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Manufacturing temperatures of severely cold-drawn hyper-eutectoid steel wires are sufficiently high to influence the mobility of dislocations and alloy elements, thereby affecting the materials’ mechanical properties. Herein, we describe the evolution of microstructure and tensile strength of the as-drawn 3.45 GPa steel wire during post-deformation annealing for 30 min at 150−450 °C. Annealing at 150 °C raised the strength to 3.77 GPa by age-hardening through activation of dislocations pinning by carbon, while further temperature rising up to 450 °C caused a severe loss of strength. It was proved that annealing at 300 and 450 °C destabilizes the lamellar microstructure, promoting the formation of carbon-deficient (Fe,Mn,Cr)3C-type cementite particles with preferentially rounded and partially faceted hetero-interfaces. Annealing at 450 °C yielded the accumulation of Mn and Cr at the ferrite/particle interfaces, and their concentrations at the interfaces were dependent on the interface structure; i.e., lower concentrations at rounded interfaces (formed through capillarity–driven coarsening of the spheroidized cementite), and higher concentrations at faceted interfaces (that are initially existing in the as-drawn state). Our proof-of-principle observations, supported by thermodynamic calculations and kinetic assessments, provide a pathway for understanding the changes in microstructural and tensile properties during manufacturing of the hyper-eutectoid steel wires.

The nanostructural hierarchy of γ and γ′ phases is characterized in Ni-8.5Al-5.4Ti (at.%) alloy, after double aging treatment at 1000 and 780 °C for 6 and 20 h. Using electron microscopy and atom probe tomography, disordered γ phase inside ordered γ′ precipitate was detected, which underwent plate-like growth during aging treatment. This unique hierarchical structure enabled the simultaneous study of direct (γ′ precipitates/γ matrix) and inverse (γ nanoparticles/γ′ precipitates) order-disorder interfaces in 6 and 20 h double-aged samples. It was found that the compositional interface width, δ, of direct interfaces decreased during aging treatment, whereas the thickness remained almost unaffected for inverse interfaces. The difference in thickening behavior is explained based on the change of the Gibbs-Thomson pressure for γ plates embedded in γ′ and cuboidal γ′ precipitates surrounded by the γ matrix. This inference suggests the possibility of prediction and control of δ(t) variations during heat treatment processes based on the current thermodynamic approach.

Order-disorder coherent interfaces determine the microstructure and mechanical properties of precipitation-hardened high-temperature alloys. The characteristics of these interfaces can be defined by a compositional width, δ, and structural width, δ′. The latter, which can be considered as the width of the ordered part of the interface, can play an important role in high-temperature mechanical behavior of precipitation-hardened alloys. This is due to the fact that diffusion in the ordered part of the interface is generally much slower than diffusion in the disordered phase, thus hindering the solid-state diffusion-based phenomena. Here, we investigate the order-disorder interface in a Ni-19Al (at.%) alloy as a model alloy for Ni-based superalloys using atomic-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. Then, we employ thermodynamic modeling to describe the interplay between the structural and compositional interface widths in binary Ni-Al and in ternary Ni-Al-Cr and Co-Al-W systems. We introduce the δ′/δ ratio as a critical parameter that varies significantly in different alloys. Our findings offer a general pathway to control the δ′/δ ratio of interfaces, which in turn affect the high-temperature properties of precipitation-hardened alloys.

The present study aims at unveiling the influence of trace amounts of phosphorus on the macro-scale delamination in a ferritic steel. Two different steels with trace amounts of phosphorus were examined to reveal the cause: One was made by long-time high temperature holding to maximize the phosphorus segregation, and the other one was made by short-time high temperature holding to minimize the phosphorus segregation. The atom probe results at the grain boundaries ahead of the delamination-related cracks provide a strong evidence that phosphorus-enrichment and carbon-depletion for the long-time high temperature holding induces a larger degree of delamination as compared to the short-time high temperature holding. Reasons for the different segregation tendency were discussed by correlating the isothermal holding time at high temperature and the competitive segregation between phosphorus and carbon.

The passive films on typical stainless steels (SS) and on a newly developed high-Cr (17 wt%)-added 18 wt%-Mn steel (HCr-HMnS) were compared by Cs-corrected scanning transmission electron microscopy and atom probe tomography. Although the passive films of all samples having similar Cr contents had the same thickness, unprecedented hexagonal wurtzite MnO inside the passive film of HCr-HMnS specimen was susceptible to corrosion cracking; this was not observed in the SS samples. This MnO caused crack formation during potentiodynamic polarization test, suggesting that reducing the harmful MnO by adding Mo and Ni facilitates the development of high-Mn base SS materials . Furthermore, higher MoO2 composition of the passive films on 316 type austenitic SS than 304 type series might would result in primarily the improved pitting resistance.

In this study, we addressed the effects of Ni-plating and nanostructuring of WC-Co powders on the microstructure-property relations and WC decarburization of WC-Co composite coatings. By high velocity oxygen fuel (HVOF) spraying, the coating materials were produced from four different powders, referred to as microstructured WC-Co (mc-WC), nanostructured WC-Co (nc-WC), Ni-plated microstructured WC-Co (Ni/mc-WC) and Ni-plated nanostructured WC-Co (Ni/nc-WC). We found that the coatings, deposited from Ni-free powders, undergo a high level of decarburization, thereby yielding W 2 C phase formation in brittle amorphous W-rich Co matrix that could be preferential site for crack propagation. On the contrary, the Ni-plating treatment could reduce the amount of decarburization of the coatings, resulting in the uniform distribution of WC phase in the W-depleted Co matrix. Consequently, we revealed the underlying mechanisms responsible for WC decarburization in the coatings, and further suggested that the coating deposited from Ni-plated and nanostructured powders exhibits a superior combination of hardness and fracture toughness as compared to other coatings.

In the present study, the effects of microstructure, chemical composition on the transformation behavior and mechanical stability of individual retained austenite (RA) with the different sizes and morphology in TRIP steels were intensively investigated. In order to characterize the property of the individual RA, various analytical techniques including atom probe tomography (APT), step-wise straining EBSD and nano-indentation were applied. The blocky type RA as we categorized have many defects and lower carbon contents (~25%) compared to film type RA. And also, step-wise straining EBSD and nano-indentation results revealed that the mechanical stability of blocky type RA was lower than that of film type RA which means blocky type RA would be easily transformed into martensite. It is considered that many defects existing in blocky type RA could work as the nucleation site of martensite transformation. In contrast, high carbon contents and hard phases enclosing film type RA would increase the resistance to shear transformation resulting in the inhibition of transformation of RA to martensite.