The effect of grain boundary chemistry on Intergranular stress corrosion cracking of Ni-Cr-Fe alloys in 50 Pct NaOH at 140 °C

The role of chromium, carbon, chromium carbides, and phosphorus on the intergranular stress corrosion cracking (IGSCC) resistance of Ni-Cr-Fe alloys in 50 pct NaOH at 140 °C is studied using controlled-purity alloys. The effect of carbon is studied using heats in which the carbon level is varied between 0.002 and 0.063 wt pct while the Cr level is fixed at 16.8 wt pct. The effect of Cr is studied using alloys with Cr concentrations between 5 and 30 wt pct. The effect of grain boundary Cr and C together is studied by heat-treating the nominal alloy composition of Ni-16Cr-9Fe-0.035C, and the effect of P is studied using a high-purity, P-doped alloy and a carbon-containing, P-doped alloy. Constant extension rate tensile (CERT) results show that the crack depth increases with decreasing alloy Cr content and increasing alloy C content. Crack- ing severity also correlates inversely with thermal treatment time at 700 °C, during which the grain boundary Cr content rises and the grain boundary C content falls. Phosphorus is found to have a slightly beneficial effect on IG cracking susceptibility. Potentiodynamic polarization and potentiostatic current decay experiments confirm that Cr depletion or grain boundary C enhances the dissolution at the grain boundary. Results support a film rupture-anodic dissolution model in which Cr depletion or grain boundary C (independently or additively) enhances dissolution of nickel from the grain boundary region and leads to increased IG cracking.     

Formation kinetics and rupture strain of Ni-Cr-Fe alloy corrosion films formed in high-temperature water

Nickel alloys such as Alloy 600 undergo stress corrosion cracking (SCC) in pure water at temperatures between about 260 °C and the critical point. Increasing the level of Cr in Ni-Fe-Cr alloys increases SCC resistance in aerated and deaerated water. The mechanism for Cr influence is not understood. The effect of Cr composition on the in-situ oxide rupture strain and corrosion kinetics of Ni-9Fe-Cr alloys was determined experimentally, to evaluate whether the rupture-dissolution model for SCC can account for the effect of Cr on SCC. The alloy corrosion rate and corrosion product oxide microstructure and mechanical properties are strongly influenced by Cr composition. As Cr concentration increases from 5 to 30 pct, oxide rupture strains measured in pressurized water at 288 °C increase from about 8×10⁻⁴ to 2×10⁻³ mm/mm. Corrosion kinetics are parabolic; the corrosion rate first increases and then decreases as Cr increases from 5 to 39 pct. These observations are qualitatively consistent with a rupture-dissolution SCC mechanism. However, parametric modeling of the SCC growth process, applying available creep, oxide rupture strain, and corrosion kinetics data, indicates that the rupture-dissolution mechanism accounts for only a fraction of the effect of Cr on SCC resistance.     

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E _a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (<0.2 wt pct) are typically ranging from 80 to 85 kJ/mol, whereas for high-copper-containing alloys (>~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.     

Effect of N addition on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels

The effect of N addition on the microstructure, tensile, and corrosion behaviors of CD4MCU (Fe-25Cr-5Ni-2.8Cu-2Mo) cast duplex stainless steel was examined in the present study. The slow strain rate tests were also conducted at a nominal strain rate of 1 × 10⁻⁶/s in air and 3.5 pct NaCl+5 pct H₂SO₄ solution for studying the stress corrosion cracking (SCC) behavior. It was observed that the volume fraction of austenitic phase in CD4MCU alloy varied from 38 to 59 pct with increasing nitrogen content from 0 to 0.27 wt. pct. The tensile behavior of CD4MCU cast duplex stainless steels, which tended to vary significantly with different N contents, appeared to be strongly related to the volume changes in ferritic and austenitic phases, rather than the intrinsic N effect. The improvement in the resistance to general corrosion in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution was notable with 0.13 pct N addition. The further improvement was not significant with further N addition. The resistance to SCC of CD4MCU cast duplex stainless steels in 3.5 pct NaCl+5 pct H₂SO₄ aqueous solution, however, increased continuously with increasing N content. The enhancement in the SCC resistance was believed to be related to the volume fraction of globular austenitic colonies, which tended to act as barriers for the development of initial pitting cracks in the ferritic phase into the sharp ones.     

The solubility and solution behavior of antimony and tin in α-lron and the effects of nickel and chromium additions

The solid solubilities of Sn and Sb in α-Fe have been determined by means of lattice parameter measurements. The Sb solubility ranges from a maximum of 11 wt pct (5.4 at. pct) at 1000°C down to 5.3 wt pct (2.5 at. pet) at 600°C; the Sn solubility ranges from a maximum of 17.7 wt pct (9.2 at. pet) at 900°C to 6.5 wt pct (3.2 at. pet) at 600°C. These solubilities are remarkably large in view of the large sizes of the Sb and Sn atoms in relation to the Fe atom. It was not possible to rationalize the variation of the α-phase lattice parameter with Sb or Sn content from the point of view of atomic diameter or atomic volume. The addition of 1 wt pct Ni lowers the Sb solubility at 600°C from 5.3 to 3.5 wt pct (2.5 to 1.6 at. pet); the effect of Cr on the Sb solubility appears to be small. The addition of 1 wt pct Ni or 1 wt pct Cr lowers the Sn solubility from 6.5 to 5.2 wt pct (3.2 to 2.5 at. pet). It was found that a substantial amount of Ni substitutes for Fe in both the FeSb phase and the Fe₅Sn₃ phase. Formerly Research Fellow, Department of Metallurgy and Materials Science and LRSM, University of Pennsylvania     

Constitutive properties of hard-alpha titanium

The flow and fracture behavior of hard-alpha Ti was studied as a function of nitrogen content, stress state, strain rate, and temperature. Hard-alpha Ti specimens with nitrogen contents ranging from 2 to 11.6 wt pct were fabricated by powder-metallurgy techniques. Stress-strain curves were obtained under various states of stress by performing uniaxial compression, indirection tension, indentation, and plane-strain compression tests at two strain rates. By varying the test technique and the specimen geometry, deformation and fracture in hard-alpha Ti was characterized for mean pressures as high as 6 times the flow stress. Most of these tests were conducted at 954 °C, but some were performed at 25 °C, 927 °C, and 982 °C. The experimental results indicated that, during compression testing at 927 °C to 982 °C, hard-alpha Ti exhibited substantial plastic deformation for nitrogen contents less than 4 wt pct, but showed brittle fracture with little plastic flow for nitrogen contents of 5.5 to 11.6 wt pct. Both the yield and fracture strengths increase with increasing nitrogen content and pressure, but decrease with increasing temperature. The yield strength increases with strain rate, while the fracture strength shows little or no rate sensitivity. The fracture strength in tension is substantially lower than that in compression. These observed deformation and fracture characteristics are explained on the basis of microcrack formation during inelastic deformation.     

Environmentally assisted cracking behavior of peak-aged 7010 aluminum alloy containing scandium

The 7010 Al alloy with and without addition of 0.25 wt pct Sc in peak-aged condition was examined for its environmentally assisted cracking (EAC) behavior. Slow strain rate testing (SSRT) per ASTM standard G129-00 was employed to investigate EAC. The base 7010 Al alloy showed 10 pct elongation, 9.9 pct reduction in area, and 561 MPa ultimate tensile strength (UTS), when tested in air. The ductility of the base alloy dropped to 3 and 3.3 pct in terms of elongation and reduction in area, respectively, when tested in 3.5 pct NaCl solution, showing its high susceptibility to EAC. On the other hand, the 0.25 wt pct Sc containing alloy showed a significant improvement in ductility not only in air but also in 3.5 pct NaCl solution, without any loss in the UTS. Thus, the 0.25 wt pct Sc containing alloy exhibited 13.4 pct elongation, 15.8 pct reduction in area, and 560 MPa UTS in air and 12.5 pct elongation, 16.4 pct reduction in area and 560 MPa UTS in 3.5 pct NaCl solution. The study for the first time shows that the high resistance to EAC of 7010 alloy can be imparted even in peak-aged condition by the addition of 0.25 wt pct Sc.     

Stress corrosion cracking of pressure vessel steels in high-temperature caustic aluminate solutions

Stress corrosion cracking (SCC) behavior of three kinds of low alloy pressure vessel steels in high-temperature (200 °C to 300 °C) caustic aluminate (AIO_-2) solutions has been studied by slow strain rate tests (SSRT). The results indicate that these pressure vessel steels are susceptible to SCC in caustic aluminate solution and that the SCC susceptibility increases with increasing temperature between 200 °C to 300 °C. Sulfide content and stringered sulfide inclusions severely and anisotrop-ically affect the caustic SCC of these low alloy steels. The inclusions in the rare-earth-treated steel are predominantly globular rare-earth sulfides or oxysulfides, resulting in improved transverse prop-erties. The effect of inclusions on SCC behavior correlates with the projected area of inclusions perunit volume at the crack tip,A _v , on the plane perpendicular to the tensile direction. The susceptibility to SCC increases with increasingA _v .     

Fatigue of Ni-Ai-Mo aligned eutectics at elevated temperatures

The elevated-temperature mechanical behavior of two aligned eutectics (Ni-8.1 wt pct Al-26.4 wt pct Mo and Ni-6.3 wt pct Al-31.2 wt pct Mo) has been investigated utilizing monotonic and cyclic testing in vacuum. Tensile yield strength and fatigue resistance increased from 25 to 725 °C, but then were reduced at 825 °C. The fatigue lives of specimens tested at 725 °C decreased sharply with decreasing frequency. A shift from surface to internal crack initiation was observed upon increasing the test temperature from 725 to 825 °C. Stage II crack propagation was observed at both temperatures, in contrast to stage I cracking at 25 °C. The test results are compared to those for other nickel and cobalt-base aligned eutectics to show that the frequency effect on fatigue life is not limited to the Ni-AI-Mo system. formerly Graduate Assistant in the Department of Materials Engineering, Rensselaer Polytechnic Institute     

Stress-corrosion cracking susceptibility of the superplastically formed 5083 aluminum alloy in 3.5 pct NaCl solution

The slow strain rate test (SSRT) method was employed to study the stress corrosion cracking (SCC) susceptibility of the superplastic 5083 Al alloy in a 3.5 pct NaCl solution after superplastic forming and various heat treatments. Experimental results showed that both superplastically formed specimens and specimens subject to the same thermal processes as that used in superplastic forming suffered severe SCC susceptibility, and obvious intergranular fracture surfaces were also observed. Furthermore, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) analyses demonstrated that the thermal processes of superplastic forming led to continuously distributed precipitation layers of β phase (Mg₂Al₃) at grain boundaries, i.e., sensitization had occurred. However, postforming annealing treatment at 345 °C for 1 hour eliminated the sensitization effect of both specimens. In this case, the SCC susceptibility was alleviated, and the fracture surfaces changed to a transgranular dimpled structure, characteristic of that found in the as-received specimen. From the metallographic observations, it was also seen that a number of cavities appeared at the grain boundaries of the superplastically formed specimen. However, the cavitation effect on SCC susceptibility is minor in comparison with the sensitization effect.     

The oxidation behavior of Ni-Cr-Al-2ThO2 alloys at 1093° and 1204°C

A pack diffusion process has been developed which permits the introduction of nearly 6 wt pct Al into solid solution in the near surface region of TDNiCr (Ni-20 wt pct Cr-2 vol pct ThO₂) and Ni-20Cr. Alumina scales, adherent under cyclic heating and cooling conditions, were produced on TDNiCr-5.86A1 upon exposure to an environment of 1.33 × 10³N/m² (10 torr) or 1.01 × 10⁵N/m² (760 torr) air at temperatures of 1093° and 1204°C. While the same oxidation kinetics were observed in isothermal tests for Ni-14.6Cr-5.86Al as were obtained for the TDNiCr-5.86A1, the dispersion strengthened alloy exhibited superior oxide scale adhesion during cyclic testing. At 1204°C continuous weight gains were observed under all test conditions for TDNiCr-5.86A1, in contrast to the weight loss with time which occurred several hours after exposure of TDNiCr to an oxidizing environment. TDNiCr with an initial aluminum surface concentration of 4.95 wt pct has nearly comparable oxidation resistance to the TDNiCr-5.86Al alloy. Specimens with 4.3 wt pct Al at the surface have inadequate aluminum to form Al₂O₃ scales, and weight losses are observed after 40 h upon exposure of these specimens to 1.01 × 10⁵N/m² (760 torr) air at 1204°C.     

The solubility of nitrogen in liquid manganese

The solubility of nitrogen in liquid manganese, in equilibrium with gas containing varying amounts of nitrogen, has been determined at 1300, 1400, and 1500°C. At one atm pressure of nitrogen, the solubility is 2.6 wt pct at 1300°C and decreases to 1.7 wt pct at 1500°C. Dissolution of nitrogen in manganese does not follow Sievert’s law. The activity coefficient of nitrogen in manganese increases with increasing nitrogen concentration. At one atm nitrogen pressure, the value of the activity coefficient, relative to the infinitely dilute solution, is 1.55 between 1300 and 1500°C.     

The solubility of nitrogen in liquid pure nickel

A constant-volume Sieverts’ method was used to determine the solubility of nitrogen in liquid nickel. This method has been used for the first time on this type of material. It was found that the solubility of nitrogen in pure nickel at 1600 °C and 1 atm is equal to 0.0020 wt pct (with an experimental error of ± 0.0002 pct). The solubility increases with increasing temperature. The temperature coefficient of nitrogen solubility within the temperature range from 1500 °C to 1750 °C is equal to 1.6 × 10⁻⁶ wt pct/°C. The solution of nitrogen in pure nickel at 1600 °C and pressures up to 1 atm was found to obey Sieverts’ law.     

The effect of nickel,chromium, and primary

Elevated temperature creep tests were performed on Ti 6242Si deformed to small (<0.002) plastic strains using a highly aligned creep testing apparatus. Specimens were solution annealed at various temperatures below the beta transus(T _β - 6 °C toT _β − 52 °C) which controlled the volume fraction of primary alpha. Decreases in the amount of primary alpha are associated with decreased primary and steady-state creep rates. The effects of trace levels of the elements Ni and Cr on the creep properties of Ti 6242Si were also studied. Relatively small additions of Ni (0.075 to 0.093 wt pct), which appeared to segregate to the bcc beta phase, substantially in- creased the creep rates of this alloy, while additions of Cr up to 0.278 wt pct had little, if any, effect on the creep rates.     

Development of austenitic nanostructures in high-nitrogen steel powders processed by mechanical alloying

In this work, mechanical alloying was employed in producing high-nitrogen Fe18Cr11Mn stainles-steel powders. It was found that the nitrogen solubility in the powder mixtures increases exponentially with milling time at room temperature. Maximum nitrogen levels of 2.47 wt pct N were achieved after milling for 170 hours. In addition, the grain size structure continually decreased and reached a plateau at nanometric grain sizes of the order of 3 nm. In addition, measured, interplanar lattice spacing, d₍₁₁₀₎, did not follow a linear trend. Apparently, initially the nitrogen tendency was to be preferentially dissolved at dislocations and grain boundaries. However, after long milling times, the crystal lattice tended to be saturated with N. Annealing at 900 °C to 1200 °C for 2 hours led to various microstructures, where the matrix was almost always γ-iron, but Cr₂N, CrN, and α-iron were also present depending on the annealing temperatures. In particular, it was found that a fully austenitic, nanometric grain structure can be achieved by annealing at 1000 °C and 1100 °C Fe18Cr11Mn alloys with 1.02 and 0.7 wt pct N, respectively.     

Effect of heat treatment on the microstructure,tensile properties,and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10_p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.     

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M₂C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.