Papers by Author: Helmut Mehrer

Abstract: Studies of tracer diffusion in silicides of iron and molybdenum mainly from our laboratory are reviewed. For three compositions of the DO3-structured Fe3Si tracer diffusion data of Fe and Ge are available. Fe diffusion is relatively fast and increases with Si content. Ge diffusion, which mimics Si self-diffusion, is fairly slow and almost independent of composition. A Mössbauer study of Fe diffusion revealed nearest-neighbour jumps. Positron annihilation and high pressure diffusion data indicate that, in addition to isolated vacancies, vacancy pairs also contribute to Fe diffusion. Ge and Si diffusion is presumably restricted to the Si sublattice. Tracer diffusion of Fe and Ge has been studied for B20-structured FeSi with the conventional tracer technique. Tracer diffusion studies after implantation of 31Si (half-life 2.6 hours) performed at the IGISOL facility in Jyväskylä, Finland, show that Ge and Si diffusion have similar diffusivities. Fe diffusion rates are considerably slower than those of Si and Ge. Tracer data of Mo, Ge and Si diffusion are available for both principal directions of the tetragonal C11b-structured molybdenum disilicide (MoSi2). For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. Mo diffusion in both principal directions is by orders of magnitude slower than Si and Ge diffusion. The huge asymmetry between Mo and Si or Ge suggests that diffusion of both constituents is restricted to their own sublattice. Positron annihilation studies show formation of thermal vacancies on the Si sublattice. This suggests that Si and Ge diffusion proceed by a vacancy mechanism in the Si sublattice of MoSi2. The anisotropy ratio of Si self-diffusion is attributed to a high mobility of vacancies in the Si double layers perpendicular to the tetragonal axis.

Abstract: Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.

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: Self-diffusion of nickel and manganese has been investigated by the radiotracer technique in Ni50Mn50 alloys over a wide temperature range. Experiments were performed on disordered fcc, B2 and L10 structure phases present in the equiatomic alloy at high, intermediate, and low temperatures, respectively. The diffusivity of manganese was found to be significantly faster (factor 3 to 5) than that of nickel in the fcc and B2 phases. More than one order of magnitude diffusivity increase was observed upon the transition from the higher temperature fcc to the intermediate temperature B2 phase. The activation enthalpy from nickel self-diffusion in the disordered fcc phase is significantly higher than the corresponding value for manganese. In the B2 phase there is only a slight difference between the activation enthalpies of the components, which indicates a coupled diffusion mechanism of the two components. A comparison of the present tracer self-diffusion data with literature data on interdiffusion in the Ni-Mn system permits to estimate thermodynamic factors by using the Darken-Manning equation. The thermodynamic factor varies from 3 to 5 depending on the structure.

Abstract: The tracer diffusivities of 22Na and 45Ca in two high-quality standard silica glasses have been measured in the temperature range between 473 and 783 K. The temperature dependences of the tracer diffusion coefficients in both glasses follow Arrhenius laws. The diffusion of 22Na is six to seven orders of magnitude faster than the diffusion of 45Ca. The ionic conductivity was determined by impedance spectroscopy and the conductivity diffusion coefficient Ds was deduced from the dc conductivity via the Nernst-Einstein relation. The temperature dependences of Ds for both glasses follow also Arrhenius functions. The activation parameters and pre-exponential factors for tracer diffusion and for conductivity diffusion were determined. The activation enthalpy of 22Na diffusion is almost equal to the activation enthalpy of the dc conductivity. We conclude that the conductivity of standard glasses is due to the motion of Na ions. The diffusivities of 22Na and 45Ca in soda-lime glasses increase with increasing Na2O content.

Abstract: We have investigated interdiffusion in iron-aluminium alloys using single-phase interdiffusion couples of FexAl1−x–FeyAl1−y for three combinations of x and y for Al contents between 18 and 49.5 at. % Al. Experimental diffusion profiles were obtained from electron-microprobe analysis of the diffusion zone. Interdiffusion coefficients were deduced via the Sauer-Freise method taking into account volume changes. A temperature interval between 997 and 1447 K was covered in our experiments. Thermodynamic factors were obtained from two theoretical models and judged by an analysis of the Kirkendall effect in the diffusion couples. The Darken-Manning equation was used to deduce self-diffusion coefficients of aluminium from the present interdiffusion coefficients, the thermodynamic factors, the vacancy-wind factors, and the iron tracer diffusivities obtained recently at the M¨unster laboratory. The results show that Al diffusion is always slightly faster than Fe diffusion. The difference never exceeds a factor of three. This small difference indicates that Fe and Al diffusion in B2 ordered iron-aluminides are closely coupled.

Abstract: Nonempirical study of the site preference occupation for Ni and V substituting in Fe3Al has been carried out in the framework of the coherent potential approximation. Obtained values of total energies show in a full agreement with experiments that Ni atoms in the equilibrium configuration occupy the iron sub-lattice for alloying with 5 at % of Ni in the Fe3Al-based alloy. Calculations of alloys with the V-doped iron aluminide in the D03 phase show differences in bonding and site occupation preferences in comparison with Ni doping. V atoms occupy aluminum sublattice.