The effect of semicoherent precipitation on void swelling in Al-Cu alloys

Aluminum-4 wt pct copper alloys previously heat treated (50 h at 473 K) to contain semi-coherent θ′ precipitate were neutron irradiated to a fast fluence of 2.8 × 10²⁶ n/m² (E > 0.1 MeV) at 328 K to determine the effect of the precipitate on void swelling relative to swelling observed in elemental aluminum under the same irradiation conditions. The ob-served void swelling values were ∼1 and ∼12 pct for the alloy and elemental aluminum, respectively. The broad faces of the θ′ precipitate underwent at least partial coherency loss, the matrix dislocation density increased somewhat, and some of the transmutation-formed silicon segregated to θ′ interfaces and some voids during the irradiation. The θ′ particle size distribution did not change significantly during the irradiation. The swell-ing behavior of the alloy under irradiation was analyzed by extending the rate theory of swelling to include reduction of irradiation-caused point defect concentration by indirect recombination, a result of trapping free point defects at θ′ interfaces. Calculated swell-ing in the alloy using the model agreed well with the experimental observation. Accord-ing to the model the suppression of swelling by θ′ precipitates under the irradiation con-ditions used occurs by extension of the knee of the swelling curve, not by changing the dose exponent for swelling. This paper is based on a presentation made at a symposium on “Radiation In-duced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8,1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

Effect of manganese on the stability of austenite in Fe-Cr-Ni alloys

The phase equilibria between austenite and ferrite in the Fe-Cr-Mn-Ni quaternary system have been computed in the temperature range of 900 ° to 1150 °, using recent thermodynamic interaction parameters. From the computed results, the effectiveness of Mn in replacing Ni as an austenite stabilizer has been evaluated as a function of composition and temperature. The results show that, for 18 wt pct Cr alloys, the computed Ni equivalent of Mn is around zero and becomes negative at higher Cr contents, which is rather surprizing. The function of Mn in 200-type stainless steels is then not so much to stabilize austenite but to supplement the role of Cr in increasing the solubility in austenite of N, which appears to be the more important austenite-stabilizing element.     

High nitrogen concentration in Fe-Cr-Ni alloys

Increasing the nitrogen concentration in iron and iron alloys significantly improves their mechanical properties. A recent technique for melting in a hot-isostatic pressure furnace using nitrogen as the pressurizing gas has been developed by United States Bureau of Mines researchers for making massive nitrogen additions to iron (up to 1.6 wt pct nitrogen) and iron-chromium-nickel alloys (up to 6.6 weight percent nitrogen). The total nitrogen concentration measured at atmospheric pressure and room temperature was determined to be the equilibrium nitrogen concentration in the molten alloy. Statistical correlations were derived to explain the effects of melt pressure and alloy composition on the resulting nitrogen concentration. Nitrogen concentrations measured in solidified alloys made by high-pressure melting technique at lower pressures are consistent with previously published data. Computer generated phase diagrams for high nitrogen-chromium concentrations are also consistent with nitride microstructure observed after high-pressure melting. Extension of existing atmospheric nitrogen concentration data to higher pressure nitrogen concentrations shows Sievert’s Law (nitrogen concentration is proportional to the square root of the nitrogen melt pressure) to be valid for pure iron. However, substantial deviations from Sievert’s Law are observed for higher alloy compositions. Statistical fits of thermodynamic concentration data to the high-pressure melt nitrogen data require evaluating element concentration terms, interaction effect terms, pressure terms, and pressure-composition effect terms. Examination of the nitrogen concentration data suggests several methods of correlation.     

原位观察电子束辐照条件下SUS316L钢中辐照肿胀行为

辐照诱导的过饱和空位聚集形成空洞,进而引起的奥氏体不锈钢辐照肿胀现象会造成核反应堆内结构材料失效,危害反应堆安全.利用超高压电子显微镜,在300~500 ℃区间内,对SUS316L奥氏体不锈钢进行电子束辐照,原位观察辐照过程中空洞演变;辐照结束后,在透射电镜下观察空洞分布,并对空洞尺寸进行测量统计;对比研究不同温度下空洞演变的差异.结果表明:不同温度时电子辐照30 min后,可观察到点缺陷簇、位错环和空洞.其中450 ℃和500 ℃时,空洞尺寸最大,分别约为13.3 nm和14.5 nm.这是因为辐照温度升高会提高空位扩散能力,最终增大空洞尺寸.研究结果可为预测不同工作温度下奥氏体不锈钢中辐照肿胀提供实验依据.      

Nitrogen solution and titanium nitride precipitation in liquid Fe-Cr-Ni alloys

The solubility of nitrogen in liquid Fe-Cr, Fe-Ni, Ni-Cr, and Fe-Cr-Ni alloys up to 20 wt pct Ni and 40 wt pct Cr was measured by the Sieverts’ method. The first and second order interactions in iron between nitrogen and chromium, and nitrogen and nickel were determined. Chromium increases the nitrogen solubility at lower chromium concentrations but the second order interaction term which is of the opposite sign becomes significant at higher chromium levels and compensates partly for the effect of the first order interaction term. Nickel decreases the nitrogen solubility in iron. Titanium nitride formation in liquid Fe-Cr, Fe-Ni, and Fe-Cr-Ni alloys also was investigated. The first and second order interactions between titanium and chromium or nickel were determined at 1600°C. Chromium increases the solubility product of TiN, principally by decreasing the activity of nitrogen in the melt. Nickel decreases the solubility product of TiN by increasing the activities of nitrogen and titanium.     

Nickel and nitrogen ion irradiation induced void swelling and defect microstructures in Ni-Al,Ni-Cr and Ni-Ti solid solutions

The effects of solute content, temperature, and irradiation dose on the void swelling characteristics of pure nickel and several nickel base solid solutions (Ni-Al and Ni-Ti containing up to 8 at. pct solute and Ni-Cr containing up to 16 pct Cr) have been iN_vesti-gated. Samples were irradiated in the temperature range 400 to 650°C to a maximum dose of 70 dpa using 3.5 MeV⁵⁸Ni⁺, 400 keV¹⁴N⁺ and 400 keV¹⁴N⁺ ₂ ions. The irradiation in-duced microstructures were studied using transmission electron microscopy. In general, the addition of Al, Cr or Ti to Ni is found to decrease the void swelling and mean void diameter and to increase the dislocation density. The behavior of the void number den-sity, N_v, as a function of solute content is found to be dependent upon the irradiation con-ditions as well as the particular solute addition. N_v passes through a maximum at approxi-mately 2 pct solute content for Ni-AI and Ni-Cr alloys irradiated at 550°C, but through a minimum at 4 pct for Ni-Ti alloys irradiated at 550 and 600°C. N_v decreases monotoni-cally as a function of Al content at 600 and 650°C. The results are discussed in terms of recent theories of void swelling suppression due to impurity or solute additions and in light of several correlations between void swelling and material parameters. The be-havior of N_v is found to be best described by the actions of two compcting processes. The first enhances void nucleation, is not strongly temperature dependent and is dominant at low solute contents. The second suppresses void swelling, is probably diffusion con-trolled and dominates in the more concentrated alloys. R.F.PINIZZOTTO, JR., formerly Graduate student, UCLA, This paper is based on a presentation made at a symposium on “Radiation Induced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of     

Phase stability and phase transformations in plutonium and plutonium-gallium alloys

The complexity of phase stability and transformations in plutonium alloys is reflected in the plutonium-gallium (Pu-Ga) phase diagram, which is perhaps the most complex of all binary systems. Although many investigations have explored phase equilibria, transformation systematics, and structure/property relations in the Pu-Ga system, many outstanding problems remain and new issues regularly appear. In this article, we describe recent dilatometry and calorimetry measurements on pure plutonium and plutonium-gallium alloys. We also present recent phase diagram modeling that attempts to unravel differences between the U.S. and Russian Pu-Ga phase diagrams. The ultimate goal of this work is to produce the first internally consistent database of thermophysical properties of this system so that a true equilibrium phase diagram can be produced and so that stability can be predicted over a range of conditions. This article is based on a presentation in the symposium “Terence E. Mitchell Symposium on the Magic of Materials: Structures and Properties” from the TMS Annual Meeting in San Diego, CA in March 2003.     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at {dy1873} K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logf_N, 1873K = 0 to -1.4 as, log (wt pct N)_T = (-247/T - 1.22) - (4780JT - 1.51) (logf_n, 1873K)-(1760/T -0.91) (logf_N,{dy1873}K )².     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at 1873 K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logJn, 1873K = 0 to −1.4 as, log(wt pct N)_T = (-247/T-1.22)-(4780/T-1.51) (logf N, 1873K)- (1760/T-0.91) (logfN,1873K)².     

Numerical modeling of solidification and subsequent transformation of Fe-Cr-Ni alloys

A computational method for the analysis of phase transformation involving solidification was developed with the assumption of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition. Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used.     

The temperature dependence of stacking fault energy in Fe-Cr-Ni alloys

The variation of intrinsic stacking fault energy (γ_i) in two austenitic Fe-Cr-Ni alloys has been determined from dislocation node measurements over the range 25‡ to 325‡C by means of high temperature transmission electron microscopy. In both alloys γ_i increases with temperature. Both reversible and irreversible effects have been observed in cyclic heating-cooling experiments. After the first heating to elevated temperature the irreversible component is removed and thereafter cyclic annealing produces essentially reversible changes in γ_i. The large reversible changes are best understood in terms of the variation in the stability of austenite with temperature. The smaller irreversible effect appears to arise from the formation of substitutional solute atmospheres around partial dislocations at elevated temperature. Formerly NAS-NAE-NBS Post Doctoral Research Associate, National Bureau of Standards, Washington, D. C.     

Atomic rearrangements in ordered fcc alloys during neutron irradiation

We describe three sets of experiments performed at Argonne National Laboratory over the past few years. These experiments deal with atomic rearrangements in the ordered alloys Ni₃Mn and Cu₃Au during fast and thermal neutron bombardment. The unique mag-netic properties of ordered Ni₃Mn are utilized to investigate radiation damage produc-tion mechanisms at low temperature (5 K) where defect migration is not possible and only disordering is observed. In the case of thermal neutron bombardment, the average recoil energy is about 450 eV and significant disordering due to (110) replacement col-lision sequences is observed. For fast neutron bombardment where typical recoil ener-gies are 20 keV, significant random disordering is observed but no evidence for sizable replacement sequences is found. The bombardment of ordered Cu₃Au by fast and thermal neutrons at higher temperature (∼150°C) is studied by electrical resistance techniques. Both ordering and disordering are observed and related to the number of migrating vacan-cies escaping from the high energy collision cascade. This paper is based on a presentation made at a symposium on “Radiation Induced Atomic Rearrangements in Ordering and Clustering Alloys” held at the annual meeting of the AIME, Atlanta, Georgia, March 7 to 8, 1977, under the sponsorship of the Physical Metallurgy and Nuclear Metallurgy Committees of The Metallurgical Society of AIME.     

In-Situ observations of oxidation and phase stability in cast nickel-based intermetallic alloys

The effects of the as-cast microstructure on the oxidation characteristics of two Ni-Al-Cr alloys with either γ or γ′ primary solidification were investigated with an in-situ, time-resolved X-ray diffraction (TRXRD) technique using synchrotron radiation. The measurements, carried out during rapid heating and cooling, showed that a segregated microstructure in these cast alloys leads to the preferential formation of zirconium oxide before the formation of aluminum oxides is detected. The oxidation leads to a change in the phase stability and to the modification of surface microstructures. Computational thermodynamic models were used to explain the preferential formation of oxides in the as-cast microstructure.     

Thermal equilibria and mechanical stability of Ti3 Al phase in Ti-Mo-Al alloys

Metallurgical and Materials Transactions A - The phase diagram of the isopleth section of the Ti-7 at. pct Mo-Al system has been improved and expanded to include alloys up to 25 at. pct aluminum....     

Equilibrium phase relationships during solidification of Fe-O-C-X alloys

Abstract

Analysis of the solidification process of alloys requires that the phase relationships at equilibrium be known. The problem is complex and difficult for commercial ferrous alloys because of the presence of iron, carbon, oxygen, a deoxidizer, manganese, sulphur, etc. This study develops the phase relationships in the iron-rich corner of four component systems and covers the Fe-O-C-Si and Fe-O-C-Al systems specifically. The equilibrium state is treated at 1 atm pressure. The effects of suppression of formation of CO and of segregation of elements between liquid and solid iron are then analyzed.

Résumé

L'analyse du procédé de solidification des alliages requiert que la relation des phases à l'equilibre soit connue. Le problème est complexe et difficile pour les alliages ferreux commerciaux à cause de la présence de fer, de carbone, d'oxygene, d'un désoxydant, de manganèse, de soufre, etc. L'étude présente développe la relation des phases dans la région riche en fer d'un système à quatre composants, en particulier les systèmes Fe-O-C-Si et Fe-O-C-Al. L'équilibre est traité à 1 atm. de pression. Les effets de la suppression de formation de CO et de ségrégation d'éléments entre le fer liquide et solide sont ensuite analysés.