Evaluation of Changes in Plastic Yield Parameters of Titanium CP-2 using Electrical Resistivity Measurements

The paper focuses on the connection between the changes in electrical resistivity and plastic yield parameters - hardening coefficient and yield stress - of Titanium CP-2 specimens subjected to quasi-static loading-unloading cycles. Such a cross-property connection possible due is made to all the mentioned properties being controlled by the same microstructural parameter – dislocation density that increases during each loading-unloading cycle. Electrical resistivity has been measured at the end of each cycle and plastic yield parameters have been obtained from the stress-strain curves. The microstructural changes were also evaluated using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Experimental measurements are in a good agreement with the theoretical estimates.     

Evaluation of Changes in Dislocation Density in TI-CP2 in the Process of Quasi-Static Loading Using Electrical Resistance Measurements

The paper focuses on the development of new methodology to evaluate increase of dislocation density in metals using electrical resistance measurements. Titanium specimens have been quasi-statically loaded with Instron testing machine till fracture and electrical resistance of the specimens have been monitored during the tests. Dislocation density was recovered from electrical resistance measurements and from SEM images before and after loading. Thus obtained results are shown to be in good agreement. The methodology developed in the present paper may be used for accurate monitoring of dislocation density in-situ.     

Cross-Property Connection between Work-Hardening Coefficient and Electrical Resistivity of Stainless Steel During Plastic Deformation

Present paper focuses on the cross-property connection between the changes in electrical resistivity and work hardening coefficient in the process of plastic deformation. The possibility of the cross-property connection is provided by the fact that both quantities are governed by the same parameter - growth of the dislocation density caused by the applied stresses. Experimental measurements on stainless steel 304 are in a good agreement with analytical estimates.     

INFLUENCE OF SHOT-PEENING ON SHORT CRACK BEHAVIOUR IN A MEDIUM CARBON STEEL

The results of research into short fatigue crack behaviour in shot peened, medium carbon steel specimens under reversed torsion are presented. Characteristic features of short crack growth were established on the basis of optical and electron-optical observations of the replicas and micro-sections of the samples. Surface crack growth analysis, supported by plots of crack development, crack growth rate and crack length distributions against cycle ratio show that significantly slower crack growth and lower crack densities occur in shot peened specimens than in non-treated samples. That results can be linked to the effect of grain distortion, the packing of laminar grains, a high dislocation density in the plastically deformed surface layer and the introduction of compressive residual stresses. A fractographic analysis of the fracture surfaces has assisted an understanding of the mechanisms of fracture in shot peened specimens.     

电阻法检测疲劳损伤及预测修复效果探讨

40Cr调质试样在低周恒应变、高周恒应力拉拉疲劳试验过程中，选择若干个特定循环周次，采用间隙回火或中温热等静压，并用QJ55型电流比较仪式电桥法，分别进行了电阻测量。发现电阻变化与疲劳损伤过程有很好的对应关系，对疲劳损伤修复延寿途径选择及其效果预测也有较好的参考价值。     

Characterization of microstructural changes in an Al-6.8 wt.% Mg alloy by electrical resistivity measurements

Microstructural changes in an AlMg6.8 alloy after different thermo mechanical (TMT) and sensitization treatments were investigated by electrical resistivity measurements. The electrical resistivity was most affected by the content of Mg solute atoms in the α-Al matrix, due to β-phase precipitation, while contribution of the dislocation density to the resistivity of the AlMg6.8 alloy was less pronounced. The amount and distribution of the β-phase precipitated during sensitization were found to be strongly affected by the microstructure developed under the previously applied TMTs, i.e. by the dislocation density and the primary β-phase particles in the dual (α + β) phase structure. During sensitization of the specimens with a recovered/recrystallized dual (α + β) phase structure, precipitation of randomly distributed, globular β-phase particles occurred. Sensitization of cold deformed and recrystallized single α-Al structures induced β-phase precipitation in the form of a continuous layer along the shear bands/grain boundaries.     

The effect of constrained groove pressing on grain size,dislocation density and electrical resistivity of low carbon steel

In this research, constrained groove pressing (CGP) technique is used for imposing severe plastic deformation (SPD) on the low carbon steel sheets. Using transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical microscopy, the microstructural characteristics of produced sheets are investigated. The results show that CGP process can effectively refine the coarse-grained structure to an ultrafine grain range. Dislocation densities of the ultrafine grained low carbon steel sheets are quantitatively calculated and it is found that the CGP can effectively enhance the dislocation density of the sheets. Measurements of their electrical resistivity values show that microstructure refinement and increasing the dislocation density can efficiently increase the electrical resistivity of the CGPed sheets up to ∼100%.     

Pre-strain effect on fatigue strength characteristics of SUH660 plain specimens

Precipitation particles in precipitation-strengthened materials are considered to be cut by pre-strain treatment, which affects the fatigue strength. In this study, fatigue tests were performed on precipitation-strengthened stainless steel SUH660 to investigate the effect of pre-strain on fatigue crack initiation and propagation characteristics. Fatigue test results showed that pre-strained specimens have a shorter fatigue crack propagation life compared to non-strained specimens; this is the opposite of results observed in carbon steel. The accelerated fatigue crack growth observed in pre-strained specimens was first assumed to be caused by precipitate cutting. A dislocation accumulation model for the fatigue crack tip in the precipitation-strengthened material was then suggested. Buff-polished specimens were also used for the fatigue tests, and the results showed that the work-hardened layer had a significant impact on the fatigue strength and fatigue life.     

Improvement of the electrical conductivity of carbon fibers through the growth of carbon nanofibers

In this work, carbon nanofibers (CNFs) were synthesized on carbon fiber (CF) surfaces precoated with metal-doped mesoporous silica films. Fe, Ni, and Co were doped in the mesoporous silica films and played the role of catalysts for the decomposition of acetylene to grow CNFs on the CF surfaces. The chemical composition and surface structure of CFs before and after the growth of the CNFs were investigated by EDX, N2 full isotherms, and SEM. The electrical property of the CFs was investigated using a four-probe volume resistivity tester. The SEM results indicated that the CNFs with diameters of 20-100 nm grew uniformly and densely on the CF surfaces. The diameter and length distributions of the CNFs were found to be dependent on the metal that was doped in the mesoporous silica films. The electrical properties of the CFs were enhanced after the CNFs' growth on the CF surfaces, and the CNFs grown over the Ni catalyst with the narrowest diameter distribution gave the lowest volume resistivity to the CFs.     

Kinetics of precipitation in 17–4 PH stainless steel

Abstract

The sequence of precipitation and its kinetics in 17–4 PH (precipitation hardening) stainless steel were studied by observing the electrical resistivity and microstructure of the alloy during isothermal aging at various temperatures in the range 320–600°C. By the absence of an incubation period for the onset of precipitation it is shown that there is no free energy barrier to nucleation. The electrical resistivity of the specimen decreased on prolonged aging approaching a steady value asymptotically with time. The alloys aged above 550°C were found to have higher final resistivity values than those aged at lower temperatures. By transmission electron microscopy, local reversion of the martensite to austenite, attributed to enhanced diffusion and concentration of copper atoms at the lath boundaries, was revealed in the specimens aged at temperatures above 550°C. The kinetics of precipitation in the system obeyed the Johnson–Mehl equation. The activation energy Q of the precipitation process was estimated to be 112·2 ± 3·6 kJ mol^?l from the resistivity measurements. This may be understood in terms of an enhanced diffusion of copper atoms in the supersaturated matrix caused by a higher dislocation density and a higher concentration of quenched-in vacancies.

MST/826     

Wechselverformung und Gefügeänderungen von Ck 15 und Ck 45

Microstructural Changes and Cyclic Deformation The crack initiation starts due to weakening and strengthening process during rotating bending. However a smaller plastic deformation amplitude is noticed at the same nominal stress compared to tension-compression stressed specimens. This results in a higher fatigue life. The different cyclic deformation behaviour was proofed by SEM (rotating bending specimens showed a lower slip line density compared to tension-compression specimens at the same nominal stress) and TEM investigations (the rotating bending specimens showed a smaller dislocation density at the same nominal stress). Furthermore it is showed, a correlation of cyclic stress strain data σ(ε_pls) between tension-compression and rotating bending specimens exists. This is also valid for the Manson-Coffin-relationship. the relation between lg ε_pls and lg N_B depends on the material (Ck 15, Ck 45) but not on the state of stress.     

Influence of microstructures on fatigue damage mechanisms in Ti-15-3 alloy

In this study, the fatigue behavior of Ti-15-3 alloy thin plate specimens with two different microstructures was determined. Two kinds of specimens were prepared with different heat treatments: solution treatment (S) and solution treatment followed by aging (S+A). The effects of the microstructures on the fatigue properties and fatigue crack growth behavior were significant in both specimens. The fatigue crack in both specimens propagated in transgranular mode. In the specimen S+A, crack propagation has occurred on non-crystallographic and was closely connected with the configuration of the α-phase platelet, which was caused by the heat treatment. The damage was characterized by dislocation debris clustering ahead of the crack tip.     

17-4PH末级长叶片叶根的疲劳强度与缺口效应

采用成组法和升降法开展了室温某17-4PH末级长叶片叶根纵、横方向光滑与缺口试样的疲劳试验,获得了两个方向上材料的S-N曲线,并对典型疲劳断裂试样进行了断口宏微观形貌观察。结果表明:叶片叶根纵、横方向的疲劳强度基本一致,在长寿命区,纵向疲劳强度略高于横向;光滑试样的疲劳断裂主要呈现单裂纹源特征,而缺口试样呈现多裂纹源汇聚的断裂形貌。结合试样缺口根部的弹塑性应变分析,进一步讨论了叶片叶根纵、横方向的疲劳缺口效应。     

PREDICTION OF FATIGUE LIFE BY X-RAY DIFFRACTION METHODS

Abstract— X-ray double crystal diffractometry and reflection topography were employed to examine the defect structure induced by fatigue of A1 2024 specimens. Analysis after various amounts of tension compression cycling revealed that the excess dislocation density in the surface layer increased rapidly early in the fatigue life, and maintained virtually a plateau value from 20 to 90% of the life. Beyond 90%, the excess dislocation density increased rapidly again to a critical value at failure. Investigation of the defect distribution in depth showed that the excess dislocation density in the bulk material, by contrast to the surface layer, increased more gradually during the life. Using deeply penetrating molybdenum radiation, the grains in the bulk could be analyzed, and thus the fatigue life and onset of failure could be predicted nondestructively. It was also shown that, after removal of the surface layer, the defect structure induced in the bulk by prior cycling was unstable. The resultant extension of fatigue life by surface layer removal was explained on the basis of this structural instability of the bulk material.     

Saturation Dislocation Microstructures of Double-slip-oriented Copper Single Crystal during the Corrosion Fatigue in a 0.5 mol/L NaCl Aqueous Solution

Copper single crystal specimens with the longitudinal axis parallel to the [013] double-slip-orientation were grown through Bridgman technique. The fatigue tests were performed using a symmetric tension-compression load mode at room temperature in an open-air and a 0.5 mol/L NaCl solution, respectively. The dislocation microstructures were observed with scanning electron microscopy (SEM) by the electron channeling contrast (ECC) and transmission electron microscopy (TEM). The results show that the saturation dislocation microstructures during the corrosion fatigue in the aqueous solution of 0.5 mol/L NaCI, mainly consisted of labyrinth, wall and vein dislocation structures, which differs from the dislocation structures of the walls and veins in an open-air environment.     

Influence of hot corrosion on low cycle fatigue behavior of nickel base superalloy SU 263

The influence of hot corrosion on low cycle fatigue behavior is studied by conducting fatigue tests at 800 °C in air on bare and salt-coated (90%Na₂SO₄ + 10%NaCl) specimens. This was followed by extensive scanning electron microscopic (SEM) examinations. Significant reduction in fatigue life is observed across all values of Δε_t/2 for the salt-coated specimens in comparison with bare specimens. SEM examination reveals that the fused salt mixture sporadically removes the protective chromium oxide layer and exposes the substrate. Subsequent SEM analysis reveals that severe grain boundary oxidation leads to grain boundary cracking and provides numerous sites for fatigue crack nucleation and growth.     

THE ELECTRON CHANNELLING CONTRAST TECHNIQUE APPLIED TO THE CHARACTERISATION OF DISLOCATION STRUCTURES IN THE VICINITY OF A FATIGUE CRACK

Abstract— In this investigation the Electron Channelling Contrast (ECC) technique in scanning electron microscopy (SEM) was applied to reveal the dislocation structures in the vicinity of surface fatigue cracks in comparison to those of cyclically-deformed recrystallized polycrystalline copper. The plastic zone around a fatigue crack was found to consist of an innermost region containing cells, followed by a region containing dense veins and PSBs, surrounded by a structure of loose veins, bundles and loop patches typical of the cyclically deformed matrix. A relation between plastic strain amplitude values deduced from cyclic stress-strain investigations and the dislocation structures near fatigue cracks are given. Typical regions of damage accumulation were identified and plastic strain contours for surface fatigue cracks established. The essentially non-destructive ECC technique is particularly suited to identify the changes in mesoscopic dislocation structures from surface layers to the interior of specimens over large specimen areas.     

Intermittent plastic flow of single crystals: central problems in plasticity: a review

Abstract

The relationships between microstructure and plastic flow properties of copper single crystals deformed at low temperatures are reviewed based on the analysis of experimental results of mechanical, structural and electrical resistivity characterisation. Electrical resistivity measurements suggest that the flow stress correlates with the average density of defects accumulated in the microstructure. The characteristic microstructural length scale that determines the flow stress corresponds closely to the size of dislocation free channels and/or cells produced during plastic flow. The correspondence between micro- and macroscopic approaches to the plastic flow has been analysed based on the behaviour of dislocation mean free path and mean slip distance and their relationship to the microstructure. Production and annihilation of nanoscale debris and point defects and their role in plastic flow and questions pertinent to the influence of microstructural instabilities on the flow stress, workhardening, storage and annihilation of lattice defects have been discussed.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

多轴低周载荷下钛合金BT9疲劳损伤的微观机理

应变控制比例及非比例载荷低周疲劳试验结果表明,非比例载荷下钛合金BT9附加强化程度很小,而疲劳寿命降低明显.采用透射电镜(TEM)对钛合金BT9的疲劳位错亚结构进行了观测和分析,结果表明,钛合金BT9比例、非比例载荷下出现的多滑移位错亚结构都呈条块状;位错密度随等效应变强度、相位角的增加而增加,且分布极不均匀;非比例载荷下钛合金BT9中的局部高密度位错是其低周疲劳损伤程度加剧及寿命降低的主要因素.