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

9904034 Shiflet该项目是一个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。 两个重要的转换进行检查。 首先是珠光体的研究，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常见的。 它在钢中形成，也在大量的有色合金中形成，在等温、连续冷却和强制速度生长条件下的转变过程中。 这项工作扩展了以前的晶体学考虑，包括特定的替代合金元素，如锰的相互作用。 解决的关键问题包括合金元素的原子扩散路径是否确实是在珠光体：奥氏体（母相）相间的边界，许多研究人员建议，或体积扩散控制的转变动力学。 一个目标是要考虑到生长模型无法解释在一定温度范围内的实验动力学。 研究的第二个领域是在中等温度下的铁素体加碳化物转变，其中当X = Cr、Mo或W时，生长动力学严重减慢。 在这些合金中，存在从约550至600 ℃的温度范围，其中对生长动力学发生极其有效的影响。 事实上，所有的铁素体生长和成核都可以停止数天。对于珠光体，同样的晶体学和溶质的扩散途径的考虑也适用于这种转变。 对于这两种转换进行了广泛的热力学计算，并与实验观察元素分配和微观结构的发展。 主要的实验方法包括在内部制造所有的三元钢，并在铅浴中进行等温热处理。 该结构的检查是与传统的倾斜和高分辨率透射电子显微镜（TEM）。 化学分区的研究，促进使用聚焦离子束铣床最近收购，允许任何相间的边界被选择用于制备成一个电子透明的TEM箔。 化学测量结果收集在一个场发射枪TEM，能够从直径小于一纳米的区域获得数据。 替代合金元素在钢的生产中很重要，因为它们可以通过延迟珠光体和铁素体在中温下的形成而大大提高合金的硬化性。 这些元素可以是强碳化物形成剂，并且可以导致低合金钢和中合金钢的强化。 他们的行为在不断增长的相间边界是很重要的理解和模型。 在这一非常活跃的研究领域中，对Fe-C-X钢中铁素体-奥氏体界面热力学的理解是必不可少的。 鉴于热力学数据库和高分辨率仪器的广泛进展，从科学和工业的角度来看，追求这一重要的研究领域是及时的。 ***

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. ***