High resolution scanning auger microscopic investigation of intergranular fracture in As-quenched Fe-12mn

Previous research in this laboratory led to the conclusion that the low temperature intergranular fracture mode in Fe-Mn alloys is microstructurally determined, and does not require metalloid segregation or other chemical contamination. That conclusion was tested in the present investigation, which used high resolution scanning Auger microscopy to study the intergranular fracture surfaces. The fracture mode at liquid nitrogen temperature was found to be intergranular fracture whenever the alloy was quenched from the austenite field, irrespective of the austenization time or temperature. High resolution chemical analyses of the intergranular fracture surfaces failed to reveal any consistent segregation of P, S, O, or N. The occasional appearance of sulfur or oxygen on the fracture surface was found to be due to a low density precipitation of MnS and MnO₂ along the prior austenite grain boundaries. Excepting these dispersed precipitates, there was no evidence of manganese enrichment of the prior austenite grain boundaries. A slight segregation of carbon was found along the grain boundaries, but does not appear to be implicated in the tendency toward intergranular fracture. The present results hence reinforce the conclusion that the low temperature intergranular fracture of Fe-12Mn is microstructurally determined.     

Hydrogen diffusion and its relevance to intergranular cracking in nickel

This investigation examines the effect of hydrogen precharging upon the fracture mode of pure nickel at 77 K. Specific attention is given to the diffusion coefficient of hydrogen along grain boundaries,D _g , the critical hydrogen concentration to cause intergranular fracture, C _g ^* , and the binding energy of grain boundaries with hydrogen,E _b . Both scanning electron microscope (SEM) observations and a newly developed grain boundary diffusion model indicate that the fracture mode changes from transgranular (TG) to intergranular (IG) when the hydrogen concentration in grain boundaries reaches a critical value, in the range of 6.5 to 9.8 at. pct. The experimental results further show that the observed increment of IG cracking depth with the precharging time could be accounted for by lattice diffusion alone, thus implying that hydrogen transport in this material is not enhanced by grain boundaries. Finally, the binding energy of grain boundaries with hydrogen is found to be 11.3 to 12.3 kJ/mol.     

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 influence of the M23 (C,N)6 compound on the mechanical properties of type 422 stainless steel

Ductility and toughness of Type 422 stainless steel was shown to be strongly influenced by either the presence or absence of large grain boundary precipitates of the M₂₃(C,N)₆ compound. A lengthy, isothermal solution heat treat cycle combined with a longer, lower temperature precipitation heat treatment was used to exaggerate the M₂₃(C, N)₆ compound along the boundaries of large austenite grains. After a subsequent conventional heat treatment, the presence of this compound along the prior austenite grain boundaries substantially reduced the ductility and the Charpy V-notch impact energy. The primary fracture mode was intergranular failure along these prior austenite grain boundaries. This is a reversible process and this compound can be completely dissolved. The M₂₃ (C, N)₆ compound was removed from these prior austenite grain boundaries by another lengthy, isothermal resolution heat treatment. After a subsequent conventional heat treatment, the ductility returned to values which are near those found in as-received and heat treated material; the Charpy V-notch impact energy increased to values above that found in commercially processed Type 422 stainless steel. The fracture appearance returned to the more normal transgranular ductile dimple mode.     

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.     

Environmental embrittlement in Ll2-ordered (Co,Fe)3 V alloys

Environmental effects on mechanical properties and fracture behavior of Ll₂-ordered (Co, Fe)₃V alloys with compositions (Co₇₈Fe₂₂)₃V and (Co₈₅ Fe₁₅)₃V were studied by tensile test in the strain range of 3.3 × 10⁻⁵ to 3.3 × 10⁻¹s⁻¹ at ambient temperatures. The (Co, Fe)₃V alloys were found to be susceptible to environmental embrittlement. The yield and flow stresses were insensitive to the test environment, while the ductility and ultimate tensile strength decreased according to the sequence of dry oxygen, vacuum, air, and distilled water. The ductility loss was closely associated with intergranular fracture, and the propensity for intergranular fracture increased in the same environmental sequence. Lower strain rate resulted in more intergranular fracture and hence lower ductility. A beneficial effect of grain refinement was observed. All these results suggest that the embrittlement was caused by moisture-induced hydrogen which diffused to grain boundaries, resulting in reduced grain-boundary cohesion and increased intergranular fracture. Grain boundaries were more intrinsically brittle in (Co₈₅Fe₁₅)₃V than in (Co₇₈Fe₂₂)₃V. A possible reason was suggested to explain the grain-boundary brittleness in (Co, Fe)₃V alloys, based on consideration of phase stability.     

Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel

Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen hardness. It is argued that the gap between the cleavage fracture stress (σ _F ^CL ) and that of intergranular fracture (σ _F ^IG ) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σ _F ^CL -σ _F ^IG ) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between temper embrittled CrMo and NiCr steels is discussed.     

Fracture at elevated temperature

We have carried out a systematic experimental study of fracture in materials which contain hard second phase particles. The principal variables in this study were the average size and spacing of the second phase particles, grain size, temperature, and the strain rate. Polycrystalline copper containing a dispersion of silica particles was the material used in these experiments. Three modes of fracture were observed: transgranular necking fracture, fracture by the propagation of intergranular cracks initiated at the surface, and intergranular fracture by grain boundary cavitation throughout the entire specimen cross-section. The transition between the fracture modes was shown to shift systematically with temperature, strain rate, and the microstructure. The intergranular fracture mode was studied in detail. The growth of cavities in the grain boundaries was determined to be the rate limiting step in the fracture process. It was determined that in the range of 10^-4 to 10^-7 s-1 in strain rate, the dominant growth mechanism of the cavities was power-law creep rather than diffusional transport. The ductility of the material in the intergranular mode of fracture was found to be strongly dependent on the area fraction of the second phase in the grain boundary and on the strain rate sensitivity of the material; it was weakly dependent on the grain size. A theoretical lower bound and a practical upper bound of the ductility in the intergranular fracture mode were established. The results are in qualitative agreement with the data on nickel-base alloys and other materials published in the literature. formerly a Graduate Student in the Department of Materials Science and Engineering at Cornell University     

Influence of austenitizing temperature on stress corrosion in 4330m steel—The role of impurity segregation in stress corrosion cracking of high strength steel

Measurements of the threshold stress intensity for stress corrosion cracking (SCC), K_ISCC, and crack growth rate,da/dt, in distilled water were made, respectively, on bolt-loaded WOL and precracked three-point-bending specimens of a 4330M steel. A significant improvement of resistance to SCC was obtained by increasing quenching temperature and it is due to a reduction of segregated impurities of P and S at prior austenite grain boundaries. Intergranular cracking tendency increases with inter-granular concentration of impurities and the fracture mode changes from intergranular separation along prior austenite grain boundaries to transgranular quasi-cleavage as the segregated impurity becomes low enough. The combined effects of hydrogen and intergranular impurities on reducing intergranular cohesion and the time for approaching the critical concentration of hydrogen are dis-cussed in terms of a dynamic model which takes into account the accumulation of hydrogen ahead of a moving microcrack. Formerly with Shanghai Jiao Tong University, Shanghai, China     

Influence of sulfur,phosphorus, and antimony segregation on the intergranular hydrogen embrittlement of nickel

The effectiveness of sulfur, phosphorus, and antimony in promoting the intergranular embrittlement of nickel was investigated using straining electrode tests in IN H₂SO₄ at cathodic potentials. Sulfur was found to be the critical grain boundary segregant due to its large enrichment at grain boundaries (10⁴ to 10⁵ times the bulk content) and the direct relationship between sulfur coverage and hydrogen-induced intergranular failure. Phosphorus was shown to be significantly less effective than sulfur or antimony in inducing the intergranular hydrogen embrittlement of nickel. The addition of phosphorus to nickel reduced the tendency for intergranular fracture and improved ductility because phosphorus segregated strongly to grain interfaces and limited sulfur enrichment. The hydrogen embrittling potency of antimony was also less than that of sulfur while its segregation propensity was considerably less. It was found that the effectiveness of segregated phosphorus and antimony in prompting intergranular embrittlementvs that of sulfur could be expressed in terms of an equivalent grain boundary sulfur coverage. The relative hydrogen embrittling potencies of sulfur, phosphorus, and antimony are discussed in reference to general mechanisms for the effect of impurity segregation on hydrogen-induced intergranular fracture.     

Temperature dependence of ductility and fracture of CuSiO2 bicrystals with [001] twist boundaries

CuSiO₂ bicrystals having [001] twist boundaries with different misorientation angles were tensile tested at various temperatures from 473 to 1023 K to investigate their deformation and fracture behavior. These bicrystals have dispersed SiO₂ particles both in grains and on grain boundaries. Most bicrystals fractured integranularly. As the misorientation angle increases, intergranular brittle fracture took place more easily at lower temperatures and the recovery of ductility at high temperatures became more difficult. Bicrystals with low-angle boundaries showed clear intermediate-temperature embrittlement with ductility minima at about 770 K. These grain boundary dependent characteristics of deformation and fracture behavior of bicrystals could be explained reasonably by considering the difference in the occurrence of grain boundary sliding and of relaxation by diffusion along the particle/matrix interfaces.     

Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

THE PREPARATION OF BERYLLIUM MAGNESIUM ALLOYS BY POWDER METALLURGICAL METHODS

Abstract

The preparation of alloys of beryllium and magnesium containing up to 20 wt.-% magnesium is described. Dense bodies may be prepared by infiltration of beryllium skeletons with molten magnesium or by liquid-phase sintering of cold-compacted mixed elemental powders. Such alloys consist of the appropriate element together with the compound MgBe₁₃, usually present as a continuous intergranular phase.     

Effect of grain boundary chemistry on the intergranular fracture of iron at cathodic potentials

The percent intergranular fracture and tensile ductility of iron tested at cathodic potentials in IN H₂SO₄ was found to depend primarily on the grain boundary sulfur concentration. Zone refined and vacuum melted iron with different bulk sulfur, oxygen, and nitrogen but similar carbon concentrations were evaluated. The grain boundary chemistry was measured by Auger Electron Spectroscopy and the fracture mode and ductility by uniaxial straining electrode tests at potentials of -0.60 to -2.0 V (SCE) in IN H₂SO₄. The tensile ductility, as measured by the total strain and reduction of area, of both irons decreased with increasing cathodic potentials. The fracture mode and ductility at a potential of - 0.6 V (SCE) was related to the grain boundary sulfur concentration with increasing sulfur resulting in an increasing percent intergranular fracture and a decreasing ductility. The fracture mode and ductility was not related to the grain boundary oxygen, nitrogen or carbon concentrations but large bulk nitrogen concentrations did promote cleavage and quasicleavage fracture.     

Experimental assessment of crack-tip dislocation emission models for an Al67Cr8Ti25 intermetallic alloy

A potential explanation for the cleavage fracture of intermetallic alloys with low or moderate critical resolved shear stress (CRSS) is the existence of an energy barrier for crack-tip dislocation emission, as described by models that analyze the energetics of dislocation emission from crack tips. In the present study, an intermetallic alloy with the Ll₂ crystal structure, Al₆₇Cr₈Ti₂₅, has been used to experimentally assess the predictions of the Rice-Thomson dislocation-emission model. The assessment is performed in two ways. First, model predictions of a fracture mode transition at elevated temperature are compared with experimental results. Bend tests performed at temperatures in the range of 293 to 1061 K reveal that the fracture mode of Al₆₇Cr₈Ti₂₅ changes from predominately cleavage fracture at room temperature to a mixed mode of cleavage and intergranular fracture at intermediate temperatures and then to predominately intergranular fracture at high temperatures. The observed cleavage-to-intergranular fracture transition tem-perature is approximately 800 K, in good agreement with the model prediction. Second, model predictions of the effect of grain orientation on the fracture mode are compared with experi-mental results. Electron backscatter patterns and fractographic techniques were used to analyze the grain orientations and fracture modes of grains on the fracture surfaces of specimens frac-tured at four temperatures in the range 439 to 1061 K. Experimental results reveal a correlation between fracture mode and slip system orientation relative to the crack, in good agreement with dislocation emission model predictions. Formerly Graduate Student with the Department of Materials Science and Engineering, University of Virginia.     

Part II. Metallurgical factors governing the H-assisted intergranular cracking of peak-aged Ti-3Al-8V-6Cr-4Mo-4Zr (beta-C)

A previous study (Part I) showed that the solution-treated and aged (STA) (i.e., peak-aged) condition of Beta-C Ti, (σ _{0.2 pct y} = 1260 MPa) possesses an enhanced hydrogen (H) embrittlement susceptibility compared to the solution-treated (ST) condition, (σ _{0.2 pct y} = 865 MPa), as measured by reductions in the fracture initiation stress with predissolved H content and the introduction of an intergranular (IG) fracture mode. It was also shown that yield-strength elevation and the subsequent enhancement in the local hydrostatic stresses within the notch root are not the controlling factors in the H-assisted* IG fracture initiation of the STA condition. Previous work (Part I) implicates a microstructural feature or condition associated with the 500 °C aging treatment. In this study, it is shown that localized internal hydride precipitation at the grain boundaries or alpha beta interfaces was not detected by a variety of experimental methods over the range of internal H contents for which IG fracture initiation was observed. It was also shown that grain-boundary alpha colonies or films are not responsible for the IG fracture initiation in the STA condition. A measured increase in hydrogen embrittlement (HE) susceptibility as a function of aging time at 500 °C is consistent with the segregation or depletion of a critical species at the grain boundary. However, grain-boundary segregation/depletion could not be detected with Auger electron spectroscopy (AES) of specimens fracturing in a vacuum. Compression tests used to characterize and compare the alloys’ slip behavior showed that plastic deformation is concentrated at or near the grain boundaries in the STA condition. Therefore, a possible intergranular fracture initiation mechanism that includes the effects of hydrogen and localized deformation is discussed.     

Grain boundary fracture of L12 type intermetallic compound Ni3Ai

AES analysis of intergranular fracture surfaces of Ni₃Al showed that grain boundaries are free from any detectable amount of impurity segregation. From this finding it was suggested that grain boundary brittleness in Ni₃Al is not due to the segregation of harmful elements. X-ray diffraction and SEM observation of the fracture surfaces detected a plastically strained layer of which thickness is compa-rable to grain size.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.     

Effect of Sulfur and Antimony on the Intergranular Fracture of Iron at Cathodic Potentials

Antimony, segregated to grain boundaries of iron, was found to be five times more effective than sulfur in promoting intergranular fracture of iron when tested in IN H₂SO₄ at cathodic potentials. A decrease in the ductility of iron accompanied the fracture mode change at increasing cathodic potentials. The effectiveness of antimony relative to sulfur was determined from straining electrode tests on iron and iron + 250 appm antimony alloys heat treated at 800 °C and 600 °C to produce different grain boundary chemical compositions. Grain boundary compositions were determined by Auger Electron Spectroscopy (AES). Similar grain boundary sulfur concentrations of 0.2 monolayers were observed by AES for the iron and iron + 250 appm antimony alloy after an anneal of 240 hours at 600 °C, while 0.08 monolayers of antimony was observed for the iron + 250 appm antimony alloy. These results suggest that sulfur and antimony do not compete for grain boundary sites.