Orowan Looping Research Articles

Microstructural evolution and tensile property of 1Cr15Ni36W3Ti superalloy during thermal exposure

1Cr15Ni36W3Ti was thermally exposed at 580 ℃ and 680 ℃, respectively, up to 3000 h. The γ’ phase and intergranular TiC carbides continuously coarsened during exposure. None of η, laves or σ phase was discovered in the exposed samples, indicating good microstructure stability under the present exposure conditions. The ripening process of the γ’ phase could be well modelled utilizing the LSW theory. The evolutions of the yield and tensile strengths were monotonous during exposure at 580 ℃. However, a transition point in strengths was detected in the tensile samples exposed at 680 ℃ for 300 h. Accordingly, the critical γ’ diameter was measured to be 13−14 nm. The γ’/dislocations interaction mechanism transformed from shearing to looping with the γ’ diameter exceeding the critical point. The combination of the weakly coupled dislocations model and the Orowan looping model yielded a critical diameter of 13.1 nm which coincided well with the measured one, indicating the applicability of these two strengthening models for 1Cr15Ni36W3Ti. The present exposure conditions did not exert a profound effect on the fracture mode. All the tensile samples underwent a typically ductile fracture with a dimple pattern dominating the fracture surface. The dispersed deformation induced by the prevalence of dislocation looping in the over-aged tensile samples retarded the propagation of intergranular cracks. The declined precipitation hardening increment and the enhanced deformation homogeneity partially recovered the tensile ductility in the over-aged samples exposed at 680 ℃.

Microstructure evolution and mechanical properties at high temperature of selective laser melted AlSi10Mg

In this study, the microstructure and tensile properties of selective laser melted AlSi10Mg at elevated temperature were investigated with focus on the interfacial region. In-situ SEM and in-situ EBSD analysis were proposed to characterize the microstructural evolution with temperature. The as-fabricated AlSi10Mg sample presents high tensile strength with the ultimate tensile strength (UTS) of ∼450 MPa and yield strength (YS) of ∼300 MPa, which results from the mixed strengthening mechanism among grain boundary, solid solution, dislocation and Orowan looping mechanism. When holding at the temperature below 200 °C for 30 min, the microstructure presents little change, and only a slight decrement of yield strength appears due to the relief of the residual stress. However, when the holding temperature further increases to 300 °C and 400 °C, the coarsening and precipitation of Si particles in α-Al matrix occur obviously, which leads to an obvious decrease of solid solution strength. At the same time, matrix softening and the weakness of dislocation strengthening also play important roles. When the holding temperature reaches to 400 °C, the yield strength decreases significantly to about 25 MPa which is very similar to the as-cast Al alloy. This might be concluded that the YS is dominated by the matrix materials. Because the softening mechanism counteracts work hardening, the extremely high elongation occurs.

Grain refinement and mechanical properties improvements in a high strength Cu–Ni–Si alloy during multidirectional forging

Abstract Microstructure evolutions and properties variations of a high-strength Cu–Ni–Si alloy during multidirectional forging have been systematically investigated using microstructure characterizations and properties measurements. Experiment results indicated that nano-scale grains formed in the block samples having been multidirectional forged (MDF) at room/cryogenic temperatures. With the increase of the MDF cycle, the sub-grain structure was further refined to nano-grains with tens of nanometers in diameter. The ultimate tensile strength of the cryogenic temperature multidirectional forged sample was 1057 MPa, the yield strength was 923 MPa, and the fracture elongation was 7.2%. Rather than the solid solution strengthening from solutes impeding the glide of dislocation motion and the second phase particle precipitation strengthening from the Orowan looping, the grain boundary strengthening from the Hall–Petch effects plays crucial roles in strengthening the samples having been cryogenic multidirectional forged. These findings indicated that multidirectional forging is a passport to an ultrafine-grained structure and high yield strength material for the nuclear industry.

An analysis of the influence of the precipitate type on the mechanical behavior of Al - Cu alloys by means of micropillar compression tests

The influence of different types of precipitates (either Guinier-Preston zones, θ″ or θ′) on the critical resolved shear stress and strain hardening was determined by means of micropillar compression tests in an Al - 4 wt% Cu alloy. The size, shape and volume fraction of the precipitates were measured in each case. It was found that size effects were negligible for micropillars with diameter ≥ 5 µm. Micropillars with Guinier-Preston zones showed strain localization due to precipitate shearing. The best mechanical properties were obtained with either a fine dispersion of the θ″ precipitates or a coarser dispersion of θ′. Both precipitate shearing and Orowan loops were observed around the θ″ precipitates and the micropillar strength was compatible with the predictions of the Orowan model. In the case of the alloy with θ′ precipitates, the strengthening contribution associated with the transformation strain around the precipitates has to be included in the model to explain the experimental results. Finally, the micropillar compression tests in crystals with different orientations were used to calibrate a phenomenological crystal plasticity. This information was used to predict the mechanical properties of polycrystals by means of computational homogeneization.

Dislocation-particle interactions in magnesium alloys

The aim of this work is to investigate the fundamentals of dislocation-particle interactions in a Mg-1%Mn-1%Nd (wt.%) (MN11) alloy in order to better understand the limited precipitation strengthening exhibited by this material. A combined approach including micromechanical testing, slip trace analysis, and high resolution transmission electron microscopy (TEM) is put in place to analyze the interaction between basal and non-basal dislocations with the Mg-Nd plate precipitates that are characteristic of the investigated alloy. It is shown that basal dislocations can easily shear the precipitates, both when their long axis lays parallel and perpendicular to the matrix c axis. This is attributed to the high coherency between the matrix and the particles, which is characterized in detail. It is additionally shown that pyramidal ⟨c + a⟩ dislocations, on the contrary, interact with precipitates both in the form of shearing along the precipitate pyramidal planes, as well as by Orowan looping. This study reveals that pyramidal <c+a> dislocations are able to shear the plate precipitates along pyramidal planes when there is a good alignment between the slip plane of the incoming matrix dislocation and the outgoing precipitate pyramidal plane. Otherwise pyramidal <c+a> dislocations sliding on matrix planes with small geometric compatibility with the precipitate pyramidal planes bypass these particles by an Orowan looping mechanism.

The effect of solid solution and gamma prime on the deformation modes in Ni-based superalloys

Understanding the deformation, strengthening and failure mechanisms in polycrystalline nickel-base superalloys is necessary to develop next generation alloys for application in highly demanding environments. Here, the aim is to examine the various ways in which solution- and γ’ precipitation-strengthening affect the deformation behaviour of three Ni-based superalloys through deformation mapping and investigation at multiple length-scales. This is achieved using high-resolution digital image correlation to quantify local strain, electron backscattered diffraction for lattice rotations and electron channelling contrast imaging to investigate dislocation-mediated mechanisms of deformation. This approach bridges the gap between nano-scale microscopy of dislocation-γ’ interactions and macro-scale measurements of mechanical properties, such as yield stress, flow stress and the strain-hardening rate. Deformation in solution-strengthened alloys progresses by a slip band refinement mechanism, which results in low levels of dislocation pileup at grain boundaries and so better grain deformation compatibility with neighbours. Deformation in the γ’-strengthened alloys evolves through a glide plane softening mechanism and the resulting high strain localisation impinges on grain boundaries, creating diffuse strain regions at the boundary. In the coarse-γ’ variant there is more Orowan looping and cross slip around larger precipitates, more slip planes are active and more grain-scale cross slip takes place, resulting in greater local friction stresses and therefore greater macroscopic flow stresses and strain-hardening rates. We provide evidence of greater interaction between intersecting non co-planar slip bands in the γ’-strengthened alloys, which contributes to the strain-hardening by progressively decreasing the slip distance. This mechanism is not observed in the solution-strengthened alloy.

Atomistic simulations of the interaction of basal dislocations with MgZn[formula omitted] precipitates in Mg alloys

The interaction between Mg edge basal dislocations and rod-shaped β1′-MgZn2 precipitates was studied by atomistic simulations using a new interatomic potential. The atomistic model was carefully built taking into account the experimental information about the orientation relationship between the matrix and the precipitate to ensure minimum energy interfaces. It was found that the dislocations initially overcame the precipitate by the formation of an Orowan loop that penetrated in the precipitate. The precipitate was finally sheared after several Orowan loops were piled-up. The number of loops necessary to shear the precipitate decreased as precipitate cross-section decreased and the temperature increased but was independent of the precipitate spacing. Precipitate shearing did not take place along well-defined crystallographic planes but it was triggered by the accumulation of the elastic energy in the precipitate which finally led to formation of an amorphous layer below and above the slip plane of the basal dislocations. The kink induced in the precipitate by this mechanism was in good agreement with transmission electron microscopy observations.

Effects of hybrid processing on microstructural and mechanical properties of thixoformed aluminum matrix composite

Abstract Hybrid processing is a promising technique that combines powder metallurgy and liquid casting to disperse reinforced particles in Al–5Si alloy matrix homogeneously. In this study, dual alumina-multiwalled carbon nanotubes (MWCNTs) and single MWCNT-reinforced particles were ball-milled and compacted into pellets. The pellets were then injected into molten Al–5Si at 650 °C, mechanically stirred for 10 min and poured into a mould via a cooling slope for semi-solid feedstock. All samples were subjected to thixoforming and standard T6 heat treatment. The effects of hybrid techniques on the distribution of reinforcements, microstructure and mechanical properties were investigated. Tensile fracture surface analyses showed that dual and single reinforcements were distributed homogeneously in the samples. However, severely damaged MWCNT structures were observed under dual reinforcement due to excessive shear stress during ball milling process. The highest yield strength, ultimate tensile strength and elongation to fracture were 262 MPa, 289 MPa and 7.2% and 316 MPa, 347 MPa and 13.3% for thixoformed-T6 alloy composites of dual and single reinforcements, respectively. The two strengthening mechanisms involved are as follows: by the matrix, microstructure evolution and precipitation; and by reinforcement, load transfer, thermal mismatch dislocation and Orowan looping. The more intact and the longer structure of MWCNTs in PB lead to better effective load transfer ability than the PA composite.

Al-matrix composite based on Al-Ca-Ni-La system additionally reinforced by L12 type nanoparticles

The structure of the quaternary Al-(2−4)wt.%Ca-Ni-La system near the aluminum corner has been studied using computational analysis in the Thermo-Calc program and experimental studies (electron microscopy, microprobe analysis and X-ray diffraction). Based on the phase equilibria data obtained, the experimental projection of the liquidus surface and solid state phase-field distribution of the Al-Ca-Ni-La system have been proposed. Microstructure studies reveal that the alloys with the 2-4 wt.% Ca, 2−4 wt.% Ni and 1-3 wt.% La ranges have an ultra-fine hypoeutectic structure with 30% volume fraction of eutectic intermetallics, which allows one to classify these alloys as natural Al-matrix composites. The ultra-fine eutectic structure produces significant strengthening, the magnitude of which can be well described using the modified Orowan looping mechanism model. Small additives of Zr and Sc (0.2 and 0.1 wt.%, respectively) lead to significant strengthening (by ~25%) due to the formation of L12 type phase (Al3(Zr, Sc)) nanoparticles during annealing of the alloy at 350-400 °C. Due to the high volume fraction of eutectic intermetallics, the new alloys have low coefficients of thermal expansion and high thermal stability of the structure and mechanical properties.

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.

On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.

Energy analysis of misfit hardening by parametric dislocation dynamics simulation

The interaction between dislocation and misfitting precipitate has been investigated by the parametric dislocation dynamics (PDD) based on Green's function method. The present research is aimed at quantitative analysis of the effect of the key parameters (geometry of precipitate, existence of cross-slip, radius of precipitate and dislocation source length) on the precipitation and dislocation hardening. Simulation results revealed that the dislocation bypassing process is varied depending on the geometry of the precipitate and slip plane, where dislocation is cutting through or leaving the Orowan loop and prismatic loop behind the precipitate. Furthermore, the cross-slip accompanied with the jog formation contributes to the dislocation strengthening because of self-energy of dislocation. The interaction energy analysis revealed that misfit hardening can be remarkably increased by the smaller radius of precipitate and also the smaller length of dislocation source. Hence simultaneous hardening by dislocation strengthening and misfit hardening is suggested by the present study.

Microchemistry-dependent simulation of yield stress and flow stress in non-heat treatable Al sheet alloys

A flow stress model which considers the processing conditions for a given alloy composition as well as the microchemistry of the alloy allows for integrated optimization of alloy composition, thermal treatments and forming operations to achieve the desired properties in the most efficient processing route. In the past, a statistical flow stress model for cell forming metals, 3IVM+ (3 Internal Variable Model), has been used for through process modeling of sheet production. However, this model was restricted to a given alloy in the state in which it was calibrated. In this work, the existing 3IVM+ model is augmented with an analytical solute strengthening model which uses input from ab initio simulations. Furthermore, a new particle strengthening model for non-shearable precipitates has been introduced which takes Orowan looping at low temperatures and dislocation climb at high temperatures into account. Hence, the present modeling approach considers the strengthening contributions from solutes, precipitates and forest dislocations. Three case studies on the alloys AA 1110, AA 3003 and AA 8014 are presented to assess the performance of the model in simulating the yield stress and flow stress of Al alloys over a wide range of temperatures and strain rates.

Atomistic mechanism and probability determination of the cutting of Guinier-Preston zones by edge dislocations in dilute Al-Cu alloys

The interaction between a ${}^{1}/{}_{2}\left[\overline{1}10\right]\left(111\right)$ edge dislocation and a (001) Guinier-Preston (GP) zone in dilute Al-Cu alloys is studied via atomistic modeling. In stark contrast to the previously reported Orowan looping mechanism where the GP zone remains intact after yield, we discover a competing mechanism where the dislocation cuts the GP zone into two pieces. We identify the key atomic process triggering the cutting mechanism and calculate its activation barrier at various strains. In further conjunction with the transition state theory, the occurrence probability of a trigger event is mapped out over a broad range of $T\text{---}\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{\ensuremath{\varepsilon}}$ parameter space. The predictions of the so-constructed mechanism map are validated by parallel MD simulations. The implications of our findings regarding the discrepancies between the existing age hardening model and experiments are also discussed.

Friction stir lap welding of AA6061 aluminium alloy with a graphene interlayer

ABSTRACTThe present study aims to enhance the strength of friction stir lap welded aluminum alloys by using an interlayer of graphene nanoplatelets (GNPs) at the weld interface. With GNP interlayer, the weld strength and percentage elongation increased by 121 and 53%, respectively, as compared to the weld without GNP interlayer. The interlayer also changes the mode of fracture from brittle in the weld without GNP to ductile mode. Grain size in the weld with interlayer decreased by ~38% as equated to the weld without GNP. The height of the hook defect (HD) and cold lap defect (CLD) decreased by 26% and 41%, respectively, in the weld with GNP interlayer as compared to the weld without interlayer. In weld with interlayer, the bottom of the top plate on the retreating side acts as the potential site for fracture due to the presence of an interfacial defect, and undeformed GNP layer. The strengthening of the weld is attributed to various primary strengthening mechanisms like thermal mismatch, grain refinement, Orowan looping, and load transfer. Moreover, GNP interlayer also prevents the formation of the Al2O3 layer at the lap interface and thus contribute significantly in the weld strengthening.

Investigation on the tensile deformation mechanisms in a new Ni–Fe-base superalloy HT700T at 750 °C

After a standard heat treatment and subsequent isothermal aging at 750 °C for 8,220 h, the tensile deformation behavior of a novel precipitation-hardened Ni–Fe-base superalloy HT700T containing around 23 vol% of γ′ precipitates is studied at 750 °C. It is found that the work-hardening rate decreases monotonously with plastic strain. Transmission electron microscopy observations on the specimens stretched to various magnitudes of plastic strain reveal that Orowan looping prevails at the very beginning of plastic deformation. Whereas, as plastic deformation proceeds, more and more isolated superlattice stacking faults are produced within the γ′ precipitates although numerous dislocation loops have surrounded the same γ′ precipitates, suggesting shearing of γ′ precipitates by {111} <112> slip systems plays a more and more important role in the plastic deformation with strain. Based on the experimental observations, the relationship between the operative deformation mechanisms and the work-hardening rate of HT700T is discussed.

On the Strengthening and Embrittlement Mechanisms of an Additively Manufactured Nickel-Base Superalloy

The γ' phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ' precipitated with various size and fraction depending on heat treatment process. For room temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built samples while dislocations bypassing the precipitates via Orowan looping in the γ' strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ' strengthened samples have higher work hardening rate than as-built sample but encountered premature failure. Experimental evidence shows that the embrittlement mechanism in the γ' strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strengthening and ductility of Nickel-base superalloy is discussed in detail.

Primary hot working characteristics of an as-cast and homogenized nickel base superalloy DMR-742 in the sub and super-solvus temperature regime

Primary deformation behaviour of a medium alloyed nickel base superalloy DMR-742 in the as-cast and homogenized condition (furnace cooled) was evaluated in the sub-solvus and super-solvus temperature regimes. The flow stress in the sub-solvus regime was found to be highly sensitive to temperature. The contribution of back stress to the flow stress sensitivity was estimated using the microstructure dependent analytical equations and compared with the back stress estimated using experimental steady state flow stress. It was established that Orowan looping or by-passing mechanism contributes to the back stress resulting in high flow stress sensitivity in the sub-solvus regime. In the super-solvus regime, the material was relatively temperature insensitive and exhibited discontinuous dynamic recrystallization (DDRX) with a characteristic power dissipation value of 38–44% in three different temperature-strain rate regimes. Disregarding the industrially non-viable low strain rate regime, the recrystallization kinetics in the moderate and high strain rate regime was estimated using grain size factor compensated grain orientation spread maps. A higher rate of recrystallization observed in the material deformed at high temperature-strain rate was attributed to relatively strong dislocation interactions and weak recovery. The true stress exponent, activation volume and energy estimated using back stress and elastic modulus compensated flow stress revealed that diffusion controlled climb and cross-slip as the rate controlling deformation mechanism. Side-pressing under high secondary tensile stress conditions was used to evaluate the workability and validate the experimental findings.

Eutectic phase strengthening and strain rate sensitivity behavior of AZ80 magnesium alloy

A Mg–8Al-0.5Zn-0.2Mn wt.% (AZ80) alloy, containing a high volume percent of the β eutectic phase, was prepared using extrusion without a homogenization pretreatment (EX-II(F)). The β-eutectic-phase contributions to grain refinement, texture tailoring, and plastic behavior were discussed. Microstructure characterization was presented along with a strengthening mechanism analysis. The β-eutectic-phase strengthening contributions to the yield strength were estimated according to the thermal mismatch, Orowan looping, load transfer, and Hall-Petch mechanisms. In addition, the temperature and strain dependence for strain rate sensitivity (SRS) were investigated, and the influence of aging on the SRS evolution was discussed. The SRS exponent, m, of the EX-II(F) was measured at 298–573 K using strain rate jumps between 10−4 s−1-10−3 s−1. The m-value increased with increasing strain and temperature, and decreased with grain coarsening and Al solute depletion. Aging precipitation exhibited a softening effect on the 473–573 K flow stress and resulted in higher m-values. The β eutectic phase cracking decreased with increasing test temperature, which was attributed to the high deformability of the β eutectic phase above 573 K. Overall, this work has shown that the β eutectic phase can serve as a means to strengthen extruded AZ80.

A comparative study on microstructural evolution and surface properties of graphene/CNT reinforced Al6061−SiC hybrid surface composite fabricated via friction stir processing

Abstract A comparative study on the surface properties of Al−SiC−multi walled carbon nanotubes (CNT) and Al−SiC−graphene nanoplatelets (GNP) hybrid composites fabricated via friction stir processing (FSP) was documented. Microstructural characterization reveals a more homogeneous dispersion of GNPs in the Al matrix as compared to CNTs. Dislocation blockade by SiC and GNP particles along with the defect-free interface between the matrix and reinforcements is also observed. Nanoindentation study reveals a remarkable ∼207% and ∼27% increment in surface nano-hardness of Al−SiC−GNP and Al−SiC−CNT hybrid composite compared to as-received Al6061 alloy, respectively. On the other hand, the microhardness values of Al−SiC−GNP and Al−SiC−CNT are increased by ∼36% and ∼17% relative to as-received Al6061 alloy, respectively. Tribological assessment reveals ∼56% decrease in the specific wear rate of Al−SiC−GNP hybrid composite, whereas it is increased by ∼122% in Al−SiC−CNT composite. The higher strength of Al−SiC−GNP composite is attributed to the mechanical exfoliation of GNPs to few layered graphene (FLG) in the presence of SiC. Also, various mechanisms such as thermal mismatch, grain refinement, and Orowan looping contribute significantly towards the strengthening of composites. Moreover, the formation of tribolayer by the squeezed-out GNP on the surface is responsible for the improved tribological performance of the composites. Raman spectroscopy and various other characterization methods corroborate the results.