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Internal stresses of pearlitic steel monitored by in-situ neutron diffraction during phase transformation and thermal aging

諸岡 聡; 川崎 卓郎; Harjo, S.; 中田 伸生*; 塚田 祐貴*

no journal, , 

Hierarchical microstructure of pearlite and martensite in steel is caused by internal stress due to phase transformation. If internal stress can be quantitatively evaluated and controlled, new microstructure control technology will be created. However, internal stress due to eutectoid transformation is not easy to measure because thermal stress and transformation stress are superimposed. The purpose of this study is to quantitatively evaluate the internal stress evolved from pearlitic transformation by In-situ neutron diffraction technique. A pearlitic steel of 0.8C-1.5Mn wt.% was used in this study, and the following thermal process was performed; the solution heat treatment at 1323K for 1.8ks followed by immediate the isothermal heat treatment at 873K for 1.8 ks to obtain a predominantly pearlitic structure included lamellar ferrite and cementite. The austenitic-to-pearlitic transformation during the thermal process was monitored with a TAKUMI neutron diffractometer at J-PARC-MLF. The Rietveld refinements of diffraction patterns were performed using Z-Rietveld software to track the phase fractions and the lattice parameters. The elastic strains state of the ferrite and the austenite phases at 873K were observed from the evolutions of the lattice constants of ferrite and the austenite during pearlitic transformation. In particular, the cubical expansion during the transformation derived the hydrostatic pressure and resulted a compressive elastic strain in ferrite. On the other hand, the elastic strains state in cementite that was predicted by an amount of internal stress relaxation during thermal aging after pearlitic transformation, was approximately -0.28%. These results show that internal stresses during transformation can be quantitatively evaluated using in-situ neutron diffraction method.

口頭

Pearlite growth kinetics in Fe-C-Mn eutectoid steels

Zhang, Y.*; 梅田 岳昌*; 宮本 吾郎*; 古原 忠*; 諸岡 聡

no journal, , 

Essential understanding of the pearlite growth kinetics is important to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, through quantitatively analyzing the microstructural features in the vicinity of pearlite growth interface, the influence of these factors and the underlying thermodynamics of pearlite growth kinetics were clarified. The pearlite growth rate and lamellar spacing were measured based on the microstructural observation via optical microscopy and scanning electron microscopy, respectively. Three-dimensional atom probe (3DAP) was used to analyze the elemental distribution in the vicinity of pearlite growth front, whereas in-situ neutron diffraction at elevated temperatures was performed at J-PARC, BL19 (TAKUMI) to quantify the elastic strain generated during pearlite transformation. Based on the proposed thermodynamic model, the influence of various factors on the pearlite growth kinetics is estimated using the experimental results obtained in this study. It was found that in most transformation conditions, solute drag effects caused by Mn interfacial segregation have the largest contribution in retarding the pearlite growth rate. In contrast, the magnitude of elastic strain in pearlite measured by neutron diffraction is quite small, which marginally affects the pearlite growth kinetics.

口頭

Effect of interstitial elements on micromechanical properties in titanium alloys monitored by in-situ neutron diffraction

諸岡 聡; Harjo, S.

no journal, , 

The interstitial elements such as hydrogen, nitrogen and oxygen have a great influence on the mechanical properties of titanium alloy. In the present work, micromechanical properties such as the phase strains, intergranular strains and dislocation densities for titanium alloys containing such interstitial elements were investigated by means of neutron diffraction technique. In-situ neutron diffraction during tensile deformation was performed with TAKUMI, a high-resolution and high-intensity TOF neutron diffractometer for engineering materials science at MLF of the J-PARC. The results show that the phase strains of nitrogen or oxygen-strengthened titanium alloys are partitioned into the grain interface between alpha phase and beta phase. In particular, the strong beta phase leads to a stress value higher than the macro-yield stress, resulting in high strengthening of (alpha + beta) dual phase titanium alloys. On the other hand, the dislocation density of titanium alloy containing hydrogen abnormally increased with an increase of the plastic strain. It is generally thought that hydrogen is trapped in the dislocations. However, our result shows that the dislocation is trapped by hydrogen through the Cottrell effect.

口頭

Magnetic order and phase transformation in Fe-Mn-C alloy at cryogenic temperature

諸岡 聡; 小山 元道*; 川崎 卓郎; Harjo, S.

no journal, , 

Medium Mn steels have been actively investigated due to their excellent balance between material cost and mechanical properties. In particular, medium Mn steel with a nominal chemical composition of Fe-5.0Mn-0.1C (mass%) fabricated by intercritical annealing 923 K for 1.8 ks after cold-rolling, was the high-strength mechanical properties at low temperature. This strengthening mechanism evaluated by means of in-situ neutron diffraction under low temperature (engineering materials diffractometer (TAKUMI) at Japan Proton Accelerator Research Complex (J-PARC)), electron back scatter diffraction (EBSD), low temperature differential scanning calorimetry (DSC) and low temperature magnetic susceptibility measurement. We found that as the sample temperature decreases, face-centered cubic (FCC) structure transferred face-centered tetragonal (FCT) structure. Namely, it suggests that austenite transformed martensite like Fe-Pd or Fe-Pt alloy. Therefore, the origin of the high-strength mechanical properties at low temperature was in the presence of FCT martensite. This study got partially support from MEXT Program: Data Creation and Utilization Type Material Research and Development (JPMXP1122684766).

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