The effects of interstitials and hydrogen-interstitial interactions on low temperature hardening and embrittlement in V,Nb, and Ta

The effects of combined C and H on the temperature dependence of the yield stress and ductility of V, Nb, and Ta have been investigated at temperatures between 295 and 78 K. Because of the limited solubility of C in these metals, there was little solid solution hardening (S.S.H.). The combined effect of C and H on ductility was similar to that of H alone. There was no correlation between the ductile-brittle transition temperature (DBTT) and the temperature where hydrides formed (T _s. Comparison of the effects of C, N, and O on strengthening in both nonhydrogenated and hydrogenated V, Nb, and Ta showed that in general the contributions of C, N, or O and H were additive. C, N, and O produced athermal S.S.H. whereas the H contribution to the strengthening was thermally activated. The effect of these interstitials on ductility in nonhydrogenated V, Nb, and Ta was minimal, while their effect in hydrogenated V, Nb, and Ta was to decreaseT _s but have very little effect on the DBTT, which was determined primarily by the H content. There was no common correlation between the DBTT andT _s or between the temperature where pronounced strengthening occurred andT _s in the different alloys. Comparison of the results indicated that current models based on either hydride precipitates or H in solution as the cause of strengthening or embrittlement are incapable of explaining the observed effects of H on both the yield stress and the ductility in V, Nb, and Ta. T. E. SCOTT, formerly with Ames Laboratory     

High temperature embrittlement of NI and Ni-Cr alloys by trace elements

The effects of Sb, Sn, and Zr additions on the creep properties of Ni and Ni + 20 pct Cr are reported. Antimony and tin additions (~1 wt pct) induce extensive grain boundary cavitation in nickel, while smaller antimony additions had little effect on Ni + 20 pct Cr. Addition of 0.11 pct Zr to Ni + 20 pct Cr greatly inhibited grain boundary cavitation and reduced its Coble creep rate. Auger electron spectroscopy of cavitated specimens provided direct evidence of impurity segregation to cavity surfaces. Residual sulfur segregated most strongly, and was observed on cavity surfaces in all cavitated specimens. Tin segregated somewhat less intensely than sulfur, and antimony segregated only slightly. Segregation of antimony and sulfur to uncavitated portions of Ni + 1 pct Sb grain boundaries was also observed. These results are discussed in terms of segregation effects on energetic and transport properties of the grain boundaries and cavity surfaces. This paper is based on a presentation made at the symposium “The Role of Trace Elements and Interfaces in Creep Failure” held at the annual meeting of The Metallurgical Society of AIME, Dallas, Texas, February 14-18, 1982, under the sponsorship of The Mechanical Metallurgy Committee of TMS-AIME.     

Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys

By using isothermally forged TiAl-based intermetallic alloys, various microstructures (of γ-grain, duplex, dual-phase, and fully lamellar microstructures) were prepared. These TiAl-based intermetallic alloys were tensile tested in vacuum and air as functions of strain rate and temperature to investigate microstructural effects on the moisture-induced embrittlement. All the intermetallic alloys with different microstructures showed different levels of reduced tensile stress (or elongation) in air at room temperature. The reduction in tensile stress (or elongation) due to testing in air diminishes as the testing temperature (or strain-rate) increases. From the fracture stress-temperature curves, it was found that the γ-grain microstructure was the most resistant to the moisture-induced embrittlement, and the dual-phase microstructure was the most susceptible to the moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloys with a fully lamellar microstructure depends on the lamellar spacing and is reduced with decreasing lamellar spacing. The possible reasons for the observed microstructural effect on the moisture-induced embrittlement were discussed, in association with hydrogen behavior and properties in the constituent phases and at some interfaces.     

The influence of microstructure on the susceptibility of titanium alloys to internal hydrogen embrittlement

Beta-processed near-alpha titanium alloys with a large colony microstructure were found to be susceptible to internal hydrogen embrittlement under conditions of sustained loading or fatigue cycling with a dwell period at peak load. The embrittlement occurs by localized increases in hydrogen content at the tips of long, blocked shear bands developed during time-dependent plastic deformation. The key microstructural features responsible for the embrittlement process have been determined to be a large transformed beta colony size and a fine, discontinuous distribution of beta phase within the colony. Alpha-beta alloys that contain thick, continuous beta platelets were determined to be immune to embrittlement. The results are consistent with a previously-proposed model for the embrittlement process.     

Hydrogen embrittlement of Ni-Cr-Fe alloys

The purpose of this work was to investigate the role of chromium on hydrogen embrittlement of Ni-Cr-Fe alloys and thus to develop a better understanding of the low-temperature stress corrosion cracking (SCC) phenomenon. The effect of chromium on hydrogen embrittlement was examined using tensile tests followed by material evaluation via scanning electron microscopy (SEM) and light optical microscopy. Four alloys were prepared with chromium contents ranging from 6 to 35 wt pct. In the uncharged condition, ductility, as measured by the percent elongation or reduction in area, increased as the alloy chromium content increased. Hydrogen appeared to have only minor effects on the mechanical properties of the low-chromium alloys. The addition of hydrogen had a marked effect on the ductility of the higher-chromium alloys. In the 26 pct chromium alloy, the elongation to failure was reduced from 53 to 14 pct, with a change in fracture mode from mixed ductile dimple and ductile intergranular failure to a brittle appearing intergranular failure. A maximum in embrittlement was observed in the 26 pct Cr alloy. The maximum in embrittlement coincided with the minimum in stacking-fault energy. It is proposed that the increased hydrogen embrittlement in the high-chromium alloys is due to increased slip planarity caused by the lower stacking-fault energy. Slip planarity did not appear to affect the fracture of the uncharged specimens.     

Hydrogen embrittlement of iron-nickel alloys

The effects of hydrogen on the deformation and fracture of alloys in the Fe−Ni alloy system were studied as a function of the alloy composition and the amount of sulfur segregated at the grain boundaries. A ductile-to-brittle transition in the tensile parameters and in the fracture mode was observed as the hydrogen fugacity was increased. The variation of this transition fugacity with segregated sulfur and with the alloy concentration was studied.     

低钴稀土系贮氢合金的研究

通过在稀土系贮氢合金中添加不同含量的Fe或Cu元素,部分替代合金中Co的方法,研究低钴贮氢合金电化学性能上的差异.结果表明:由于合金组成和化学计量比的不同,同样是添加了Fe或Cu的合金在电化学性能上表现不同,但总体说,含Cu低钴贮氢合金容量和倍率放电性能较好,含Fe低钴贮氢合金有较好的循环稳定性,过化学计量比的合金循环稳定性要好于欠化学计量比的合金.     

Dislocation transport and hydrogen embrittlement

Tensile tests of several austenitic stainless steels show that when smooth bar samples are exposed to very high pressure hydrogen significant degradation in mechanical properties is observed. This degradation is accompanied by fracture along interfaces such as grain and twin boundaries although microvoid coalescence remains a prominent fracture feature. The interface cracking and loss in ductility increases as the exposure pressure increases to 172.5 MPa and at the higher pressures the adverse effects of hydrogen are enhanced by warm working operations. Rationalization of these data show that dislocation transport of hydrogen is not required in the embrittlement process; constraint, local lattice dilation, hydrogen content of interfaces are the dominant variables influencing the hydrogen affected fracture processes.     

Selective adsorption and hydrogen embrittlement

This paper presents a definite correlation between hydrogen embrittlement and adsorption. The effects of the presence of gaseous additives on hydrogen embrittlement and hydrogen adsorption were studied. Those gaseous additives which halt a running crack in 4340 steel loaded in a hydrogen atmosphere also halt hydrogen adsorption. Those gaseous additives which accelerate the crack growth increase the supply of hydrogen atoms at the metal surface. It is concluded that the effect of gaseous additives to inhibit or promote crack growth is a consequence of their ability to increase or decrease the supply of hydrogen atoms at the metal surface by some chemical process.     

Selective adsorption and hydrogen embrittlement

This paper presents a definite correlation between hydrogen embrittlement and adsorption. The effects of the presence of gaseous additives on hydrogen embrittlement and hydrogen adsorption were studied. Those gaseous additives which halt a running crack in 4340 steel loaded in a hydrogen atmosphere also halt hydrogen adsorption. Those gaseous additives which accelerate the crack growth increase the supply of hydrogen atoms at the metal surface. It is concluded that the effect of gaseous additives to inhibit or promote crack growth is a consequence of their ability to increase or decrease the supply of hydrogen atoms at the metal surface by some chemical process.      

Microstructural effects on hydrogen embrittlement in a high purity 7075 aluminum alloy

A high-purity 7075 alloy was heat treated to produce a variety of precipitate sizes and populations in grain interiors and at grain boundaries. Specimens were then evaluated for slip character and hydrogen embrittlement susceptibility following cathodic precharging or by simultaneous cathodic charging and straining. Embrittlement susceptibility was found to correlate very well with the size and type of matrix precipitates, consistent with the rationale that slip character controls the extent of dislocation transport of hydrogen and thereby may control and dominate the distribution of hydrogen in the material and the degree of hydrogen embrittlement. The results also indicate that a smaller role is played by grain boundary precipitates in controlling hydrogen embrittlement susceptibility.     

Secondary work embrittlement of cold-rolled 05CuPCrNi sheet at low temperature

Due to the rapid development of China's rail transportation equipment manufacturing industry and related international distribution,important material suppliers such as Baoshan Iron Steel Co.,Ltd.carries on investigating the stability of material under extremely cold conditions.In this study,mechanical properties and secondary work embrittlement of weathering steel 05 CuPCrNi were tested at low temperature.Compared to the mechanical properties at room temperature,the yield and tensile strength increase slightly with decreasing temperature.However,the variation of elongation is not obvious.The experimental results also show that the secondary work embrittlement transition temperature of 05 CuPCrNi is lower than-60 ℃.These results provide the basis for the use of this train body material in extremely cold regions.     

The effect of copper content and microstructure on the hydrogen embrittlement of AI-6Zn-2Mg alloys

Two commercially-processed Al-6Zn-2Mg alloys, 7050 and a “low copper” 7050, were tested for susceptibility to embrittlement by precharged hydrogen and by simultaneous cathodic charging and straining (SET procedure). Specimens were heat treated to underaged, peak-strength aged, and overaged conditions. In 7050, the peak strength and overaged conditions were not embrittled by hydrogen, though underaged material showed marked embrittlement. All microstructures tested for the low-copper alloy were embrittled. The results agree with the microstructural rationale established through earlier work on 7075 and 2124 aluminum alloys, particularly with respect to the susceptibility of underaged material to hydrogen. As in earlier work, the extent of dislocation transport of hydrogen, and local hydrogen accumulation at grain boundaries, evidently controlled the extent and degree of brittle fracture. These three important alloys can now be ranked in the order 7050, 2124, 7075 of increasing relative susceptibility to theonset of stress corrosion cracking.     

Evaluation of hydrogen embrittlement mechanisms

The incubation time which precedes the initiation of slow crack growth in the delayed failure of high-strength steel containing hydrogen was reversible with respect to the applied stress. The kinetics of the reversibility process indicated that it was controlled by the diffusion of hydrogen and had an activation energy of approximately 9000 cal per mole. Reversible hydrogen embrittlement studies were also conducted at liquid nitrogen temperatures where diffusional processes should not occur. The previously reported low temperature embrittlement behavior was confirmed indicating a basic interaction between hydrogen and the lattice. The experimental results could be satisfactorily explained by the lattice embrittlement theory proposed by Troiano.     

Strain hardening during low temperature creep of 304 stainless steel

A study has been done of the strain hardening of 304 stainless steel during low temperature creep. Hardening is measured by additional loading (above the creep stress) after various creep times. Both the level and the shape of the strain versus stress curve are altered by creep; higher creep strain is linked to a higher offset yield stress and to a longer inelastic transient during the early stages of deformation during reloading. A theory is described in which the mobile dislocation density is determined by a competition between stress rate dependent injection and velocity dependent trapping. This theory predicts both the creep curve and the hardening effects of creep with good quantitative accuracy.     

Impurity-defect interactions and radiation hardening and embrittlement in BCC metals

The interaction of interstitial impurity atoms (IIA’s) with radiation-produced defects in bcc metals and its influence on radiation hardening and embrittlement are reviewed. Special emphasis is placed on the role of oxygen in vanadium and niobium and of nitrogen and carbon in iron and steel. Upon postirradiation annealing at temperatures where the IIA’s are mobile (about 50 °C to 250 °C), resistivity stages and decreases in internal friction (Snoek damping) are observed. Evidence is examined that leads to the conclusion that IIA’s are trapped at radiation-produced defects upon postirradiation annealing,which removes the IIA’s from solid solution. The consequences of this IIA trapping on mechanical properties are summarized, particularly in terms of the phenomena of radiation anneal hardening, static strain aging, and dynamic strain aging (DSA). Static and dynamic strain aging are shown to be retarded or suppressed in irradiated metals. Recent investigations, which demonstrate that the suppression of DSA in steel can lead to an increase in ductility upon irradiation, are described. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. For the materials discussed in this paper, the displacement cross section for fission neutrons may be estimated as 500 to 1500 barns. Thus, a fluence of 10¹⁸ neutrons/cm² corresponds roughly to 10^-3 displacements per atom (DPA). For electron irradiations, the indicated DPA’s are based on the displacement cross sections given by Oen^[80] and the threshold displacement energies in the1986 Annual Book of ASTM Standards;^[81] i.e., T _d = 40 eV for Fe and V and 60 eV for Nb.