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

9904034本项目是对替代合金元素对Fe-C-X钢中奥氏体分解影响的分析和实验检验。研究了两个重要的变换。首先是对珠光体的研究，珠光体是共析分解的片状产物，可能是所有金属组织中最常见的。在等温、连续冷却和强迫速度生长条件下的相变过程中，它在钢和大量有色合金中形成。这项工作扩展了以前的结晶学考虑，包括与替代合金元素，如锰的特定相互作用。所解决的关键问题包括合金元素的原子扩散路径是否真的像许多研究人员所说的那样位于珠光体：奥氏体(母相)界面，或者体积扩散是否控制着相变动力学。一个目标是解释生长模型无法解释在一定温度范围内的实验动力学。第二个研究领域是铁素体+碳化物在中温下的转变，当X=Cr，Mo或W时，生长动力学严重减慢。在这些合金中，大约550-600℃的温度对生长动力学产生极大的影响。事实上，所有铁素体的生长和形核都可以停止数天。与珠光体一样，扩散路径的结晶学和溶质考虑同样适用于这种转变。对于这两种转变，都进行了广泛的热力学计算，并与元素分配和微结构发展的实验观测相关联。主要的实验方法包括在室内制造所有三元钢，并在铅浴中进行等温热处理。用常规倾斜和高分辨率透射电子显微镜(TEM)对结构进行了检查。使用最近购买的聚焦离子束研磨机促进了化学分配的研究，该机允许选择任何相间边界以制备成用于透射电子显微镜的电子透明箔。化学测量是在场发射枪瞬变电子显微镜中收集的，它能够从直径不到一纳米的区域获得数据。%替代合金元素在钢铁生产中很重要，因为它们可以在中温延缓珠光体和铁素体的形成，从而极大地提高合金的淬透性。这些元素可以是较强的碳化物形态，并能在低合金钢和中合金钢中导致强化。它们在生长相界面上的行为对于理解和模拟是很重要的。在这个非常活跃的研究领域，了解Fe-C-X钢中铁素体：奥氏体界面上的热力学是至关重要的。鉴于热力学数据库和高分辨率仪器的广泛进步，从科学和工业的立场来看，现在进行这一重要领域的研究是及时的。***

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. ***