Dense Dislocation Tangles Research Articles

Contribution of dislocation substructures developed during cold rolling to the formation of rolling textures in Al–Mg alloys

Abstract In order to investigate the mechanism of the development of rolling textures in Al–Mg alloys, hot bands of high-purity Al, Al-3 wt. % Mg and Al-5 wt. % Mg alloys were solution-treated at 450 °C and cold-rolled by varying rolling reductions up to 97 %. Rolling textures of these specimens were investigated with the orientation distribution function analysis. Comparisons of the results of the texture analysis with those of the hardness measurement and transmission electron microscopy observations revealed that, in all specimens, the texture development occurred mainly at the later half of work hardening, i. e., at rolling reductions above 80 %, where dislocation substructures were well developed. In pure Al, in which well defined, elongated cell structures were developed, {112} and {123} components developed remarkably, whereas in the Al-5 % Mg alloys, in which only dense dislocation tangles and ill defined small cells were developed, the development of {112} and {123} components...

An investigation of surface nanocrystallization mechanism in Fe induced by surface mechanical attrition treatment

By means of surface mechanical attrition (SMA), a nanostructured surface layer was formed on a pure Fe plate. Microstructure features of various sections in the surface layer, from the strain-free matrix to the treated top surface, were systematically characterized by using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the SMA treatment in Fe was proposed. It involves formation of dense dislocation walls (DDWs) and dislocation tangles (DTs) in original grains and in the refined cells (under further straining) as well, transformation of DDWs and DTs into subboundaries with small misorientations separating individual cells or subgrains, and evolution of subboundaries to highly misoriented grain boundaries. Experimental evidences and analysis of the grain refinement mechanism indicate that high strains with a high strain rate are necessary for formation of nanocrystallites during plastic deformation of metals.

CRACK GROWTH MORPHOLOGY AND MICROSTRUCTURAL CHANGES IN 316 STAINLESS STEEL UNDER CREEP‐FATIGUE CYCLING

Abstract— A study into microstructural effects and crack growth behaviour of AISI type 316 stainless steel under creep‐fatigue conditions at 550°C within the high strain ranges of 0.9–2.5%, including a 60min hold time, was undertaken on a high‐temperature reverse‐bending rig. Throughout the tests, surface cracks on both the tensile‐hold and the compressive‐hold sides were monitored by means of a plastic‐strip replication technique. Additional investigations were conducted on failed specimens to examine the crack morphology in the depth direction, and to examine the function of oxidation; also to study changes of fracture surface morphology, changes in dislocation structures and precipitate configurations corresponding to the different strain ranges. These detailed analyses revealed that the predominantly intergranular long cracks on the tensile‐hold side and transgranular short cracks on the compressive‐hold side are dominant aspects of the investigation. The dislocation structures under creep‐fatigue conditions are strain‐range dependent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduction in creep‐fatigue endurance can be attributed to early crack growth and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many minor surface short cracks.

The cyclic deformation and fatigue behaviour of the low carbon steel SAE 1045 in the temperature regime of dynamic strain ageing

The cyclic deformation behaviour of normalized SAE 1045 steel (german steel grade Ck 45) has been investigated over a range of temperatures between 20 and 375°C. Special attention has been paid to the effects of dynamic strain ageing, which are most pronounced around 300°C. Different types of deformation tests (tension tests, incremental step tests, and constant amplitude cyclic deformation tests under stress control with a stress amplitude of 400 MPa as well as under plastic strain control with a plastic strain amplitude of 0.5%) were carried out to observe the influence of temperature on the macroscopic mechanical behaviour. These tests were followed by TEM studies on microstructural features. In the temperature range of maximum dynamic strain ageing, the material was found to show maximum strength in unidirectional as well as in cyclic deformation tests. While the fatigue life is maximum at the temperature of maximum dynamic strain ageing in stress-controlled tests, it is minimum in plastic strain controlled tests. At the temperature of maximum dynamic strain ageing around 300°C, the dislocations are arranged in dense dislocation tangles and parallel dislocation walls, whereas at room and at higher temperatures (375°C) mainly dislocation cell structures are observed.

Trial production and strength assessment of titanium alloy connecting rods: Toyama, K. and Kuwayama, T. Sumitomo Met. Apr. 1989 41, (2), 75–80 (in Japanese)

An expanding ring technique has been employed to study the influence of microstructure on the uniform strain and the initiation of fracture in precipitation-type alloys subjected to dynamic loading. Significant increases in the average uniform strain were observed in solution-treated and aged copper cobalt alloys subjected to strain rates in the range (103−104) s−1. The deformation structures observed after dynamic loading were characterized by incompletely developed dislocation cell structures in solution-treated alloys and the loss of precipitate cohenrency in peak-aged alloys. Dynamically loaded overaged alloys wxhibited microvoids and dense dislocation tangles connecting dispersed incoherent precipitates; dynamic fracture occurred by void interconnection along grain boundaries. The increase in ductility under the dynamic loading conditions is attributed to the limited time available for void growth and interconnection, plus the reduced level of strain localization owing to incompletely formed low energy cell configurations.

Substructure evolution and deformation modes in shock-loaded niobium

An electron microscopy study has been conducted of the substructure evolution of polycrystalline niobium shock loaded to 6 and 37 GPa. It was found that dense dislocation tangles and loops dominate the substructure of samples shocked at 6 GPa, whereas slip bands and microbands on {110} and twins on {112} in addition to tangles and loops are observed in samples shocked at 37 GPa. The twins observed possess the characteristic lenticular shape similar to those seen in shock-loaded molybdenum and are consistent with the shape predicted by existing twin models. The twin elements and characters do not appear to be different from those induced by conventional deformation at low strain rates. Slip bands on {110} are considered to result from an independent deformation mode which is not a consequence of the termination of twin growth inside the crystal. Microbands, which are the most predominant feature, were developed within 10° of {110} planes with a slight misorientation (1° or less) across the bands. Macroscopic shear bands were not found in either of the shock-loaded samples. Comparison between the microstructures in shock-loaded and slowly deformed samples shows no drastic difference. It is argued that the prevalent deformation modes in shock-loaded niobium are not extraordinarily distinct from conventional deformation.

The cyclic stress-strain behavior of nickel-base superalloys—I. Polycrystals

The cyclic stress-strain behavior of a wrought, nickel-base superalloy has been investigated at ambient temperature and 843°C with a constant cyclic ramp rate of 10 −2/s. At ambient temperature, the derived cyclic stress-strain curve exhibits three stage behavior, as has been reported under similar conditions for f.c.c. metals. At elevated temperature, the curve is reduced to a power law function with no indication of three stage behavior. The dislocation substructure produced in the intermediate strain range region consists, at both temperatures, of strong persistent slip bands shearing the strengthening γ′ precipitates in the grain interiors and dense dislocation tangles necessitated by near grain boundary strain accommodation. The shapes of the cyclic stress-strain curves are shown to be consistent with the observed dislocation substructures. The cyclic hardening response and details of hysteresis loop shapes were found to be substantially unchanged with temperature.

Elevated Temperature Slow Plastic Deformation of NiAl/TiB2 Particulate Composites

AbstractTo enhance the high temperature strength of aluminides, NiAl–iB2 composites with particulate contents up to 30 vol. pct. were made by XDTM synthesis and hot pressed to full density. Microstructures of these composites were characterized by optical, scanning and transmission electron microscopy (TEM). The average size of the TiB2 particles was about 1 μm, and the average grain size of the NiAl matrix was on the order of 10 μm. Elevated temperature compression testing was conducted on these composites in air at 1200 and 1300 K with strain rates varying from −10−4 to −10−7 s−1. Flow strengths were found to increase with increasing TiB2 content; for example, the 20 vol. pct. TiB2 composite was three times stronger than unreinforced NiAl. Post test TEM analysis showed that the primary feature of the dislocation substructure of deformed NiAl was well defined subgrain boundaries, whereas the structure of the higher volume fraction composites consisted of a very high density of tangled dislocations, loops and subgrain boundaries connecting particles. These observations suggest that TiB2 particles can stabilize a completely different dislocation structure than that normally found in NiAl.

Microstructural damage and ductility of CuCo alloys under dynamic loading conditions

An expanding ring technique has been employed to study the influence of microstructure on the uniform strain and the initiation of fracture in precipitation-type alloys subjected to dynamic loading. Significant increases in the average uniform strain were observed in solution-treated and aged copper cobalt alloys subjected to strain rates in the range (10 3−10 4) s −1. The deformation structures observed after dynamic loading were characterized by incompletely developed dislocation cell structures in solution-treated alloys and the loss of precipitate cohenrency in peak-aged alloys. Dynamically loaded overaged alloys wxhibited microvoids and dense dislocation tangles connecting dispersed incoherent precipitates; dynamic fracture occurred by void interconnection along grain boundaries. The increase in ductility under the dynamic loading conditions is attributed to the limited time available for void growth and interconnection, plus the reduced level of strain localization owing to incompletely formed low energy cell configurations.

Microstructural effects of the cyclic and monotonic hardening of Al5Mg

The cyclic and monotonic hardening mechanisms in Al5Mg (where the composition is in approximate weight per cent) have been characterized by transmission electron microscopy and by differential scanning calorimetry (DSC). After constant-plastic-strain-amplitude tests to cyclic saturation, a high density of fine dislocation loops is generated. The corresponding DSC plots reveal a characteristic exotherm at 150°C (ΔH x in the range from −0.2 to −0.5 J g −1) which is shown to result from the annihilation of the fine dislocation loops. This behavior is not found after monotonic deformation by cold rolling or drawing to equivalent total plastic strains, for which dense dislocation tangles rather than loops develop. The 150°C exotherm occurs at saturation after fatigue of Al5Mg in both the annealed and the cold-worked states and also of Al5Mg0.5Ag aged to form homogeneous T precipitates. At lower temperatures in the range 50–110°C, an endotherm (probably due to Guinier-Preston (GP) zone dissolution) is also detected ( ΔH x ≈ 0.5 J g −1). The variation in the endothermic peak temperature is consistent with a GP zone size which depends on the aging or fatigue treatment.

Se + ion implantation into encapsulated GaAs

PECVD Si3N4 was deposited on undoped SI (100) GaAs. Se+ ions were implanted with a dose of 1×1014 ions cm−2 at an ion energy of 400 keV through the Si3N4 layer. Annealing was carried out on a dual graphite strip heater at temperatures up to 900°C for 100 s. Transmission electron microscope studies show a large concentration of damage in the form of dense dislocation tangles and long-range strain centres at the Si3N4/GaAs interface. Below this sheet of gross damage the material contains a relatively low density of dislocation loops. Electrical measurements indicate that, for a similar implant condition directly into GaAs, a relatively high electrical activity is measured compared with that of the implants through the Si3N4 encapsulating layer.

A dislocation link length model for strain hardening in stage II of polycrystalline metals of high stacking fault energy

A theoretical strain hardening model is developed. The model takes the distribution of dislocation link lengths into account. During plastic deformation this distribution is assumed to change by three different subprocesses. (1) Under the action of a continuously increasing applied stress, dislocation links are released and start gliding. (2) The released links expand into loops by glide and thereby produce strain. New dislocation links are formed when the loops are eventually arrested in the network. (3) Stress-induced annihilation of screw dislocations occurs in the network. To verify the model experiments on polycrystalline nickel were performed. It is shown that the model is able to simulate the experimentally determined stress-strain curve, the increase in the dislocation density and the behaviour of the distribution of dislocation link lengths. The following important results were obtained. (1) The value predicted by the model for the diameter of an expanded dislocation loop is in agreement with the experimentally measured distance between dense dislocation tangles. (2) The model predicts that dynamic recovery processes are very active.

The deformation behaviour of AlAl 6Fe eutectic alloys

The deformation by rolling of fibre reinforced AlAl 6Fe eutectic material has been studied. During rolling the Al 6Fe fibres fracture into progressively smaller pieces and dense dislocation tangles extend from one fibre to another. These concentrated tangles ultimately form dislocation domains which polygonize after further deformation to form low angle sub-grain boundaries. The polygonization is promoted by stored energy levels which are considerably higher than those found in particle-free aluminium.

Elastic moduli and internal friction of cold worked copper single crystals

The change of the elastic moduli and the internal friction of copper single crystals of various orientations during deformation at 78 K was measured. First the elastic moduli change due to the fact that the crystal orientation changes upon plastic extension. An analogous effect is the change of the moduli due to the change of orientation of part of the crystal by mechanical twinning. Finally the dislocations produced by the deformation give rise to a decrease of the moduli and an increase of the internal friction. The contribution of these mechanisms to the observed effects is discussed. The dislocation contribution is analyzed in terms of the Koehler-Granato-Lücke model yielding the density and line lengths of the contributing dislocations. The result is compared with literature data from transmission electron microscopy, leading to the conclusion that the dislocation part of the modulus effects and internal friction is due to the small fraction of total dislocation density lying between the dense dislocation tangles and having line lengths of the order of 1 μm.