Dislocation-precipitate Interactions Research Articles

Deformation of a nanocube with a single incoherent precipitate: role of precipitate size and dislocation looping

ABSTRACTPrecipitate hardening is a key strengthening mechanism in metallic alloys. Classical models for precipitate hardening are based on the average behaviour of an ensemble of precipitates, and fail to capture the complexity of dislocation-precipitate interactions that have recently been observed at individual precipitates in simulations and in-situ electron microscopy. In order to achieve tailored mechanical properties, detailed deformation mechanisms at specific precipitates that account for precipitate size, crystallography, and defect structure must be understood, but has been challenging to achieve experimentally. Here, in-situ scanning electron microscope mechanical testing is used to obtain the compressive stress–strain behaviour at an individual, incoherent Au precipitate within a Cu nanocube, and determine the influence of precipitate and cube size on yield strength and strain hardening. TEM imaging and strain mapping of the initial structure shows misfit dislocations at the Au precipitate, threading dislocations that traverse the Cu shell, and localised and anisotropic strain near the precipitate and threading dislocation. These nanocubes have yield strengths of 800–1000 MPa and strain hardening rate of 1–4 GPa. Yield strength is found to depend on the distance from the precipitate interface to the cube edge, while strain hardening depends on both cube size and precipitate size. An analytical model is developed to quantify the contribution of Orowan looping, Orowan stress, back stress and image stress to plasticity at the Au precipitate. Orowan stress is found to be the largest contributor, followed by back stress and image stress.

The combined effects of hydrogen and aging condition on the deformation and fracture behavior of a precipitation-hardened nickel-base superalloy

The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.

Prediction of the shear strength of aluminum with θ phase inclusions based on precipitate statistics, dislocation and molecular dynamics

We propose a three-stage technique for predicting the flow stress of aluminum alloy combining atomistic calculations, parameterizing the dislocation–precipitate interaction model, and 2D dislocation dynamics. The precipitates of θ phase in aluminum matrix are considered as the object of atomistic calculations in this work. It is shown that, in contrast to the previously studied precipitates of the aluminum-copper system (GP zones, θ'' and θ' phases), θ phase appears as uncuttable inclusion during our atomistic study demonstrating only significant bending of its shape. The obtained rate and temperature dependencies of the average stress in the system are used to find the parameters of the previously proposed model of the dislocation–precipitate interaction (Krasnikov and Mayer, 2019; Krasnikov et al., 2020) in the case of θ phase. After fitting of the model parameters, the equation of motion of the dislocation in the presence of inclusions of θ phase is used in 2D dislocation dynamics. The dislocation dynamics model employs the experimental data on the size distribution of precipitates (Zuiko and Kaibyshev, 2018). The obtained values of the flow stresses of alloy demonstrate good agreement with the experimental results, and the dislocation dynamics model predicts the correct value of the thermal softening of alloy. The model predicts a strict dependence of the flow stress of alloy on the dispersion of the size distribution of precipitates at the constant average size.

High-Temperature Deformation Behavior of 718Plus: Consideration of γ′ Effects

The hot deformation behavior of 718Plus is modeled through a physically based hybrid dislocation density model, which includes the effects of precipitating particles. It is well known that the service performance and hot flow characteristics of this alloy are strongly dependent on the microstructure, particularly the grain size and second-phase particles. Thus, comprehension and modeling of the hot flow behavior is an important task. In precipitation hardening alloys (superalloys, microalloyed steels, etc.), it is particularly challenging to model the microstructural evolution in the processing windows, where material softening and precipitation processes take place concurrently. In this work, the initial stages of the deformation are studied. A model based on dislocation density evolution is presented. As in conventional approaches, the dislocation density is considered as a competition between dislocation generation and dynamic recovery at the early stages of deformation. Recovery is assumed to be driven by glide and climb of dislocations, which are considered to be proportional to the strain rate and damped by the existence of second-phase particles. It is known that under high-temperature deformation conditions, 718Plus may undergo dynamic precipitation. Second-phase particles in the material may impede the grain boundary motion and contribute to an increase in flow stress. To account for the dynamic precipitation, the present model combines experimental results and precipitation models to predict volume fraction and particle size. The effect of aging is studied through transmission electron microscopy (TEM) characterization of the specimens from interrupted tests at the onset of dynamic recrystallization. Stress contribution due to different modes of dislocation precipitation interaction are modeled and integrated to the phenomenological dislocation density–based hardening models.

A Molecular Dynamics-Informed Probabilistic Cross-Slip Model in Discrete Dislocation Dynamics

We present here a quantitative study of dislocation cross-slip, an essential thermally activated process in deformation of metals, in discrete dislocation dynamics (DDD) simulations. We implemented a stress-dependent line-tension model in DDD simulations, with minimal information from molecular dynamics (MD) simulations. This model allows reproducing in DDD simulations the probabilistic cross-slip rate calculated in MD simulations for Cu in a large range of stresses and temperatures. This model opens new horizons in modelling cross-slip related mechanisms such as deformation softening, dislocation-precipitate interaction and dislocation patterning in realistic strain rates.

Measurement and Theoretical Calculation Confirm the Improvement of T7651 Aging State Influenced Precipitation Characteristics on Fatigue Crack Propagation Resistance in an Al–Zn–Mg–Cu Alloy

Precipitation characteristics influencing fatigue crack propagation contained matrix precipitate, grain boundary precipitate and precipitate free zone for Al–Zn–Mg–Cu alloys. Over-aging treatment could effectively regulate precipitation and then to be able to change fatigue crack propagation behavior compared with the peak aging state. In the current work, typical T651 and T7651 aging tempers of the alloy were extracted via hardness, electrical conductivity and mechanical properties under one-step and two-step aging treatments. Fatigue crack propagation (FCP) rate under them was tested and corresponding precipitation characteristics and fracture morphology were observed. The results indicated that fatigue crack propagation resistance for the T7651 temper possessed an obvious improvement on the side of that for the T651 temper, which was also supported by fracture appearance, including tearing ridge, tearing dimple and fatigue striation. The precipitation observation showed that the T651 alloy contained GPI zone, GPII zone and ηʹ phase while the T7651 alloy possessed ηʹ phase and η phase. Compared with the T651 temper, matrix precipitate for the T7651 temper distinctly owed an expanding of size distribution and an enlargement of average size while cuttable phase still remained the dominance for both tempers. Grain boundary precipitate and precipitate free zone manifested no obvious difference between the two aging tempers. Cut and bypass mechanisms of dislocation–precipitate interactions were used for explanation and it revealed the reinforced cuttable phase was in favor of enhancing fatigue crack propagation resistance. A theoretical model which directly correlated FCP rate with matrix precipitate characteristics was employed to calculate FCP rate by substituting quantitative precipitate characteristics and the calculation results were vaguely consistent with the experimental measurement, which proved its reliability and feasibility.

Effects of precipitates versus solute atoms on the deformation-induced grain refinement in an Al–Cu–Mg alloy

With prior knowledge of alloying additions beneficial to grain boundary (GB) stability of nanostructured metals, here we demonstrate that an artificially aged Al–Cu–Mg alloy exhibits superior grain refinement capability compared to its solution-treated counterpart after surface severe plastic deformation processing. The concurrent precipitate dissolution and GB solute segregation, together with the strong precipitate-dislocation interaction enhanced dislocation multiplication but reduced dislocation mobility, is the fundamental reason for improved grain refinement. This result offers a novel insight into the role of precipitate in deformation-induced grain refinement and a new design pathway towards nanostructured materials with smaller grains and higher hardness.

Investigating ultrasound-induced acoustic softening in aluminum and its alloys

Ultrasonic vibration has been observed to lower the flow stress necessary to initiate plastic deformation, a phenomenon known as “acoustic softening”. This unique effect of ultrasound has been extensively applied in welding, machining, forming of metals, and ultrasonic additive manufacturing to lower the yield stress necessary to initiate plastic deformation, it nevertheless lacks fundamental investigation. Some prior studies showed experimental errors due to the design of experimental setups and the associated testing methods that have been introduced, leading to questions about their observations and conclusions. Therefore, an experimental setup described in this paper is designed to minimize the constraints identified from the setups in prior studies. Three types of aluminum are studied: Al 1100-O a commercially pure aluminum, Al 6061-O an aluminum alloy without precipitate strengthening, and Al 6061-T6 a precipitate-strengthened aluminum alloy. The acoustic softening and residual effect are compared based on the similarities and differences in microstructures of the three types of aluminum. In both acoustic softening and residual effect, linear relations are obtained between stress change and ultrasound intensities. The slope defined by the linear relations, i.e. the acoustic softening factor, depends on the microstructure of the specific material. The underlying mechanism of acoustic softening is associated with the activation of dislocations by ultrasonic energy and subsequently their interactions with other dislocations and precipitates, whereas the residual effects are attributed to the permanent changes in dislocation density due to dislocation annihilation, dynamic annealing, and dislocation-precipitate interaction.

Atomistic phase field chemomechanical modeling of dislocation-solute-precipitate interaction in Ni–Al–Co

Dislocation-precipitate interaction and solute segregation play important roles in controlling the mechanical behavior of Ni-based superalloys at high temperature. In particular, the increased mobility of solutes at high temperature leads to increased dislocation-solute interaction. For example, atom probe tomography (APT) results [1] for single crystal MC2 superalloy indicate significant segregation of solute elements such as Co and Cr to dislocations and stacking faults in γ′ precipitates. To gain further insight into solute segregation, dislocation-solute interaction, and its effect on the mechanical behavior in such Ni-superalloys, finite-deformation phase field chemomechanics [2] is applied in this work to develop a model for dislocation-solute-precipitate interaction in the two-phase γ-γ′ Ni-based superalloy model system Ni–Al–Co. Identification and quantification of this model is based in particular on the corresponding Ni–Al–Co embedded atom method (EAM) potential [3]. Simulation results imply both Cottrell- and Suzuki-type segregation of Co in γ and γ'. Significant segregation of Co to dislocation cores and faults in γ′ is also predicted, in agreement with APT results. Predicted as well is the drag of Co by γ dislocations entering and shearing γ'. Since solute elements such as Co generally prefer the γ phase, Co depletion in γ′ could be reversed by such dislocation drag. The resulting change in precipitate chemistry may in turn affect its stability and play a role in precipitate coarsening and rafting.

Revealing the atomistic nature of dislocation-precipitate interactions in Al-Cu alloys

Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocation-precipitate interactions scenarios for both θ′ and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ′ Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as un-shearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts.

Hardening mechanisms and impact toughening of a high-strength steel containing low Ni and Cu additions

Aging treatments at 400–550 °C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300–600 °C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittlement regime can improve the impact toughness but at a great expense of strength. In this work, the strengthening mechanisms as well as the toughening of a low cost weldable HSLA steel with a low content of carbon (C ∼0.08 wt.%), nickel (Ni = 0.78 wt.%), and copper (Cu = 1.3 wt.%) were carefully investigated. Our findings show that the low-C-Ni-Cu HSLA steel is insensitive to the aging temperatures and can achieve a yield strength (YS) and ultimate tensile strength (UTS) over 1000 and 1100 MPa, respectively, with tensile ductility >10% (reduction of area >60%) at a heat-treat temperature of 640 °C through multiple strengthening mechanisms. Besides, a good low-temperature (−40 °C) impact performance (∼200 J) with high YS (∼900 MPa) and UTS (∼1000 MPa) can be obtained by seeking a strength balance among the fine grain size (∼2.5 μm), medium-sized (∼14 nm) overaged Cu-rich precipitates, tempered martensite, and fresh martensite (or carbides). Moreover, a relatively lower YS (∼800 MPa) and UTS (∼900 MPa) useful for steel manufacturing can be attained by a prolonged aging at 640 °C. In addition, the dislocation-precipitate interactions were also explored based on the dislocation theories in this study.

Microstructure, thermo-mechanical properties and Portevin-Le Chatelier effect in metastable β Ti-xMo alloys

The microstructure, mechanical properties and Portevin-Le Chatelier (PLC) effect in Ti-xMo alloys (x = 10, 12, 15 and 18 wt%), in the temperature range of 250–350 °C with strain rates from 10−3 s−1 to 10−4 s−1, are systematically investigated in tension by using transmission electron microscopy and Gleeble 3500 testing machine combined with a digital image correlation technique. Results show that Young modulus decreases with increasing Mo contents, which is related to a more stable β phase matrix and a decrease of ω phase fraction. Moreover, the values of Young modulus and 0.2% offset yield strength at elevated temperature are higher than the ones at room temperature in all the Ti-xMo alloys, except the Ti-18Mo alloy which shows an opposite result. These macroscopic features are consistent with the ω phase precipitation in deformed Ti-xMo alloys, due to the combined effects of ω phase strengthening and temperature softening. Furthermore, the serration type transforms from A to A + B, then to B and eventually to C as increasing temperature and decreasing strain rate as well as Mo contents, which mainly depends on the spatial cohesion of PLC bands influenced by the intensity of ω precipitate-dislocation interactions. Finally, the peak value of maximum stress drop magnitude appears in Ti-12Mo alloy and increases with decreasing the strain rate, which is attributed to a stronger intensity of ω precipitate-dislocation interactions caused by reducing dislocations movement and providing a longer ageing time for the precipitation of ω phase particles. Besides, the average stress drop magnitude increases in Ti-18Mo alloy and decreases in the other compositions as increasing engineering strain, which is related to the variation of ω phase fraction.

Microstructural design for advanced light metals

Abstract

A new approach to synthesise high strength nano-oxide dispersion strengthened alloys

High oxygen content is usually harmful for metals and alloys. However, in this study, we propose a new approach, which involves direct additive manufacturing of a powder alloy that contains a high oxygen level, to synthesise nano-oxide dispersion strengthened (ODS) alloys. Thus, by selectively laser melting a FeNi alloy powder with an oxygen level of 3000 ppm, a new ODS alloy has been in-situ synthesised. The as-fabricated samples were also heat treated at different temperatures to understand their microstructural stability. The nano-sized oxides in the as-fabricated sample were identified to be face-centered cubic Fe3O4 and are entangled with dislocations. During deformation, dislocations loop around these particles suggesting that they have acted as effective dislocation barriers, which leads to enhanced tensile strengths. Heat treatment at or above 950 °C leads to depletion of oxides in grain interior and to segregation of oxides along grain boundaries (GBs). Transmission electron microscopy study reveals that at increased temperatures dislocations tend to act as bridges between oxides and GBs, promoting dissolution of oxides in grain interior and nucleation and coarsening of oxides at GBs. The results indicate that the proposed approach is an effective new way of synthesising high performance ODS alloys and also give a strong implication that the precipitate-dislocation interaction plays a significant role in microstructural evolution during annealing.

Meso-scale modeling and simulation for reduced activation ferritic/martensitic steel

Reduced activation ferritic martensitic (RAFM) steel has been regarded as a candidate for blanket material of nuclear fusion plant. Microstructure of RAFM steel, featured by precipitates and hierarchical structure of martensite, contribute to its excellent irradiation resistance. For better understanding of mechanical responses of RAFM steels, microstructure-based simulation has been developed for describing detailed elastic-plastic behavior in grain (or meso-) scale. In this study, a dislocation density based crystal plasticity model including dislocation-precipitate interactions was proposed and implemented in the finite element simulations of meso-scale micro-pillar compression and macro-scale uniaxial tensile tests. For the macroscale simulations, a representative volume element approach with periodic boundary condition was applied for predicting mechanical response of investigated polycrystalline RAFM steel based on measured mesoscale parameters and microstructure. The presented crystal plasticity model could reproduce the mesoscopic local deformation and macroscale flow stress strength in good agreement.

Creep Behavior and Microstructural Evolution of a Fe-20Cr-25Ni (Mass Percent) Austenitic Stainless Steel (Alloy 709) at Elevated Temperatures

Understanding creep properties and microstructural evolution for candidate materials of the next-generation nuclear reactors is essential for design and safety considerations. In this work, creep tests were carried out at temperatures ranging from 973 to 1073 K and stresses 40 to 275 MPa followed by microstructural examinations of a Fe-20Cr-25Ni (mass pct) austenitic stainless steel (Alloy 709), a candidate structural material for the Sodium-cooled Fast Reactors. The apparent stress exponent and activation energy were found to be 6.8 ± 0.4 and 421 ± 38 kJ/mole, respectively. The higher activation energy relative to that of lattice self-diffusion together with the observation of dislocation-precipitate interactions in the crept specimens was rationalized based on the concept of threshold stress. The threshold stresses were estimated using a linear extrapolation method and found to decrease with increased temperature. By invoking the concept of threshold stresses, the true stress exponent and activation energy were found to be 4.9 ± 0.2 and 299 ± 15 kJ/mole, respectively. Together with the observation of subgrain boundary formation, the rate-controlling mechanism in the Alloy 709 was conclusively determined to be the high-temperature dislocation climb. Three types of precipitates were identified in the crept samples: Nb(C, N), Z-phases of sizes between 20 and 200 nm within the matrix and M23C6 with sizes between 200 and 700 nm within the matrix and on grain boundaries. Further, the analysis of creep rupture data at high stresses indicated that the Alloy 709 obeyed Monkman–Grant and modified Monkman–Grant relationships with creep damage tolerance factor of ~ 5. Using the Larson–Miller parameter, it was concluded that the Alloy 709 exhibited superior creep strengths relative to the other advanced austenitic steels.

Origin of the low precipitation hardening in magnesium alloys

In this work electron backscattered diffraction (EBSD)-assisted slip trace analysis and transmission electron microscopy have been utilized to investigate the interaction of basal dislocations with precipitates in the Mg alloys Mg-1%wt.Mn-0.7%wt.Nd (MN11) and Mg-9%wt.Al-1%wt.Zn (AZ91), with the ultimate aim of determining the origin of their poor precipitation hardening. Precipitates in these alloys have a plate-shaped morphology, with plates being, respectively, perpendicular (MgxNdy) and parallel (Mg17Al12) to the basal plane of the magnesium matrix. Mechanical tests were carried out in solid solution and peak-aged samples, in tension and compression, both at RT and at moderate temperature (250 °C). EBSD-assisted slip trace analysis revealed a clear dominance of basal slip under a wide range of testing conditions in the peak-aged MN11 and AZ91 alloys. At room temperature, the origin of the low precipitation hardening lies at the easiness with which precipitates are sheared by basal dislocations, which is promoted by the excellent lattice matching at the precipitate-matrix interface. At high temperature, dislocation-precipitate interactions are highly dependent on the deformation mode. In tension, enhanced basal slip localization gives rise to high stress concentrations at the intersection between coarse slip traces and particle interfaces, leading to precipitate fracture; in compression, a more homogenous distribution of basal slip leads to the dominance of particle shearing. Our study demonstrates experimentally that basal dislocations are able to shear, and even fracture, the MgxNdy and Mg17Al12 plates when, for appropriate testing conditions, the local stress due to dislocation accumulation at particle interfaces exceeds the precipitate strength.

Origin of double-peak precipitation hardening in metallic alloys

Whereas conventional precipitation hardening is well-known to feature a single hardness peak, recently, double-peak precipitation hardening was observed, where the first peak hardness is higher than the second conventional one, thus offering a new approach to strengthen materials. Yet, classical precipitation strengthening models fail to predict such high strengthening in the early aging stage. In this work, molecular dynamics simulations were firstly performed to obtain a realistic dislocation-precipitate interaction law at the nano-scale, which was introduced into the discrete dislocation dynamics (DDD) method so as to investigate the precipitation hardening effects at the micro-scale. The DDD simulations correctly predict the double-peak hardening, namely, the critical resolved shear stress (CRSS) for a dislocation passing through a precipitate field first decreases, then increases, and finally decreases, as the precipitate radius rp increases. Then, a precipitate shearing model was developed, which agrees well with the DDD simulations and experimental observations. Based on the DDD simulations and theoretical analysis, the three CRSS regimes were found to be controlled by coherency strengthening (CRSS∝rp−1/2), chemical strengthening (CRSS∝−rp−1) and Orowan mechanism (CRSS∝rp−1), respectively. Finally, a universal law for the inverse relation between the CRSS and precipitate size at the second, conventional peak was unveiled, while the first peak was found to occur favorably for rapid precipitation in the early aging stage. This work provides new insights into precipitation hardening in general and double-peak hardening in particular, which are of great importance for alloy design.

Stress relaxation behavior of an aluminium magnesium silicon alloy in different temper condition

In the present investigation, stress relaxation tests were carried out on an aluminium magnesium silicon alloy to determine rate controlling mechanism in different temper conditions. Single and multiple relaxation tests were carried out to determine apparent and physical activation volume. It was found that in the presence of solutes and incoherent precipitates, dislocation-dislocation interaction is the rate controlling mechanism, whereas dislocation-precipitate interaction is the rate controlling mechanism in the presence of semi-coherent precipitates. Moreover, the nature of deformation was found to be more viscous (higher strain rate sensitivity) in the presence of semi-coherent precipitates compared to other microstructural features (solutes, incoherent precipitates). Special emphasis is placed upon understanding exhaustion rate of mobile dislocation density during relaxation period. It was found that the exhaustion rate of mobile dislocation density is negligible in the presence of incoherent precipitates, whereas a significant reduction in mobile dislocation density was observed in the presence of solutes and semi-coherent precipitates. Furthermore, thermal and athermal components of the flow stress were determined to develop a comprehensive understanding regarding deformation behavior in different temper conditions.

The constitutive model and threshold stress for characterizing the deformation mechanism of Al0.3CoCrFeNi high entropy alloy

Isothermal compression experiment of Al0.3CoCrFeNi high entropy alloy were performed at high temperatures. The effects of temperature and strain rate on the deformation behavior are investigated. A constitutive model incorporates the effects of strain rate, strain and temperature is developed to describe the flow stress. Experimental analysis show that the stress exponent n is generally greater than five, which decreases rapidly with increasing of temperature at the initial stage and changes slower subsequently. Structure characterization by the transmission electron microscopy demonstrates that dislocation–precipitate interaction exists during high temperature deformation and the precipitate is B2-type NiAl phase.