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.     

Strengthening of copper by dislocation substructures

Substructural strengthening in pure copper deformed by cold rolling is shown to depend on both the mean free path and the level of nonuniform strain associated with the dislocation substructure. Contributions from the nonuniform strain are particularly important because the mean free path becomes constant after deformations of ∼50 pct R.A., whereas the material continues to work harden. The behavior of the nonuniform strains at large deformations is also found to be closely related to dynamic recovery. Development of an empirical expression for the strengthening in terms of the substructural parameters is shown to lead to a modified Hall-Petch type relation. W. G. TRUCKNER formely Graduate Student, Michigan Technological University is now at Alcoa Laboratories, Alcoa Center, PA 15069.     

Cyclic work hardening and formation of dislocation substructures during low-cycle fatigue of tempered dual-phase steels

A C-Mn-Si dual-phase steel, containing 33.8% lath martensite of 0.45% C content (called steel 1), was tempered at 200, 460 and 650°C and designated steels J, K and L, respectively. Sheet specimens of 3 mm thickness and 10 mm gauge length pertaining to steels I, J, K and L were subjected to tensile and low cycle fatigue testing at room temperature. The steels I, K and L show cyclic softening while the steel J exhibits a small cyclic hardening. The cyclic stress response varies significantly at higher and lower applied strain amplitudes for each of the steels which has been discussed in terms of competing processes of hardening and softening during strain cycling. The value of the cyclic strain hardening exponent decreases continuously in the order steels I>J>K>L. Varied dislocation substructures form at higher and lower strain amplitudes during cyclic deformation.     

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.     

Low cycle fatigue,fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.     

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables. AXENA, formerly Graduate Student, Dept. of Materials Science and Metallurgical Engineering, University of Cincinnati is This paper is based on a thesis submitted by Ashok Saxena in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Cincinnati.     

The dislocation and martensite substructures of a fatigued,polycrystalline, pseudoelastic Cu-AI-Ni alloy

Polycrystalline Cu-AI-Ni specimens, subjected to pulsating compression fatigue, while capable of pseudo-elastic deformation, nevertheless exhibit cyclic hardening and fatigue fracture, as reported inMet. Trans. A, 1977, vol. 8A, p. 955. In order to interpret this behavior, transmission electron microscopy has been used to study the microstructure. Increasing amounts of martensite and deformation substructure result from decreasing the test temperature and raising the stress level. Martensite of different morphologies has been observed and identified. Large plates ofβí, common to all samples in varying amounts, were determined to be of the 18R structure, with lattice parameters ofa = 4.382Å,b = 5.356Å, andc = 38.0Å. Samples deformed at low temperature or high stresscontain not only β₁, but two distinct forms of γ both with lattice parameters ofa = 4.41Å,b = 5.31Å, andc = 4.222Å, and of the 2H crystal structure. The largeβí and γ plates seem to be characterized by interfacial dislocations between the matrix and plates. In all samples, antiphase boundaries (APB’s) can be imaged, even in heavily dislocated areas, indicating that the deformation has not destroyed the matrix order. It is concluded that hardening results from interactions between matrix dislocations and the stress-induced martensite, as well as by collisions between groups of martensite plates having different habits.     

Effect of fatigue damage on the dynamic fracture toughness of En-8-grade steel

Machine components normally experience fatigue cycling during operation. Failure of these components is mostly due to fatigue. So, it is important to know the fatigue damage behavior and fatigue life of the material before selecting these steels for making different machine components. The En-8-grade (equivalent to SAE/AISI 1040) steel is generally used as a machine component in the annealed or hardened-and-tempered condition. The fatigue life (fatigue/endurance limit) is also dependent upon the tensile properties of any material. By suitable heat treatment, one can manipulate the tensile properties of any steel. The present work reports the effect of fatigue damage in En-8-grade heattreated steel (annealed and hardened and tempered), under different cyclic loading conditions at room temperature (25 °C), on the impact and dynamic fracture-toughness properties. The results indicate higher fracture toughness and impact toughness in hardened-and-tempered steel than in annealed steel. Cyclic hardening and softening occurs in both the hardened-and-tempered as well as the annealed steel. With the increase of peak stress and number of fatigue cycles, the K _ID and CVN values decrease in hardened-and-tempered steels. The results are discussed in terms of dislocations, slip bands, and their density, microstructure, and fracture morphology.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600 °C because of dynamic recovery and interlamellar separation. The plastic strain energy-stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy-stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Hydride precipitation and dislocation substructures in Ti-5 pct Al-2.5 pct Sn

Hydrides do not precipitate in Ti-5 pct Al-2.5 pct Sn stressed at room temperature at levels less than 80 pct of the yield stress. Above this level basal and γ-hydrides occur, the relative amounts of which are probably related to the relative amounts of basal and prism slip intersecting the stressed specimen surface.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600°C because of dynamic recovery and interlamellar separation. The plastic strain energy—stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy—stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Influence of differential dislocation mobility on the fatigue behavior of alloyed

Fatigue tests, using both stress and strain control, have been performed on alloyed Ni₃Al (γ′-phase). The fatigue life in stress controlled tests is independent of temperature below 800°C, which correlates with the temperature independent microyield strength of the γ′ phase. On the other hand, the fatigue life in strain controlled tests decreases with increasing temperature, correlating with a marked increase in the 0.2 pet offset yield strength of γ′ with temperature. This difference in behavior is attributed to a temperature dependent differential dislocation mobility. Thus, a temperature independent mobility for edge dislocations is considered to be responsible for the fatigue behavior under stress cycling, whereas it is proposed that a strongly temperature dependent mobility for screw dislocations (due to pinning by cross slip from octahedral planes into cube planes) controls the fatigue behavior under strain cycling. A comparison is made of the fatigue properties of alloyed γ′ and a typical γ′ precipitation hardened alloy, showing that the relative fatigue strength (endurance limit/yield strength) of the γ + γ′ superalloy is superior to that of single phase alloyed γ′ at elevated temperatures.     

A microstructural study of dislocation substructures formed in metal foil substrates during ultrasonic wire bonding

A study has been conducted on the deformation mechanisms in metal substrates subject to aluminum ultrasonic wire bonding (UWB). Aluminum wires were bonded to copper, nickel, stainless steel, and aluminum bronze foil substrates and then removed in aqueous sodium hydroxide to permit thin sections of bonded areas to be examined in the transmission electron microscope (TEM). The results showed a variety of dislocation substructures formed during bonding, including dislocation cells, subgrains, and planar arrays. Aluminum and copper showed evidence of thermal effects on microstructural evolution during bonding, such as dislocation annihilation at cell walls in copper and complete recrystallization in aluminum. In the nickel and stainless steel substrates, which have higher recrystallization temperatures, thermal effects on microstructure were not observed. In addition, it was found that low stacking-fault energy (SFE) materials, such as aluminum bronze, were less likely to undergo cell formation, and only planar dislocation arrays formed. In general, it is clear that the process of UWB induces cyclic stresses in the substrates, which exceed the yield strength of the metals examined. In addition, there is some heat generated during the bonding process, which can influence the resultant deformation microstructure.     

Trapezoid dislocation with a Galeazzi fracture

Significant airway stenosis occurs in 7% to 14% of lung transplant recipients. The use of permanent, nonadjustable, wire mesh stents can be of concern in the transplant recipient with nonmalignant stricture. We report the replacement and repositioning of an expandable wire mesh stent in a double lung transplantation with distal bronchial stenosis.     

Vertical fracture dislocation of the tarsal navicular

An isolated vertical fracture dislocation of the left tarsal navicular with some comminution was successfully treated with open reduction and internal fixation with a screw. The surgical treatment is described, and early mobilization strongly recommended.     

疲劳失效机理及热轧H型钢疲劳断口分析

介绍了疲劳断口类型、形貌及形成机理,并对热轧H型钢疲劳断口进行了分析,为今后型钢产品疲劳研究奠定基础。     

The combined effect of grain size and strain rate on the dislocation substructures and mechanical properties in pure aluminum

The effect of grain size on the development of dislocation substructures has been studied as a function of strain rate. Pure aluminum rods with grain diameters of 70, 278, and 400 μm were deformed in tension at room temperature to various percent strains at strain rates of 0.01, 0.25, 2.5, and 5/min. It has been confirmed that the smaller grain size results in higher flow stress in this strain-rate range. The cell size strengthening described by the modified Hall-Petch (MHP) equation is applicable to samples with 70 and 278 μm grain sizes at all four strain rates used in this study, while 400 μm grain sizes show deviation from this because of inhomogeneities developed in the microstructure. The influence of strain rate on the slope of the MHP plots, for a grain size of 70 μm, is such that at lower strain rates, the slope does not change much, but at higher strain rates, there is an increase in the slope value. At all strain rates, the values of slopes from the MHP plots of the smaller grains are higher than for the larger grains.