H. Mughrabi

Low energy dislocation structures produced by cyclic deformation

Abstract Cyclic deformation in push-pull straining provides ideal conditions for achieving low energy dislocation structures because large cumulative strains give rise to high dislocation densities and the to-and-fro dislocation motions enhance entrapment probabilities. At low strain amplitudes the structures are dominated by clusters of prismatic loops; evidence for the dislocation content and interaction of the clusters is reviewed and is shown to be consistent with the establishment of low energy arrays. At higher amplitudes the dislocation structures are dominated by dislocation walls which are strongly dipolar. The deformation which occurs in these structures at low numbers of cycles does not seem to be consistent with low energy arrays, or at least only with those of moderately low energy. However, with sufficient cycling, lower energy arrays seem to become established and to promote homogeneity of the strain. At still higher amplitudes, complex cellular structures are observed and the complications of these structures can be interpreted convincingly in terms of low energy dislocation structures.

A two-parameter description of heterogeneous dislocation distributions in deformed metal crystals

Abstract A two-parameter model of heterogeneous dislocation distributions (cell structures) in deformed metals has been developed. The model permits a description of the effective mechanical and energetic properties of (one-dimensional) cell structures in terms of the local properties (dislocation densities and volume fractions) of cell walls and cell interiors and includes the limiting case of the homogeneous dislocation distribution. Long-range internal stresses that arise unavoidably in heterogeneous dislocation distributions during deformation are an important feature of the model. The model is consistent with the well-established empirical relationships between the macroscopic flow stress, the mean dislocation density and the cell wall spacing. It is shown that, for a given mean dislocation density, the macroscopic flow stress is always lower for a heterogeneous than for a homogeneous dislocation distribution. This result and the fact that the total elastic strain energy in the stress-applied state (with contributions from the dislocations, the long-range internal and the applied stress) decreases progressively with increasing heterogeneity of the dislocation distribution are considered to be the major reasons for dislocation cell formation.

Microstructural study of the parameters governing coarsening and cyclic softening in fatigued ultrafine-grained copper

Abstract The cyclic deformation behaviour of ultrafine-grained (UFG) copper produced by equal-channel angular pressing was investigated. Special attention was paid to the parameters governing cyclic softening and cyclic grain coarsening. UFG copper shows significant cyclic softening for the tests performed at intermediate plastic strain amplitudes Δϵp1/2, that is in the range 2 × 10−4 ≤ Δϵpl/2 ≤ 1.0 × 10−3. Within this range, the cyclic softening as well as the intensity of grain coarsening increase with decreasing plastic strain amplitude. By contrast, under stress control, corresponding to a plastic strain amplitude range 2.4 × 10−5 ≤ Δϵpl/2 ≤ 1.2 × 10−4, cyclic softening as well as the intensity of grain coarsening decrease with decreasing plastic strain amplitude. Furthermore, cyclic softening and grain coarsening were also found to be enhanced by decreasing the deformation rate (and thus increasing the test time) and/or by increasing the temperature. These findings indicate that the responsible microstructural processes are thermally activated. Based on the experimental results, a dynamic ‘recrystallization’ mechanism is proposed and suggested to be responsible for the cyclic grain coarsening and to some extent also for the cyclic softening.

An X-ray study of creep-deformation induced changes of the lattice mismatch in the γ′-hardened monocrystalline nickel-base superalloy SRR 99

Abstract X-ray line profile measurements were performed on monocrystalline specimens of the γ′-hardened nickel-base superalloy SSR 99 with axes close to the [001] direction. The aim was to measure the local lattice parameters in the γ-matrix and in the γ′-particles and to obtain information on the lattice mismatch and internal stresses. For this purpose, a special high-resolution double crystal diffractomoter with negligible instrumental line broadening was used. Measurements were performed on specimens in the initial state with cuboidal γ′-particles and on creep-deformed specimens containing the so-called γ/γ′-raft structure. In several cases the line profiles were measured as a function of the rocking angle, and the intensity distributions were mapped in reciprocal space around the (002) and (020) Bragg reflections. In the undeformed state these intensity distributionsindicate that the local lattice parameter varies spatially in the γ-phase. The line profiles of specimens in the initial state were asymmetric. A remarkable result obtained on creep-deformed specimes was that, whereas the asymmetry of the (020) line profiles was enhanced to the extent that a hump or a second peak appeared, the asymmetry of the (002) line profiles was reversed in sign. A quantitative evaluation yielded mean values of the constrained lattice mismatch which, in the case of creep-deformed specimens, differ significantly for the (001) and (010) lattice plane spacings. It is concluded that the orientation-dependent lattice spacings represent a triaxial state of residual stress which has its origin in the superposition of the originally present coherency stresses and the deformation-induced internal stresses. All observed features could be explained in detail in terms of a composite model of plastic deformation, which takes into account the dislocation networks deposited at the γ/γ′-interfaces during deformation.

Plasticity-induced martensitic transformation during cyclic deformation of AISI 304L stainless steel

Specimens of AISI 304L stainless steel were deformed cyclically at room temperature at various plastic strain amplitudes Δϵpl/2. In addition to tests at constant plastic strain amplitudes, incremental step tests were also carried out. The cyclic deformation behaviour was investigated and related to microstructural changes observed by transmission electron microscopy (TEM). At low values of the plastic strain amplitude a saturation state was reached, whereas specimens fatigued at plastic strain amplitudes Δϵpl/2 > 0.3% exhibited, after initial cyclic hardening, an extensive secondary hardening stage which persisted until fracture. The reasons for this behaviour were clarified by TEM observations. Planar dislocation arrays with faults were observed after cycling at Δϵpl/2 = 0.02%. However, martensitic phases were not found. The specimens fatigued at Δϵpl/2 = 0.5% or in the incremental step test (0.02%<Δϵpl2<0.5%) displayed cell structures with a considerably higher number of stacking faults. Moreover, deformation-induced martensitic phases were observed and identified as ϵ- and α′-martensite by electron diffraction pattern analysis.

A model of extrusions and intrusions in fatigued metals II. Surface roughening by random irreversible slip

Abstract A model of the evolution of the surface roughness of persistent slip bands (PSBs) is presented. The physical background of the model is the existence of a dynamic equilibrium between dislocation multiplication and annihilation in PSBs which leads to irreversible microscopic positive and negative slip steps at the surface. The fluctuation of a random distribution of these slip line segments leads to a surface roughness which increases with the square root of the cycle number. The relation between the parameters which characterize the surface relief and the microscopic parameters which control the dislocation glide in PSBs is investigated by computer simulations. Fair agreement is obtained between observed and simulated surface reliefs where such a comparison is possible. The limits of our purely statistical treatment are pointed out.

Electron channelling contrast as a supplementary method for microstructural investigations in deformed metals

Abstract Electron channelling contrast is obtained in the scanning electron microscope by detecting back-scattered electrons created by a parallel incident electron beam. The intensity of the back-scattered electrons depends on the orientation of the incident beam with respect to the crystal lattice. In this report, several examples for the application of channelling contrast as a means for investigating the microstructure of metals after deformation are given. First, the change in dislocation glide mechanism during fatigue of the austenitic stainless steel AISI 304L as a function of deformation temperature is demonstrated by means of a comparative study by transmission electron microscopy and scanning electron microscopy (SEM) channelling contrast. The efficiency of channelling contrast in imaging dislocation arrangements is shown. In the next example, referring also to AISI 304L, it is shown that electron channelling contrast can also be applied advantageously to the study of subgrain structures in the ...

Dependence of the high-temperature low-cycle fatigue behaviour of the monocrystalline nickel-base superalloys CMSX-4 and CMSX-6 on the γ/γ′-morphology

Abstract Strain-controlled high-temperature (950 and 1050°C) low-cycle fatigue (LCF) experiments were conducted on monocrystalline specimens of the γ′-hardened nickel-base superalloys CMSX-4 and CMSX-6. The goal was to investigate the effect of different γ/γ′-morphologies on the LCF behaviour. For this purpose, the LCF tests were performed on specimens with three different initial γ/γ′-morphologies, namely cuboidal γ′-particles, γ/γ′-rafts perpendicular and γ/γ′-rafts parallel to the stress axis, respectively. The raft structures had been introduced by a small high-temperature creep pre-deformation (plastic strain ≤0.4%) in tension or compression. The LCF tests showed that the fatigue lives were reduced for specimens with γ/γ′-rafts perpendicular to the stress axis and extended for specimens with γ/γ′-rafts parallel to the stress axis. Specimens with cuboidal γ/γ′-particles exhibited intermediate fatigue lives. Fractographic investigations revealed the reasons for this behaviour. The fatigue cracks usually followed along the γ-channels or the γ/γ′-interfaces and avoided cutting the γ′-phase. Thus, fatigue cracking was facilitated when the γ/γ′-rafts were perpendicular to the stress axis and obstructed, when the γ/γ′-rafts lay parallel to the stress axis.

Annealing treatments to enhance thermal and mechanical stability of ultrafine-grained metals produced by severe plastic deformation

Ultrafine-grained (UFG) metals produced by techniques of severe plastic deformation, such as equal channel angular pressing (ECAP), exhibit extraordinary strength properties. However, in the as-ECAP-processed state, the heavily deformed microstructure of such UFG metals is rather unstable and is prone to undergo grain coarsening (recrystallization) at moderate temperatures. This microstructural instability is enhanced in the presence of modest mechanical stressing as, for example, in cyclic deformation. Thus, all measures to enhance the thermal stability are also considered as beneficial for the improvement of the mechanical stability. One main objective of the present work is to analyse the thermal and mechanical stability of ECAP-processed metals during specific annealing and cyclic deformation tests. As a by-product, some conclusions relating to the separate effects of dislocation density, grain size (in the UFG regime) and internal stresses on the (micro)yielding behaviour will be drawn. Another goal is to explore the potential of different annealing treatments with respect to the stabilization of the microstructure and the optimization of the mechanical properties of ECAP-processed UFG metals in terms of an optimal combination of strength and ductility. In order to demonstrate the potential and the limitations of this approach, experimental work performed on UFG copper, aluminium and α-brass produced by ECAP will be reported and discussed. The results presented indicate strongly that a heat treatment leading to a bimodal grain size distribution provides the best compromise between strength and ductility.

Crystallographic features of intergranular crack initiation in fatigued copper polycrystals

Abstract In order to study the mechanism of grain boundary (GB) cracking in fatigued polycrystalline copper, specimens were fatigued in symmetrical push-pull at an intermediate constant plastic strain range at room temperature in dry air. The intergranular cracks were examined under the scanning electron microscope. Many GB cracks were found to have been formed by the impingement of persistent slip bands (PSBs) against the grain boundaries (PSB-GB cracks). The orientations of the grains adjacent to the cracks were determined by electron back-scattering patterns. The misorientations of the cracked boundaries were calculated and the boundary plane orientations were also determined. High-energy grain boundaries were found to be preferred sites for cracking. The activated slip systems in the component grains adjacent to the cracks were determined and analyzed. With these data, the cracking stresses due to the interaction between the PSBs and the boundaries were calculated for the observed PSB-GB cracks in a pile-up type dislocation model in a three-dimensional analysis. The results confirmed that, with reasonable assumptions, the estimated minimum theoretical shear stresses which are required to act in the PSBs for causing PSB-GB cracks were always smaller than the real shear stresses operating in the PSBs.