676
M. C. Bouleklab et al.
The DSC dependence, at the same time, showed another blurred peak near 700 °C
and shoulder at ~200 °C. Comparing the T extr values with the data about the temperature ranges selected by us above for estimation of the thermal expansion coefficient
average values <α i (T)>, it is easy to see that the T extr values correspond to the temperature ranges in which there are changes in the <α i (T)> values. This result confirms the
above made conclusions regarding the main role in the dilatometric manifestations
of changing their composition (range of low temperatures up to 200 °C) and phase
transformations of the structure at higher temperatures. The origin of the described
wide band is directly related to the iron component in the composite matrix. This band
is quite wide, and its half-width is ~350 °C, which lies from 400 to 750 °C. Within
this temperature range, iron is in α-Fe allotropic form. However, as we have already
shown above, structural transformations of α-Fe → γ-Fe occur in the same temperature range for Fe–Cu and Fe–Cu-MWCNT nanocomposites and this range depends
on the composites content and the state (size of the particles, etc.) and the interaction
of their components, which is reflected in the position and intensity of the main peak.
The position of other high-temperature features (T > 200 °C) on the DSC curves
also correspond to the structural transformations discussed above and to the role
of carbon material (starting MWCNT) in the Fe–Cu-MWCNT composites thermal
characteristics determination. So, we have already accentuated, above, on the role of
particle size and grain components of investigated composites in their dilatometry.
In addition, we note that the small size and homogeneity of particles and grains
contribute to the increase of surface energy and energy of interface layers, which are
the main sources of storage energy accumulated under milling. The release of this
energy during heating, as our data show, should increase the activity of composite
components by increasing the speed and ways of their atoms transport and, thus,
affecting the processes of phase transformation, sintering, etc. Here, we must add
that the influence of carbon forms on the size of the structural components and on the
oxidation processes of the metallic components of composites is also important. As
it had been shown earlier, the kinetics of oxidation on the stage of such composites
heating depends on the allotropic carbon and their structure, and the formation of
an amorphous form with a disordered defective structure weakens such reactions
[30]. The various carbon forms and their structure appear at peaks of varying position and intensity on the DSC curves [30]. Consequently, the carbon component, in
addition to the “blocking” role described above, through its influence on oxidation
processes determines the relation between the “carbide” and “oxide” mechanisms
of influence on the temperature behavior of the characteristics of the investigated
Fe–Cu-MWCNT composites. The cumulative effect of all processes described in
this work is reflected, in particular, in the temperature dependences described herein
for dilatometric and calorimetric characteristics.
M. C. Bouleklab et al.
The DSC dependence, at the same time, showed another blurred peak near 700 °C
and shoulder at ~200 °C. Comparing the T extr values with the data about the temperature ranges selected by us above for estimation of the thermal expansion coefficient
average values <α i (T)>, it is easy to see that the T extr values correspond to the temperature ranges in which there are changes in the <α i (T)> values. This result confirms the
above made conclusions regarding the main role in the dilatometric manifestations
of changing their composition (range of low temperatures up to 200 °C) and phase
transformations of the structure at higher temperatures. The origin of the described
wide band is directly related to the iron component in the composite matrix. This band
is quite wide, and its half-width is ~350 °C, which lies from 400 to 750 °C. Within
this temperature range, iron is in α-Fe allotropic form. However, as we have already
shown above, structural transformations of α-Fe → γ-Fe occur in the same temperature range for Fe–Cu and Fe–Cu-MWCNT nanocomposites and this range depends
on the composites content and the state (size of the particles, etc.) and the interaction
of their components, which is reflected in the position and intensity of the main peak.
The position of other high-temperature features (T > 200 °C) on the DSC curves
also correspond to the structural transformations discussed above and to the role
of carbon material (starting MWCNT) in the Fe–Cu-MWCNT composites thermal
characteristics determination. So, we have already accentuated, above, on the role of
particle size and grain components of investigated composites in their dilatometry.
In addition, we note that the small size and homogeneity of particles and grains
contribute to the increase of surface energy and energy of interface layers, which are
the main sources of storage energy accumulated under milling. The release of this
energy during heating, as our data show, should increase the activity of composite
components by increasing the speed and ways of their atoms transport and, thus,
affecting the processes of phase transformation, sintering, etc. Here, we must add
that the influence of carbon forms on the size of the structural components and on the
oxidation processes of the metallic components of composites is also important. As
it had been shown earlier, the kinetics of oxidation on the stage of such composites
heating depends on the allotropic carbon and their structure, and the formation of
an amorphous form with a disordered defective structure weakens such reactions
[30]. The various carbon forms and their structure appear at peaks of varying position and intensity on the DSC curves [30]. Consequently, the carbon component, in
addition to the “blocking” role described above, through its influence on oxidation
processes determines the relation between the “carbide” and “oxide” mechanisms
of influence on the temperature behavior of the characteristics of the investigated
Fe–Cu-MWCNT composites. The cumulative effect of all processes described in
this work is reflected, in particular, in the temperature dependences described herein
for dilatometric and calorimetric characteristics.