The effect of surface laser treatment on the creep of the magnesium-lithium alloy MA21

Conclusion Surface laser irradiation with fusion of the surface leads to reduced creep rate of the alloy MA21 at normal temperature.Production Association "Ura1AZ." Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 39–40, September, 1986.     

The kinetics of duplex scale formation on 18/8 type stainless steel

A previous model of duplex scale growth on 18/8 stainless steels based on the formation of an impervious barrier at hemispherical alloy grain boundaries has been extended to consider the influence of a finite healing layer growth rate. The analysis shows that true primary and secondary or healing layer parabolic rate constants can best be calculated from weight gain vs. the square root of time plots. Accurate determinations of the secondary rate constant requires oxidation exposures a factor of 3 or 4 times greater than the time to form a complete healing layer at the oxide-metal grain boundary interface. Good agreement is found between the present theory and expeimental data and the model gives a simple extrapolation method for the evaluation of long-term metal losses or oxide thicknesses.     

Microstructural formation in Ti alloys: In-situ characterization of phase transformation kinetics

The prediction of microstructure during processing needs to characterize the phase transformation occurring during the thermal treatments and their kinetics. In-situ high-energy synchrotron x-ray diffraction experiments performed during temperature variations allow the characterization of the phase evolution. For some transformation conditions, the continuous recording of diffraction diagrams evidences clearly intermediate phases. The quantitative analysis of the diffraction diagrams gives the transformation kinetics of each phase as well as their cell parameters. Transformation kinetics obtained by this method are compared to results obtained by electrical resistivity.     

The formation of protective films on iron-silicon alloys

The oxidation of high purity FeSi alloys was studied over the temperature range 765–965°C by means of a micro-balance. In hydrogen with 2.5% water vapour only silicon was oxidized. Specimens with 1% Si showed mostly internal oxidation. External layers of SiO₂ formed on specimens with 2 and 3.4% Si. These layers also hindered the oxidation of iron. The oxidation of Fe could be distinguished from oxidation of Si by reduction of the Fe oxides. The low oxidation rates reached at 865° after long times of exposure were ascribed mainly to the oxidation of Si.     

The kinetics of growth and the critical conditions for the formation of the most-stable oxide in the oxidation of binary alloys

During the growth of the most-stable oxide BO _v in the oxidation of binary alloys containing nonnoble components A and B, the oxygen pressure prevailing at the alloy-scale interface is higher than the corresponding value for equilibrium between BO _v and pure B. The effects of this change on the rate constant for the growth of BO _v and on the critical concentration of B in the bulk alloy required for the stability of BO _v on its surface are examined and discussed. The general treatment is then applied to the growth of NiO on Cu–Ni alloys and of Cr₂O₃ on Fe–Cr, Co–Cr, and Ni–Cr alloys by using appropriate defect models for these oxides, considering also the possibility of oxide doping.     

Features of substructure formation on tempering Kh23K15T type alloys

Conclusion Formation of structural morphology in alloys of the Kh23K15T type is accomplished in two heat treatment stages: from ITMT to the final tempering stage at 550–500°C. Depending on the conditions for carrying out stepwise heat treatment at different stages of forming a highly coercive condition it is possible to form different structural imperfections: composition inhomogeneity and secondary decomposition within ₁ - and ₁-phases. Secondary decomposition arises primarily in the case when with a change over from one tempering stage to another overcooling t exceeds 20°C. The most intense secondary decomposition develops at temperatures below 610°C with which the most intense separation of phases in chemical composition probably starts.Moscow Institute of Steels and Alloys. Factory Higher Technical School at ZIL. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 14–16, February, 1990.     

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective —Al₂O₃ scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.     

The effects of nitrogen on the kinetics and mechanisms of oxidation of Titanium-Tantalum alloys

The oxidation kinetics of Titanium alloys containing nominally 5, 40, and 60 weight percent Ta were measured for 16 hr in nitrogen-20% oxygen. In all three alloys at each temperature tested oxidation rates in the nitrogen-oxygen mixture were significantly lower than those previously measured in argon-20% oxygen or pure oxygen. Using X-ray diffraction and electron microprobe a nitrogen rich layer was identified in the scale near the scale/metal interface. This layer apparently acts as an oxygen-diffusion barrier, as demonstrated by the reduced depth of oxygen penetration in the metal relative to identical specimens oxidized in nitrogen free atmospheres.     

The influence of partial cycling on the martensitic transformation kinetics in shape memory alloys

The martensitic transformation kinetics during partial cycling and the so-called “hammer” effect has been carefully characterized in Cu–Al–Ni shape memory alloys. These temperature memory effects were measured by adiabatic calorimetry and analyzed within the frame of a new thermodynamic model. A straightforward study of the nucleation processes explains on quantitative grounds the shift of the reverse transformation to higher temperatures and the presence of a secondary C_p peak associated to these phenomena. The optical observations support the calorimetric results. Finally, the release of the elastic energy in the martensitic state due to the different thermal cycles has been determined.     

Impurities influence on oxidation kinetics of Fe-Ni-Cr-Al alloys

The influence of both additional and constitutional impurities on the oxidation kinetics of Fe-Ni-Cr-Al alloys was investigated. It appears that Al₂O₃ growth never follows a parabolic rate. A cubic law is observed only with pure alloys which develop pure Al₂O₃ scale. For industrial alloys the kinetic law is always complex and is related to the presence of constitutional and/or additional impurities in the oxide scale and to the continuous variation of the Al₂O₃ scale purity. Yttrium or hafnium additions do not change the kinetics.     

镁合金AZ31锰系磷化膜的生长过程及形成机理

以磷酸二氢锰的盐溶液为研究体系,采用SEM和EDS和电化学分析手段,研究镁合金AZ31锰系磷化膜的生长过程和形成机理。结果表明:磷化膜的生长过程分为5个阶段:基体溶解-成核阶段(0～130 s)、基体和磷化膜溶解阶段(130～630 s)、磷化膜快速生长阶段(630～1 300 s)、磷化膜稳态生长阶段(1 300～2 000 s)和磷化终止阶段(2 000 s以后)。磷酸盐晶核在镁合金AZ31浸入溶液的初始阶段一次形成,并优先在β相表面经过成核—长大—分裂—细化—增厚5个过程,沿表面方向生长和外延,最终形成致密的磷化膜。并且,磷化膜有两层,第一层是以Mg3(PO4)2和AlPO4以及MnHPO4为主的沉积薄膜;第二层则是在β相表面成核—长大的MnHPO4磷化膜。     

A study of the film formation kinetics on zinc in different acidic corrosion inhibitor solutions by quartz crystal microbalance

Chromates conversion coatings provide very effective corrosion protection for many metals. However, the high toxicity of chromate leads to an increasing interest in using non-toxic alternatives such as molybdates, silicates, rare earth metal ions and etc. In this work, quartz crystal microbalance (QCM) was applied as an in-situ technique to follow the film formation process on zinc (plated on gold) in acidic solutions containing an inorganic inhibitor, i.e. potassium chromate, sodium silicate, sodium molybdate or cerium nitrate. Using an equation derived in this work, the interfacial mass change during the film formation process under different conditions was calculated, indicating three different film formation mechanisms. In the presence of K₂CrO₄ or Na₂SiO₃, the film growth follows a mix-parabolic law, showing a process controlled by both ion diffusion and surface reaction. The apparent kinetic equations are 0.4t = −17.4 + 20Δm_f + (Δm_f)² and 0.1t = 19.0 + 8.4Δm_f + 10(Δm_f)² respectively (t and Δm are in seconds and μg/cm²). In solutions containing Na₂MoO₄, a logarithmic law of Δm_f = −24.7 + 6.6 ln t was observed. Changing the inhibitor to Ce(NO₃)₃, the film growth was found to obey an asymptote law that could be fit into the equation of Δm_f = 55.1(1 − exp(−2.6 × 10⁻³t)).