A Comparison of hydrogen and mercury embrittlement in aisi 4142 steel

Slow strain rate tensile tests, and some fatigue tests, were run at room temperature in the environments of air, liquid mercury and hydrogen. The hydrogen was generated electrolytically, with the charging and testing commencing simultaneously. The specimens were quenched and tempered to a range of hardness levels. The embrittlement by hydrogen and by mercury differed in several aspects. In hydrogen alone, there was a loss of tensile strength, a marked strain rate sensitivity, and a crack progression of transgranular to intergranular to microvoid coalescence. In mercury, the crack initiation was intergranular. It is believed that the hydrogen induced transgranular cracking is plasticity related with hydrogen penetration occurring at an enhanced rate. This situation would not arise with mercury.     

Near- threshold fatigue crack growth in copper and alpha- brass: Grain- size and environmental effects

The fatigue propagation rates and fatigue threshold ( ΔK _th) values were studied (R = 0.1 and frequency = 20 Hz) on copper and 70-30 α-brass of two different grain sizes in laboratory air and dry argon. With decreasing grain size, the threshold increased in copper, while it decreased in α-brass. These results suggest that in copper, crack tip plasticity considerations were more important in determining the threshold values than crack closure effects. Dry argon increased ΔK _th slightly in copper and more significantly in α-brass. A transition from completely transgranular to partially intergranular and back to completely transgranular cracking was observed with decreasing crack growth rates in both materials and environments. The growth rates for which intergranular cracking was obtained were found to be consistent with a hydrogen embrittlement mechanism, associated with adsorption of water molecules and dislocation transport of hydrogen.     

The improved microstructures and properties of 7075 alloys produced by a water-cooling centrifugal casting method

A centrifugal water-cooling casting method was used to cast a 7075 alloy with the aim of refining the grain and inclusion size and improving mechanical properties in the wrought condition. Con-ventional ingot casting methods were also used and investigated for comparison with the centrifugal casting method. The results show that by the centrifugal casting method, a small equiaxed grain size, 17 μm, is obtained in as-cast condition. Only 50 minutes are required for material homogenization. After rolling to obtain sheet, a grain size of 15 × 8 × 6 μm and an inclusion size of 2 to 3 μm are achieved. Fine-grained centrifugal-cast 7075 alloy exhibits higher strength than the ingot-cast one in the early stages of aging but poorer in the latter stages. However, its ductility and combination of strength and ductility is superior to the ingot-cast ones at all aging times. The reduction in strength in the latter aging stages for the fine-grained structure arises from its higher volume fraction of soft precipitate free zones. The improved ductility is attributed to the higher fraction of transgranular fracture, higher transgranular fracture strain, and intergranular fracture strain. Fine-grained 7075 alloy also displays significant improvements in the exfoliation corrosion resistance. These improvements are related to the increased density of attacking sites on the surface and the increased turns for crack propagation along grain boundaries. SHANG-HAW JONG, formerly Graduate Student with the Department of Materials Science and Engineering, National Tsing Hua University     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 MPa√m. Charging with hydrogen decreased the fracture toughness, K _Ic , to 52 MPa√m at a rapid loading rate and further decreased the toughness to 42 MPa√m for a slow loading rate. This is consistent with the rate-limiting step for the embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

晶粒尺寸对高锰奥氏体TWIP钢氢脆行为的影响

主要研究了晶粒尺寸对Fe- 17Mn- 1Al- 0.6C TWIP钢的氢脆行为的影响。原始材料经过不同的热处理制度,得到晶粒尺寸为17和45μm的材料。通过慢拉伸试验研究氢质量分数在0～0.001%材料的氢脆敏感性。试验结果表明,充氢后的试验材料比未充氢试验材料易发生氢脆,充氢后的试验材料断裂强度和断裂应变均降低。随着晶粒尺寸的增大,试验材料的氢脆敏感性增强。在氢质量分数为0.001%,晶粒尺寸增加到45μm时,应变损失率为17%,随着晶粒尺寸的增大,氢脆敏感性增加的原因是晶粒尺寸较大的材料孪晶较早出现,孪晶密度较大,同时单位晶界氢质量分数增加。     

Grain size and hydrogen concentration effects on the ductility return in a refractory alloy

The effects of grain size (in the range 30 to 220 μm) and hydrogen concentration on the ductility return of a hydrogen doped 25 at. pct V-75 at. pct Nb alloy was investigated at low strain rates. Since this alloy has a large terminal hydrogen solubility and since the hydrogen additions were kept low, hydride formation seems not to contribute to the alloy’s embrittlement. It was found that the ductility return is present only for small grain sizes and low hydrogen concentrations. Cracks leading to failure were initiated intergranularly. It appears to be unlikely that the ductility return, if present, is caused by the low diffusivity of hydrogen in that temperature range. A qualitative model is proposed to explain the observations. Formerly a Graduate Student at Ames Laboratory     

碳含量对Mn-B钢氢致延迟断裂行为的影响

采用电化学阴极充氢、氢热分析(TDS)和慢应变速率拉伸等试验方法,研究了4种不同碳含量Mn-B钢经不同热处理制度处理后的氢致延迟断裂行为。结果表明,在低于400℃回火时,随着碳含量的增加,试验钢的氢脆敏感性升高,当碳的质量分数高于0.3%后,试验钢的氢脆敏感性几乎不再增加;碳含量一定时,试验钢的氢脆敏感性随回火温度的升高而降低,且以20MnB试验钢的降低趋势最为明显;当回火温度达到600℃时,各试验钢对氢几乎不再敏感;TDS分析表明,试验钢充氢后的氢含量明显增加,其中以可扩散性氢量的增加为主;随碳含量的增加,试验钢充入的氢量增加;当碳含量一定时,随回火温度的升高,试验钢充入的氢量减少;SEM断口观察表明,试验钢充氢后的脆性断裂倾向性增加;随着碳含量的升高,试验钢的断裂方式由韧性断裂向脆性断裂转变;碳含量一定时,随回火温度的升高,试验钢由淬火态的脆性断裂向高温回火态的韧性断裂转变。     

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.     

Influence of Microstructure on the Spall Failure of Aluminum Materials

Laser-shock-induced spall failure is studied in thin aluminum targets at strain rates from 2 to 5 × 10⁶ s⁻¹. Targets were prepared from high-purity aluminum in the recrystallized condition and a low-impurity aluminum alloy containing 3 wt pct magnesium in both recrystallized and cold-rolled conditions. The effects of material and microstructure on spall fracture morphology are investigated. Recrystallized pure aluminum produced spall fracture surfaces characterized by transgranular ductile dimpling. Recrystallized aluminum-magnesium alloy with a 50-μm grain size produced less ductile spall surfaces, which were dominated by transgranular fracture, with some isolated transgranular ductile dimpling at fast strain rates. Transgranular ductile dimpling regions disappeared in recrystallized alloy specimens with a 23-μm grain size tested at faster rates. Cold-rolled alloy material produced spall failure surfaces consisting of brittle intergranular and transgranular fractures. Measured spall strength increases with increasing ductile fracture character. Spall failure preferentially follows grain boundaries, making grain size an important factor in spall fracture surface character. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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Influence of nitrogen alloying on hydrogen embrittlement in AISI 304-type stainless steels

Hydrogen embrittlement of AISI 304-type austenitic stainless steels has been studied with special emphasis on the effects of the nitrogen content of the steels. Hydrogen charging was found to degrade the mechanical properties of all the steels studied, as measured by a tensile test. The fracture surfaces of hydrogen charged specimens were brittle cleavage-like whereas the uncharged specimens showed ductile, dimpled fracture. In sensitized materials transgranular cleavage mode of fracture was replaced by an intergranular mode of fracture and the losses of mechanical properties were higher. Nitrogen alloying decreased the hydrogen-induced losses of mechanical properties by increasing the stability of austenite. In sensitized steels the stability of austenite and nitrogen content were found to have only a minor effect on hydrogen embrittlement, except when sensitization had causedα′-martensite transformation at the grain boundaries. Formerly with Helsinki University of Technology, Laboratory of Physical Metallurgy, SF-02150 Espoo 15, Finland.     

Slip observations in 70–30 alpha brass

Observations have been made on slip nucleation in 70–30 alpha brass for plastic strains ranging from 0.05 pct to 0.7 pct in four grain sizes: 33, 84, 126, and 163 μm. Slip was seen to start primarily at grain and twin boundaries and occasionally within grains. In grains in which slip commenced first, slip tended to traverse the entire grain and multiple slip tended to develop as well. The average length of the longest slip band, compared to the available slip path, was longer in large than in small grains. The volume fraction of grains covered by slip was greater at a given strain the larger the grain size. However, when the volume fraction of multiple slip was separated from single slip the volume fraction of multiple slip increased more slowly with increasing strain for the 33, 84, and 126 μm grain sizes than for the 163 μm grain size. Evidence was adduced to indicate that slip nucleation caused by elastic interaction is more significant than slip nucleation by slip impingement.     

The fatigue of pseudoelastic polycrystalline β-cuznsn

The fatigue of polycrystalline pseudoelastic β-CuZnSn has been studied by cycling specimens to fixed stress. The fatigue life was found to decrease with increasing initial strain and decreasing specimen grain size. In both cases the results gave similar stress -vs - fatigue life curves, indicating that stress is the primary parameter controlling fatigue life. The results fitted a curve of the form △ε.N^B _f = constant, whereβ = 0.32 for the total initial strain, andβ = 0.29 for the initial elastic strain. The fatigue life appeared to be independent of strain rate. Fatigue cracks nucleated in the first cycle at three grain intersections and grew along grain boundaries until adjacent cracks linked up. In the later stages of crack growth, some intergranular cracking occurred when there were no suitably oriented grain boundaries. Both the intergranular and transgranular regions showed somewhat ill-defined fatigue striations.     

Hydrogen pressure dependence of the fracture mode transition in nickel

A relationship between fracture mode, grain boundary composition, and hydrogen pressure has been determined for nickel straining electrode samples tested at cathodic potentials. This relationship can be expressed asCs/* αCH2/− nwhereCs/* is the critical grain boundary sulfur concentration corresponding to 50 pct transgranular and 50 pct intergranular fracture andP _{H 2} is the hydrogen pressure. The value ofn was found to be between 0.34 and 0.9. This expression was derived by relatingCs/* to the hydrogen overpotential with the Nernst equation. At a cathodic test potential of −0.3 V (SCE),Cs/* was equal to 0.20 monolayers of sulfur and at higher cathodic potentials or higher hydrogen pressures,Cs/* decreased such that at −0.72 V (SCE)Cs/* was equal to 0.045 monolayers of sulfur. The inverse hydrogen pressure dependence observed with cathodic hydrogen is similar to that for the hydrogen permeation rate or a critical hydrogen concentration derived by Gerberichet al.6 for gaseous hydrogen. This similarity between gaseous and cathodic hydrogen suggests that grain boundary impurities contribute to the hydrogen embrittlement process without altering the embrittlement process although this result does not indicate whether decohesion or plasticity dependent processes are responsible for the combined sulfur-hydrogen effect on the intergranular fracture of nickel.     

Slow strain-rate embrittlement of niobium by Oxygen

The embrittlement of niobium by oxygen has been studied over the temperature range from 77 to 1100 using tensile specimens containing between 0.09 and 2.31 at. pct oxygen. A slow strain-rate embrittlement was observed in specimens containing more than 0.5 at. pct oxygen that occurred inside the approximate temperature range from 500 to 1000 K. In analogy with the slow strain-rate embrittlement due to hydrogen, a ductile-to-brittle-to-ductile fracture transition was observed; the ductility (reduction in area) decreased with decreasing strain-rate; the ductility decreased with increasing oxygen concentration; and a fracture morphology involving both intergranular and transgranular modes was observed. The transgranular mode, which was the predominant one, resembled cleavage.     

The effect of hydrogen on the fracture toughness of alloy X-750

The effect of hydrogen on the fracture toughness behavior of a nickel-base superalloy, Alloy X-750, in the solutionized and aged condition was investigated. Notched bend specimens were tested to determine if the fracture process was stress or strain controlled. The fracture was observed to initiate at a distance between the location of maximum stress and maximum strain, suggesting that fracture required both a critical stress and strain. The effect of hydrogen was further investigated and modeled using fracture toughness testing and fractographic examination. The fracture toughness of the non-charged specimen was 147 . Charging with hydrogen decreased the fracture toughness, K _lc, to 52 at a rapid loading rate and further decreased the toughness to 42 for a slow loading rate. This is consistent with the rate-limiting step forthe embrittlement process being hydrogen diffusion. The fracture morphology for the hydrogen-charged specimens was intergranular ductile dimple, while the fracture morphology of noncharged specimens was a mixture of large transgranular dimples and fine intergranular dimples. The intergranular failure mechanism in Alloy X-750 was a microvoid initiation process at grain boundary carbides followed by void growth and coalescence. One role of hydrogen was to reduce the void initiation strain for the fine intergranular carbides. Hydrogen may have also increased the rate of void growth. The conditions ahead of a crack satisfy the critical stress criterion at a much lower applied stress intensity factor than for the critical fracture strain criterion. A model based on a critical fracture strain criterion is shown to predict the fracture behavior.     

Slow strain-rate embrittlement of niobium by Oxygen

The embrittlement of niobium by oxygen has been studied over the temperature range from 77 to 1100 using tensile specimens containing between 0.09 and 2.31 at. pct oxygen. A slow strain-rate embrittlement was observed in specimens containing more than 0.5 at. pct oxygen that occurred inside the approximate temperature range from 500 to 1000 K. In analogy with the slow strain-rate embrittlement due to hydrogen, a ductile-to-brittle-to-ductile fracture transition was observed; the ductility (reduction in area) decreased with decreasing strain-rate; the ductility decreased with increasing oxygen concentration; and a fracture morphology involving both intergranular and transgranular modes was observed. The transgranular mode, which was the predominant one, resembled cleavage.     

Environmental hydrogen embrittlement of an α-β titanium alloy: Effect of microstructure

Environmental hydrogen embrittlement of a Ti-6 Al-4 V alloy has been studied as a function of test displacement rate and of variations inα- β microstructure. Embrittlement in low pres sure (∼1 atm) gaseous hydrogen was inversely dependent on test displacement rate and strongly dependent on microstructure. At a given displacement rate, microstructures having a continuous α-phase matrix were less severely embrittled than those having a continuous β-phase matrix. Further, brittle fracture occurred in the former microstructures by transgranular cleavage and in the latter microstructures by intergranular separation. These observations are consistent with previous studies made on slow strain-rate embrittlement of hydrogen-charged titanium alloys and are explained in terms of relative hydrogen transport rates within the α-phase and β-phase titanium.     

Crack size effects on the chemical driving force for aqueous corrosion fatigue

Small crack size accelerates corrosion fatigue propagation through high strength 4130 steel in aqueous 3 pct NaCl. The size effect is attributed to crack geometry dependent mass transport and electrochemical reaction processes which govern embrittlement. For vacuum or moist air, growth rates are defined by stress intensity range independent of crack size (0.1 to 40 mm) and applied maximum stress (0.10 to 0.95 Φ_ys). In contrast small (0.1 to 2 mm) surface elliptical and edge cracks in saltwater grow up to 500 times faster than long (15 to 40 mm) cracks at constant δK. Small cracks grow along prior austenite grain boundaries, while long cracks propagate by a brittle transgranular mode associated with tempered martensite. The small crack acceleration is maximum at low δK levels and decreases with increasing crack length at constant stress, or with increasing stress at constant small crack size. Reductions in corrosion fatigue growth rate correlate with increased brittle transgranular cracking. Crack mouth opening, proportional to the crack solution volume to surface area ratio, determines the environmental enhancement of growth rate and the proportions of inter- and transgranular cracking. Small cracks grow at rapid rates because of enhanced hydrogen production, traceable to increased hydrolytic acidification and reduced oxygen inhibition within the occluded cell.     

The influence of grain size on the erosion rate of metals

The objective of this work was to understand the influence of grain size on solid impingement erosion behavior characterized by deformation at high strain rates and large strains. Experiments were carried out at a velocity of 40 m/s, impact angle of 90 deg with 300 to 450 μm steel shot as erodent on iron, copper, and titanium with varying grain sizes. The results indicate that the erosion rate is independent of grain size in iron and copper while it is apparently grain size dependent in titanium. The results are rationalized in terms of the negligible contribution of the Hall-Petch component to the flow stress at large strains in the case of copper and iron. The decreasing erosion rate in titanium with increasing grain size was due to the increased interstitial content picked up during thermal treatment and consequent increase in strain hardening and strain rate hardening and not due to increased grain sizeper se. Adiabatic shear bands were observed in coarse-grained iron under actual erosion conditions.     

The effect of grain size and strain rate on the

The substructural developments taking place in nickel 200 with grain diameters of 47, 108, 141, and 274 μm have been studied at four different strain rates of 0.01, 0.25, 2.5, and 5/min during tensile testing at room temperature. The percent strain necessary to develop well-defined cell boundaries increases with an increase in grain size at a given strain rate. The cell size refinement takes place throughout the entire range of percent strains (up to 30 pct) in tension for the nickel samples with grain diameters of 47, 108, and 141 μm at all four strain rates used in this article. However, nickel, with the largest grain diameter of 274 μm, shows refinement and then sat- uration for tensile strains greater than 25 pct. The cell size strengthening described by the mod- ified Hall-Petch (MHP) equation at the selected four strain rates of this article indicates that the flow stress is higher for smaller grain size samples at a given cell size. The effect of strain rate on the slope from the MHP plots is such that even though it does not change with an increase in strain rate up to 0.25/min for the four grain sizes, the actual value of the slope decreases with an increase in grain size at a given strain rate. Beyond this strain rate, even though an increase in the slope value as a function of strain rate has been observed for all four grain diameter samples, the influence of grain size on the slope of the MHP plots is so small that it can be assumed that they may become grain size independent at extremely high strain rates. JYOTHI G. RAO, formerly Graduate Student, JYOTHI G. RAO, formerly Graduate Student, JYOTHI G. RAO, formerly Graduate Student,