of fast grain growth and are not cost-effective. Therefore, in this
work, only the sintering temperature of 1050 C is focused on, albeit
with different additive amounts. Fig. 1 presents the effect of the
additive content, from 0 to 6 wt%, on the porosity features, suggesting
the evolution of densification with increasing the additive
content. Sintered densities measured by the water Archimedes
method have been reported in Ref. [6]. Note that, as indicated in
Fig. 2, the addition of 8 wt% sintering aid leads to the detrimental
phenomenon of spoiling due to the high level of liquid formed
during sintering; thus, the higher contents were not tested.
Fig. 3 shows the XRD pattern of the sintered sample containing
6% additive. The Rietveld analysis of the XRD data depicted the
formation of a single-phase austenitic structure after sintering, as
confirmed with ferritoscopic measurements. Note that to have an
austenitic structure, after sintering at 1050 C, the samples were
water-quenched. In addition, the mean crystallite size was
measured to be less than 50 nm, where mechanical alloying had
been previously created the nanostructured powders [10–15]. This
nanoscale structure, even after the sintering process, have been also
verified by microscopic studies [6,7], reflecting a significant resistance
to grain growth. The solute drag effect combined with the
contribution of carbon and nitrogen are expected to be responsible
for the retarded grain growth in this material. The solubility of
nitrogen and carbon atoms in crystals is limited; therefore, they
segregate at grain boundaries [16–18] and retard grain boundary
mobility at high temperatures [18–21].
Fig. 4 represents the true compressive stress–strain curve of the
sintered samples. The yield and compression strengths extracted
from the curves are also summarized in Fig. 5. As can be seen, both
increases by increasing the sintering aid content, due to the
progress in densification and the decrease of retained porosity.
Moreover, an increase in elastic modulus by decreasing the pore
level is observable in the curve, which is in agreement with
previously-reported models [22]. It is also noticeable that, despite
the presence of some porosity in the samples, considerable
compressive strengths were found. This is due to the strengthening
contribution of the interstitially dissolved atoms of carbon and
nitrogen, on the one hand, and the obtained nanometric structures,
on the other hand. Note that the presence of retained porosity in
implants can be advantageous for medical applications. Porosity
decreases the elastic modulus mismatch of the bone and implant,
and subsequently decreases the probability of loosening [23]. Also,
surface porosity can help the mechanical fixation of implants [24].
As well as the better strength of these materials, their higher biocompatibility
of these nickel-free stainless steels is another benefit,
as compared to conventional stainless steels like AISI 316L [8].
The absorbed energy of the samples during the Charpy impact
tests is indicated in Fig. 6, showing an increase in the energy by
increasing the amount of the additive and thereby reducing the