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Zhang, Y.*; 丸澤 賢人*; 工藤 航平*; 諸岡 聡; 宮本 吾郎*; 古原 忠*
no journal, ,
As-quenched martensite in carbon steels needs to be tempered to restore its ductility and toughness for practical applications. During tempering, a series of reactions relevant to carbon diffusion are known to occur sequentially, causing changes in microstructure in tempered martensite. In this study, multi-aspect characterization using various advanced characterization techniques were performed, with an aim to clarify the low-temperature tempering behaviors of high-carbon martensitic steels. An Fe-0.8 mass% binary alloy was mainly used in this study, and 4 ternary alloys with further 2 at% addition of Mn, Si, Cr or Al, were also investigated for comparison. All the alloys were water quenched after austenitization to obtain the as-quenched martensite as the starting microstructure. Tempering processes were performed either by continuous heating or isothermal holding under various conditions. Afterwards, the changes in physical properties of tempered martensite were analyzed via calorimetry, dilatometry, and resistometry, whereas the microstructural evolutions were characterized via transmission electron microscopy, in-situ neutron diffraction, and three-dimensional atom probe tomography. The experimental results revealed the continuous occurrence of different tempering stages, including carbon clustering, precipitation of metastable iron carbide, decomposition of retained austenite, and precipitation of cementite. In addition, the tetragonality of martensite became continuously lowered due to the reduction in solute carbon content by tempering. Among all the investigated elements, the addition of Al was found to have the largest retardation effects on the tempering kinetics, which was caused by its suppression effect on carbon diffusivity.
都留 智仁
no journal, ,
体心立方(BCC)構造を持つ耐火ハイエントロピー合金(HEA)は優れた機械的特性を有し、その特徴は構成元素によって大きく異なる。例えば、特定の元素を含むBCC-HEAは、高強度と伸びのバランスに優れ、独特のすべり挙動を示す。したがって、HEAの特異な力学特性の起源を解明するためには、構成元素の影響を考慮した欠陥構造の原子レベルの描像を理解することが不可欠である。特に、転位コアの構造と運動を詳細に調べることは、力学特性を明らかにする鍵となる。本研究では、異なる特徴を持つ2つのBCC-HEAに着目し、広く議論されている合金系としてVNbMoTaWとTiZrNbHfTaを選んだ。機械的性質の違いの起源は、局所的な格子歪み、弾性的性質、および短距離秩序(SRO)形成にあると予想し、第一原理計算を用いて2つのBCC-HEAの転位コアの特徴とこれらの性質の違いを調べた。その結果、TiZrNbHfTaの平均二乗変位は、高濃度の第IV族元素によって非常に大きくなることがわかった。さらに、TiZrNbHfTaの転位芯は大きく広がる傾向があり、特異な転位芯構造とスリップ挙動にも高濃度の第IV族元素が大きく影響を及ぼすことが明らかになった。
Gong, W.; 川崎 卓郎; Mao, W.; 伊東 達矢; 相澤 一也; Harjo, S.
no journal, ,
Mechanical properties, as the principal properties of structural metallic materials, are governed by the microstructures. To track the structure-property relationship associated with various solid-solid reactions such as phase transformation and plastic deformation, is crucial for the development of structural metallic materials. TAKUMI [1], an engineering materials neutron diffractometer at MLF, J-PARC, covering a large d-range with a good balance between high neutron flux and high resolution is reliable to investigate various dynamic phenomena associated with microstructure evolution and deformation behavior. In addition, a variety of specifical sample environments at TAKUMI allow the in-situ studies under extreme environments such as cryogenic deformation (10K~), thermomechanical controlled process (~1473K), fatigue, etc. Crystallographic information including phase fraction, lattice strain (stress), texture, dislocation density, etc. as obtained by neutron diffraction can uniquely provide insights into microstructure evolution and deformation mechanisms in bulky materials. In the presentation, several recent studies on microstructure evolution and deformation mechanisms in metallic materials including steels, high-entropy alloys, magnesium alloys using in-situ neutron diffraction via TAKUMI will be introduced.
Gong, W.; Harjo, S.; 川崎 卓郎; 相澤 一也; 辻 伸泰*
no journal, ,
Extension twinning and detwinning play crucial roles in deformation, particularly in cyclic loading deformation of magnesium (Mg) and its alloys. The present study investigated the cyclic loading-unloading behavior in a commercial AZ31 alloy at 20K and 298K using in-situ neutron diffraction. The yield stress and flow stress in the plastic regime up to 0.06 strain at 20K were slightly higher than those at 298K. However, the hysteresis loops (i.e., recovery strains) of the loading-unloading at 20K were significantly larger compared to those at 298K. The alternate increase and decrease in the integrated intensity of diffraction reflections confirmed the activity of twinning and detwinning corresponding to compressive deformation and unloading steps. During compression, twinning was more prevalent at 20K than that at 298K. A decrease in the volume fraction of twins and lattice stress relaxation were observed in each unloading step, which can be attributed to detwinning. The amount of detwinning and lattice stress relaxation were found to be higher at lower temperatures, which is considered to be the origin of the large recovery strain observed at 20K.