granular networks, semi-transparent blobs and large, single spheres, the first mentioned being the predominant. The most frequent populations in the MIG fumes were granular networks and clusters of globules. Other popula- tions included small particles in long, linear chains and large, single spheres.

The particle size varied from below 0.01 pm (granular networks) up to 10 urn (large spheres) in fumes from both weld- ing methods, although the average size was consistently smaller in the MIG than in the MMA fumes, due to the differences in occurrence of various particle popu- lations.

The chemical composition of most pop- ulations varied to such an extent from particle to particle that they obvious- ly are consisted of several different compounds, and probably even of meta- stable, non-stoichiometric components in varying ratios.

ELECTRON MICROPROBE STUDY OF THE IRON AND IRON-MANGANESE SILICATES OF THE HIGH-GRADE IRON FORMATIONS OF SOUTHERN KARNATAKA, INDIA

K. Laajoki

Department of Geology, University of

OUlU, Linnamaa, SF-90570 Oulu,Finland

The high-grade Archean quartz-banded iron formations of the Halaguru-Satnur area in southern Karnataka can be divid- ed into Mn-poor (BIF) (MnO less than 1%) and Mn-bearing (MnBIF) (MnO up to 10%) types. Their main Fe and Fe-Mn sili- cates are pyroxenes and garnet with pyr- oxenoid in the Mn-richest varieties and occasional hornblende, This paper re- ports microprobe results based on 48 samples collected from 28 occurrences.

The orthopyroxenes are ferrohyper- sthenes and manqano-ferrohypersthenes in the BIF and MnBIFs with MnO contents of 0.5 - 1% and 4.5 - 18.5%, respective-

ly- As far as the author knows, the MnO content of 18.5% is the highest so far reported for orthopyroxenes. The chemical compositions of the clinopyrox- enes lie close to the border between the auqite-ferroauqite and salite-ferrosal- ite fields. The MnO contents of the BIF and MnBIF clinopyroxenes are about 0.2 and 2.5 - 8.5%, respectively. The two pyroxenes mostly occur intergrown inti- mately with each other.

The occurrence of garnet is most com- mon in MnBIFs and its amount seems to

increase with increasing Mn content. The MnBIF garnets are almandine-spessartite (MnO content 5 - 24%), while the few BIF garnets are almost pure almandine (MnO < 2%).

The pyroxenoid belonging to the pyroxmanqite-pyroxferroite series has been analysed from six occurrences. The FeO:MnO ratios are close to 1:l and so they are either calcian manqanoan pyrox- ferroites or calcian ferroan pyroxman- gites with 34% CaO and 2 - 5% MqO.

LONG- AND SHORT-RANGE ORDERING IN THE a-AlFeSi PHASE IN Al-Fe-Si

Ping Liu and G. L. Dunlop

Department of Physics, Chalmers Uni-

versity of Technology, S-412 96

Gdteborg, Sweden

The bee a-AlFeSi phase (Im3 a=12.56B) often precipitates during solidification of subsequent heat treatment in many Al alloys. Extra reflections have been found in SAD patterns from this phase in dilute Al-Fe-Si alloys. A previous ex- planation in terms of a hexagonal super- structure for these extra reflections does not agree with diffuse scattering which is observed for certain beam dir- ections. In the present work the crystal structure was analyzed by a combination of SAD, CBED, EDX and HREM.

The perfect stoichiometric structure of bee ol-AlFeSi should contain 114 Al plus Si atoms and 24 Fe atoms per unit cell. In the present work, however, analysis by EDX showed that rr-AlFeSi con- tained 14.5 at% Fe. This implies that there are 4 vacancies in sites which are reserved for Fe atoms.

CBED pattern along a <ill> beam dir- ection showed that the 4 vacancies are ordered along this axis of the a-AlFeSi structure. Ordering along <ill> results in a triqonal superlattice with the fol- lowing parameters:

a1=30.76% a2=30.76a c = 32,638

a=900 8=900 y = 1200

For some adjacent points in this suuerlattice, atom sites,

the same corresponding Fe of the 12 available, may be

preferred as vacant sites. This gives rise to simultaneous short-range order and explains the observed diffuse scat- tering patterns associated with the su- perlattice reflection in certain beam directions.

Abstracts of the Scandtnavian Society for Electron Microscopy 405

Electron diffraction data was in agreement with structure factor calcula- ted for the trigonal superlattice and lattice images were also in agreement with this structure. No ordering was observed in a-loys where Cu replaces Fe in the a-precipitate phase.

SYMMETRY DETERMINATION BY CBED OF TWO RECENTLY DISCOVERED Al-Fe-Si PHASES

Ping Liu and G. L. Dunlop

Department of Physics, Chalmers Uni- versity of Technology, S-412 96 Gateborg, Sweden

The crystal structures of two inter- metallic phases which have been recently discovered in a dilute Al-Fe-Si alloy have been determined by convergent beam electron diffraction.

ql-AlFeSi (c-centered orthorhombic, a=12.7R. b=36.2g, c=12.78), which forms during fairly fast solidification (%lO°C/s) of the alloy, was found by CBED to have the Cmmm space group. During heat treatment at 600°C, q -AlFeSi transforms to q2--AlFeSi (mono- clinic !3=109o;.

a=12.5g, b=12.3W, c=19.38, This phase was found to have

the Pm space group.

AN ANALYTICAL ELECTRON MICROSCOPIC INVESTIGATION OF A NICKEL-ALUMINIUM- BRONZE ALLOY

G. W. Lorimer

Department of Metallurgy and Mater- ials Science, University of Manches- ter/UMIST, Grosvenor Street, Manchester Ml 7HS, England

The phases present in cast and heat treated nickel-aluminium-bronze (NAB) alloy containing lOwt%At-5wt%Fe and -5wt%Ni have been characterised using a variety of metallographic techniques, including optical microscopy, scanning electron microscopy and analytical elec- tron microscopy.

The as-cast microstructure consists of f.c.c. a phase, Fe3Al precipitates, a eutectoid decomposition product con- sisting of NiAl and cx and martensitic 6 phase. Upon tempering the martensitic B phase disappears, the microstructure coarsens and precipitation of NiAl oc- curs within the a matrix and on pre- viously formed Fe3Al particles.

The scale and complexity of the mi- crostructural features are such that a complete characterisation of the alloy is only possible by analytical electron microscopy.

QUANTITATIVE ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY OF INORGANIC MATERIALS

G. W. Lorimer

Department of Metallurgy and Mater- ials Science, University of Manches- ter/lJMIST, Grosvenor Street, Manchester Ml 7HS, England

Modern analytical transmission elec- tron microscopes can be used to obtain morphological, crystallographic and chemical information from a single, thin specimen. The morphological information, with a resolution of less than 0.5 nm, can be correlated with crystallographic data and chemical analyses with a spa- tial resolution of less than 20 nm. The chemical analyses can be obtained by a number of techniques which include X-ray spectrometry, usually with an energy- dispersive X-ray detector (EDS), and electron energy loss spectrometry (EELS). At the present time X-ray spectrometry has an advantage over EELS in that tech- niques have been developed to enable quantitative analyses to be carried out very simply, while quantification is not as straightforward using EELS.

Quantification of the X-ray data is based on the simple relationship

cA __=k IA

cB AB q ’

where IA and I istic X-ray in B

are measured character- ensities from elements A

and B, CA and CB are weight fractions and kAB is a constant which may be cal- culated or determined experimentally. The major correction which may need to be made to the equation is for X-ray absorption with the specimen. Criteria for specifying the spatial resolution for the microprobe analysis of thin spe- cimens, the minimal detectable weight fraction and the minimum detectable mass have been derived and are discussed.