Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part I. Behavior in as-processed state

Increased interest has been paid to grain boundary segregation in alloy K-500 due to severe intergranular cracking recently observed in forged bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessary condition for C grain boundary segregation determination was established. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bulk Ni and high bulk Al significantly enhanced Cu segregation to grain boundaries. The depth profiles of intergranularly segregated elements also showed different features for high and low Ni content alloys. In high Ni alloys, C and Al levels dropped continuously as a function of distance from the grain boundaries but the Cu level dropped only slightly. In low Ni alloys, the Al and C levels rose from relatively low grain boundary levels to a peak at a certain distance from the grain boundary where the high grain boundary Cu level dramatically dropped. Transmission electron microscope (TEM) observation revealed a grain boundaryγ′-depleted zone followed by a region with coarser and denserγ′ particles in low Ni and high Al alloys but quite uniformly distributedγ′ particles with no depleted zone in high Ni and low Al alloys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theories about surface segregation in binary Ni-Cu alloys, and the segregation of C and Al to grain boundaries is suggested to be probably due to their interaction with Ni and Cu.     

Behavior of grain boundary chemistry and precipitates upon thermal treatment of controlled purity alloy 690

Grain boundary composition and carbide composition and structure were characterized for various microstructures of controlled purity alloy 690. Heat treatments produced varying degrees of grain boundary chromium depletion and precipitate distributions which were characterizedvia scanning transmission electron microscopy (STEM). Convergent beam electron diffraction revealed that the dominant carbide is M₂₃C₆, and energy dispersive X-ray analysis (EDAX) determined that the metallic content was about 90 at. pct chromium. A discontinuous precipitation reaction was observed and is attributed to a high degree of carbon supersaturation. Grain boundary composition measurements confirm that chromium depletion is controlled by volume diffusion of chromium to chromium-rich grain boundary carbides in the temperature range of 873 to 1073 K. Grain boundary chromium levels as low as 18.8 at. pct were obtained by thermal treatment at 873 K for 250 hours and 973 K for 1 hour. A thermodynamic and kinetic model developed for alloy 600 was modified to describe the development of the chromium depletion profile in alloy 690 during thermal treatment. Experimentally measured chromium profiles agree well with the model results for the dependence of the chromium depletion zone width and depth on various input parameters. The establishment of the model for alloy 690 allows the chromium depletion zone width and depth to be computed as a function of alloy composition, grain size, and temperature. The chromium depletion profiles and the precipitate structure and composition of controlled purity 690 are compared to those of controlled purity 600. A thermodynamic analysis of the carbide stability indicates that other factors, such as favorable orientation relationships, play an important role in controlling the precipitation of Cr₂₃C₆ in nickel-base alloys.     

Investigation of the segregation behavior of boron in Ni3Al alloys by PTA technique

The particle-tracking autoradiograph (PTA) technique has been used, combined with quantitative statistical analysis using the image processing system, to study the effect of such factors as aluminium content of base alloy, the bulk boron concentration and different heat treatments on the grain boundary segregation behaviors of boron in Ni₃Al alloys. Moreover, the mechanical properties of Ni₃Al alloys subjected to different heat treatments and therefore with various quantities of segregated boron have been tested. Their fracture surfaces have also been observed in SEM. The relation between the level of boron segregation and ductility has been inspected.     

Grain boundary segregation of boron and sulfur and its effect on ductility in rapidly solidified Ni-base L12 compounds

This paper reports a study of boron segregation in Ni₃Al, Ni₃Si, Ni₃Ge, and Ni₃Ga. Sulfur segregation in Ni₃Al is also considered. The results show that boron segregates in Ni₃Al and that the amount of segregation tends to increase as the bulk concentration of boron in the alloy increases. However, because the samples had not reached equilibrium during the heat treatment, there was some scatter in this correlation. Boron also segregated in Ni₃Si and Ni₃Ge, but there appeared to be no dependence of the amount of segregation on the bulk concentration. This result is not surprising because in these alloys the boron concentration was above the solubility limit. Boron segregation also occurred in Ni₃Ga and it appeared to increase with increasing bulk concentration. However, only a small amount of data was obtained for this system. Sulfur segregated in Ni₃Al and its concentration on the grain boundaries increased with increasing bulk concentration. It did not appear to compete with boron for grain boundary sites. Aluminum also segregated in Ni₃Al, but there was a large amount of scatter in the data. The plastic strain to failure measured for the samples of Ni₃Al did not correlate with the amount of boron segregation. In particular, we could not explain the fact that boron additions enhance grain boundary cohesion more effectively in Ni-rich alloys by an increase in boron segregation in these alloys. Stoichiometric alloys and Ni-poor alloys that were very brittle had boron segregation in equivalent amounts to that found in ductile Ni-rich alloys.     

Superplastic-like behavior in as-extruded AlZnMg alloy matrix composites reinforced with Si3N4 whiskers

For as-extruded AlZnMg alloy without reinforcement of Si₃N_4ω, a maximum elongation is 140%, which was measured near a relatively high strain rate 2 × 10⁻²s⁻¹ at 818 K. On the other hand, a maximum elongation of 160% for α-Si₃N_4ω/AlZnMg composites, whose grain size was less than 4 μm, was obtained at 798 K and at the highest end in strain rate range of 8 × 10⁻¹s⁻¹ evaluated in this work, and for β-Si₃N_4ω/AlZnMg composites with grain size of about 4 μm, a maximum elongation of 230% was obtained at 818 K and a high strain rate of 2 × 10⁻¹s⁻¹. In these AlZnMg system alloys the composites reinforced with α-Si₃N_4ω exhibited a high superplastic elongation at higher strain rates than the composites reinforced with β-Si₃N_4ω.     

Grain boundary segregation of an Al-Zn-Mg ternary alloy

Auger electron spectroscopy (AES) has been used to measure the grain boundary concentration profiles of alloy additions in an A1-5.5 pct Zn-2.5 pct Mg ternary in as-quenched, under-, peak-, and over-aged conditions. The AES depth profiles show marked segregation of Mg and Zn to the grain boundary, in contrast to that reported previously on similar A1 alloys. It is found that this apparent contradiction can be resolved by exploiting the plasmonloss features of the AES spectra to help elucidate the grain boundary segregation. With the AES/plasmon-loss measurements, one can determine not only the concentration of Mg and Zn at the grain boundary, but also the metallurgical environments surrounding the alloy additions. It is shown that, for over-aged specimens of the Al alloy, only a fraction of the total Mg at the grain boundary is incorporated in MgZn₂ precipitates, the remainder being segregated to within a few atomic layers of the boundary.     

微量添加晶界合金对烧结Nd-Fe-B力学性能及微观结构的影响

采用双合金法制备系列烧结Nd—Fe—B磁体(保持其主合金成分不变：Ndl4．1Dy0．5Fe79.0B6.4(原子分数)，所添加的晶界合金中的B含量从0．95％(原子分数)逐步增加到6．95％(原子分数))，研究了微量添加晶界合金对烧结Nd—Fe—B力学性能及微观结构的影响。研究结果表明：微量添加晶界合金所制备的磁体，其抗弯强度值普遍高于单合金法制得的磁体；前者的抗弯强度最高可达397MPa，高于铸造，热压磁体的抗弯值，而后者的抗弯强度仅为309MPa。由相结构分析可知，当添加的晶界合金中的B含量为O．95(原子分数)，主相晶格的四方度减小，这时磁体具有最高的抗弯强度。另外，微量添加晶界合金，可使磁体中晶界相的分布更加均匀，从而基本上消除了主相晶粒直接接触的现象，使晶粒的不规则长大得到抑制。这也是微量添加晶界合金后磁体具有较高抗弯强度的原因之一。对磁性能的研究结果表明，微量添加晶界合金几乎不影响烧结Nd—Fe—B磁体的磁性能。     

Rhodium segregation in dilute silver-rhodium alloys

Segregation of Rh in Ag-based alloys has been studied using the perturbed angular correlation of γ-rays emitted in the nuclear decay of radioactive ¹¹¹In. The formation of impurity complexes, consisting of an ¹¹¹In probe atom and one or more Rh atoms, was observed as a function of annealing time and temperature. Rhodium atom aggregation starts at about 600 K. From the fraction of ¹¹¹In bound to isolated Rh atoms the solute rhodium atom concentration was determined. It increases with the nominal alloy concentration up to about 0.04 at.% and then it is essentially constant for the nominal Rh concentration varying between 0.1 and 0.5 at.%. The solute rhodium atom concentration is 3 times larger at the melting point than at 750 K.     

A method of determining the diffusion coefficient of vacancy-solute atom complexes during the segregation to grain boundaries

A new method of determining the diffusion coefficient of vacancy-solute atom complexes by measuring the critical time for non-equilibrium grain boundary segregation of solute atoms was established in this work. For a sample cooled quickly enough from a high temperature to a lower temperature and isothermally held there, the critical time is defined to be the holding time of the sample at the lower temperature required for the level of segregation to reach a maximum. According to this method, the Arrhenius parameters of the diffusion of vacancy-boron atom complexes in a B-doped Fe-28 at. % Ni alloy were determined to be 10.0 · 10⁻⁵ m²/s for the pre-exponential factor and 0.94 eV for the activation energy. As a by-product of this work the numerical constant δ was determined to be 0.48 for the diffusion of vacancy-boron atom complexes in the above alloy.     

Effect of oxygen addition on mechanical properties of TiAl at 293–1273 K

Mechanical properties of Ti−50, −53, and −56 at.% Al alloys doped with 0, 0.1, 0.2 and 0.3 mass% oxygen have been studied in compression tests at temperatures from 293 K to 1273 K in a vacuum. The yield stress in Ti−50 at.% Al alloy increased by about 1.5 times owing to the addition of 0.3 mass% oxygen at all testing temperatures. In Ti−53 and −56 at.% Al alloys the increases of yield stress due to the addition of oxygen were not as pronounced as those in Ti−50 at.% Al alloys. The work hardening coefficients increased at 293–873 K but they decreased at 1073–1273 K with increasing oxygen and aluminum content. Insoluble particles extracted isolated by a mixed acid from TiAl alloys were α-Al₂O₃ phase and the amount of α-Al₂O₃ phase increased with increasing aluminum and oxygen content. The increase of yield stress at all testing temperatures occurred owing to the solid solution hardening of oxygen in Ti−50 at.% Al alloy and the dispersion strengthening of α-Al₂O₃ in Ti−53 and Ti−56 at.% Al alloys.     

Low-temperature superplasticity of ultra-fine-grained Ti-6Al-4V processed by equal-channel angular pressing

The low-temperature superplasticity of ultra-fine-grained (UFG) Ti-6Al-4V was established as a function of temperature and strain rate. The equiaxed-alpha grain size of the starting material was reduced from 11 to 0.3 μm (without a change in volume fraction) by imposing an effective strain of ∼4 via isothermal, equal-channel angular pressing (ECAP) at 873 K. The ultrafine microstructure so produced was relatively stable during annealing at temperatures up to 873 K. Uniaxial tension and load-relaxation tests were conducted for both the starting (coarse-grained (CG)) and UFG materials at temperatures of 873 to 973 K and strain rates of 5 × 10⁻⁵ to 10⁻² s⁻¹. The tension tests revealed that the UFG structure exhibited considerably higher elongations compared to those of the CG specimens at the same temperature and strain rate. A total elongation of 474 pct was obtained for the UFG alloy at 973 K and 10⁻⁴ s⁻¹. This fact strongly indicated that low-temperature superplasticity could be achieved using an UFG structure through an enhancement of grain-boundary sliding in addition to strain hardening. The deformation mechanisms underlying the low-temperature superplasticity of UFG Ti-6Al-4V were also elucidated by the load-relaxation tests and accompanying interpretation based on inelastic deformation theory.     

Kinetics of cellular dissolution in an AlZn alloy

Dissolution of the lamellar precipitate by cell boundary migration has been studied in an Al-18.9at.% Zn alloy in the temperature range 554–637 K. Microstructural observations have revealed that the process of dissolution in this alloy is cellular mode of transformation in the early stages. The boundary diffusivities were calculated by using the theory of Petermann and Hornbogen modified for cellular dissolution. The diffusivities calculated from the experimental data are four orders of magnitude higher than the volume diffusivities. From the Arrhenius plots, activation energies of 51.5 ± 2 and 47.5 ± kJ mol⁻¹ were obtained from the temperature dependence of diffusivity and mobility respectively. These values are about half the activation energy required for volume diffusion of Zn in Al and compare very well with the activation energy of 60 kJ mol⁻¹ for the cellular precipitation in this alloy system. There exists a range of temperature between 502 and 532 K where the two kinetic processes, cellular precipitation and dissolution, are equally probable. The forward migration of the grain boundary during cellular precipitation is acted upon by the back pull of dissolution and the migrating grain boundary remains motionless.     

Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part II. Behavior in slow strain rate tensile test

An auger electron spectroscopy (AES) study was performed to characterize grain boundary segregation behavior and grain boundary precipitation occurring during slow strain rate tensile tests (SSRTTs) of alloy K-500. It was discovered that flake-like precipitates formed on grain boundaries of some test alloys which had a high bulk Cu level. Auger electron (AE) spectra taken from precipitates and AE mapping of grain boundary facets containing precipitates indicated that these precipitates were most likely titanium carbides. The grain boundary segregation that existed in as-processed alloys was also changed in the course of SSRTTs. A significant reduction in C segregation and a noticeable increase in Cu segregation were observed in test alloys after SSRTTs. Graphite precipitation was not detected in any of the test alloys before or after SSRTTs. The carbide precipitation and change in segregation pattern are qualitatively understandable on the basis of available thermodynamic data, but further study is necessary to fully explain the observed phenomena.     

Superplasticity and internal friction of a superplastic Zn-22% Al eutectoid alloy

Temperature spectrums of internal friction, in other words, specific points have been investigated and discussed in the wide temperature range from room temperature to equilibrium eutectoid isotherm for Zn-22% Al eutectoid alloy in order to measure grain boundary peaks. Three large grain boundary peaks of Pα, Pαβ and Pβ which are associated with superplasticity have been observed over the range of temperatures from 447K (174°C) to 525K (252°C). The Activation energies of Pα, Pαβ and Pβ have been calculated to be 109, 93 and 62 kJ·mol⁻¹ respectively in internal friction measurements. These peaks seem due to the grain boundary sliding which can be accommodated by the diffusive flux on a boundary between like phases, α/α, for Pα and β/β for Pβ, and on an interphase boundary, α/β for Pαβ. Furthermore, it has been indicated that a utilization of Pα as the damping alloys would be possible.     

An evaluation of the flow behavior during high strain rate superplasticity in an Al-Mg-Sc alloy

An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10⁻² s⁻¹. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673 K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys.     

Hydrogen embrittlement in ductile Ni3Al—Effects of hydrogen content,strain rate and pre-deformation

Hydrogen embrittlement has been studied in continuous cast sheet of an Ni₃Al alloy (Ni_77.83Al_21.73Zr_0.34B_0.1). When tensile tests were performed at the initial strain rate of 5.8 × 10⁻⁵ s⁻, the elongation decreased from 32.7% for no charging to 1.9% for 330 min of cathodic charging with 50 mA/cm² current, but the yield stress did not change. The fracture mode changed partially from dimple to intergranular and cleavage modes. At a faster strain rate of 5.8 × 10⁻³ s⁻¹, hydrogen embrittlement was less pronounced, but the yield stress increased with hydrogen content and multiple cracks were generated. Plastically pre-deformed (2–26% elongations) and subsequently charged specimens failed after yielding, which occurred at the final pre-deformation stress. Our results suggest that the grain boundary or the interior of the grain was not weakened by hydrogen, rather hydrogen-enhanced localized plasticity caused the loss of ductility in B-doped Ni₃Al alloy.     

A simultaneous analysis of segregation and diffusion of impurities in the grain boundary of a bicrystal. Application to sulphur segregation and diffusion in nickel bicrystals

A new technique is described to study diffusion and segregation in a bicrystal; it is based on a radiochemical microanalysis through a narrow slit, slowly moving on the surface. It is used to study sulphur (³⁵S) behaviour in a Ni bicrystal at 700°C (〈100〉 tilt grain boundary; angular desorientation: 20°). The segregation coefficient (α = 7000), the bulk diffusion coefficient (D_b = 4.10⁻¹²cm² · s⁻¹) and the grain boundary diffusion parameter αδD_gb are obtained from one sole experiment in one sample.     

The strength,hardness and ductility of Ni3Al with and without boron

Experiments were performed to investigate the effects of grain size (2–1100 μm) and temperature (77–1023 K) on the strength, hardness and ductility of stoichiometric Ni₃Al with (0.35 at.%) and without boron. The results show that grain refinement strengthens and hardens the alloys at low temperatures (< 873 K), but weakens them at high temperatures; lowers the magnitude of the anomalous thermal strengthening, leading to thermal weakening; and effects a “ductile to more ductile” transition in Ni₃ Al + B at room temperature and a brittle to ductile transition in both alloys at elevated temperatures (⩾ 673 K). Grain refinement has little effect on the room-temperature ductility of Ni₃ Al and on the work hardening rate. The addition of boron hardens coarse-grained material but softens fine-grained aggregates. These results are explained in terms of deformation and fracture mechanisms.     

Deformation and microstructure in L10 type Ti-Al-V alloys

The addition of vanadium to polycrystalline, single-phase γ-TiAl significantly enhances stiffness from room temperature (RT) to 1173 K. The maximum yield stress in Ti-55A1-10V alloy is centered near 1073 K, in contrast with those of single-crystal Ti-54A1 at 873 and 1073 K, depending upon deformation orientation. The effect of strain rate on yield stress is present over the entire temperature range, but it is more prominent above 873 K, while the effect becomes significantly small between 673 and 873 K, similar to that observed in Ll₂ type Ni₃Al. The microstructure of deformed Ti-55A1-10V and Ti-49.5A1-10V alloys evolved by slip, twinning, and formation of stacking faults. Twinning is a major deformation mode in Ti-49.5-10V alloy and a minor mode in Ti-55A1-10V alloy. It appears that the amount of twins for a given deformation decreases with increasing Al content. The twin structures in both alloys were found to be true twins of {111}〈112]. No pseudotwinning was observed in the alloys deformed in the temperature range of RT to 1273 K. The stacking faults found in the deformed alloys had an extrinsic character. No intrinsic type of stacking faults were found in these alloys. In addition to the cross slip of superdislocations, the anomalous hardening up to 1073 K appears attributable to an ordinary superdislocation pinning structure, dislocation loops, and nonglissile “square shaped dislocations,” which are products of super-superdislocation reaction. These nonglissile or pinning structures are unwinded in the deformed Ti-55A1-10V alloy at 1273 K, consistent with the softening behavior at this temperature.     

Effect of dispersed Al<Subscript>3</Subscript>Sc particles on dynamic recrystallization in P/M 7000 series alloys

Al₃Sc particles are well known to be a recrystallization inhibitor in Al alloys. In this study, 0.4, 0.8 and 1.5 mass% Sc were added to 7000 series Al alloys in supersaturation by using powder metallurgy. By subjecting rapidly solidified powders of these alloys to a heat treatment at 773 K, Al₃Sc particles were precipitated in the matrix. Subsequently, hot extrusion was carried out at 773 K. During extrusion, continuous dynamic recrystallization (DRX) occurred and fine DRX grains with a diameter of 1 mm were formed. The number of DRX grains was the largest in the 1.5Sc alloy. Even though the amount of Sc added in this case was two times larger, the number of DRX grains in 0.4Sc was almost the same as that in 0.8Sc. Since continuous DRX occurs only under conditions where the grain boundary mobility is low, the number of DRX grains is strongly related to the pinning force on the initial grain boundary exerted by Al₃Sc particles. The pinning force varied with the diameter and volume fraction of Al₃Sc particles. When the pinning force was calculated from the diameter of Al₃Sc particles, which was measured by TEM, it was proven that the number of DRX grains was proportional to the calculated pinning force. Except in the case of 0.4Sc, the Al₃Sc particle diameter was twice that obtained at the maximum pinning force (d _max ). It is possible to promote continuous DRX by decreasing the Al₃Sc particle diameter in 0.8 and 1.5Sc alloys. Lowering the heating rate before heat treatment for degassing reduced the critical nucleus size for precipitation of Al₃Sc particles as well the diameter of Al₃Sc particles. Thus, the pinning force increased in 0.8Sc and 1.5Sc alloys, and the number of DRX grains also increased, as expected.