Influence of the Carbon Allotropes on Dilatometric Properties …
647
high concentrations are often located along particle and grain boundaries [8, 25].
The evidence from these two studies appears to show that the carbon content of the
sinter-hardened compact has a strong effect on the diffusion of the added copper,
and, that at higher carbon contents, the diffusion of the copper is inhibited by the
diffused carbon. This situation causes a higher local hardenability at the particle/pore
surfaces and promotes the formation of retained austenite in the high-copper regions
[8, 25].
The hard metallic balls used at the MA are contaminated by powders under milling.
So, in order to avoid excessive sticking of the powdered sample to the balls some
liquid organic (e.g., ethyl or methyl alcohol) are added to the powder mixes. The effect
of those organic on the product of MA (Fe–Cu system) was particularly studied in
the paper [21].
The sintering behavior and sintered microstructures of mechanically alloyed
Fe-9.5 mass%Cu–0.5 mass% P system have been examined in the work [34].
Started materials for MA were reduced pure iron powder, water atomized pure
copper powder, and crushed Cu-14 mass% P alloy powder. MA was performed
in argon gas atmosphere by using a planetary ball mill. The change in the crystallite
size of the powders with MA time almost finished for 36 ks and the yielded size
was estimated to be about 10 nm. Sintering of MA powders at 1073 K for 36 ks
gave a consolidated alloy, the density of which is comparative to that of an atomized
powder alloy sintered at 1473 K. The microstructure of the sintered MA powder
alloy is composed of copper particles of a few micrometers in diameter distributed
in the matrix the grain size of which is less than about 5 μm [34].
The characteristics of the Fe–Cu alloys prepared by ball milling nominally pure
(99%) Fe and Cu powders and compacted (at 300 °C) were measured in the [35].
Surprisingly, however, calorimetric measurements indicate the presence of a large
endothermic peak for these nanocrystalline composites on heating near 600 °C and
an exothermic peak near 400 °C on cooling. Furthermore, heating to temperatures
above 600 °C results in the dissolution of Cu into the iron oxides which does not
reprecipitate on cooling. The differential scanning calorimetry data for this sample,
shown in Fig. 6, further support the likelihood that the large features in the M versus T
data are due to phase transformations. In this figure, endothermic events are indicated
by “upward” deviations, while exothermic events are the reverse. It is worth to note
the endothermic reaction near 600 °C and the exothermic reaction near 425 °C, which
roughly repeat with thermal cycling. The similarity in the temperatures of these two
arrests to the temperatures of the largest magnetic effects is suspiciously coincidental.
For a first-order phase transformation between equilibrium crystallographic phases
(the common type), an endothermic heat effect will occur during heating and the
opposite effect will occur during cooling, just as observed for this sample [35].
647
high concentrations are often located along particle and grain boundaries [8, 25].
The evidence from these two studies appears to show that the carbon content of the
sinter-hardened compact has a strong effect on the diffusion of the added copper,
and, that at higher carbon contents, the diffusion of the copper is inhibited by the
diffused carbon. This situation causes a higher local hardenability at the particle/pore
surfaces and promotes the formation of retained austenite in the high-copper regions
[8, 25].
The hard metallic balls used at the MA are contaminated by powders under milling.
So, in order to avoid excessive sticking of the powdered sample to the balls some
liquid organic (e.g., ethyl or methyl alcohol) are added to the powder mixes. The effect
of those organic on the product of MA (Fe–Cu system) was particularly studied in
the paper [21].
The sintering behavior and sintered microstructures of mechanically alloyed
Fe-9.5 mass%Cu–0.5 mass% P system have been examined in the work [34].
Started materials for MA were reduced pure iron powder, water atomized pure
copper powder, and crushed Cu-14 mass% P alloy powder. MA was performed
in argon gas atmosphere by using a planetary ball mill. The change in the crystallite
size of the powders with MA time almost finished for 36 ks and the yielded size
was estimated to be about 10 nm. Sintering of MA powders at 1073 K for 36 ks
gave a consolidated alloy, the density of which is comparative to that of an atomized
powder alloy sintered at 1473 K. The microstructure of the sintered MA powder
alloy is composed of copper particles of a few micrometers in diameter distributed
in the matrix the grain size of which is less than about 5 μm [34].
The characteristics of the Fe–Cu alloys prepared by ball milling nominally pure
(99%) Fe and Cu powders and compacted (at 300 °C) were measured in the [35].
Surprisingly, however, calorimetric measurements indicate the presence of a large
endothermic peak for these nanocrystalline composites on heating near 600 °C and
an exothermic peak near 400 °C on cooling. Furthermore, heating to temperatures
above 600 °C results in the dissolution of Cu into the iron oxides which does not
reprecipitate on cooling. The differential scanning calorimetry data for this sample,
shown in Fig. 6, further support the likelihood that the large features in the M versus T
data are due to phase transformations. In this figure, endothermic events are indicated
by “upward” deviations, while exothermic events are the reverse. It is worth to note
the endothermic reaction near 600 °C and the exothermic reaction near 425 °C, which
roughly repeat with thermal cycling. The similarity in the temperatures of these two
arrests to the temperatures of the largest magnetic effects is suspiciously coincidental.
For a first-order phase transformation between equilibrium crystallographic phases
(the common type), an endothermic heat effect will occur during heating and the
opposite effect will occur during cooling, just as observed for this sample [35].