Calorimetric Studies of Precipitation and Dissolution Kinetics in Aluminum Alloys 2219 and 7075

Differential scanning calorimetry (DSC) was used to study the kinetics of precipitation and dissolution of metastable and stable phases in aluminum alloys 2219 and 7075. A comparison of DSC scans obtained at heating rates of 1, 5, 10, and 20 K per minute showed that, during a DSC scan, the rates of precipitation of θ′ and θ in 2219 and η′ and η in 7075 were limited by their reaction kinetics. Likewise, the rates of dissolution of GP zones, θ′ and η′, were found to be dominated by kinetics. In contrast, the dissolution of θ and η was dominated by the thermodynamic equilibrium between these phases and the matrix. Analysis of the kinetically dominated reaction peaks and their dependence on heating rate and particle size showed that the GP zone dissolution reaction could best be described by a three-dimensional volume diffusion limited rate expression with an activation energy equal to that for diffusion. The rate of formation of θ′ was best described by an Avrami expression withn = 1.1, indicating that nucleation was not the rate controlling step. A pronounced dependence of the θ′ formation rate on prior plastic deformation was observed and ascribed to the influence of the matrix dislocation density on diffusivity.     

Studies of retrogression and reaging behavior in a 1441 Al-Li-Cu-Mg-Zr alloy

A 1441 Al-Li-Cu-Mg-Zr alloy in the peak-aged T8 temper was subjected to retrogression treatment and immediately reaged to various tempers. Transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), hardness measurements, tensile testing, scanning electron microscopy (SEM) fractography, and electrochemical polarization studies have been made to characterize the retrogression and the retrogression and reaging (RRA) behavior of the alloy. Retrogression of the T8 temper causes dissolution of δ′ (Al₃Li) precipitates into solid solution, resulting in a decrease of hardness and tensile strength and an increase of ductility. Reaging the retrogressed state causes restoration of strength and ductility properties because of the reprecipitation of the δ′ phase in the matrix, confirmed by TEM, XRD, and DSC studies and image analysis. The SEM fractographs revealed predominantly intergranular fracture in all the tempers due to strain localization at the grain boundaries. The gross aging time of the RRA tempers approaches that of the over-aged T7 temper, which would contribute to an improvement of the stress-corrosion cracking (SCC) resistance while maintaining the T8 temper mechanical properties.     

The Kinetics of Slag-Metal Reactions

Abstract

The experimental techniques which are available for the study of the kinetics of slag-metal reactions are considered. The possible rate controlling mechanisms are briefly reviewed. An analysis is made of the various studies of slag-metal reaction kinetics and attempts are made to correlate the results of different investigations. Of particular interest in this context are the studies of silica reduction by carbon in iron and sulphur transfer studies between slag and metal. Studies of manganese, phosphorus and chromium transfer in ferrous systems are briefly reviewed as well as the reduction of ferrous oxide from slags.

Résumé

Dans cet article, les méthodes expérimentales disponibles pour l'etude de la cinétique des réactions métal-scorie sont exposées. Les mécanismes possibles pouvant contrôler les taux de réaction sont briévement revus. Une analyse des diverses études sur la cinétique des réactions métal-scorie est faite. De plus, des corrélations entre les résultats des différentes études ont tente d'etre obtenues. Un intérét dans ce domaine est l'etude de la réduction de la silice par le carbone present dans le fer, ainsi que les études du transfert du soufre entre le métal et la scorie. Les études sur le transfert du manganése, du phosphore et du chrome en présence de fer sont briévement revus ainsi que la réduction de l'oxyde de fer contenu dans la scorie.     

Mechanism and Kinetics of Solid-State Transformation in High-Temperature Processed Linepipe Steel

A relatively new class of linepipe steels with yield strength greater than 500 MPa created for thermomechanical processing at temperatures in excess of 1473 K (1200 °C) has established a firm foothold in the market for modern, large diameter, and high-pressure gas transmission systems. The design concept for the steels takes advantage of the enhanced role which higher levels of niobium can play in very low carbon steels, during the plate manufacturing process. The transformation products observed after cooling have been interpreted in conflicting ways in the literature, using ambiguous terms which are not established rigorously. Revealing characterization experiments have therefore been conducted to establish that the principal transformation product grows by a displacive transformation mechanism, and that it is properly identified as bainite. The implications of this, on both the interpretation of microstructure and on the processing of the steel, are discussed.     

Cooling Curve Analysis to Determine Phase Fractions in Solid-State Precipitation Reactions

This work presents a new method of cooling curve analysis to make in situ measurements of the amount of precipitate formed in solid-state phase transformations. The presented technique is based on a first-principles analysis of thermodynamics and heat flow to develop equations that relate cooling curve data to the amount transformed. The precipitation of Ag₂Al in a binary Al-Ag alloy was examined both as a practical example of this technique and to obtain metallographic measurements of the amount of precipitation for comparison purposes.     

Thermodynamic Properties of Magnesium-Boron Binary Alloys Determined Using Solid-State Electrochemical Measurements

Metallurgical and Materials Transactions B - This study provides the thermodynamic properties of the Mg-B binary alloys. Experiments were performed to measure the electromotive force (emf) as a...     

Transient Kinetics of Slag Metal Reactions

The kinetics of slag metal reactions are complex and often transient, in the sense that interfacial area, the equilibrium driving force, temperature gradients, and fluid properties are changing with time. This highly transient behavior is challenging to model using simple ordinary differential equations, and new theoretical approaches must be developed to deal with the complexity associated with these systems. Three examples from recent studies are described to illustrate methods of analyzing transient behavior. The first is desulfurization of steel in ladle metallurgy where the equilibrium driving force is changing with time, and the second is the case of reacting droplets in oxygen steelmaking where “bloating” of the droplet has a dramatic effect on the kinetics. The third is in the case of reactions between an alloy droplet and slag that result in large changes in interfacial area due to surface tension driven flows. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France.

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Solid-State Oxidation Kinetics of a Lanthanum-Modified Al + 6% Li Alloy

This article describes the results of thermogravimetric analysis of the oxidation kinetics of Al + 6% Li alloy modified by lanthanum in amount up to 0.5%. It is demonstrated that the addition of lanthanum to the alloy or an increase in temperature provides an increase in the oxidation rate of the initial alloy in the solid state. The apparent oxidation activation energy of the initial (basic) alloy (35.2 kJ/mol) decreases to 17.3 kJ/mol for the alloy with 0.5% La. The oxidation curves of Al + 6% Li + La alloys in the solid state are described by equations evidencing a hyperbolic mechanism of this process.     

Kinetics of the ζ to μ, Transformation in Ag-Al Alloys

The kinetics of the ζ → μ polymorphic transformation in Ag-24.0, 25.0, 26.0, and 27.0 at. pct Al alloys have been examined. The quenched ζ-phase transformed completely to the μ-phase on isothermal annealing within the μ-phase field. The transformation process exhibited an activation energy comparable to a short-range-diffusion controlled reaction. Decomposition of the quenched ζ within the (μ + ζ) two-phase field occurred by two processes. The surface transformed by the development of a product comparable to that obtained by transforming within the μ single phase field. The remaining bulk of the product transformed by the nucle-ation and growth of a long-range-diffusion controlled Widmanstätten product. No metastable extension of the boundary of the μ-phase was observed. The growth of the surface transfor-mation product terminated prior to the visible appearance of the Widmanstätten morphology.     

Kinetics of Martensite Formation in Substitutional Fe-Al Alloys: Dilatometric Analysis

High-resolution differential dilatometry was employed to study the kinetics of the martensite formation upon isochronal cooling/quenching of substitutional Fe-(0.5, 0.7, and 1.0) at. pct Al alloys at fast cooling/quenching rates in the range of 17 K (17 °C) through 100 K (100 °C) s^?1, with an emphasis on the as-yet unexpected influence of cooling/quenching rate. The martensite transformation initiated at nearly the same temperature (i.e., the $ M_{\text{S}} $ temperature) in the ferrite-phase region for all cooling/quenching rates applied, which indicates athermal nucleation: the chemical driving force governs the initiation of the nucleation of the martensite plates. Variation of the cooling/quenching rates revealed two principal kinetic features: both the temperature ranges passed during transformation and the grain size of the product martensite increase with the increase of cooling/quenching rates. A modular phase-transformation model, incorporating a classic partitioning analysis for nucleation and anisotropic growth for impingement, has been employed to extract the velocity of the migrating martensite/austenite interface from the dilatometric data. The thus obtained velocity of the martensite/austenite interface as function of temperature indicates a thermally activated growth governed by relatively lower activation energy, as determined by evaluation of the martensite-formation-rate maximum as function of cooling/quenching rate.     

Crystallization Kinetics of Pr_8Fe_(86-x)Zr_xB_6 Amorphous Alloys

Ｎａｎｏｃｏｍｐｏｓｉｔｅｍａｇｎｅｔｓ ,ｗｈｉｃｈｃｏｎｓｉｓｔｏｆａｔｗｏ ｐｈａｓｅｄｉｓｔｒｉｂｕｔｉｏｎｏｆｈａｒｄ ａｎｄｓｏｆｔ ｍａｇｎｅｔｉｃｇｒａｉｎｓ,ｈａｖｅａｔｔｒａｃｔｅｄｃｏｎｓｉｄｅｒａｂｌｅｉｎｔｅｒｅｓｔｓｓｉｎｃｅｔｈｅｙｃｏｕｌｄ ,ｂｙｅｘｃｈａｎｇｅｃｏｕｐｌｉｎｇ ,ｐｏｔｅｎｔｉａｌｌｙｐｒｏ ｖｉｄｅａｍａｘｉｍｕｍｅｎｅｒｇｙｐｒｏｄｕｃｔ ,(ＢＨ ) ｍａｘ,ｉｎｅｘ ｃｅｓｓｏｆ 10 0ＭＧＯｅ[1] ,ｗｈｉｃｈｉｓｍｕｃｈｌａｒｇｅｒｔｈａｎａｎｙｓｉｎｇｌ…     

Fe—P—C—Cu—Mo—Si非晶材料的晶化动力学

用不同加热速度下差热扫描量热卡计研究了Ｆｅ７９．５Ｐ１８－ｘＣｘＣｕ０．Ｍｏ０．５Ｓｉ１．５非晶材料的晶化动力学。实验结果表明：磷Ｆｅ（ｂ．ｃ．ｃ）起始晶化温度升高。对Ｆｅ７９Ｐ１８－ｘＣｘＣｕ０．５Ｍｏ０．５Ｓｉ１．５非晶材料，随着碳含量增多含量的减少，ＤＳＣ曲线出现双峰，并且两峰峰值温差ΔＴ＝ＴＰ２－Ｔｐ１加宽。     

AgCu_4Ni_(0.3)合金的内氧化动力学、组织结构和性能的研究

内氧化Ag基合金由于具有优良的导电性和导热性而成为一种重要的电接触材料。利用X射线衍射仪(XRD)、光学显微镜(OM)和显微硬度仪对AgCu4Ni0.3合金在大气条件下,700℃时的内氧化行为、组织结构以及性能进行研究,并从热力学和动力学的角度对合金的内氧化机制进行了分析。研究结果表明,在700℃氧化8.5 h后,AgCu4Ni0.3合金的氧化层主要由CuO以及NiO所组成,并没有明显的Ag的氧化物形成。Cu,Ni元素在合金中发生了明显的选择性氧化。横截面形貌表明厚度为0.8 mm的AgCu4Ni0.3合金在大气退火8.5 h后已完全氧化,CuO和NiO颗粒均匀弥散分布于氧化层基体中。内氧化后合金的硬度值(HV)高达126,远高于退火态初始合金的硬度。综上结果表明AgCu4Ni0.3合金在大气条件下的内氧化是可行的,且其内氧化动力学符合抛物线规律。这与AgCuNi合金的内氧化热力学计算结果完全一致,为AgCuNi合金的内氧化生产提供了可靠的理论和实验依据。     

块体Pd77.5Cu6Si16.5非晶合金的晶化动力学研究

应用变温条件下的JMA方程和Kissinger方程研究长体Pd77.5Cu6Si16.5非晶合金的晶化动力学行为。计算了其玻璃转变温度Tg，晶化起始温度Tx，晶化峰值温度Tpl的激活能E，频率因子和Avrami指数等晶化动力学参数。并将这些参数与非晶条带的相应参数做对比，发现一些描述晶化过程的参量发生了变化。实验结果表明，块体非晶合金的原子活动性小于传统非晶条带，而具有更宽的过冷液相区。     

固态铅、锡及其合金表面腐蚀动力学

用增重法和俄歇能谱(AES)纵向溅射法,对冷轧Pb、Sn及其常量配比的Pb-Sn合金用3种不同腐蚀方法测定和计算了其腐蚀速度。实验证明,Pb在实验室大气腐蚀、155℃干热大气腐蚀和100℃水蒸气腐蚀下的腐蚀速度符合抛物线关系式。同时也证明,Sn和Pb-Sn合金在100℃水蒸气下的腐蚀速度也符合抛物线关系式。并给出了155℃干热腐蚀288h和大气腐蚀2.5年的腐蚀层厚度数据。