The initiation of intergranular failure in inconel-600

The nature of slip interaction with the grain boundaries was examined on the polished surface of Inconel-600 pulled in tension at 370°C and at the initial strain rate of 1.65 × 10^?6 sec^?1. It was shown that the grain boundaries could not always accommodate slip taking place at the boundary regions. This led to the separation of grains at the boundary. Depending on the detailed morphology of the carbides, these intergranular cracks could form at axial strains as low as 10 pet. Since the cracking takes place in the inert atmosphere, it is concluded that the grain boundaries are inherently embrittled by the formation of carbides and/or by solute segregation. There was no evidence of grain boundary sliding, but when carbides were absent highly localized slip in the boundary region gave the appearance of grain boundary offsets. The implications of these observations to stress corrosion cracking are discussed.     

Grain boundary segregation of boron in INCONEL 718

The segregation behavior of boron at grain boundaries in two INCONEL 718⁺ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.     

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy during isothermal annealing after down and up quenching from an equilibrated state at 620°C in the α + liquid phase field has been studied by light and scanning electron microscopy. Down quenching to 520, 540 and 560°C resulted in grain boundaries migrating by DIGM against their curvature from one grain into another leaving behind alloyed zones. Down quenching to 590 and 605°C resulted in liquid films migrating against their curvature from one grain into another leaving behind alloyed zones. Boundaries were also observed to migrate at these temperatures. Up quenching to 630 and 640°C resulted in liquid films migrating from one grain into another leaving behind dealloyed zones. The migration distance, s, for both boundaries and liquid films was observed to decrease monotonically with annealing time (t) and to obey a power law relationship, s = Ktⁿ, with annealing time. The exponent n falls between 0.20 and 0.25. For a given annealing time the migration rates of liquid films by LFM were about twice as fast as those of grain boundaries by DIGM. With respect to driving force for boundary and liquid film migration, the coherency strain energy developed in the grain being consumed does not seem to be great enough to drive the reactions against the curvature observed.     

Investigation of Interfaces by Atom Probe Tomography

We investigated the thermodynamic and transport properties of buried interfaces with atom probe tomography. Owing to the 3D subnanometer resolution and single atom sensitivity of the method, it is possible to obtain composition profiles with high accuracy both along or normal to the interfaces. We have shown that the width of the chemical interface between the Fe and Cr system follows the Cahn–Hilliard relation with a gradient energy coefficient of 1.86 × 10^?22 J nm². Sharpening of the Ni/Cu interface as a result of kinetic control was directly observed. We investigated the grain boundary and triple junction transport in Fe/Cr and Ni/Cu. Cr segregation enthalpy into Fe triple junctions was found to be 0.076 eV, which falls in between the surface (0.159 eV) and grain boundary (0.03 eV) segregation enthalpies. In the investigated 563 K to 643 K (290 °C to 370 °C) range, Ni transport is 200 to 300 times faster in the triple junctions of Cu than in the grain boundaries. The diffusion activation enthalpy in the triple junctions is two-thirds that of the grain boundaries (0.86 and 1.24 eV, respectively). These investigations have shown that triple junctions are defects in their own right with characteristic segregation and diffusion properties: They are preferred segregation sites and can be considered as a diffusion shortcut in the grain boundary network.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel-II. Fine scale segregation behaviour

A combination of TEM, FIM, AP and IAP has been used to study boron grain boundary segregation in austenitic stainless steels of the types 316L (with 40 ppm or with <1 ppm boron) and “Mo-free 316L” (23 ppm boron). High resolution segregation profiles were determined for cooling rates from 0.29 to 530°C/s for three starting temperatures: 800, 1075 and 1250°C. Boron grain boundary segregation was found after all heat treatments. The segregation behaviour was mainly of the nonequilibrium type after cooling from 1075 or 1250°C whereas equilibrium segregation dominated after rapid cooling from 800°C. The influence of the relative grain orientation on the amount of non-equilibrium segregation was small for general boundaries. However, no segregation was detected at coherent twin boundaries. The binding energy of boron to austenite grain boundaries was estimated at 0.65 ± 0.04 eV for both types of steels. The influence of the composition and boron content of the steels on the segregation behaviour is discussed and the experimental techniques used are presented.     

Effect of homogenization heat treatment on the microstructure and heat- affected zone microfissuring in welded cast alloy 718

The effect of homogenization temperature on microfissuring in the heat-affected zones of electronwelded cast INCONEL 718 has been studied. The material was homogenized at various temperatures in the range of 1037 ° to 1163 ° and air-cooled. The homogenized material was then electron-beam welded by the bead-on-plate welding technique. The microstructures and microfissuring in the heat-affected zone (HAZ) were evaluated by analytical scanning electron microscopy (SEM). The grain boundary segregation of various elements was evaluated by secondary ion mass spectroscopy (SIMS). It was observed that the total crack length (TCL) of microfissures first decreases with homogenization temperature and then increases, with a minimum occurring in the specimen heat treated at 1163 °. This trend coincides with the variation in segregation of B at grain boundaries with homogenization temperature and has been explained by equilibrium and nonequilibrium segregation of B to grain boundaries during the homogenization heat treatment. No other element was observed to segregate at the grain boundaries. The variation in volume fraction of phases like δ-Ni₃Nb, MC carbide, and Laves phases does not follow the same trend as that observed for TCL and B segregation at the grain boundaries. Therefore, microfissuring in HAZ of welded cast INCONEL 718 is attributed to the segregation of B at the grain boundaries.     

Influence of an electric field on the plastic deformation of fine-grained Al2O3

The influence of an electric field E = 300 V/cm on the plastic deformation at 1450 °C to 1600 °C of fine-grained alumina with ∼300 ppm MgO and d _o=1.4 to 2.5 μm was investigated. After removal of the effect of Joule heating, the field reduced the flow stress by ∼4 MPa relatively independent of temperature, representing a 25 to 70 pct reduction in the flow stress. The field had no effect on the stress exponent n=2.2, nor on the grain size exponent p=1.9. It did however have an appreciable effect on the combined constant AD _o of the Weertman-Dorn equation and the activation energy Q. The latter increased from 492 kJ/mole without the field (typically that for Al³⁺ ion lattice diffusion) to 880 to 1070 kJ/mole with the field, which is more typical of sub-boundary or grain boundary diffusion of either Al³⁺ or O²⁻-ions in alumina. It therefore appears that the field changed the rate-controlling, grain boundary sliding accommodation mechanism from the lattice diffusion of Al³⁺ ions to either sub-boundary or grain boundary diffusion of Al³⁺ or O²⁻-ions. The electric field caused isotropic swelling of the specimen when applied for an extended period of time without application of stress. Pores responsible for the swelling occurred along the grain boundaries. They may have resulted from electrotransport of Al³⁺ ions, leaving behind an Al-deficient layer from which free oxygen bubbles formed along the grain boundaries. Electric field-enhanced gas reactions at the pores may have also contributed to the swelling. It is proposed that swelling due to electrotransport may possibly be avoided by the addition of solutes, which increase the electronic or oxygen ion conductivity transference numbers compared to the aluminum ions.     

Equilibrium grain-boundary segregation and the effect of boron in B-doped Fe−3wt%Ni austenitic alloy

The equilibrium segregation of boron at grain boundaries and the relation between grain boundary hardening and segregation in B-doped Fe−30 wt%Ni austenitic alloy were studied with the methods of microhardness measurement and Particle Tracking Autoradiography (PTA). It was found that equilibrium segregation of boron at grain boundaries appeared in the alloy as annealed between 650 and 960°C and no segregation of boron appeared above 1050°C. It could be concluded that an excess grain boundary hardening by addition of boron to the alloy was caused by the grain boundary segregation of boron.     

The thermodynamics of interactive co-segregation of phosphorus and alloying elements in iron and temper-brittle steels

The thermodynamics of co-segregation and precipitation of P and alloying elements (transition metals M and carbon) involved in temper embrittlement of steels are studied quantitatively on the basis of the regular solution model for co-segregation. The equations of this model are fitted to the available Auger data for grain boundary segregation in high purity iron-base alloys and commercial steels, allowing the determination of the intrinsic segregation energies ΔG_i ^o and of the binary β_P ^gb, β_c ^gb and ternary β_PC ^gb, ß_MP ^gb interaction coefficients in the grain boundaries. This analysis shows that Ni, Cr, and Mo do not segregateper se in iron whereas Mn does weakly, and that the segregation of these elements is essentially driven by that of P through the strongβ_MP ^gb attractive interaction energyat the boundaries. This energy, which increases in the order Ni, Mn, Cr, Mo, is remarkably close to the bulk values β_MP ^B in the corresponding phosphides as calculated on the basis of solubility data. The scavenging of P by M elements with largebulk M-P interactions is shown to play a determining role in low Mo and high (12 pct) Cr steels. The beneficial role of carbon is complex since it drives Mo to the grain boundaries due to the large Mo-C attraction, but it also strongly opposes P segregation due to the large repulsive P-C interaction.     

Effect of ageing on properties of pressure vessel steels

Manganese-molybdenum-nickel steels are used in the fabrication of nuclear pressure vessels operating at temperatures up to 350°C. In this work the effects of thermal ageing in the temperature range 300–550°C for durations up to 2 × 10⁴ h have been studied in conventionally quenched and tempered, and simulated heat-affected-zone (HAZ) microstructural conditions. Quantitative fractography and Auger spectroscopy have been used to relate changes in mechanical properties with associated changes in fracture mode and grain boundary chemistry. The results show that ageing increases the ductile-brittle transition temperature by an amount dependent on material, prior heat treatment, ageing temperature and time. Embrittlement was associated with the segregation of phosphorus to grain boundaries and was successfully modelled using McLean's approach to equilibrium segregation. The embrittling potency of phosphorus was highly dependent on prior heat treatment; largest in the coarse-grained, higher hardness simulated HAZ condition, and least in the conventionally quenched and tempered conditon.     

The microstructural response of mill-annealed and solution-annealed INCONEL 600 to heat treatment

Samples of INCONEL* 600 were examined in the mill-annealed and solution-annealed states, and after isothermal annealing at 400 °C and 650 °C. The corrosion behavior of the samples was examined, analytical electron microscopy was used to determine the microstructures present and the chemistry of grain boundaries, and Auger electron spectroscopy was used to measure grain boundary segregation. Samples of different alloys in the mill-annealed state were found to have quite different microstructures, with Cr-rich M₇C₃ carbides occurring either along grain boundaries or in intragranular sheets. The corrosion behavior of the samples correlated well with the occurrence of grain boundary chromium depletion. Solution annealing at 1190 °C caused dissolution of all carbides, whereas at 1100 °C the carbides either dissolved or the grain boundaries moved away from the carbides, depending upon alloy carbon content. Low-temperature annealing at 400 °C had little effect on millannealed or fully solutionized samples, but in samples with intragranular carbides present, the grain boundaries moved until intersecting or adjacent to the carbides. Isothermal annealing at 650 °C caused carbide nucleation and growth at grain boundaries in fully solutionized samples. Chromium depletion at grain boundaries accompanied carbide precipitation, with a minimum chromium level of 6 wt pct achieved after 5 hours. Healing was found to occur after 100 hours. Solution-annealed samples with intragranular carbides present had more rapid corrosion kinetics since the grain boundaries moved back to the existing carbides. Thermodynamic analysis of the chromium-depletion process showed good agreement with experimental measurements. The Auger results found only boron present at grain boundaries in the mill-annealed state. Aged samples had boron, nitrogen, and phosphorus present, with phosphorus and nitrogen segregating to the greatest extent. The kinetics of phosphorus segregation are much slower at 400 °C compared with 650 °C.     

Observations concerning the β→αm massive transformation in Cu-Zn alloys

The occurrence and progress of the massive transformation in copper-rich Cu-Zn alloys, have been studied during quenching and heating by metallographic and electron-microscopic techniques. The transformation occurred predominantly at compositions within the single-phase α field in samples containing a diffusion gradient, and in two-phase samples containing both the α and β phases. α_m particles produced during quenching from the β-phase field have been found to grow massively following a rapid reheating into the α-phase field. Equilibrium α, or Widmanstätten α particles, have not been observed to continue by massive growth, nor did they act as preferred sites for nucleation of the massive phase. These observations are consistent with a growth model suggested recently by Karlyn, Cahn, and Cohen.³ However, in samples rapidly quenched from the β phase, the onset of the massive transformation occurred in the two-phase field some 10° to 20°C above the equilibrium a-phase boundary. This observation is not consistent with a model that requires the predominant growth of the massive phase to occur within the α single-phase field, and includes delay times that increase on approaching the α/(α + β) phase boundary. No low index orientation relationship was found to exist across the faceted interfaces between the massive and the retained β(β′) matrix. In addition, no regular interfacial dislocation arrays were evident at these boundaries. These observations add further support to the notion that the advancing interface is structurally comparable to a high-angle, incoherent boundary.     

Humidity sensing properties of La~(3+)/Ce~(3+)-doped TiO_2-20 wt.% SnO_2 thin films derived from sol-gel method

The humidity sensing properties of La3+/Ce3+-doped TiO2-20 wt.%SnO2 thin films were studied.Sol-gel method was employed to prepare the films on alumina substrates.By constructing a humidity-impedance measuring system,the sensing behaviors were inspected for the films sintered at different temperatures.Experimental results showed that,0.5 wt.% of La2O3 or Ce2O3 doped films sintered at 500 °C for 2 h had the best humidity sensing properties,the impedance decreasing from 109 ? to below 104 ? in the humidity range of 15-95 RH%.Moreover,Ce3+-doping had better humidity sensing properties than La3+-doping.The doping mechanism was discussed in terms of phase composition,granularity of crystalline and segregation of rare earth ions at grain boundaries.     

Grain boundary pest of boron- doped Niin3Al at 1200 °C

A ductile Ni_77.4Al₂₂Zr_0.6B_0.2 alloy suffered severe intergranular embrittlement after air exposure at 1200 °C for 100 hours. However, the material survived after 1038 °C air exposure for 100 hours. Auger analysis showed enormous oxygen segregation on the grain boundaries in the 1200 °C, air exposed, boron-doped Ni₃Al, while the boron segregation remained unchanged. To elucidate this type of grain boundary damage, a modified fracture mechanism was proposed. Finally, anomalous grain growth was found in this alloy after 1200 °C air exposure, and an explanation for this phenomenon was suggested.     

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.     

The wetting transition in high and low energy grain boundaries in the Cu(In) system

Wetting of two symmetrical tilt grain boundaries, 77° 〈110〉 and 141° 〈110〉, in synthetic copper bicrystals with a Cu(In) melt was studied in the temperature range 690°990°C. The contact angle at the site of GB intersection with the solid-melt interface was measured. A wetting transition occurred at T_w = 960 ± 6°C for the 77° 〈110〉 grain boundary and at T_w = 930 ± 5°C for the 141° 〈110〉 grain boundary. The contact angle approached zero for this transition. The relative surface energies of the two boundaries were measured using the thermal grooving technique. The energy of the 77° 〈110〉 grain boundary is about 40% lower than that of the 141° 〈110〉 grain boundary. Therefore, it has been shown experimentally that the lower the grain boundary energy, the higher the wetting transition temperature. This is in agreement with the thermodynamic model of wetting transitions on grain boundaries.     

Grain boundary segregation of antimony in iron base alloys and its effect on toughness

The equilibrium grain boundary segregation of antimony was investigated in iron base alloys (Fe-Sb, Fe-C-Sb, Fe-Ni-Sb) after annealing at temperatures between 550 and 750°C. Utilizing Auger electron spectroscopy (AES) the concentration of antimony at intergranular fracture faces was determined as a function of bulk concentration and equilibration temperature. The segregation of antimony in Fe-Sb alloys with mass contents of between 0.012 and 0.094 % Sb was described by the Langmuir-McLean equation. The evaluation leads to the free enthalpy of segregation ΔG_segr = ?19 kJ/mol - T 28 J/mol K. The relatively low value for the segregation enthalpy ΔH = ?19 kJ/mol indicates a rather small tendency for grain boundary segregation of Sb. However, its embrittling effect is strong, scanning electron micrographs (SEM) of fractured samples show that the percentage of intergranular fracture strongly increases with an increasing coverage of antimony at the grain boundaries. The data for Fe-0.93% Sb and Fe.1.91% Sb (mass contents) do not fit in the thermodynamic evaluation obviously due to formation of antimonide precipitates in the grain boundaries. The addition of carbon to Fe-Sb alloys results in a higher grain boundary cohesion which is caused by two effects of carbon, displacement of antimony from the grain boundaries by carbon and enhanced grain boundary cohesion. In the Fe-Ni-Sb alloys additional segregation of nickel was found at the grain boundaries but no enhanced antimony segregation, as expected from previous models of other authors, assuming Ni-Sb cosegregation.     

Embrittlement of 21/4 CrMoV steel bolts after long exposure at 540 °C

Bolts made of2 ^1/4 CrMoV steel became gradually embrittled and exhibited an increasing fraction of intercrystalline fracture after long exposure (more than 25,000 h) at 540 °C. This has been found to be caused by the segregation of phosphorus to the prior austenite grain boundaries. This was accompanied by the depletion of molybdenum in the ferrite down to about 0.3 pct owing to the formation of a molybdenum-containing carbide Fe₃Mo₃C(M₆C). Reheating of the embrittled bolts to 680 °C removed the segregation of phosphorus at the grain boundaries and therefore also removed the intercrystalline embrittlement. However, the molybdenum content of the ferrite was lowered further to about 0.2 pct; hence the scavenging of phosphorus by molybdenum was further reduced, and therefore the embrittling tendency of these bolts during reuse was increased. Even after reaustenitizing, quenching and tempering, the service life of these bolts was found to be shorter than in the original condition; this is presumed to be due to an inhomogeneity of molybdenum in the austenite not removed by the reaustenitizing treatment.     

The non-equilibrium segregation of boron during the recrystalization of Nb-treated HSLA steels

The effect of boron segregation on recrystallization was investigated in two Nb and two Nb-free steels. The results indicate that the addition of B to a Nb steel retards the recrystallization of austenite after high temperature deformation to a significant degree. By contrast, the effect of B when added alone is not of commercial importance. At deformation temperatures above 950°C, the influence of boron is mainly due to grain boundary segregation and solute drag; below 900°C, the retardation results from the precipitation of Nb carbonitride. During isothermal holding, there are two types of non-equilibrium segregation on boundaries. Strain-induced segregation occurs on the original boundaries; it increases and attains a maximum rapidly, then decreases and disappears after 60 s of holding at 1000°C. A second type of non-equilibrium segregation occurs on newly formed boundaries during recrystallization. It develops quickly as the new boundaries are formed and begin to move, and can persist until grain impingement occurs and the motion is halted. The enhanced effect of B in the presence of Nb is attributed to the formation of (Nb, B) complexes, which appear to increase the solute drag force and decrease the boundary velocity.