Collective interstitial ordering is at the core of martensite formation in Fe-C-based alloys, laying the foundation for high-strength steels. Even though this ordering has been studied extensively for more than a century, some fundamental mechanisms remain elusive. Here, we evidence the unexpected effects of two correlated phenomena on the ordering mechanism: anharmonicity and segregation. The local anharmonicity in the strain fields induced by interstitials significantly reduces the critical concentration for interstitial ordering, up to a factor of three. Further, the competition between interstitial ordering and segregation results in an effective decrease of interstitial segregation to extended defects for high interstitial concentrations. The mechanism and corresponding impact on interstitial ordering identified here enrich the theory of phase transitions in materials and constitute a crucial step in the design of ultrahigh-performance alloys.Alloying is one of most efficient ways to improve the structural and electronic properties of materials. The alloying atoms enter the host lattice as interstitials or substitutionals. Due to differences in the chemistry and atomic sizes, each interstitial or substitutional atom creates a local strain field, displacing its neighboring host atoms away from their original lattice positions. At high alloying concentrations, the interstitial or subsitutional atoms strongly interact with each other both chemically and elastically, leading to ordering/disordering phenomena and severe lattice distortions. This concept is often employed in designing, e.g., phase-change materials [1], battery electrode materials [2], and high-entropy alloys [3]. However, even at dilute alloying concentrations the interstitial or substitutional atoms can still interact via the host lattice, mediated by long-range strain induced interactions between the local distortion fields. An interplay between short-range chemical interaction and long-range strain induced interaction may lead to an ordering of interstitial or substitutional atoms, significantly impacting the performance of materials [4][5][6][7]. For instance, the presence of ordered oxygen complexes may simultaneously enhance the strength and ductility of alloys by changing their microscopic deformation mechanism [7]. Interstitials interact not only with each other, but, in real materials, also with extended defects, leading to interstitial segregation and a competition between interstitial segregation and ordering. Understanding the mechanism of the collective interstitial ordering is thus key in designing ultrahighperformance alloys.
The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer. This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions. Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer (DFL) with nano-sized grains (48–67 nm) at the topmost surface, a DFL with submicron-sized grains (66–158 nm) and micron-sized laminated structures at the subsurface, and a dislocation accumulated layer extending to the bulk material. The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process, which were as high as 6.67 and 8.17 × 107 s−1, respectively. In the current study, the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and, simultaneously, from one orientation of a single crystal to random orientations of polycrystals, during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy.
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