Nano B2 FeCu ordered clusters and multiphase microstructure consisting of intercritical ferrite, tempered martensite and nano-γ phase (reverted austenite) were obtained by two-step heat treatment involving intercritical annealing and intercritical tempering. The experimental steel with nano Cu precipitates and nano-γ phase exhibited high strength and high ductility combination. The yield strength and total elongation of the experimental steel increased from 758 MPa and 16.8% to 984 MPa and 29.5% after the second step intercritical tempering for 5 min. High resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe (3DAP) studies provided evidence to support that high density of nano B2 FeCu ordered clusters contributed to high strength. First principle calculations suggested that the feasibility of B2 FeCu nano-ordered clusters, and the stability of B2 structure is related to the coherent stress field at the interface between clusters and the BCC-Fe matrix. The average size of B2 FeCu nano-ordered clusters was 4 nm with a lattice constant of 0.2893 nm and an orientation relationship of (1 1 0)_B2//(1 1 0)_α and [0 0 1]_B2//[0 0 1]_α. 9R Cu without twinned structure was discovered at different tempering times. The proportion of Cu in Cu precipitates varied from 24.4 at% to 61.2 at% with change in crystal structure and increase in the size of precipitates. The significant increase in yield strength is discussed in terms of ordered domain strengthening, modulus strengthening and lattice misfit strengthening mechanisms. Bright field TEM image and selected area electron diffraction (SAED) pattern discovered and proved that spindle nano-γ phase was present at phase boundary between tempered martensite and intercritical ferrite. The size of them were among 242–375 nm in length and 52–80 nm in width. The two needle tip ends of nano-γ phase were just at phase boundaries, indicating the preferred growth direction. (1 −1 1)_γ plane of nano-γ phase was parallel to (1 −1 0)_α plane of the adjacent tempered martensite. The effect of nano-γ phase on enhancing the plasticity was revealed by instantaneous work hardening index curve.

通过包括临界退火和临界回火的两步热处理，获得了由临界铁素体，回火马氏体和纳米γ相（还原奥氏体）组成的纳米B2 FeCu有序簇和多相组织。具有纳米铜沉淀物和纳米γ相的实验钢表现出高强度和高延展性的组合。在第二步进行临界回火5分钟后，实验钢的屈服强度和总伸长率从758 MPa和16.8％增加到984 MPa和29.5％。高分辨率透射电子显微镜（HRTEM）和三维原子探针（3DAP）研究提供了证据，证明纳米B2 FeCu纳米有序簇的高密度有助于高强度。第一性原理计算表明，B2 FeCu纳米级团簇的可行性以及B2结构的稳定性与团簇与BCC-Fe基体界面的相干应力场有关。B2 FeCu纳米有序簇的平均尺寸为4 nm，晶格常数为0.2893 nm，取向关系为（1 1 0）_B2 //（1 1 0）_α和[0 0 1] _B2 // [0 0 1] _α。在不同的回火时间发现了没有孪晶结构的9R Cu。随着晶体结构的改变和沉淀物尺寸的增加，Cu沉淀物中Cu的比例从24.4原子％变化至61.2原子％。从有序畴增强，模量增强和晶格失配增强机制方面讨论了屈服强度的显着提高。发现了亮场TEM图像和选择区域电子衍射（SAED）模式，并证明在回火马氏体和临界铁素体之间的相界处存在纺锤体纳米γ相。它们的大小在长度242-375 nm和宽度52-80 nm之间。纳米γ相的两个针尖位于相界处，表明了优选的生长方向。（1 -1 1）_γ纳米γ相的平面平行于相邻的回火马氏体的（1 -1 0）_α平面。瞬时加工硬化指数曲线揭示了纳米γ相对增强塑性的作用。