508
Y. Yoshida
in the defects at low temperature, while they can diffuse overcoming the trapping
barriers at high temperature. The MSM image seems to be quite helpful in studying
such diffusion and segregation processes in such an inhomogeneous matrix.
9.6.3 Carbon Diffusion in Fe Steel
Here we are going to show another potential of the MSM imaging technique, i.e.,
direct observation of the carbon diffusion and segregation processes in Fe steel, which
is a vital material in our modern world. The system contains a complex microstructure
consisting of Martensitic and Austenite phases, which will change drastically as a
function of the annealing temperature. Finally, it will be followed by the phase
separation to ferrite and cementite phases. These phenomena are indeed a longstanding research theme for Mössbauer Spectroscopy: Already in 1971, F. E. Fujita
reported his pioneering works on Fe carbon alloys in a review book edited by Gonser
[4]. In Sect. 9.3, we have already shown the experimental results on Fe-7.8 at. % C
alloy using the thermal scanning combined with the spectrum measurements.
After half-century, low- to medium-carbon martensitic steels have been one of the
essential subjects due to their potential for saving the weight as well as for improving
the crash resistance of structural parts in automobiles. It is, therefore, crucial to
understand not only the phase evolution during quenching and tempering but also
the relationship between the microstructure and mechanical properties. The strength
of martensite dominates by its carbon contents, distributing and even diffusing in
the solid. Fe steel is a mixture consisting of carbides, retained austenite, carbon
at dislocations and grain boundaries in martensite. It is, however, still not fully
understood how all the phases and the lattice defects contribute to the mechanical
properties. The main reason could be due to the lack of an evaluation technique
to determine the concentration and their distribution of carbon in martensite under
operation. Especially crucial is to obtain atomistic information. We expect the MSM
imaging technique enables us to follow the development of the microstructure at
high temperatures. As a first step toward an operand measurement on the carbon
diffusion and segregation processes at high temperature, we measured the Mössbauer
spectra and their mapping images of Fe-1Mn-2.7C foils [at%] with a martensitictransformation start-temperature (Ms) of 210 °C after different heat treatments [98].
We first annealed the samples at 1100 °C, and water-quenched to room temperature
with a cooling rate of 500–2000 °C/s, and subsequently put into liquid nitrogen. We
tempered five foils separately at various temperatures from 100 to 700 °C. The
Mössbauer spectra were measured at room temperature with a 1.85 GBq
57 Co-inRh source. Figure 9.62(a) and (b) show the Mössbauer spectra and their successive
subtractions, respectively. After quenching, there are four components for martensite
phase and one singlet for retained austenite [4]: The green one corresponds to α-Fe
(bcc), the light blue to the Fe atom on the 1st Nearest Neighbour of Octahedral-site
carbon atom, the dark green to Fe atom on the 2nd NN of O-site carbon, and the
yellow to the 1st NN of Mn, and the red singlet to Austenite phase. The successive
Y. Yoshida
in the defects at low temperature, while they can diffuse overcoming the trapping
barriers at high temperature. The MSM image seems to be quite helpful in studying
such diffusion and segregation processes in such an inhomogeneous matrix.
9.6.3 Carbon Diffusion in Fe Steel
Here we are going to show another potential of the MSM imaging technique, i.e.,
direct observation of the carbon diffusion and segregation processes in Fe steel, which
is a vital material in our modern world. The system contains a complex microstructure
consisting of Martensitic and Austenite phases, which will change drastically as a
function of the annealing temperature. Finally, it will be followed by the phase
separation to ferrite and cementite phases. These phenomena are indeed a longstanding research theme for Mössbauer Spectroscopy: Already in 1971, F. E. Fujita
reported his pioneering works on Fe carbon alloys in a review book edited by Gonser
[4]. In Sect. 9.3, we have already shown the experimental results on Fe-7.8 at. % C
alloy using the thermal scanning combined with the spectrum measurements.
After half-century, low- to medium-carbon martensitic steels have been one of the
essential subjects due to their potential for saving the weight as well as for improving
the crash resistance of structural parts in automobiles. It is, therefore, crucial to
understand not only the phase evolution during quenching and tempering but also
the relationship between the microstructure and mechanical properties. The strength
of martensite dominates by its carbon contents, distributing and even diffusing in
the solid. Fe steel is a mixture consisting of carbides, retained austenite, carbon
at dislocations and grain boundaries in martensite. It is, however, still not fully
understood how all the phases and the lattice defects contribute to the mechanical
properties. The main reason could be due to the lack of an evaluation technique
to determine the concentration and their distribution of carbon in martensite under
operation. Especially crucial is to obtain atomistic information. We expect the MSM
imaging technique enables us to follow the development of the microstructure at
high temperatures. As a first step toward an operand measurement on the carbon
diffusion and segregation processes at high temperature, we measured the Mössbauer
spectra and their mapping images of Fe-1Mn-2.7C foils [at%] with a martensitictransformation start-temperature (Ms) of 210 °C after different heat treatments [98].
We first annealed the samples at 1100 °C, and water-quenched to room temperature
with a cooling rate of 500–2000 °C/s, and subsequently put into liquid nitrogen. We
tempered five foils separately at various temperatures from 100 to 700 °C. The
Mössbauer spectra were measured at room temperature with a 1.85 GBq
57 Co-inRh source. Figure 9.62(a) and (b) show the Mössbauer spectra and their successive
subtractions, respectively. After quenching, there are four components for martensite
phase and one singlet for retained austenite [4]: The green one corresponds to α-Fe
(bcc), the light blue to the Fe atom on the 1st Nearest Neighbour of Octahedral-site
carbon atom, the dark green to Fe atom on the 2nd NN of O-site carbon, and the
yellow to the 1st NN of Mn, and the red singlet to Austenite phase. The successive