The role of dislocation substructures in fatigue crack propagation in copper and alpha brass

The effect of dislocation substructures on fatigue crack propagation (FCP) behavior in copper and alpha brass was studied. Various dislocation substructures were obtained by prestraining in tension. Dislocation cells were formed by this prestraining in copper and 90/10 brass and when they formed the resistance to FCP at intermediate propagation rates (5×10⁻⁹ to ∼10⁻⁷ m/cycle) increased with increasing prestrain. Planar dislocation arrays were observed in 70/30 brass instead of cells, and the effect of prestraining on the FCP resistance was insignificant. From the FCP data for each material it was observed that, regardless of the difference in the dislocation substructures and grain sizes, the two constantsC andm in the Paris equation,da/dN=C(ΔK) ^m, were interrelated. Possible relations between the cyclic strain hardening exponent andm are discussed. The influence of both prestrain and grain size on threshold behavior was also studied.     

The production and utility of recovered dislocation substructures

The production of dislocation substructures by cold working and recovery, fatigue, creep and hot working are reviewed. The relationships of subgrain size and dislocation density to the causal parameters of strain, strain rate, strain amplitude, temperature, stress and time (as applicable) are presented for each process. The importance of dislocation mechanisms such as climb, cross-glide, annihilation and subboundary formation are explained. The relative capabilities and limitations of each mode of creation with respect to both external processing and internal mechanisms are explored. The effects of the metal's stacking fault energy, of solid solution and of particle dispersion on structure and behavior are presented. The properties of the different kinds of substructures for room temperature and creep service are examined. The need for modification of the Petch relationship between yield strength and subgrain size is explored. The thermal stability is shown to be an important factor for creep service. It is concluded that the most suitable modes of substructure preparation are either cold working and recovery or hot working both from the view point of fitting into current industrial practice and from that of dependable, useful service properties. This paper is based on a presentation made at a symposium on “Mechanical-Thermal Processing and Dislocation Substructure Strengthening”, held at the Annual Meeting in Las Vegas, Nevada, on February 23, 1976, under the sponsorship of the TMS/IMD Heat Treating Committee.     

Developing Dislocation Subgrain Structures and Cyclic Softening During High-Temperature Creep–Fatigue of a Nickel Alloy

The complex cyclic deformation response of Alloy 617 under creep–fatigue conditions is of practical interest both in terms of the observed detriment in failure life and the considerable cyclic softening that occurs. At the low strain ranges investigated, the inelastic strain is the sole predictor of the failure life without taking into consideration a potentially significant environmental influence. The tensile-hold creep–fatigue peak stress response can be directly correlated to the evolving dislocation substructure, which consists of a relatively homogenous distribution of subgrains. Progressive high-temperature cycling with a static hold allows for the rearrangement of loose tangles of dislocations into well-ordered hexagonal dislocation networks. The cyclic softening during tensile-hold creep–fatigue deformation is attributable to two factors: the rearrangement of dislocation substructures into lower-energy configurations, which includes a decreasing dislocation density in subgrain interiors through integration into the subgrain boundaries, and the formation of surface grain boundary cracks and cavity formation or separation at interior grain boundaries, which occurs perpendicular to the stress axis. Effects attributable to the tensile character of the hold cycle are further analyzed through variations in the creep–fatigue waveform and illuminate the effects of the hold-time character on the overall creep–fatigue behavior and evolution of the dislocation substructure.     

Creep behavior of Al-Al3Ni eutectic composites

Specimens of Al-Al₃Ni, directionally solidified at 11 cm per hr, were creep-tested at a single set of conditions to plastic strains in the first, second, and third stages, and to rupture. Specimens from these interrupted tests were sectioned and examined by transmission electron microscopy. Due to the small plastic strains encountered in the first and second stages of deformation(εt < 0.3 pct) relatively little substructure buildup was observed in the matrix of these specimens. In specimens tested to rupture, dense dislocation cells keyed on the whiskers were found. As a result, a time-dependent fracture mechanism was identified which differed from normal tensile fracture, and was related to the substructure buildup. The effects of solidification rate on creep resistance, macrohardness, and compressive yield strength were also investigated and showed that by increasing the solidification rate a more creep-resistant, harder, and stronger material was produced. This has suggested that additional strengthening of this material by a dispersion effect of the whiskers on the matrix is possible. Formerly with United Aircraft Research Laboratories     

Low-Cycle fatigue of dispersion-strengthened copper

The cyclic deformation behavior of a dispersion-strengthened copper alloy, GlidCop Al-15, has been studied at plastic strain amplitudes in the range 0.1 pct ≤Δε _p/2 ≤ 0.8 pct. Compared to pure polycrystalline copper, the dispersion-strengthened material exhibits a relatively stable cyclic response as a consequence of the dislocation substructures inherited from prior processing and stabilized by the A1₂O₃ particles. These dislocation structures remain largely unaltered during the course of deformation; hence, they do not reveal any of the features classically associated with copper tested in fatigue. At low amplitudes, the fatigue lifetimes of the dispersion-strengthened copper and the base alloy are similar; however, the former is more susceptible to cracking at stress concentrations because of its substantially greater strength. This similarity in fatigue lifetimes is a consequence of the dispersal of both deformation and damage accumulation by the fine grain size and dislocation/particle interactions in the GlidCop alloy. The operation of these mechanisms is reflected in the fine surface slip markings and rough fracture surface features for this material. Formerly Graduate Research Assistant, University of California, Davis, CA     

The Study of Fatigue Behaviors and Dislocation Structures in Interstitial-Free Steel

There are three types of cyclic hardening for cyclically deformed interstitial-free (IF) steels. The magnitude of cyclic hardening was unobvious and dislocation cells smaller than 2 μm were very hard to find when total strain amplitude (Δε/2) was controlled to within 0.1 pct. When Δε/2 is increased to 0.125 to 0.3 pct, secondary cyclic hardening takes place prior to fatigue failure. Δε/2 = 0.6 pct, following an initial rapid-hardening stage. Dislocation cells smaller than 2 μm tend to develop near grain boundaries and triple junction of the grains while cycling just above Δε/2 = 0.125 pct. Such dislocation development results in secondary hardening. However, no failure occurs if cycling just below Δε/2 = 0.1 pct; hence, the fatigue limit for IF steel should be very close to Δε/2 = 0.1 pct.     

Low cycle fatigue of as-HIP and HIP+forged rené 95

The continuous cycling and hold time low cycle fatigue properties of the Ni base superalloy René 95 were studied at 649°C using powder products (−60 mesh) in the as-HIP and HIP + forged conditions. It was shown that cracks were initiated by pores, by ceramic particles and by a classical stage I mechanism for both materials and for both cycle characters. For the continuously cycled as-HIP material, deformation was restricted to well defined bands at low strains and became homogeneous as the strain level increased. The total energy to fracture increased abruptly in the low strain regime and this was also reflected by a break in the Coffin-Manson plot. In all cases cracks initiated at pores. The hold time specimens exhibited an extremely high dislocation density and surface connected initiation yet without a significant life reduction. The observations were essentially similar for the HIP + forged material except that deformation tended to be confined to well defined slip bands even at high strains and to some extent even for the hold time tests. This behavior was attributed to the fact that theγ′ were smaller, more coherent and more readily sheared by dislocations which were strongly paired. There was a marked tendency for crack propagation to change from transgranular to intergranular (also observed for the as-HIP material) at a unique combination of crack length and plastic strain. The transition occurred at shorter crack lengths for the HIP + forged material except when crack initiation was subsurface. In this case the transition was delayed and the life was greatly enhanced, indicating that the environment plays a major role in determining the fatigue life. PHILIPPE TAUPIN formerly Research Associate, Department of Materials Science, University of Cincinnati.     

Ductile fracture initiation in pure α-Fe: Part I. Macroscopic observations of the deformation history and failure of crystals

Single crystalα-Fe whiskers, grown by the reduction of ferrous chloride by hydrogen have been strained to fracture in an Instron tensile testing machine and in a bench straining device at various elongation rates at room temperature. Whiskers were found to exhibit macroscopic slip behavior strongly dependent upon elongation rate while the geometric reduction in area and the fracture mode remained in all cases identical. Ductile rupture of iron whiskers produces a characteristically shaped chisel-edge fracture whose geometry is sensitive to crystal orientation, due to the geometry of active slip systems, but which isnot a function of strain rate. The micromechanisms of ductile rupture of these single crystals are strongly affected by dislocation dynamics. The development of dislocations necessary to accomodate an extensive reduction in area appears to be independent of the nature of surface slip observed. Dislocation structures form small volume elements which are separated from one another by dislocation cell walls. The accommodation of large strains as well as the reduction in area is determined by the movement of dislocations on the order of a distance equal to that of the dislocation cell size. The boundaries of the cell and/or the cell volume could then be expected to be specifically related to the site where the initiation of fracture occurs.     

Fatigue deformation-induced response in a superduplex stainless steel

The cyclic deformation behavior of SAF 2507 superduplex stainless steel (SDSS) was studied under constant plastic-strain amplitudes. The cyclic hardening/softening curves show initial hardening, followed by softening and, finally, saturation behavior. Two regimes can be differentiated in the cyclic stress-strain curve (CSSC) of SDSS. The transition point at which the cyclic strain-hardening rate changes is identified to be ɛ _p/2=7 × 10⁻³. Transmission electron microscopy (TEM) results on dislocation structures suggested that there is a close relationship between the CSSC, hardening/softening curves, and the dislocation substructure evolution. In the low-plastic-strain-amplitude regime of the CSSC, the dislocation activity in the austenite grains is found to be higher than that in the ferrite grains. At higher plastic strain amplitudes, low-energy dislocation structures are found in the ferrite grains, while clusters and bundles of dislocations can be observed in the austenite grains. Strain localization due to formation of these structures resulted in a decrease in the cyclic strain-hardening rate within the high-plastic-strain-amplitude regime. Dislocation substructure evolution is also used to explain the shape of the hardening/softening curve.     

The formation of cell structures in fatigued iron crystals

Single crystals of α-Fe have been fatigued in constant stress amplitude reversed plane bending. TheS-N curve, the plastic strain amplitude variation, and the dislocation substructures developed in the crystals during fatigue have been studied. The formation of a cell structure was found to occur once a critical dislocation density had accumulated, the nature of which was influenced by the crystallography of the specimen and underwent changes with continued testing. The evolution of a continuous cell structure with testing coincided with a large decline in the level of plastic strain amplitude, and it appears that it is these substructures which account for fatigue hardening. From the observations of crack formation and the mode of dislocation movement during saturation, a mechanism for crack initiation is suggested.     

The effect of alloy chemistry and heat treatment on the low cycle fatigue behavior of 7000 series aluminum alloys

The purpose of the present investigation is to determine the relative importance of minor variations in alloy chemistry and thermomechanical treatment on the low cycle fatigue behavior of 7000 series aluminum alloys. Two types of alloying variations are considered: changing the alloy purity level by controlling the iron and silicon content, and changing the grain refiner from chromium to zirconium. The effects of these alloying variations, with regard to mechanical properties other than low cycle fatigue, have been discussed elsewhere.^1-4The purpose of thermomechanical processing is to provide increased strength over 7075-T7351 with equivalent fracture toughness and corrosion properties.^5-7 The effect of the dislocation substructure introduced by thermomechanical processing (TMP) on the high cycle fatigue behavior of 7075 was documented by Reimann and Brisbane.⁸ The present work was undertaken to determine the relative importance of purity level, dispersoid type, and dislocation substructure (TMP) on the low cycle fatigue behavior of 7000 series aluminum alloys. formerly with the Air Force Materials Laboratory, Wright-Patterson AFB, OH     

Misoriented dislocation substructures and the fracture of polycrystalline Cu–Al alloys

The evolution of the dislocation substructure in polycrystalline Cu–Al alloys with various grain sizes is studied during deformation to failure. A relation between the fracture of the alloys and the forming misorientation dislocation substructures is revealed. Microcracks in the alloy are found to form along grain boundaries and the boundaries of misoriented dislocation cells and microtwins.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Dislocation substructure evolution in torsion of pure copper

The evolution of dislocation substructures in pure copper during torsion deformation at strains ranging from 0 to 440 pct has been investigated using transmission electron microscopy. The study reveals that checkerboard patterns have formed and shrunk in size at strains ranging from 10 to 60 pct. This was followed by the development of laminar dislocation structures consisting of paired sheets which evolved from short double walls delineating the checkerboard patterns. Linear strain hardening was found to be maintained in the paired sheets at strains from 120 to 330 pct. Dislocation wall annihilation and microbands located along the wall of paired sheets were observed in stage IV of the work-hardening curve. At higher strains, another set of wall formation intersects with the paired sheets. The strain hardening of copper under torsional loading from the checkerboard pattern to the laminar structure is described by the mesh length theory of dislocation structures.     

Characterization of Fatigue Substructure of Incoloy Alloy 800 at Elevated Temperature

Transmission electron microscopy has been employed to study the fatigue substructure of Incoloy 800 at elevated temperatures. Substructure parameters such as cell or subgrain intercept size, dislocation density, and misorientation angle between adjacent cells or subgrains were evaluated and correlated with the observed fatigue properties. Observations, results and analysis of substructure data are presented and also discussed in terms of existing theories. Incoloy 800 push-pull fatigue specimens were examined after being tested at various test conditions at 538, 649, 704 and 760 °C. Subboundary intercept size and dislocation density were found to be related to both the saturation stress and the plastic strain amplitude, in accord with the “modified” theoretical predictions and the experimental findings. A critical dislocation density for cell formation was experimentally shown to exist and was found to be strongly temperature dependent. Subboundaries were also found to lie on the low index planes such as {100}, {110}, and {111}.     

Low cycle fatigue of René 80 as affected by prior exposure

Low cycle fatigue of the Ni base alloy René 80 was studied at 871°C. Prior to testing, specimens were exposed for 100 h at 982°C either stress-free or at 97 MPa (1/3 the yield). The LCF studies were supplemented by tensile and creep tests. The prior exposure caused significant microstructural changes and life reductions which were most pronounced for stress exposed specimens tested at high rates. Dislocation substructures were extensively studied. The deformation mode was constant for all smooth bar fatigue tests and the dislocation arrangements resembled those seen in creep far more closely than those characteristic of tensile deformation. The observed behavior was attributed to time-dependent changes and environmental interactions. The presence of classical creep/fatigue interactions was considered to be of only minor importance in these experiments.     

The Bauschinger effect and cyclic hardening in copper

The Bauschinger Effect and cyclic hardening were studied during the first few cycles in copper single crystals of an “easy glide” orientation. Dislocation etch pitting was used to augment the mechanical measurements. It was found that significant rearrangements of the dislocation distribution occurred during stress reversal. Sharply defined kink bands which formed during forward straining broadened, and the overall dislocation distribution became more uniform during subsequent cyclic deformation. For cycling at constant strain amplitude, it was found that hardening above the prestress level occurred almost as rapidly as during unidirectional deformation. However, the absence of hardening during increments of Bauschinger strain yielded anomalously low hardening in terms of the cumulative strain. A cyclic hardening model is proposed in which the Bauschinger strains below the prestress level result from relaxation of internal stresses whereas “hardening strains” occur only above the prestress level. The non-symmetry, which causes higher Compressive stresses than tensile stresses during cycling at constant strain amplitude, is shown to result from larger Bauschinger strains below the prestress levelduring tensile half-cycles. Cell sizes and bundle spacings reported in the literature for cyclic tests have been correlated with the corresponding flow stresses and are consistent with predictions of the Long Range Stress Theory. Formerly Graduate Student, Carnegie-Mellon University, Pittsburgh, Pa. Formerly Associate Professor at Carnegie-Mellon University, This paper is based on a thesis submitted by R. C. DANIEL in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Carnegie-Mellon University.     

Substructure strengthening mechanisms

The mechanisms which give rise to substructural strength originate in the dislocation character of the substructure boundary, in the size of sources for continued slip which the boundaries permit, and in the size of the cells or subgrains. The interrelation of these factors must be considered in obtaining a general view of the strengthening. Anthony W. Thompson, formerly with the Science Center, Rockwell International, Thousand Oaks, CA This paper is based on a presentation made at a symposium on “Mechanical-Thermal Processing and Dislocation Substructure Strengthening,” held at the Annual Meeting in Las Vegas, Nevada, on February 23, 1976, under the sponsorship of the TMS/IMD Heat Treating Committee.     

Dislocation structures in single-crystal tungsten and tungsten alloys

Deformation of tungsten single crystals as a function of strain, temperature, and alloying was studied by transmission electron microscopy. Single crystals oriented for (?101)[lll] slip were grown by electron beam zone refining. Compression specimens of tungsten, W-l and 3 pct Re and W-l and 3 pct Ta were deformed to 2 pct strain at 150°, 300°, and 590°K (0.04, 0.08, and 0.16T _m). Specimens were also strained to 0.5 and 5.0 pct strain at 300°K. Transmission microscopy revealed that the dislocation substructures in single-crystal tungsten are similar to substructures in other refractory metals when compared on a homologous temperature basis. At temperatures greater than 0.1T _m, the substructure is characterized primarily by edge dipoles. At temperatures less than 0.1T _m, long screw dislocations lying parallel to the primary [111] slip direction characterize the substructure. Rhenium additions to tungsten promote formation of edge dipoles at temperatures of 300° and 150°K and increase dislocation density at all three temperatures. In addition, dislocations consistent with (1?12)[?111] slip were observed in the W-Re single crystals after deformation at 150°K. Tantalum additions had a lesser effect on the dislocation substructure compared to rhenium additions. The W-l and 3 pct Ta alloys exhibited higher dislocation densities than unalloyed tungsten after similar strains and, at 150°K, W-3 pct Ta contained a few dislocations consistent with (1?12)[?111] slip. It is concluded that the reduction in ductile-brittle transition temperature of poly crystalline tungsten containing dilute rhenium additions, 1 to 5 pct, can be attributed to an increase in dislocation mobility at temperatures less than 0.1 T_m.     

Cyclic Deformation Response of β-Annealed Ti-5Al-5V-5Mo-3Cr Alloy Under Compressive Loading Conditions

This article reports the cyclic deformation behavior of the β-annealed metastable Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy under the condition of pure compressive fatigue stress. The following three aspects, namely, the mechanical response, the surface morphology evolution, and the dislocation structures, were systematically investigated. Under all testing conditions, the material demonstrated cyclic softening in the initial cycles followed by saturation. The progressive observation of surface morphology at fixed locations, but after different numbers of cycles, elucidated typical planar slip behavior and the early appearance of fatigue microcracks, which were found often to be induced by the highly localized planar slip bands. The transmission electron microscopy (TEM) study revealed dislocation annihilation upon cycling, i.e., the reduction of dislocation density as well as the simplification of dislocation configurations. In addition, detwinning and changed twin boundary structures upon cycling were also detected. Such activities, together with the intersection of coherent ω precipitates by moving dislocations, are considered to be responsible for the initial softening, whereas the dislocation dipole flip-flop mechanism is presumably responsible for the cyclic saturation behavior. An attempt was made to explain the strain-localized planar slip behavior by considering the stacking fault energy (SFE) as well as the free-electron-to-atom (e/a) ratio. The nanoscaled ω and α precipitation in the β matrix may also contribute to the planar slip behavior. The effect of the microstructure in the as-received material was also analyzed for the strain localization and planar-slip mode.