AbstractThe nucleation (to a limited extent), growth and coarsening behavior of Cu-rich precipitates in a concentrated multicomponent Fe–Cu-based steel aged at 500 °C from 0.25 to 1024 h is investigated. The temporal evolution of the precipitates, heterophase interfaces, matrix compositions and precipitate morphologies are presented. With increasing time, Cu partitions to the precipitates, Ni, Al and Mn partition to the interfacial region, whereas Fe and Si partition to the matrix. Coarsening time exponents are determined for the mean radius, 〈R(t)〉, number density, NV(t), and supersaturations, which are compared to the Lifshitz–Slyzov–Wagner (LSW) model for coarsening, modified for concentrated multicomponent alloys by Umantsev and Olson (UO). The experimental results indicate that the alloy does not strictly follow UO model behavior. Additionally, we delineate the formation of a Ni–Al–Mn shell with a stoichiometric ratio of 0.51:0.41:0.08 at 1024 h, which reduces the interfacial free energy between the precipitates and the matrix.]]>

High strength low alloy (HSLA) steels are widely used in the construction of ship structures, oil pipelines, offshore platforms and so on because of their good strength, toughness and weldability. HSLA steel is generally designed with low carbon and Cu alloying. Tempered lath bainite or martensite and nano-precipitate phase of Cu can be obtained by quenching and ageing process after rolling to ensure the excellent matching of strength, low temperature toughness and weldability of HSLA steel. At present, increasing attention has been focused on the precipitation behavior and strengthening mechanism of Cu particles in HSLA steel which was aged at the peak hardness of ageing curve. However, in practical engineering applications, overageing heat treatment is generally used to make HSLA steel achieve a good match of strength and toughness. In this work, the microstructure and nano-sized Cu precipitates of an industrial production HSLA steel plate with thickness of 35 mm were characterized by SEM, EBSD, HRTEM and APT. Meanwhile, the strengthening mechanism of the tested steel was investigated. The results show that Cu precipitates in the tested steel processed by overageing are mainly in the range of 6~50 nm, Cu particles exhibiting short rod or spherical shape within 30 nm are 9R structure, and other particles size larger than 30 nm exhibiting long rod or spherical shape are fcc structure. The segregation of trace Mn and Ni in rod particles on the interface between Cu particles and matrix is more obvious. After ageing at a higher temperature range, the yield strength of the tested steel decreases linearly with the increase of tempering temperature. The main strengthening mechanism of the HSLA steel is fine grain strengthening, followed by dislocation strengthening and precipitation strengthening. The calculated results show that every 1%Cu added in the tested steel can produce about 90 MPa precipitation strengthening increment under the condition of overageing heat treatment. The strength difference between the surface and the center of the tested steel plate is about 40 MPa, which is mainly due to the difference of grain size and dislocation density of steel.

对含Cu低合金高强度钢板淬火并经高温时效后,采用SEM、EBSD、HRTEM和APT等手段对其微观组织和纳米尺度Cu的析出相进行了表征,对其厚度截面的室温拉伸性能进行了测定,并对钢板厚度方向近表面和心部的强化机理进行了分析。结果表明,高温时效后钢中Cu的析出相尺寸在6~50 nm范围内,30 nm以内的为9R结构的短棒状或球状粒子,30 nm以上的为fcc结构的长棒状粒子,棒状粒子中微量的Mn、Ni在Cu粒子与基体界面上的偏聚更明显。在较高温度范围内进行时效后,钢的屈服强度随着时效温度的升高而呈大致线性下降趋势,钢的主要强化机制为细晶强化,其次为位错强化和析出强化,经计算,钢中每1% (质量分数)的Cu在过时效状态下能够产生约90 MPa的析出强化增量。钢板厚度截面存在强度差异,表面与心部强度相差约40 MPa,这主要是由于晶粒尺寸及位错密度差异所导致。

The Cu precipitation in a quench-aged high-strength low-alloy steel is studied at the atomic scale by atom probe tomography and high-resolution transmission electron microscopy. The results indicate that the Cu precipitates greatly correlate with carbides in the aspect of distributional character, i.e., the two phases are prone to coprecipitate on the dislocations and/or interfaces (low angle boundaries of the martensite laths). The crystallographic defects have a significant effect on the sizes, morphology and composition of Cu precipitates with Ni and Mn segregation shell.

γ' [Ni3(Al,Ti)], precipitated during ageing, is thought to have the main contribution on the strength. Thus, it is very important to understand the characterization of the precipitation. However, few previous studies are focused on atomic scale evolution of the precipitated phase. Atom probe tomography (APT) is a unique microscopy technique that provides 3D analytical mapping of materials at near atomic resolution and a high detection sensitivity for all elements. The present research is focused on the microstructure evolution at ageing temperature at different time scales using APT. A Fe-Ni base austenite alloy were aged at 620 ℃ for different time after solution treated at 980 ℃ for 2 h. Hardness testing indicates that a sharp increase is observed when the ageing time is less than 6 h. The hardness is up to 205 HV from the initial 145 HV at the ageing time 6 h. After that the hardness increases slowly. The hardness is 251 HV at 120 h. APT results reveal that Ti-rich nanoclusters precipitate obviously at the initial stage of ageing, which contain Fe, Cr, Ni, Mo and Al elements. As the ageing time increases, more Ni and Al atoms are segregated in the Ti-rich nanoclusters while the Fe, Cr and Mo are ejected from the nanoclusters. When the ageing time is up to 120 h, the Ni/(Ti+Al) ration is approximately close to 3. The precipitates can be identified as γ' phase. The results reveal that the formation of γ' involves nucleation and growth. Effect of the number density and the size of the γ' precipitates on the hardening of the alloy has been estimated.]]>

γ'相的析出行为及其对材料硬度的影响。结果表明,当时效时间小于6 h时,合金硬度增加较快,由时效前的145 HV迅速增加至205 HV,随后硬度增加速率缓慢;时效120 h时,硬度为251 HV。APT结果表明,合金经固溶处理后,合金元素均匀分布在基体中。在时效最初阶段,Ti发生了较为明显的偏聚,形成含有Fe、Ni和Al等元素的富Ti纳米团簇。随着时效时间延长,富Ti纳米团簇中的Ni和Al原子的含量逐步增多,而Fe、Cr及Mo等原子的含量不断减少;当时效至120 h时,团簇中Ni与Ti+Al比值近似于3,即已完全形成γ'相,表明γ'相析出是形核-长大过程。合金硬度的变化与时效过程中γ'相的数量密度和体积分数有关。]]>

The technique of atom probe tomography (APT) is reviewed with an emphasis on illustrating what is possible with the technique both now and in the future. APT delivers the highest spatial resolution (sub-0.3-nm) three-dimensional compositional information of any microscopy technique. Recently, APT has changed dramatically with new hardware configurations that greatly simplify the technique and improve the rate of data acquisition. In addition, new methods have been developed to fabricate suitable specimens from new classes of materials. Applications of APT have expanded from structural metals and alloys to thin multilayer films on planar substrates, dielectric films, semiconducting structures and devices, and ceramic materials. This trend toward a broader range of materials and applications is likely to continue.

followed by water quenching, and tempered isochronally for 60 min at different temperatures. The hardness was conducted, the microstructure and Cu precipitate were analyzed by HRTEM and APT. During tempering, Cu precipitation happened, Cu precipitate Moire fringe formed and the Cu precipitate transformed to fcc structure; the lath boundary gradually bulged out and migrated, a repeat of bulging and migration of local parts of lath boundary resulted in migration of the whole boundary, and lath martensite transformed to equiaxed ferrite finally. At 500℃, the strengthening peaked by Cu precipitates. During 400-500℃, the number of Cu clusters changed greatly when the Cu isoconcentration set at different values, this indicated that the Cu precipitates  were on the stage of nucleation; while the number of Cu clusters changed little during 500-650 ℃, this indicated that the Cu precipitates were on the stage of coarsening. The Cu, C, Mo and P segregated at the grain boundary. The boundary could provide Cu solutes and nucleation sites for Cu precipitation, leading to the segregation of Cu clusters at the grain boundary. The Ni, Mn and Al segregated at the heterophase interface between Cu precipitate and ferrite matrix forming a core-shell structure.]]>

HRTEM和三维原子探针(3DAP)对含Cu和Ni低碳高强度钢等时回火析出的富Cu相进行了研究.结果表明: 回火过程中,基体发生软化, 富Cu相析出, 板条状马氏体逐渐转变成多边形状铁素体; 在500 ℃时富Cu相强化作用达到最大值; 设置不同的Cu等浓度值时, 在400-500 ℃富Cu相的数量变化幅度大, 在500-650 ℃富Cu相的数量基本不变; 在晶界处发生C, Mo, P和Cu的偏聚; 晶界处Cu浓度高于基体, 为富Cu相的形核和长大提供了有利条件; 在析出的富Cu相与基体的过渡层上发生Ni, Mn和Al的偏聚, 这些偏聚元素与富Cu相核心共同形成核-壳结构.]]>

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 +/- 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 10(24) per cubic metre) and small size (about 2.7 +/- 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.