Defect and Diffusion Forum Vols. 258-260

Abstract: The science of diffusion had its beginnings in the 19th century, although the blacksmiths and metal artisans of antiquity already used diffusion phenomena to make such objects as iron swords and gilded bronze wares. Diffusion as a scientific discipline is based on several corner stones. The most important ones are: (i) The continuum theory of diffusion originated from the work of the German physiologist Adolf Fick, who was inspired by elegant experiments on diffusion in gases and of salt in water performed by the Scotsman Thomas Graham. (ii) The Brownian motion, observed for the first time by the British botanist Robert Brown, was interpreted decades later by Albert Einstein and almost at the same time by the Polish physicist Marian Smoluchowski. Their theory related the mean square displacement of atoms to the diffusion coefficient. This provided the statistical cornerstone of diffusion and bridged the gap between mechanics and thermodynamics. The Einstein-Smoluchowski relation was verified in tedious experiments by the French Nobel laureate Jean Baptiste Perrin and his coworkers. (iii) The atomistics of solid-state diffusion had to wait for the birthday of solid-state physics heralded by the experiments of the German Nobel laureate Max von Laue. Equally important was the perception of the Russian and German scientists Jakov Frenkel and Walter Schottky, reinforced by the experiments of the American metallurgist Ernest Kirkendall, that point defects play an important role for properties of crystalline substances, most notably for those controlling diffusion and the many properties that stem from it. This paper is not meant as systematic history of diffusion. It is devoted to some major landmarks and eminent pioneers of diffusion including also people from recent decades until today.

Abstract: In the present study, solidification and segregation of Al-Li-Cu-Zr alloys have been investigated. Results show that shape of ingots and amount of lithium can change solidification essence. Increasing solidification rate, not only eliminates grain-refining effect of zirconium but also causes coarse segregation of copper in grain boundaries (negative segregation). Increasing amount of lithium in Al-Cu-Li-Zr alloys can change the solubility of other alloying elements (Cu, Zr) and increase solidification range and microsegregation of copper in structure.

Abstract: Aluminum-Lithium alloys are attractive choices for weight saving of aerospace structures. By addition of Lithium to Aluminum alloys, the mechanical and physical properties are improved, density decreases up to 10% and module of elasticity increases up to 15%. As-cast Al-Li ingots include some continues brittle phases in grain boundaries that decrease formability by crack initiation during hot rolling. In the present work, influence of homogenization on hot rolling of Al- Li alloys was studied. Homogenization temperature determined based on DTA test and homogeneity properties were characterized by microstructures observation. One specimen was cast in conventional method and the other was cast by direct casting (DC) method. Homogenization was performed at four different times: 10, 24, 50 and 100 hours at 500°C also no cracking has been observed in both ingots in 100 hr and 24 hr at 500°C respectively due to eliminate the brittle phases.

Abstract: In this study, diffused carbon in workpiece surface with Plasma Electrolysis Carburizing (PEC) treatment applied to harden SAE 1015 steel is investigated. The workpiece is connected cathodically to a high current DC power supply that applies high voltage and uses stainless steel as the anode. After water ionization with this voltage, a hydrogen film forms around the workpiece. This film acts as a thermal and electrical insulation layer which increases the electric resistance around the workpiece and makes it being heated. The surface of the workpiece is transformed to austenite and after the disconnection of the current, the workpiece is quenched in aqueous solution. By the aid of this method, the workpiece can be hardened to 1200HV or more. In this paper, the effect of time and solution concentration on the depth of diffused carbon was studied. The hardness of workpiece was measured and the variation of the diffusion depth versus time and solution concentration was investigated.

Abstract: Understanding the atomic movements of simultanous diffusion of dopant (B) and point defects (V, I) in silicon is of great importance for both experimental and theoretical diffusion studies. This paper presents the atomistic dynamic diffusion modelling of boron (B), self-interstitial (I) and vacancy (V) process in silicon based on simultaneous diffusion of boron dopant and point defects based on a previous developed theory. The simulation is based on the random walk theory with three main diffusion mechanisms: namely vacancy, interstitial and interstitialcy mechanism. The migration frequencies of dopant and point defects have been programmed based on the experimental diffusion data of boron, vacancy and Si self-interstitial. This simulation procedure can be seen very clearly about the atomic movements, the interactions between dopant and point defects via three diffusion mechanisms. The diffusion depth of B, V, I in very short time can be estimated from the simulation picture on the screen. The simulation results reflect the simultaneous diffusion as well as the interaction of boron and point defects via the three diffusion mechanisms. The point defects (V, I) were generated during the dopant diffusion and they diffused further into the depth as shown in the results of the simulation as well as in the previous published experimental findings.

Abstract: The effect of Si and Al diffusion from a coating in the microstructure of electrical steels have been investigated for three different processing routes. In general the final texture is not affected by the diffusion of Si or Al from the coating whereas the grain size and mor- phology can be affected if the silicon content of the substrate is low enough to allow phase transformation. The gamma to alpha phase transformation caused by the diffusion of Si and Al determines the grain size and morphology resulting in columnar grain growth. The evolu- tion of the microstructures during the diffusion annealing for the production of high Si steels shows some common features with the microstructure evolution during the grain growth in conventional low silicon (Si < 3 wt.%) electrical steels.

Abstract: Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive matrix for nuclear applications, such as inert matrix for the destruction of excess plutonium or good host material for nuclear waste storage. Some actinide elements in high-level radioactive wastes can be simulated by cerium as tetravalent actinide, and gadolinium as trivalent actinide or neutron absorber. The present work is focused on the diffusion study of Ce and Gd in YSZ single crystal and high density polycrystals. A thin film of Ce or Gd was deposited either by spin-coating method or by physical vapour deposition on the surface of polished samples. The diffusion experiments were performed from 1173 to 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. The experiments led to the determination of effective diffusion coefficient, Deff, bulk and grain boundary diffusion coefficients, DB and DGB. The dependence of these diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with literature.

Abstract: Traditional theories of interdiffusion in solids based on Fick’s first and second laws and Darken’s equations can not describe the relationship between the diffusion fluxes and the diffusion-induced stresses, because the subject matter of the traditional theories is the diffusing atom or atomic flux, not the volume unit within the interdiffusion field. For this reason, it is suggested that the concept of flow point in the interdiffusion field should be constructed to describe the diffusion-induced stresses and the phase growth.

Abstract: More than twenty years were needed for people to understand the formation mechanism of the periodic-layered structures not only because of the complications of pattern formation, but also due to the limits of traditional diffusion theories. Based on the general theory of interdiffusion growth proposed by Y.C. Chen et al. [1], the quantitative model of periodic layer formation during solid state reactions has been succeeded [2]. The experimental results shown in this paper proved one of the model’s predicts that the reactive diffusion system Zn/Fe3Si with clamp press annealed at 663K should be mixed controlled. Also, the important influence of the uniaxial compressive stress on the diffusion coefficient was emphasized and the reasons are discussed.

Abstract: Diffusion processes play a dominant part in the macro kinetics of Fe, Ni and Co oxidation by calcium and sodium sulfates. Here, the reaction product forms a compact covering which spatially divides the reagents on the surface in the same way as in the oxidation and sulfidization of metals by oxygen and sulfur. Therefore, it is possible to assume in advance that interaction of metals with calcium and sodium sulfates will be determined not by the actual chemical reaction properly but by the diffusion transport processes.