Volume 89, Issue 5

Solid state sintering of cemented carbides is studied experimentally by means of dilatometry and SEM. Surface analysis of the powders is performed by means of ESCA, Auger and SIMS. It is concluded that for pure WC–Co mixtures the major part of densification occurs in the solid state and that high heating rates yield higher densification rates. 200 ppm of Ti, preferentially located at grain surfaces, yield a drastic decrease in sintering rate. Melting of the binder, i. e. transition to liquid phase sintering, only has a minor effect on densification rate.

Solid state sintering of cemented carbides is studied experimentally by means of dilatometry. Both an ordinary dilatometer and a dilatometer where an axial load was applied on the specimen were used. It is concluded that increased axial load, due to its hydrostatic component, gives larger volumetric shrinkage and that the decrease in the shrinkage rate at isothermal treatment is a result of a rapidly increasing rigidity of the powder compact at isothermal temperatures.

Extrusion of the alloy 6063 through bridge dies producing hollow shapes is a problem common in the aluminium industry. This communication reviews previous work on the extrusion of both hollow and solid shapes and presents methods to determine the pressure requirements and assess the difficulty of extrusion of shaped extrusions in general. Material flow through the bridge die is investigated by optical macroscopy and by use of a finite element program. It is shown that the pressure may be predicted by upper bound techniques and that the process has the same thermally activated features as that for rod extrusion. The finite element program appears to predict all the major characteristics of the flow observed macroscopically.

Titanium Grade 2 was used to study the influence of grain size gradients on the shape of short surface cracks. Round bars with a deformation gradient were used to produce microstructures with increasing and decreasing grain sizes in radial direction by suitable recrystallisation treatments. It is shown that the shape of a surface crack in a gradient material changes during crack growth and thus differs significantly from that in a homogeneous microstructure where the crack proceeds with a constant ellipticity. This change must be taken in consideration for the calculation of crack propagation rates in order to avoid errors in the d a /d N vs ΔK relationship and in the estimation of fatigue life times.

Superelastische NiTi-Legierungen werden aufgrund ihrer hervorragenden biokompatiblen, korrosiven und mechanischen Eigenschaften zunehmend in der Medizintechnik, Mikro-Handhabungstechnik oder Mikrosystemtechnik eingesetzt. Kommt es in ungeschmierten Gleitkontakten zu Reibung und Verschleiß, so kann die Funktionsfahigkeit insbesondere miniaturisierter Systeme stark beeintrachtigt werden. Es wurden daher die Reibungs- und Verschleißeigenschaften von selbstgepaartem, superelastischem NiTi unter ungeschmierter Gleitbeanspruchung in Abhängigkeit von der Belastung untersucht. Als Referenzmaterialien wurden die reinen Metalle Ni und Ti in die Messungen einbezogen. Die Ergebnisse zeigen, daß für alle drei Materialpaarungen die Verläufe der Reibungszahlen und Verschleißintensitäten durch natürliche Oxidschichten, die Bildung oxidischer Deckschichten und Metall–Metall-Kontakt bestimmt werden. Unterschiede im tribologischen Verhalten der einzelnen Materialpaarungen werden mit den Eigenschaften der metallischen Grundmaterialien und ihrer Oxide erklärt und Schlußfolgerungen für den praktischen Einsatz gezogen.

The effect of copper addition in the range from 0 to 3 % on the tensile creep behaviour of gravity cast alloy ZA-8 has been investigated at a temperature of 120 °C and an applied stress of 40 MPa. It was found that copper additions up to approximately 2 % had a profound effect in decreasing the amount of primary creep and secondary creep rate, and therefore substantially increased the creep performance measured as times to attain 1 % creep strain, but copper contents above 2 % produced very little further creep resistance. Transmission electron microscopy work carried out to examine the microstructure of the alloys suggested that substantial improvement in the creep resistance of the alloys brought about by addition of copper was due to the presence of uniformly distributed, fine and dense dispersion of ε (CuZn 4 ) phase precipitates in the eutectic η matrix. The ε phase was also present as discrete particles, much bigger in size and irregularly distributed among the eutectic cell boundaries in the alloys containing high copper content. These bigger particles probably did not have much effect in increasing the creep resistance of the alloys, and therefore increasing the copper content above 2 % produced very little further creep resistance.

The manifestations of serrated yielding behavior at room temperature accompanied with the microstructural transitions in the 8090 alloy were systematically studied. Differential scanning calorimetric analyses in combination with transmission electron microscopic observations were employed to identify the microstructures for specimens tempered under various conditions. Experiments revealed that two types of serration in the stress–strain curves of the 8090 alloy existed, which were clearly related to their microstructural changes and dislocation interactions. The type A serration was mainly due to the work softening of the shearing slip of δ' (Al 3 Li) particles which was found in the specimens of as-quenched, natural-aged and under-aged conditions. Otherwise, the type B serration, which was observed in the stress–strain curves of all the specimens except for over-aged condition, was primarily attributed to the interaction of Cu and/or Mg atoms with dislocations in the solid solution, i. e., resulting from dynamic strain aging.

Catalytic lead (IV) dioxide coatings have been produced on titanium and stainless steel 316 and 316L anodes. Good adherence of the lead (IV) dioxide coatings was obtained by depositing thin intermediate tin and nickel layers on titanium and steel respectively; this was achieved by procedures which have not been previously reported. On both the pretreated titanium and stainless steel substrates, the formation of α-lead (IV) dioxide (orthorhombic) and β-lead (IV) dioxide (tetragonal) takes place, the β form being the most abundant. Catalytic surfaces were produced at different current densities on both substrates. The lead (IV) dioxide surface stability was good in all cases. Anode performance was tested by means of the cyanide to cyanate oxidation reaction. The oxidation efficiency for the steel substrate anodes was better than the efficiency exhibited by titanium substrate anodes.

A thermodynamic model has been derived for the surface tension of molten salt mixtures in common ion alkali- halide systems considering the relaxation structure of the surface and the ionic sizes. The composition dependence of the surface tension of those systems has been discussed on the basis of the calculated results which have been obtained using thermodynamic databases.

A penning discharge micro-sputtering technique using a specially designed outer-touch type apparatus has been applied for non-destructive etching of a creep tested specimen of SUS316H steel and long-term serviced boiler tubes of 2.25Cr-lMo and SUS321H steels. The vacuum chamber of the micro-sputtering apparatus was attached to the outer surface of the boiler tube through an O-ring. Replicas were sampled from the micro-sputtered surfaces and then observed by optical and scanning electron microscopy. For both the micro-sputtered surfaces and their replicas high quality micrographs have been obtained as good as those produced by optimized chemical etching. The quality of the micrographs was not sensitive to sputtering conditions or the surface roughness before sputtering.