Dislocation-solute Atom Interactions Research Articles

High-temperature low cycle fatigue behavior of an equiatomic CoCrFeMnNi high-entropy alloy

In the present work, low cycle fatigue (LCF) behavior of an equiatomic CoCrFeMnNi high entropy alloy (HEA) is correlated to the microstructural evolution at 550 °C. The fully reversed strain-controlled fatigue tests were conducted in air under strain amplitudes ranging from 0.2% to 0.8%. The measured cyclic stress response showed three distinct stages which include initial cyclic hardening followed by a quasi-stable cyclic response until failure. The rate and amount of cyclic hardening increased with the increase in strain amplitude. In comparison to common austenitic stainless steels, CoCrFeMnNi HEA shows comparable strength and improved LCF lifetime at similar testing conditions. Electron-microscopy investigations after failure reveal no noticeable change in grain size, texture and annealing twins density. Initial cyclic hardening is attributed to the dislocations multiplication and dislocation-dislocation as well as dislocation-solute atom interactions. The quasi-stable cyclic response is associated with the equilibrium between dislocation multiplication and annihilation, which also leads to the formation of complex dislocation structures such as ill-defined walls and cells, particularly at higher strain amplitudes. Besides, the material exhibits serrated plastic-flow due to interactions between mobile dislocations and diffusing solute atoms (such as Cr, Mn and Ni). Lastly, segregation in the form of Cr- and NiMn-enriched phases were observed near grain boundaries, which appears to have a detrimental effect on the fatigue life.

Negative Strain Rate Sensitivity in Two-Phase Nanocrystalline CoCrFeMnNi High-Entropy Alloy with Broader Grain Size Distribution Studied by Nanoindentation

Two-phase microstructure was realized in CoCrFeMnNi system using a combination of ball milling and spark plasma sintering at 1273 K. Microstructure consisted of finer average grain size with broader grain size distribution, complex solid solution phases (a major face-centered cubic phase and a minor body-centered cubic phase) with very close lattice constants, dislocations and nano twins. A high hardness of 6.3 GPa and a negative strain rate sensitivity (with no associated serrated flow) of − 0.0206 were observed. Various interactions between dislocations and grain boundaries/interphase boundaries/twin boundaries, prevailing atomic level complex chemistry in the lattices, interfaces and dislocation cores might be the reasons for the observed flow characteristics. Dislocation–solute atom interactions observed in conventional dilute alloys that have resulted in negative strain rate sensitivity and dynamic strain aging may not be operating in the present alloy system under the given set of test conditions.

Influence of dislocation–solute atom interactions and stacking fault energy on grain size of single-phase alloys after severe plastic deformation using high-pressure torsion

Several pure metals (magnesium, aluminum, iron, cobalt, nickel, copper, zinc, palladium and silver) and single-phase Al–Mg, Al–Ag, Al–Cu, Cu–Al, Cu–Zn, Pd–Ag, Ni–Fe and Ni–Co alloys were processed by severe plastic deformation using high-pressure torsion (HPT). The steady-state grain size was decreased and hardness increased by alloying in all the systems. It was shown that the dominant factor for extra grain refinement by alloying was due to the effect of solute–matrix atomic-size mismatch and modulus interaction on the mobility of edge dislocations. For the selected alloys, unlike pure metals, the grain size was almost insensitive to the melting temperature, and like pure metals, no systematic correlation was established between the grain size and stacking fault energy (chemical interaction) or between the grain size and valence electrons (electrical interaction). The presence of a power-law relation, with n≈0.56, between the hardness normalized by the shear modulus and grain size normalized by the Burgers vector signified the large contribution of grain boundaries to the hardening. The contribution of the solid-solution effect to the total hardening appeared to be <15%.

Non-linear anelasticity due to dislocation–solute atom interactions in solid solutions

A theory for the non-linear, strain-amplitude-dependent internal friction (ADIF) in solid solutions due to dislocation–solute atom interactions has been developed. The suggested model accounts for different modes of dislocation–solute atom interactions: • solute atoms, distributed in the dislocation glide plane, represent short-range obstacles for the dislocation motion; • solute atoms, situated away from the dislocation glide plane, create diffuse weak long-range elastic stress fields, also impeding dislocation motion. Dislocations overcome localised obstacles under the combined action of the applied stress and thermal energy, whereas diffuse long-range obstacles can be surmounted only athermally. Numerical calculations of the ADIF, strain-amplitude-dependent modulus defect, their ratio, and of the fraction of the athermal ADIF component have been performed. The model predicts a complicated multistage behaviour of the parameters of the non-linear anelasticity in the strain amplitude–temperature–solute concentration domain which is in excellent agreement with recent experimental data.

Theory of dislocation-solute atom interactions in solid solutions and related nonlinear anelasticity

A theory for dislocation-solute atom interactions in solid solutions has been developed which allows one to calculate the nonlinear dislocation strain-amplitude-dependent internal friction. The suggested model accounts for different modes of dislocation-solute atom interactions: (i) Solute atoms distributed in the dislocation glide plane interact with the dislocation core and represent short-range obstacles for the dislocation motion; (ii) Solute atoms situated away from the dislocation glide plane create relatively weak long-range elastic stress fields, also impeding dislocation motion. We assume that dislocations move in a two-component system of obstacles which differ with respect to the thermodynamics of dislocation--point-defect interactions. Namely, dislocations overcome short-range obstacles under the combined action of applied stress and thermal energy, whereas relatively weak long-range obstacles are surmounted athermally. The model predicts a complicated multistage behavior of the nonlinear internal friction in the strain amplitude--temperature--solute concentration domain, which is in excellent agreement with recent experimental data.

Effects of Li concentration and a Mg addition on serrated flow in AlLi alloys

Serrated flow phenomena have been reported in a variety of precipitation-strengthened aluminum alloys. In the particular case of precipitation-strengthened Al-Li alloys, serrated flow effects of similar character have been reported in binary Al-Li alloys and in commercial-type Al-Li alloys containing multiple alloying elements. Observations of serrated flow in binary Al-Li alloys indicate that the presence of Li alone is sufficient to produce serrated flow. Aging time has been used to probe the mechanisms that cause serrated flow in individual Al-Li alloys, and several investigators have noted that serrated flow disappears when Al-Li alloys are aged to peak strength or overaged. Much of the available experimental evidence supports dislocation-[delta][prime] interactions as the cause of serrated flow in Al-Li alloys, rather than dislocation-solute atom interactions to which serrated flow phenomena are traditionally attributed. Additional support for this conclusion could be provided by comparison of stress-strain curves for a solid solution Al-Li binary alloy of the same composition as the matrix phase of a precipitation-strengthened Al-Li binary alloy. The purpose of the present paper is to show stress--strain curves for Al-1.38Li, Al-1.80Li and Al-1.39Li-1.0Mg alloys, and to interpret the results in terms of the interactions proposed to account for serrated flow in Al-Limore » alloys.« less