Martin Eggersmann

Diffusion in intermetallic phases of the Fe–Al and Fe–Si systems

Abstract Self-diffusion of Fe and impurity diffusion of In in Fe–aluminides of 26.5, 34 and 50 at.% aluminium have been investigated over wide temperature ranges. For Fe diffusion in Fe 3 Al an influence of A2–B2 and B2–D0 3 transitions has been observed. The activation enthalpy increases with increasing order. Diffusion in alloys with higher Al content shows linear Arrhenius behaviour. Impurity diffusion of In is faster than Fe diffusion in Fe 66 Al 34 and Fe 50 Al 50 by about a factor of two. In Fe 3 Al the ratio between the diffusion coefficients of In and Fe varies between two and ten. Indium is homologous to Al and considered as substitute of Al, for which an affordable radiotracer is not available. Self-diffusion of Fe (and Si) and impurity diffusion of Ge in D0 3 -type Fe–Si alloys (24, 21 and 18 at.% Si) published in detail elsewhere are summarized. Diffusion of the majority component Fe is very fast. It occurs via nearest neighbour jumps into vacant sites on the Fe sublattice, which are available in high thermal concentrations. Fe diffusion is fastest for the stoichiometric alloy (highest Si content) and decreases with increasing Fe content. Ge (and Si) diffusion is slower than Fe diffusion by orders of magnitude. Some preliminary results on Fe diffusion in the B20 structured compound FeSi are also reported. Diffusion is by orders of magnitude slower than in Fe 3 Si indicating a strong influence of structure and/or decreasing metallicity on diffusion in this silicide. The results will also be discussed in connection with available studies of Mosbauer spectroscopy, positron annihilation experiments and ab-initio electron theory calculations of defect properties.

A universal ion-beam-sputtering device for diffusion studies

An apparatus for ion-beam-sputtering is described which offers for the first time the possibility of measuring radiotracer diffusion profiles with mean diffusion length (D is the tracer diffusion coefficient and t is the diffusion time) in the nano- as well as in the micrometre range. It is also possible to use the device for ion milling, especially for the deposition of thin layers of radiotracer onto diffusion samples. Investigations of diffusion in pure metals, in a metallic glass, in a compound semiconductor and in intermetallic compounds are presented as examples.

Interface diffusion and amorphous intergranular layers in nanocrystalline Fe90Zr7B3

Iron tracer diffusion was studied in soft-magnetic nanocrystalline Fe90Zr7B3 without any influence of porosity, relaxation, or grain growth. The interfacial diffusion characteristics differ substantially from grain boundaries in metals due to the presence of an intergranular amorphous phase. The reduced diffusivity in thin amorphous layers compared to in the initial amorphous phase indicates the effect of confinement. The indication of a second, fast interfacial diffusion path is found and quantitatively analyzed within the framework of a two interface-type model.

Diffusion in Metals, Quasicrystals, and Intermetallic Compounds

In this paper recent developments of diffusion in three related areas will be reviewed: The first part is devoted to self- and solute diffusion in metals with particular emphasis on Al. Contrary to most other metallic solvents, diffusion of transition elements in Al is anomalous in several respects: diffusion is very slow, activation enthalpies, pre-exponential factors and activation volumes are unusually high. By contrast, non-transition elements in Al show more or less normal solute diffusion behaviour. The anomalous behaviour is attributed to a strong repulsive interaction between transition metal solutes and vacancies. Ab-initio calculations could help to understand this well-documented diffusion problem in detail. In the second part very recent diffusion studies on single crystals of the Al-base quasicrystalline intermetallic compound Al-Pd-Mn will be discussed. Diffusion of Zn, Ge, Mn, Fe, Co, Pd and Au has been studied by various groups. At least in the high-temperature regime diffusion in the quasicrystal – despite some differences in detail – shows striking similarities to diffusion in Al, for which a vacancy-type mechanism is generally accepted. The activation volumes of +0.67 and +0.74 atomic volumes measured for Mn- and Zn-diffusion in Al-Pd-Mn strongly favour a vacancy mechanism as well. For the low temperature regime of Pd and Au diffusion the possibility of a phason-related mechanism is discussed. The third part deals with recent investigations of iron-aluminides and iron-silicides for which Fe self-diffusion and diffusion of selected foreign elements (Ge in Fe–Si, In and Zn in Fe–Al) has been investigated. There is no doubt that vacancies mediate the diffusion process. Within this general mechanism a number of factors like the crystal structure, the state of order, the composition, and the type of bonding have strong influence on diffusion. Such factors are discussed also in connection with results from positron annihilation and M6ssbauer spectroscopy.

Self-diffusion in nanocrystalline Fe and Fe-rich alloys

The present work aims at a comparison of the self-diffusion behaviour of nanocrystalline (n-)Fe produced by cluster condensation and compaction with that of Fe-rich n-alloys made by crystallization of melt-spun amorphous ribbons. In cluster-synthesized Fe (relative density higher than 91 %), a decrease of the (59)Fe tracer diffusivity upon annealing indicates interface relaxation. The diffusion coefficients in the relaxed grain boundaries are similar to those extrapolated from high-temperature data of conventional grain boundaries. Substantially lower interface diffusivities in crystallized n-Fe(90)Zr(10) and n-Fe(90)Zr(7)B(3) presumably arise from residual intergranular amorphous layers. Due to the reduced amorphous fraction, in n-Fe(90)Zr(10) additional fast diffusion paths exist like in conventional grain boundaries.

Intergranular melting of ultrafine grained Nd2Fe14B studied by means of radiotracer diffusion

Grain-boundary (Gb) diffusion was studied in ultrafine grained Nd2Fe14B-based permanent magnets below and above the melting transition of the Nd-enriched intergranular phase using the radiotracer technique with the isotope 59Fe. The product δDGb of interface diffusion coefficient and interface thickness shows a substantial increase above the intergranular melting transition. Assuming a volume self-diffusivity as in α-Fe, an analysis in the framework of grain-boundary diffusion kinetic of type B yields an Arrhenius-type behaviour δDGb = 1.53 × 10−11 exp(−1.74 eV/kT) m3 s−1 below the intergranular melting transition. Similar values δDGb are observed for ultrafine grained Nd-Fe-B with reduced Nd excess in the grain boundaries. The diffusion characteristics are compared with the kinetics of the hot-deformation which is of technical relevance for the processing of high-performance permanent magnets.

Diffusion and induced magnetic anisotropy in nanocrystalline Fe73.5Si13.5B9Nb3Cu1

71Ge tracer diffusion was studied to gain insight into the atomistic transport processes underlying the formation of magnetic anisotropy in nanocrystalline soft-magnetic Fe73.5Si13.5B9Nb3Cu1. The interfacial diffusion characteristic was determined by the residual intergranular amorphous phase, which gives rise to a strongly reduced interface diffusivity compared with grain boundaries in metals. Ge diffusion in the nanocrystallites, which is considered to characterize Si self-diffusion, is much slower than Fe diffusion owing to the D03 order of the Fe3Si nanocrystallites. Slow Si diffusion in the nanocrystallites is identified as the rate-controlling process for the generation of the field-induced magnetic anisotropy.

Self-diffusion behaviour and microstructure of ultrafine-grained Nd2Fe14B with intergranular melting transition

The interface diffusion was studied in ultrafine-grained Nd 2 Fe 1 4 B which shows an intergranular melting transition owing to an excess amount of Nd. Below the intergranular melting transition, the interface diffusion of 5 9 Fe is similar to grain boundary diffusivities in bcc Fe. The microstructure and intergranular melting transition show characteristic variations with the Nd excess which sensitively affects the diffusivity at and above the melting transition. In Nd 2 Fe 1 4 B with high Nd excess, which contains extended Nd-rich triple junctions, the melting transition occurs at ca. 950 K in one step leading to an abrupt increase of the diffusion coefficient. In Nd 2 Fe 1 4 B with low Nd excess, which is free from additional phases at triple junctions, the diffusion coefficient above the melting transition increases gradually. Interface diffusion is found to have no rate-controlling influence on the hot-deformation used for the generation of magnetic anisotropy.