3. Results and discussion
The average grain sizes on the cross-section of the wires were
determined from TEM photographs of the microstructures (Table 1). It
has been found that the grain size decreases with decreasing wire
diameter.
By way of example, Figs. 2 and 3 show the microstructures of the
0.4 and 0.08 mm wires, as observed on longitudinal and transversal
cross-sections by TEM. We can see that the shapes of the grains visible
on the cross-sections of the samples are varied. On the longitudinal
cross-sections, the grains are elongated in the direction consistent
with that of the drawing operation, which obviously evidences the
effect of this treatment on the sample microstructure. The morphological
texture becomes more pronounced as the wire diameter
decreases. The porosity was determined using the stereological
methods. As expected, it decreases with decreasing wire diameter.
The greatest reduction of porosity (by 0.35% — Table 1), was observed
when drawing operation reduced the wire diameter from 0.5 to
0.4 mm. This reduction of porosity coincides with the substantial
decrease of the average grain size. A porosity of 0.08% occurring in the sample with a diameter of 0.08 mm is a satisfactory result — porosity
of this order of magnitude does not affect adversely the mechanical
and catalytic properties of the wires. The micro-pores visible in this
sample are marked in the TEM image shown in Fig. 3a. The reduction
of porosity with increasing deformation degree is accompanied by an
improvement of the mechanical properties of the wire represented by
its hardness (Table 1).
To assess the suitability of the 0.08 mm in diameter wire produced
by isostatic pressing+sintering (powder metallurgy technique) for
the fabrication of catalytic nets, the wire was subjected to tensile tests.
It appears that the tensile strength of the wire is higher by a factor of
2.5 compared to that of wires produced by casting (Table 2).
Examinations of the microstructure, microhardness, porosity, and
tensile strength of the wires produced by the proposed technology,
namely isostatic pressing+sintering, have shown that this treatment