|Abstract: ||鐵基麻田散鐵有三種不同外形：板條狀、透鏡狀和板片。板條狀的麻田散鐵通常 出現在低合金或低碳鋼中，具有較高的Ms 溫度，內部含有密度很高的差排。板片狀的 麻田散鐵則是出現在高合金或高碳鋼中，具有最低的Ms 溫度,其板片之內部組織則含 許多平形且厚度極細小的奈米雙晶(twins)，這些雙晶不會延伸超過板片邊界，而是終止 於板片之邊緣。當含碳量及合金成份介於板條狀和板片狀麻田散鐵合金成份之間時， 則產生透鏡狀麻田散鐵，其Ms 溫度也是介於兩者之間，透鏡狀麻田散鐵外形像透鏡的 形狀，其內部組織則較為複雜，可分為三個區域:中脊區、雙晶擴展區和非雙晶區。已 有不少文獻對中脊區域研究分析，但是其細部的結構和形成的機制仍未被完全澄清。 Shimizu 等人研究Fe-Ni 與-Ni-Co 合金之麻田散鐵而聲稱 “中脊區域和其旁的雙晶擴展 區(extended twinned region)在方位(orientation)上有差異＂，然而詳細的原因不清楚，有待 後續實驗的觀察。另一方面，Shibata 等人將Fe-31Ni-10Co-3Ti 合金中的板片狀麻田散鐵 在200K 下施加1%的拉伸變形，發現少部分在板片狀麻田散鐵內的雙晶有向外延伸的 現象，變形的板片狀麻田散鐵之形貌和Fe-31Ni、Fe-33Ni 和Fe-20.5Ni-35Co 合金中的透 鏡狀麻田散鐵類似，因此，他們認為透鏡狀麻田散鐵內的中脊區域本身就是板片狀麻 田散鐵。雖然Shibata 等人做了很多相關的實驗嘗試去證實中脊區域本身就是板片狀麻 田散鐵，但用板片狀麻田散鐵歷經變形而成透鏡狀麻田散鐵的機制，是否就可以完全 解釋透鏡狀麻田散鐵真正的成長機制，有待更進一步地討論。關於中脊區域的形成或 透鏡狀麻田散鐵相變時的初期產物等相關議題，仍需做更近ㄧ步的研究，以進一步的 闡釋透鏡狀麻田散鐵相變態之現象學(Phenomenological theory)。主持人實驗室團隊研究 高碳高鉻合金鋼AISI 440C (合金成份Fe-1C-17Cr, wt%)，發現:在夜態氮(–196℃)深冷過 程，片狀麻田散鐵與透鏡狀麻田散鐵可以共存，而擬以電子顯微鏡來解析，以釐清 透 鏡狀麻田散鐵之中脊區域所含的雙晶是否與相鄰的雙晶擴展區有相同的方位，並進一 步研究非雙晶區，以闡明 透鏡狀麻田散鐵之整個相變態機制。 本研究計畫擬以三年時間來研究AISI 440C麻田散鐵之奈米結構。第一年研究重 點：透鏡狀麻田散鐵 “中脊區、雙晶擴展區＂之奈米組織；第二年研究重點：透鏡狀 麻田散鐵“非雙晶區(untwinned region)＂ 之奈米組織。另有鑑於AISI 440C是工業常用零 主件材料，因此研究 AISI 440C 於深泠處理後，其麻田散鐵與殘留沃斯田鐵之回火特 性，則規畫為第三年研究重點。期望本研究計畫在參年時間獲得一系列成果，對透鏡狀 麻田散鐵相變態之現象學進一步釐清，另對AISI 440C深泠處理後回火，提出微結構演化 供工業應用參考。|
In ferrous martensite, there are primarily three different morphologies, namely lath, thin plate, and lenticular, depending on the alloy composition and martensite start (Ms) temperature. Lath martensite forms in the highest temperature range and contains a high density of dislocations. Thin plate martensite, which forms in the high chemical composition alloys in the lowest temperature range, is composed of a set of uniformly spaced transformation twins crossing throughout the plate. Lenticular martensite forms at an intermediate temperature between lath martensite and thin plate martensite; it takes on a lens-like morphology and contains three regions: the midrib, extended twinned region and untwinned region. The substructures of lenticular martensite are much more complicated than those of other types. Although there are considerable reports on the martensite midrib, the detailed substructures and formation of the midrib have not been clarified yet. Shimizu et al. studied Fe-Ni and Fe-Ni-Co alloys, and asserted that the midrib region had a different orientation from its surrounding extended twin region. On the other hand, Shibata et al. investigated the thin plate martensite in Fe-31Ni-10Co-3Ti (wt%) alloy, which grew into a lenticular shape by 1% tensile deformation at a temperature slightly higher than its Ms temperature. The deformed thin plate martensite had the substructures similar to the lenticular martensites in Fe-31Ni, Fe-33Ni and Fe-20.5Ni-35Co (wt%) alloys. The effect of the deformation on thin plate martensite caused the originally existing twins in the thin plate martensite to be extended outwards to a range of about several hundred nm. From the result of a comparative elaboration, they claimed that the midrib in lenticular martensite was thin plate martensite itself at the earliest stage formation of lenticular martensite. However, their explanation for the transition from thin plate martensite to lenticular martensite is still debatable. Direct TEM observation in the same alloy, without resorting to mechanical treatment, is required. Although there are several fundamental difficulties in assessing the substructures of lenticular martensite, such as sectioning and accommodation-distortion effects, TEM investigation continues to assume greater significance in research. In this work, it has been found that thin-plate and lenticular martensites co-existed in the specimens of AISI 440C stainless steel. This condition facilitated the efforts of TEM investigation. In this three-year proposed project, the nanostructures of martensites in AISI 440C stainless steel will be investigated. After subzero-treatment in liquid nitrogen (–196℃) for different time intervals, the corresponding specimens will be examined. Via transmission electron microscopy the substructures of thin-plate martensites and lenticular martensite will be studied, focusing on the details of the midrib. The aim of the first year work is to reveal the midrib and the extended twinned region in lenticular martensite. The objective of the second year job is to disclose the dislocation structures and dislocation density in the untwined region in in lenticular martensite. From the first and second year results, the martensite phenomenological theory for lenticular martensite will be discussed. The purpose of the third year job is to study the decompositions of retained austenite in the subzero-treated samples by multiple tempering cycles and by a single long-time tempering cycle. It is hoped that through this project, valuable results can be obtained to elucidate the lenticular martensite transformation in AISI 440C stainless steel.