Metastable grain boundary segregation in copper containing tellurium

Evidence for metastable grain boundary segregation has been experimentally obtained in Cu alloyed with 25 to 100 ppm Te. The solution-treated material fractured in a ductile intergranular mode. After aging for a short time; i.e., one day at 540°C, the fracture is intergranular and is attributed to the observed intergranular segregation of Te. The magnitude of the tellurium segregation and its local nature are established using Auger electron spectroscopy. Further aging results in precipitation of a stable phase throughout the grains and grain boundary fracture no longer takes place.     

Study of Aging-Induced Degradation of Fracture Resistance of Alloy 617 Toward High-Temperature Applications

For the Alloy 617, the effect of aging on the fracture energy degradation has been investigated after aging for different time periods at 1023 K (750 °C). A sharp reduction in impact energy (by ~55 pct vis-à-vis the as-received material) after 1000 hours of aging, as evaluated from room-temperature Charpy impact tests, has been observed. Further aging up to 10,000 hours has led to a degradation of fracture energy up to ~78 pct. Fractographic examinations using scanning electron microscopy (SEM) have revealed a change in fracture mode from fibrous-ductile for the un-aged material to intergranular mode for the aged one. The extent of intergranular fracture increases with the increasing aging time, indicating a tendency of the material to undergo grain boundary embrittlement over long-term aging. Analysis of the transmission electron microscopy (TEM) micrographs along with selected area diffraction (SAD) patterns for the samples aged at 10,000 hours revealed finely dispersed γ′ precipitates of size 30 to 40 nm, rich in Al and Ti, along with extensive precipitation of M₂₃C₆ at the grain boundaries. In addition, the presence of Ni₃Si of size in the range of 110 to 120 nm also has been noticed. The extensive precipitation of M₂₃C₆ at the grain boundaries have been considered as a major reason for aging-induced embrittlement of this material.     

Effect of quenching temperature on microstructure and fracture toughness of high carbon steel

The effect of quenching temperatures on microstructure and fracture toughness of high carbon steel was investigated. Plane strain fracture toughness was tested with compact tension specimen. Microstructure and fracture morphology of KIC samples after quenching and tempering treatment were examined by scanning electron microscope (SEM).The results show that the residual carbides of steel in hardened state decreasea with the quenching temperature increasing and totally disappear after quenched at 920??;the grain size grows up obviously when the quenching temperature is more than 960??. The microstructure in high temperature tempered state is composed of residual carbides, precipitated carbides and ferrite matrix;plasticity decreases monotonically; the fracture toughness gradually decreases in the range from 800?? to 960??,and then almost invariant; the fracture type of KIC specimens is gradually changed from cleavage fracture to intergranular fracture. The main reason for the changes of fracture toughness has close relationship with the plasticity.     

Effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy

The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been investigated by means of tensile tests in air and with hydrogen-charging in 1N-H₂SO₄ solution. The annealed specimen has exhibited intergranular fracture by hydrogen-charging, resulting in a marked reduction in tensile elongation and ultimate tensile strength. The mode of fracture was changed by aging at 773 K, and the transgranular fracture has been found to be dominant in the aged specimens. The susceptibility to hydrogen embrittlement, as identified by the test method used in this study, seems to be reduced by short-term aging, though it turns out to be increased again by further aging. The fractured boundaries have been characterized using electron channeling pattern (ECP) analysis of adjacent grains. It is found that the misorientation of grain boundaries plays an important role in fracture, and ∑3 boundaries, twin boundaries in a face-centered cubic (fcc) lattice, are most likely to fracture in the aged specimens. Transmission electron microscopy (TEM) observation has shown that a short-range ordering reaction from a disordered fcc lattice into an ordered Ni₂(Cr, Mo) (Pt₂Mo type) super-lattice takes place by aging, and hence, superdislocation triplets with APB (antiphase boundary) become predominant when deformed. It is also seen that in the aged specimens, deformation twinning is another mode of deformation, and this leads to the transgranular fracture at twin boundaries by hydrogen-charging. These results suggest that a change in the mode of deformation after aging plays a major role in fracture due to hydrogen embrittlement as a consequence of the heterogeneous interaction between slip dislocations and twin boundaries.     

The relation between microstructure and toughness in 7075 aluminum alloy

Electron microscopy, fractography, and notched tear tests have been used to investigate the effects of heat treatment upon the fracture behavior of aged 7075 aluminum alloy sheet. Toughness, as measured by crack propagation energy, decreases as the yield stress increases; the toughness of an overaged structure is inferior to that of an underaged structure at the same yield stress. The decrease of toughness with increased aging time is accompanied by a change in fracture mode from predominantly transgranular to intergranular. Transgranular fracture proceeds by dimple rupture and is facilitated by chromium-rich particles which are dispersed throughout the microstructure. Intergranular fracture proceeds by the fracture of grain boundary precipitate particles. The variation of fracture mode with aging time is attributed to a steady decrease of the intergranular fracture stress relative to the transgranular fracture stress, due to increasing grain boundary particle size. A possible explanation of this effect is discussed using the stress concentration due to colinear crack arrays as an analogy. The effects of quenching variations and two-step aging are discussed. It is shown that, in aged 7075, microstructural variables such as the width of precipitate-free zones and the nature of the matrix precipitate do not have a controlling effect on toughness.     

HR3C不锈钢时效过程中铌的析出规律及其对性能的影响

通过金相、SEM、EDS、TEM、电化学分析及硬度测量的方法,研究了固溶处理HR3C不锈钢在800℃加速时效不同时间后,含铌析出相的种类、数量和形状大小及其对性能的影响。结果表明,HR3C中主要的析出相为MX相和Z相(NbCrN)。随着时效时间的延长,析出相尺寸逐渐增大,弥散强化效果先快速增加,后逐渐减弱。Z相的析出能提高HR3C钢的耐晶间腐蚀性能。     

The structure of tempered martensite and its susceptibility to hydrogen stress cracking

A series of 4130 steels modified with 0.50 pct Mo and 0.75 pct Mo were tempered at temperatures between 300 and 700 °C for one hour. The changes in the carbide dispersion and matrix substructure produced by tempering were measured by transmission electron microscopy. These measurements were correlated with resistance to hydrogen stress cracking produced by cathodic charging of specimens in three-point bending. Scanning electron microscopy showed that specimens tempered between 300 and 500 °C failed by intergranular cracking while those tempered at higher temperatures failed by a transgranular fracture mode. Auger electron spectroscopy showed that the intergranular fracture was associated with hydrogen interaction with P segregation and carbide formation at prior austenite grain boundaries. Transgranular cracking was initiated at inclusion particles from which cracks propagated to produce flat fracture zones extending over several prior austenite grains. The 4130 steels modified with higher Mo content resisted tempering and showed better hydrogen stress cracking resistance than did the unmodified 4130 steel. The transition in fracture mode is attributed to a decohesion mechanism in the low temperature tempered samples and a pressure mechanism in the highly tempered samples.     

]Microstructure and hydrogen effects on fracture in the alloy A-286

The commercial iron-base superalloy A-286 was solution-treated and aged at 993 K for times ranging from 0. 5 to 450 hours, including the peak-strength age for 16 hours. Microstructural examination showed that the equilibriumη phase formed first at grain boundaries after about 6 hours of aging and increased in amount with additional aging. Tensile tests on specimens which were uncharged or hydrogen charged from the gas phase revealed in all cases that ductile fracture took place by microvoid coalescence. The path of ductile fracture changed with aging, however, becoming increasingly intergranular with increased aging time after 50 hours. It was found that hydrogen decreased ductility, particularly for aging times greater than 4 hours, although the fracture with hydrogen remained ductile in character. The amount of loss in ductility was approximately constant beyond 4 hours of aging and was independent of fracture path. Fracture was evidently nucleated by carbide inclusion particles by they’ strengthening particles (particularly from 4 to 16 hours of aging) and, at aging times of 50 hours or more, by grain boundaryη phase. These observations were consistent with fractographic observations, including dimple size measurements, and with previous work.     

Nonequilibrium grain-boundary segregation and ductile-brittle-ductile transition in Fe-Mn-Ni-Ti age-hardening alloy

Nonequilibrium segregation kinetics of alloying elements and a ductile-brittle-ductile transition behavior have been investigated in an Fe-8.4Mn-7.4Ni-l.7Ti alloy. The alloy experienced a ductilebrittle-ductile (DBD) transition during isothermal aging. In the brittle region, the alloy showed a decrease in intergranular fracture strength and a subsequent increase with aging time. This is due to the segregation of titanium to the grain boundaries and its desegregation into the matrix. The intergranular fracture strength was higher on the zero tensile elongation finish curve than on the start curve. This is because the grain-boundary segregation level of titanium is relatively lower on the finish curve. The lowest intergranular fracture strength increased with increasing aging temperature, which was attributed to a lower grain-boundary segregation level of titanium at higher temperature. Manganese caused an overall reduction in intergranular fracture strength and, as a result, the delayed occurrence of the zero tensile elongation (ZTE) finish curve in a temperature and log-time plot. formerly Graduate Student, Department of Metallurgical Engineering, Seoul National University, Seoul, 151-742 Korea     

Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

Influence of Aging and Thermomechanical Treatments on the Mechanical Properties of a Nanocluster-Strengthened Ferritic Steel

This study investigated the effect of aging and thermomechanical treatments on the mechanical properties of a nanocluster-strengthened ferritic steel, Fe-1.5Mn-2.5Cu-4.0Ni-1.0Al (wt pct). The effect of thermomechanical treatments on the microhardness and tensile properties were measured at room temperature and correlated with microstructural features. Cu-rich precipitates were characterized by transmission electron microscopy and were found to coarsen slowly during long-time aging. The microhardness measurements indicate a typical precipitation hardening behavior during aging at 773 K (500 °C). Tensile tests showed that thermomechanical treatments can improve the mechanical strength and ductility of the nanocluster-strengthened ferritic steel significantly compared with those without the treatments. Fractography results indicated that the high yield strength resulted from precipitation hardening makes the steel more susceptible to grain-boundary decohesion, which can be suppressed by grain refinement. Atmosphere adsorption and diffusion along grain boundaries were found to intensify brittle intergranular fracture, and this embrittlement can be avoided by vacuum heat treatment.     

Fatigue of Cu-1 Pct Cd

The fatigue properties of high conductivity Cu-1 pct Cd are reported. In contrast to Cu, the fatigue strength of Cu-Cd is grain-size dependent in high cycle fatigue. Strain aging is responsible for the superior fatigue properties of Cu-Cd, and the effect of dislocation pinning is carried over to the cold work state, giving a work hardened structure which is stable in fatigue loading. A notable feature of the results is that fatigue fracture in Cu-Cd is almost entirely intergranular at long fatigue lives, in contrast to the fracture in tensile loading and in contrast to the transgranular fracture which was observed in Cu. The effect is due to segregation of Cd at the grain boundaries and to the effect of Cd on the cohesive strength of the boundary. Moderate cold work suppresses intergranular fracture. Formerly Department of Metallurgy and Engineering Materials, The University of Newcastle upon Tyne     

Gallium-induced magnesium enrichment on grain boundary and the gallium effect on degradation of tensile properties of aluminum alloys

By applying a controlled amount of gallium (3 mg or 5 mg) to double-notched samples, the effects of the gallium on the grain boundary chemistry and tensile properties of AA6061-T4 alloy were investigated. Commercial-purity aluminum AA1050 was used for comparison to determine whether alloying elements would correlate with Ga-induced embrittlement and to elucidate the physical reason that governed the occurrence of intergranular fracture in the AA6061 Al-Mg-Si alloy. The AA6061 and AA1050 samples wetted by 3 mg or 5 mg of Ga were held statically for 7 days before tensile tests were conducted. The 6061 Al-Mg-Si samples with gallium were fractured intergranularly. However, the Ga-treated AA1050 samples had a mixed fracture mode, showing better strength and ductility than the Ga-treated AA6061 alloy, independent of whether the samples had their longitudinal axis parallel or perpendicular to the rolling direction, or the holding temperatures before tensile tests. Auger electron spectroscopy scanning the intergranular facets on fracture surfaces showed that the Auger peak-to-peak ratio I_Ga/I_Al of 6061 samples was similar to that of 1050 samples, but the high intensity of Mg signal was detected from the intergranular fracture surface of the AA6061 alloy. Magnesium being induced by Ga to enrich on the grain boundary and free surface of the AA6061 alloy was confirmed. The intergranular embrittlement of the 6061 T4 Al-Mg-Si alloy wetted by small amount of Ga involves the combination of the following two effects: Ga metal on grain boundary embrittlement, and Ga-induced magnesium enrichment on grain boundary that further decreases the strength of the grain boundary.     

Deformation and fracture behavior of two Al-Mg-Si alloys

Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.     

高能球磨制备WC/Al2O3-Cr-Mo-Ni金属陶瓷及其组织和力学性能

以亚微米级WC粉、Al2O3粉、Cr粉、Mo粉与Ni粉为原料,采用高能球磨+热压工艺制备WC/Al2O3-Cr-Mo-Ni金属陶瓷材料,利用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)分析材料的物相组成和显微组织结构以及断裂方式,研究粘结相Ni和陶瓷相Al2O3的含量(均为质量分数)对该材料力学性能和微观结构的影响。研究表明:金属陶瓷的抗弯强度及断裂韧性随Ni含量增加而提高,随Al2O3含量增加而降低,硬度的变化趋势则相反。当Ni含量为7%、Al2O3含量为10%时,该金属陶瓷具有良好的综合性能,抗弯强度为567 MPa,断裂韧性为7.46(MPa.m1/2),维氏硬度为15.24 GPa,基本达到现用模具材料的水平。随着Ni含量增加,金属陶瓷的断裂方式由沿晶断裂向沿晶断裂与穿晶断裂相混合的方式转变。     

Structure-property relations in a metastable β Ti alloy containing Si

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)_xSi_y transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)_xSi_y precipitates, b) the equilibrium silicide, and c) the α-phase.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

Effect of Carbon on Grain Boundary Segregation of Phosphorus and Phosphorus-Induced Intergranular Fracture in High Purity Iron with Phosphorus

Ａｓｗｅｌｌｋｎｏｗｎ ,ｔｈｅａｄｄｉｔｉｏｎｏｆｃａｒｂｏｎｃａｎｐｒｅ ｖｅｎｔｔｈｅｐｈｏｓｐｈｏｒｕｓ ｉｎｄｕｃｅｄｉｎｔｅｒｇｒａｎｕｌａｒｆａｉｌｕｒｅｉｎｉｒｏｎ .ＢｕｔｉｔｉｓｔｈｅｒｅｓｕｌｔｏｆｕｓｉｎｇＡＥＳ (Ａｕｇｅｒｅｌｅｃ ｔｒｏｎｓｐｅｃｔｒｏｓｃｏｐｙ)ａｎａｌｙｓｉｓｉｎｆｒａｃｔｕｒｅｄｓｕｒｆａｃｅａｎａｌ ｙｓｉｓ .ＥｒｈａｒｔＨｅｔａｌ[1] ,ｉｎｏｒｄｅｒｔｏｅｘｐｌａｉｎｔｈｅｍｅｃｈ ａｎｉｓｍｏｆｃａｒｂｏｎｓｕｐｐｒｅｓｓｉｎｇｔｈｅｉｎｔｅｒｇｒａ…     

聚合物B位前驱体法制备锆钛酸铅纳米复相陶瓷微观结构

采用前驱体法合成了钙钛矿型B位离子氧化物固溶体,以此作为B位先驱体与碳酸铅通过固相反应在740℃合成A位缺铅的亚稳态钙钛矿型锆钛酸铅(PZT)固溶体.烧结过程中纳米级四方和单斜ZrO₂纳米粒子从固溶体中析出.借助X射线衍射仪、扫描电镜和透射电镜对物相、组成和微观结构进行了分析.随ZrO₂的加入量增加,断口从沿晶穿晶混合断裂变为穿晶断裂.研究表明,采用聚合物B位前驱体法成功制备出内晶型锆钛酸铅纳米复相陶瓷.     

Role Of M23C6 Carbide Precipitation and Phosphorus Grain Boundary Segregation in Intergranular Fracture and Corrosion of 18Cr-12Ni Austenitic Stainless Steel

Abstract

The effects of intergranular precipitation (size of precipitates) and phosphorus grain boundary segregation on fracture and corrosion behaviours of the 18Cr-12Ni steel were investigated. In the investigation, techniques of light microscopy, SEM, TEM, AES, EDX, electron diffraction, and the Huey test were used. A good correlation was found between the average size of intergranular particles and the portion of intergranular fracture for samples impact loaded at 173 K. The equilibrium values of phosphorus grain boundary concentration showed the close correlation with the values of weight loss (penetration depth) measured during the Huey test.