Formation Of Lomer-Cottrell Locks Research Articles

A novel L12-strengthened single crystal high entropy alloy with excellent high-temperature mechanical properties

In this work, a novel and low-cost L12-strengthened single crystal high entropy alloy (HEA) was prepared through the seeding method of directional solidification. The microstructures, tensile properties and deformation mechanisms of the single crystal HEA were systematically investigated. The volume fraction of L12 precipitates after heat treatment was almost 80%, which was higher than that of conventional single crystal superalloys. The maximum yield strength, ultimate tensile strength and elongation, which were separately as 897 MPa, 1038 MPa and 37%, were obtained at the intermediate temperature of 800 °C. Moreover, at higher temperature (900 °C and 1000 °C), the HEA was able to maintain excellent performance, which was comparable to the commercial René N5 second-generation Ni-based single crystal superalloy. Finally, the high performance was examined by the model of strengthening contribution, which proved that the high-volume fraction of L12 precipitates in single crystal HEA was the primary reason for their significant precipitation and solid solution strengthening. Additionally, the formation of Lomer-Cottrell locks and stacking faults could enhance the high-temperature properties of the single crystal HEA.

A mortise-and-tenon structure inspired high strength-ductility 3D printed high-entropy alloys with mechanically interlocked network

Designing special heterogeneous structures has been proven to be an effective strategy for breaking the trade-off between strength and ductility. In this work, we drew inspiration from the architectural integrity and mechanical ingenuity of the mortise-and-tenon joint, a hallmark of traditional craftsmanship, to engineer a novel mechanically interlocked network (MIN) within a 3D printed high-entropy alloy (HEA). The sub-grains crosslink not only mimics the joint in structure, but also reproduces its function in modifying the mechanical properties of the MIN. The MIN structure provides the excellent structural stability and disperses the stress concentration at the grain boundary during the deformation process, which thus avoids the fracture failure caused by the crack propagation. The alloy's remarkable performance, characterized by a tensile strength of approximately 1152 MPa and an elongation of 28%, is attributed to a symphony of underlying mechanisms, including hetero-deformation induced strengthening and hardening, dislocation tangling, stacking faults, and the formation of Lomer-Cottrell locks. These findings demonstrate the feasibility of introducing MIN heterogeneous structures to enhance the mechanical properties of HEA and promote the development of high-performance materials with manufacturing flexibility, enabling customization of their multi-scale microstructure using additive manufacturing techniques.

A spatial decomposition parallel algorithm for a concurrent atomistic-continuum simulator and its preliminary applications

This paper presents the development of a spatial decomposition parallel algorithm and its implementation into a concurrent atomistic-continuum (CAC) method simulator for multiscale modeling of dislocations in metallic materials. The scalability and parallel efficiency of the parallelized CAC are tested using up to 512 processors. With a modest computational resource, a single crystalline f.c.c. sample containing 10.6 billion atoms is modeled using only 4,809,108 finite elements in a CAC model at a fraction of the cost of full molecular dynamics (MD). The simulation demonstrates a nearly ideal scalability of the newly parallelized CAC simulator. The parallel efficiency of the newly parallelized CAC is shown to be higher than 90% when using 512 processors in the high performance computing cluster at Iowa State University. This parallel efficiency is comparable to the state-of-the-art atomistic simulator. Moreover, the newly parallelized CAC simulator employing a uniform coarse mesh is capable of capturing important atomistic features of dislocations, including dislocation nucleation, migration, stacking faults as well as the formation of Lomer-Cottrell locks, in a billion-atom system. The spatial decomposition-based parallelization algorithm developed in this work is general and can be transferable to many other existing concurrent multiscale simulation tools.

Transient minimum creep of a γ′ strengthened Co-base single-crystal superalloy at 900 °C

A characteristic creep behavior, with a transient minimum creep rate region, was exhibited in a Co-Al-W-Ta-Ti single-crystal superalloy at 900°C and 420MPa. Interrupted creep tests were performed at various creep regions, the investigation of substructural evolution indicated the configurations of stacking faults were different during various creep regions. The extension of stacking faults and the formation of Lomer-Cottrell locks are hypothesized to be responsible for the accelerating region and steady-state region, respectively.

Cyclic deformation behavior and dislocation structures of [001] copper single crystals—I Cyclic stress-strain response and surface feature

Cyclic deformation behavior of [001] copper single crystals was investigated in symmetrical pull-push fatigue tests at the ambient temperature and with constant plastic shear strain amplitudes ( γ pl) in the range of 1.0 × 10 −4 to 3.0 × 10 −3. The formation and development of surface slip bands (SBs) were examined in situ by a light microscope. A rapid initial hardening stage followed by a pronounced overshooting or significant softening was found with the crystals tested at γ pl ≥ 4.8 × 10 −4. The cyclic stress-strain curve of [001] crystals does not show any plateau behavior, instead it follows, if corrected with the Taylor factor, the power law function for copper polycrystals. The fatigue limit, defined as the critical strain amplitude below which SBs do not form, was found to be approximately 1.7 × 10 −4, a value much higher than that for single slip oriented crystals, but very close to that for polycrystals modified by the Taylor factor. In situ observation revealed that the primary SBs appeared at the very beginning of cyclic deformation and frequently they were not persistent. The secondary (critical) SBs usually formed at the overshooting stage after rapid hardening. Deformation bands with the characteristics of kink bands were detected in the final stage of the cyclic deformation. Analysis indicated that the rapid initial cyclic hardening is caused by the formation of Lomer-Cottrell locks, while the overshooting/softening behavior is related to the operation of secondary slip or the formation of dislocation labyrinth structure. The preferred combination of primary and critical slip during cyclic deformation is also discussed in terms of dislocation reactions.

Lomer-cottrell locks in a Cu-15 at. % Al alloy

Abstract A discussion is given of the direct observation of the formation of Lomer-Cottrell locks by electron transmission microscopy in a Cu-15 at. % Al alloy single crystal tested in tension. The Burgers vectors of the interacting dislocations as well as the geometry of the final configurations are in accord with that predicted by theory for the formation of Lomer-Cottrell locks.