Crystallography and Solute Effects in Fe-C-X Austenite Decomposition

Fe-C-X 奥氏体分解中的晶体学和溶质效应

• 批准号: 9904034
• 负责人: Gary Shiflet
• 金额: $ 30.73万
• 依托单位: University of Virginia Main Campus
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-07-01 至 2002-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9904034&HistoricalAwards=false
• 关键词: Crystallography; Solute; Effects; Fe; Austenite

9904034ShifletThe project is an analytical and experimental examination of substitutional alloying element effects on austenite decomposition in Fe-C-X steels. Two important transformations are examined. The first is a study of pearlite, which is a lamellar product of eutectoid decomposition and probably the most familiar of all metal microstructures. It forms in steels and also in a large number of non-ferrous alloys during transformation under isothermal, continuous cooling and forced-velocity growth conditions. This work extends previous crystallographic considerations to encompass specific interactions with substitutional alloying elements, such as Mn. Key questions that are addressed include whether the alloying element atomic diffusion path is indeed at the pearlite: austenite (mother phase) interphase boundary, as many researchers suggest, or is volume diffusion controlling the transformation kinetics. One goal is to account for the inability of growth models to account for experimental kinetics over a range of temperatures. The second area of investigation is the ferrite plus carbide transformation at intermediate temperatures where growth kinetics are severely slowed when X = Cr, Mo or W. In these alloys there is a temperatures range from about 550 to 600 C where an extremely potent effect on growth kinetics occurs. Indeed, all ferrite growth and nucleation can be halted for days. The same crystallographic and solute considerations of diffusion paths are applied to this transformation as for pearlite. For both of these transformations extensive thermodynamic calculations are performed and correlated with experimental observations as to element partitioning and microstructure development. The primary experimental approach includes making all ternary steels in house and performing heat treatments isothermally in lead baths. Examination of the structure is with conventional tilting and high-resolution transmission electron microscopy (TEM). Chemical partitioning studies are facilitated using a Focused Ion Beam milling machine recently acquired that allows any interphase boundary to be selected for preparation into an electron transparent foil for TEM. Chemical measurements are collected in a Field Emission Gun TEM that is able to obtain data from areas less than one nanometer in diameter. %%%Substitutional alloying elements are important in steel production because they can greatly increase the hardenablity of the alloy by delaying the formation of pearlite and ferrite at intermediate temperatures. These elements can be strong carbide formers and can lead to strengthening in low and medium alloy steels. Their behavior at the growing interphase boundary is important to understand and model. The understanding of thermodynamics of the ferrite:austenite interface in Fe-C-X steels is essential in this very active research area. Given the extensive advances in thermodynamic data bases and high resolution instrumentation, it is timely from both a scientific and industrial position, to pursue this important area of research. ***

9904034Shiflet 该项目是一项分析和实验研究替代合金元素对 Fe-C-X 钢中奥氏体分解的影响。 研究了两个重要的转变。 第一个是珠光体的研究，珠光体是共析分解的层状产物，可能是所有金属微观结构中最熟悉的。 它是在等温、连续冷却和强制速度生长条件下的转变过程中在钢和大量有色合金中形成的。 这项工作扩展了之前的晶体学考虑，涵盖了与替代合金元素（例如锰）的特定相互作用。 需要解决的关键问题包括合金元素原子扩散路径是否确实位于珠光体：奥氏体（母相）界面，正如许多研究人员所建议的那样，还是体积扩散控制着转变动力学。 一个目标是解决生长模型无法解释一定温度范围内的实验动力学的问题。 研究的第二个领域是中间温度下的铁素体加碳化物转变，当 X = Cr、Mo 或 W 时，生长动力学严重减慢。在这些合金中，温度范围约为 550 至 600 C，对生长动力学产生极其有效的影响。 事实上，所有铁素体生长和成核都可以停止数天。与珠光体相同的晶体学和溶质扩散路径考虑因素也适用于这种转变。 对于这两种转变，进行了广泛的热力学计算，并将其与元素分配和微观结构发展的实验观察相关联。 主要实验方法包括在内部制造所有三元钢并在铅浴中进行等温热处理。 使用传统的倾斜和高分辨率透射电子显微镜（TEM）检查结构。 最近购买的聚焦离子束铣床有助于化学分配研究，该铣床允许选择任何相间边界来制备用于 TEM 的电子透明箔。 化学测量结果在场发射枪 TEM 中收集，该场发射枪能够从直径小于 1 纳米的区域获取数据。 %%%替代合金元素在钢生产中很重要，因为它们可以通过延迟中间温度下珠光体和铁素体的形成来大大提高合金的淬透性。 这些元素可以是强碳化物形成元素，并且可以导致低合金钢和中合金钢的强化。 它们在不断增长的相间边界处的行为对于理解和建模非常重要。 在这个非常活跃的研究领域中，了解 Fe-C-X 钢中铁素体：奥氏体界面的热力学至关重要。 鉴于热力学数据库和高分辨率仪器的广泛进步，从科学和工业角度来说，开展这一重要的研究领域是及时的。 ***