Thixoforming : Semi-solid Metal Processing

290j 8 Tool Technologies for Forming of Semi-solid Metals
Figure 8.44 XRD spectra of the contact zone of Si3N4/X210CrW12 contact corrosion at 1300 C/2 h.
The above-mentioned reactions are superimposed by chemical interaction with
steel and scale in the areas in contact to steel. On top of the SiO2 layer a thick scale
layer is observed under oxidizing conditions, consisting of ferrous oxides with
varying amounts of chromium in solid solution. XRD analysis of this layer yields
magnetite, haematite and chromite as major constituents in addition to cristobalite
(Figure 8.44).
Enclosed in this layer metallic precipitations are detected that exhibit a high
amount of steel alloying elements, for example, chromium, manganese and tungsten,
significantly exceeding the element content of the respective steel grade applied.
Thus, alloying elements are mobilized in the bulk of the steel and diffuse to the
contact zone. The entire scale layer is permeated by channels, cracks and pores
resulting from the nitrogen gas released during decomposition of Si3N4 trying to
escape from the contact zone.
As expected, chemical interaction of Si3N4 and steel in an argon atmosphere differs
from the behaviour in air: depletion of the grain boundary phase in the upper Si3N4
layers is less pronounced. Likewise, precipitations containing a high amount of the
sintering additive elements Yand Al are observed only locally in the contact zone. On
top of the Si3N4 ceramic a metallic layer is detected, consisting mainly of iron and
silicon. Embedded in this layer metallic precipitations are found, showing high
amounts of steel alloying elements, and also large pores that are ascribed to the
nitrogen gas emerging during decomposition of Si3N4. Although this gas can readily
escape from the contact zone through the powdery and brittle scale layer in the case of
oxidizing conditions, the lack of such a layer in an argon atmosphere leads to
entrapment of the gas in the contact zone, eventually resulting in very large cavities
formed due to the gas pressure. While more pronounced in an argon atmosphere,
interdiffusion of silicon and iron is observed independently of the experimental
parameters.
Examination of silicon nitride die parts after application in small-scale forming
series revealed that under real thixoforming conditions a combination of the
aforementioned mechanisms observed in model tests in air and in an argon
atmosphere is present. Although the inner die surfaces showed a mirror finish