses the differences between mobilities of screw and non-screw dislocations, increases the flow stress level and consequently the density of primary dislocations. As a result of this the latent hardening in the secondary slip system increases and stage I extends to a large strain.     

Temperature dependence of development of slip bands and of strain hardening in AgCl crystals

The dependence of the flow stress and the slip band density on the plastic strain has been measured at 201 K, 293 K and 363 K. The growth of deformation concentrated in an average slip band has been stated. The types of obstacles acting against the rise and development of a slip band and the temperature dependence of the strain hardening in AgCl crystals are discussed. An equation stating the dependence of the flow stress on the slip band density is presented. The hardening in AgCl crystals is classified as the stage III — hardening.     

在疲劳载荷下含铜4%的铝合金中的位错钉扎和解脱

【摘　要】用Al-4％Cu合金进行了恆应变扭转疲劳试验,观察到ΔE在起始阶段下降直至接近零值,经过一定的疲劳循环数以后又重新上升。金相观测的结果指出,ΔE的重新上升对应着滑移区的突然变得集中。这有力地证明了我们以前所作的假设,即局部化滑移区的出现是引起ΔE的一种基本过程。在疲劳载荷下的T_m在超始阶段上升,但上升至最高值并保持此值经过一定的疲劳循环数以后,出现骤然的下降,这正对应着ΔE的重新上升。这种现象的出现可能是反映着在前期疲劳载荷下被钉紥的位错得到了骤然的解脱。初步认为,位错的被钉紥和随后得到解脱的过程,是由于在疲劳初期所形成的溶质铜原子气团沿着位错线的某些有利地段富集成核,同时使位错线上的另一些地段从溶质原子中解脱出来。随着富集核的逐惭增大,和其间的得到了解脱的位错段的长度的逐渐增加到某一临界值,便在疲劳载荷下出现了位错“雪崩”的现象,产生了大量的新位错,这使T_m骤然降低,并且导致局部滑移地区的形成,从而使ΔE重新增加。     

Cyclic deformation and fatigue cracking behaviors of Cu–28wt%Ag binary alloy

The cyclic deformation and fatigue cracking behaviors of coarse-grained Cu–28wt%Ag binary alloy were investigated under axial plastic strain amplitudes ranging from 10^?4 to 7.5 × 10^?4. It was found that the cyclic stress of the Cu–Ag alloy increased rapidly in the initial tens of cycles and became saturation with further cyclic deformation. The cyclic saturation stress increased with increasing the plastic strain amplitude. The interfaces are classified into two categories based on the orientations of the eutectic and the dendrites, i.e. type I and type II interfaces. The surface damage morphologies show that fatigue cracks normally nucleated either along the type I interfaces or along the slip bands (SBs), while no cracking occurred along the type II interface. Fatigue striations with different spacings appeared on the fracture surface, and secondary cracks along the striations were also observed. Based on the experimental results, the cyclic deformation and fatigue cracking behavior of the Cu–Ag binary alloy were discussed in detail.     

Formation and propagation of cracks during cyclic deformation

Summary Starting from observation of the geometric features of formation and propagation of cracks in cycled single crystals of copper a model was developed for fatigue cracking. The only and experimentally well established assumption of the model is that the slip during cyclic deformation tends to be coarse. By the cooperation of stress concentrations at the slip steps and the hardening of slip planes activated locally two slip systems (with different slip planes and Burgers vectors) are activated alternatively so that a crack develops from the slip step. It propagates without monotonically increasing the hardening at the crack tip. The coarse slip produces sharp slip steps at the surface for crack formation and prohibits crack blunting during propagation. In contrast to other models the one described can show how the irreversible process of crack formation and propagation can take place despite completely symmetrical push-pull stresses. The whole crack is formed merely by the motion of dislocations present in the material so that but comparatively small stresses are needed. As, moreover, no thermally activated processes are necessary, fatigue at 4·2°K can be explained too. The strong dependence of fatigue on the state of the surface can also be accounted for since the cracks form at the surface steps. Materials which tend to coarse slip even in unidirectional tests are expected to fatigue easily. This is corroborated experimentally. Finally, many details of crack geometry can be explained in terms of the model.Published in Z. f. Metallkunde58 (1967), 780.     

Precipitation in solution-treated aluminium–4 wt% copper under cyclic strain

Solution-treated Al–4 wt% Cu was strain-cycled at ambient temperature and above, and the precipitation and deformation behaviours investigated by TEM. Anomalously rapid growth of precipitates appears to have been facilitated by a vacancy super-saturation generated by cyclic strain and the presence of continually refreshed dislocation density to provide heterogeneous nucleation sites. Crystallographic texture appears to be responsible for latent hardening in specimens tested at room temperature. Increasing temperatures lead to a gradual hardening throughout life due to precipitation. Specimens machined at 45° from the rolling direction, which exhibit rapid precipitation hardening, show greater texture hardening due to increased axial stress required to cut precipitates in specimens. In the temperature range 100–200°C, precipitation of Θ″ is suppressed by cyclic strain, and precipitation of Θ′ promoted. The rapid growth of precipitates generated by cyclic strain operates with diminishing effect at higher temperatures due to faster recovery of non-equilibrium vacancy concentrations. Θ′ precipitates generated under cyclic strain are smaller and more finely dispersed than those produced via quench-ageing due to heterogeneous nucleation on dislocations and possess a low aspect ratio and rounded edges of the broad faces caused by the introduction of ledges into the growing precipitates by dislocation cutting. Frequency effects indicate that dislocation action is responsible for the observed reduction in aspect ratio. Accelerated formation of grain-boundary precipitates appears partially responsible for rapid inter-granular fatigue failure at elevated temperatures, resulting in coexistent fatigue striations and ductile dimples on the fracture surface.     

Influence of ageing on the low cycle fatigue behaviour of an Al–Mg–Si alloy

Abstract

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–Si alloy at under-aged (UA), peak-aged (PA) and over-aged (OA) conditions has been examined to understand the micromechanism of fatigue and the associated dynamic structural changes in this alloy. The LCF behaviour of the differently aged AA6063 alloys has been studied at strain amplitudes ranging between 0.2 and 1.0% under strain control mode. The UA state exhibits pronounced cyclic hardening unlike the PA and the OA states at strain amplitudes greater than 0.4%. The PA and the OA states show hardening only for a few cycles followed by prolonged softening. Characterisations of the micro- and the sub-structural alterations due to LCF establish that the phenomenon of dynamic precipitation results in cyclic hardening the UA alloy. The softening of PA alloy occurs due to shearing of precipitates and that in the OA alloy takes place owing to reversibility of slip by the formation and annihilation of the Orowan loops around the β (Mg₂Si) precipitates. Analyses of the hysteresis loops reveal Masing, nearly-Masing and non-Masing behaviour in the UA, OA and PA states, respectively. Analyses of the asymmetry factor of the hysteresis loops assist to infer that the Masing behaviour in the UA alloy is due to dislocation–dislocation interactions, whereas the nearly-Masing behaviour in the OA alloy and the non-Masing behaviour in the PA alloy are the consequence of varying degrees of dislocation–precipitate interactions associated with inhomogeneous deformation.     

Hardening and softening during fatigue fracture

Summary The comparison of the change of hardness and plastic deformation amplitude at a constant stress loading or stress amplitude at a constant deformation loading during the fatigue process shows some singularity of the hardening and softening effects. These effects were investigated on mean carbon and low-alloyed steel and on globular cast iron.The fatigue fractures at cycle numbers 10⁴÷10⁶ under stresses below the yield strength predominate in the softening process, which arises after an inconsiderable hardness increase extends in the region to 0·2 from the fracturing cycle number. Under the stresses above the yield strength, which in some cases for annealed and coarse-grained states are below the fatigue limit, the hardening process predominates, followed by a hardness increase in the field up to 0·25 and above the fracturing cycle number.At low cycle fatigue fractures with cycle numbers < 10⁴ depending on the cyclic plastic properties of steels the fatigue process can be followed by a continuous hardening or softening till fracture. This process is characterized by the change of the deformation amplitude and a one-sided accumulation of plastic deformations at a constant amplitude of active stresses. The one-sided accumulation of deformations commonly ends in a quasistatic failure. Under loading with a constant deformation amplitude during softening a fatigue fracture takes place as a result of damage accumulation under the alternating stresses with amplitudes decreasing with cycle number.     

Stimulation and Suppression of Grain Boundary Sliding by Intragranular Slip in Zinc Bicrystals

The influence of intragranular slip on grain boundary sliding is studied in originally compatible zinc bicrystals with symmetric tilt boundary. The experiment is designed to separate different effects of intragranular slip on the boundary sliding and establish their mechanisms. Grain boundary sliding with and without development of intragranular slip is observed. The rate of sliding accompanied by slip is more than five times of that without slip. A good correlation between the boundary sliding and intragranular slip prior to slide hardening is established. Slide hardening followed by the negative sliding near one end of the boundary and strain hardening in the boundary vicinity, are observed at the last stages of deformation. For the case of formation of slip induced glissile grain boundary dislocations of opposite signs the possibility of their contribution to total grain boundary sliding, is analyzed. The effect of the increase in the rate of sliding is explained in terms of the accommodation of sliding by slip and appearance of additional glissile grain boundary dislocations of one sign due to strain incompatibility. Contribution of these different dislocation mechanisms to the increase in the sliding rate is determined for the stage of deformation preceding slide hardening. It is supposed that the effect of slide hardening and negative sliding as well as boundary curving is created by non-smooth boundary and small degree of incompatibility caused by straining.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 195K and 77K

Measurements of the coercive field, the initial susceptibility and the reversible susceptibility in the approach to ferromagnetic saturation show that during low-temperature deformation of iron single crystals mainly screw dislocations are created. Long-range internal stresses are found to be significantly smaller than in crystals deformed at room temperature. Macroscopic slip occurs on several slip systems. In the parabolic region of the work-hardening curves at 195 K the relation is valid, where τ isthe shear stress andN is the dislocation density. In the region of saturation of the shear stress the dislocation density further increases. After room-temperature prestrain the relation appears to hold for 77K-deformation also. Exhaustion hardening of edge dislocation is found at the beginning of the low-temperature deformation.     

疲劳载荷作用下铝镁合金中的粗滑移区的形成

过去关于铝和铝铜合金的研究工作指出,在疲劳载荷过程中的能量消耗(△E)所发生的变化可以分成两个不同的阶段。第一阶段相当于位错的被钉札,在第二阶段里△E的再上升则表示已有粗滑移区出现。为了进一步验证这种看法,在本文中用含镁量为0.52,0.91,3.46和5.15％的铝合金进行了扭转疲劳试验,测定了经过各种应力循环数N以后的滞后迴线的面积,从而算出在每次循环中的能量消耗△E。在疲劳载荷经过不同循环数后,试样表面进行金相观测的结果指出,对于所用的合金而言,滑移痕迹的变化都表现出两个明显不同的阶段。在第一阶段里,常常观察到几组细而直的滑移线均匀分布在一个晶粒内。在第二阶段里,某一组滑移线变得集中而粗化成簇。一般而言,在镁含量较低(0.52,0.91％)的合金里,以及当扭应变较大时,粗滑移区出现得较早。将所观察到的△E-N曲线的变化与粗滑移区的出现做了比较,并且考虑到在疲劳载荷过程中第一阶段和第二阶段的△E可能发生重迭的情况,指出了试样里出现粗滑移区可以引起△E在疲劳后期的再上升。这与过去关于铝和铝铜合金所得结果相合。本文还讨论了位错在疲劳载荷第一阶段里被溶质原子气团所钉札的状态与粗滑移区的随后形成的联系。     

Study of the mechanical properties of alloy 36NKhTYu in relation to the mechanisms underlying the precipitation of the γ' and η phases

The nature of the dispersion (precipitation) hardening of the alloy 36NKhTYu is examined. It is shown that the increase observed in the resistance of the alloy to the motion of dislocations inside the grains during the continuous precipitation of the γ' phase is due to the presence of long-range order in the hardening particles. In addition to this, there is also a considerable increase in that part of the yield stress which is associated with the grain boundaries; this is because of the precipitation of carbides at grain boundaries and the corresponding sharp increase in slip localization. Overaging is characterized by a reduction in the resistance to the motion of the dislocations inside the grains, and a diminishing influence of the grain boundaries (ultimately to no influence at all). These results are in satisfactory agreement with calculations based on the Orowan theory, and also with the assumption that, as overaging progresses, the range of the dislocations is reduced from a distance equal to the grain size to a distance equal to that separating the particles ofγ′ phase, while localized slip is replaced by uniform slip. When the precipitation of theη phase reaches a well-developed stage, the yield stress is reduced. The result of the present investigation confirm our earlier conclusion as to the substantial hardening (strengthening) of the meterial which occurs as a result of the discontinuous precipitation of theγ′ phase.     

铝-银合金在疲劳过程中所表现的一些特点

本工作进行了淬火Al-7.27％Ag合金的扭转疲劳试验,测定了各种扭应变下的△E-N曲线,并且观察了经过各种循环数以后试样的表面金相变化。实验结果指出,当扭应变较小时,△E随着循环数N的增加而逐渐下降,△E-N曲线的变化类似Al-Cu和Al-Mg合金在较低扭应变下的情况。但当扭应变较大时,△E开始略有下降,随后上升到某一较高值后再下降,直至试样断裂。△E-N曲线的形状与Al-Cu和Al-Mg合金完全不同。试样表面的金相变化分为两个明显不同的阶段。在疲劳的起始阶段,滑移痕迹细而均匀,但经过一定循环数后,少数滑移痕迹变得集中而深化。随着循环数的增加,新的滑移带在原有滑移带之间不断地出现,没有纯Al和Al-Mg合金中滑移带变宽的情况。还看到了裂纹沿晶界的形成和发展。根据溶质银原子与位错的电交互作用和位错切割银原子簇的观点,对所得到的结果进行了讨论。 关键词：     

A discussion of the structure and behaviour of dipole walls in cyclic plasticity

Assuming that cross-slip by thermally activated migration of jogs can cause annihilation of screw dislocation dipoles without macroscopic crystallographic confinement of cross-slip to the cross-slip plane, an attempt is made to re-derive earlier equations for the saturation stress and the plastic strain amplitude in persistent slip bands. These equations had been based on the assumption that cross-slip could occur only on a cross-slip plane making an obtuse angle with the slip plane, an assumption which limits the mean free path of screw dislocations. The key new assumption is that the walls of edge dislocation dipoles which dominate the structure of persistent slip bands are penetrable obstacles, which increases the mean free paths of the mobile dislocations. Agreement with experiment is obtained if the penetration probability in cyclic saturation is on average one third, a value for which there is a simple rationalization. Estimates can be made of the wall width, which is independent of temperature, in agreement with recent observations by Tippelt et al. However, the main unresolved difficulty is the role of the very fine dipoles, particularly the faulted dipoles, in the walls. A further weakness in the theory is that it ignores the cutting of dipoles by the cross-slipping screw dislocations. Despite these problems, the distribution of dipole heights can be worked out and is found to be in reasonable agreeement with experiment.     

Work-hardening of metal crystals

Some recent problems relating to work-hardening of metal crystals, such as the contribution of dislocation stress fields to the flow stress, the mechanism of the second stage, or linear, hardening, and the transition to the strongly temperature dependent third stage are discussed. It is shown that linear hardening can be explained by recourse to the strain invariance of the geometrical pattern of the dislocation network. The thermodynamic origin of this invariance is considered. Experimental data on the temperature and strain-rate dependence of the stress at which transition to the third stage occurs are shown to accord best with the view that at temperatures at which diffusion effects are negligible the third stage begins with the onset of appreciable dynamic recovery, involving the formation of point defects, probably through non-conservative movement of elementary intersection jogs.     

Effects of size on the dynamics of dislocations in ice single crystals

Single crystals of ice subjected to primary creep in torsion exhibit a softening behavior: the plastic strain rate increases with time. In a cylindrical sample, the size of the radius affects this response. The smaller the radius of the sample becomes while keeping constant the average shear stress across a section, the softer the response. The size-dependent behavior is interpreted by using a field dislocation theory, in terms of the coupled dynamics of excess screw dislocations gliding in basal planes and statistical dislocations developed through cross slip occurring in prismatic planes. The differences in the results caused by sample height effects and variations in the initial dislocation microstructure are discussed.