Showing papers on "Grain boundary strengthening published in 2017"

Grain boundary stability governs hardening and softening in extremely fine nanograined metals

Abstract: Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni–Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.

Grain Structure Control of Additively Manufactured Metallic Materials.

Abstract: Grain structure control is challenging for metal additive manufacturing (AM). Grain structure optimization requires the control of grain morphology with grain size refinement, which can improve the mechanical properties of additive manufactured components. This work summarizes methods to promote fine equiaxed grains in both the additive manufacturing process and subsequent heat treatment. Influences of temperature gradient, solidification velocity and alloy composition on grain morphology are discussed. Equiaxed solidification is greatly promoted by introducing a high density of heterogeneous nucleation sites via powder rate control in the direct energy deposition (DED) technique or powder surface treatment for powder-bed techniques. Grain growth/coarsening during post-processing heat treatment can be restricted by presence of nano-scale oxide particles formed in-situ during AM. Grain refinement of martensitic steels can also be achieved by cyclic austenitizing in post-processing heat treatment. Evidently, new alloy powder design is another sustainable method enhancing the capability of AM for high-performance components with desirable microstructures.

High-temperature tensile deformation behavior and microstructure evolution of Ti55 titanium alloy

Abstract: High temperature tensile and electron backscatter diffraction (EBSD) techniques were combined to perform a systematic investigation for the hot deformation behavior and microstructure evolution of the Ti55 alloy Under temperatures ranging from 885 to 935 °C and stain rates of 83×10−4 s−1−133×10−2 s−1, all the flow curves of the Ti55 alloy exhibit similar behaviors: after reaching the peak flow stress, the curves enter into a softening stage and then remain a steady level, where the maximum superplastic elongation of 987% indicates good superplasticity Detailed microstructure characterizations under different deformation stages show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries (>15°) increase rapidly at the softening stage These observations are attributed to the dynamic recrystallization, in which low angle grain boundaries (

Reconciling grain growth and shear-coupled grain boundary migration.

Abstract: Conventional models for grain growth are based on the assumption that grain boundary (GB) velocity is proportional to GB mean curvature. We demonstrate via a series of molecular dynamics (MD) simulations that such a model is inadequate and that many physical phenomena occur during grain boundary migration for which this simple model is silent. We present a series of MD simulations designed to unravel GB migration phenomena and set it in a GB migration context that accounts for competing migration mechanisms, elasticity, temperature, and grain boundary crystallography. The resultant formulation is quantitative and validated through a series of atomistic simulations. The implications of this model for microstructural evolution is described. We show that consideration of GB migration mechanisms invites considerable complexity even under ideal conditions. However, that complexity also grants these systems enormous flexibility, and that flexibility is key to the decades-long success of conventional grain growth theories.

Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy

Abstract: The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.

Phase transformation behaviors and properties of a high strength Cu-Ni-Si alloy

Abstract: High strength and high electrically conductivity Cu-Ni-Si alloys are important candidate materials for extending the life of currently used elastic-conductor materials. Phase transformation behaviors and properties of a synthesized Cu-6.0Ni-1.0Si-0.5Al-0.15Mg-0.1Cr alloy were investigated systematically. After homogenized for 4 h at 940 °C, hot rolled by 80% at 850 °C, solution treated for 6 h at 970 °C, cold rolled by 50%, and annealed for 60 min at 450 °C, the studied alloy approached physical properties of 1097.5 MPa in tensile strength and 26.4%IACS in electrical conductivity. Transmission electron microscope observations showed that four precipitation phases including δ-Ni2Si, β-Ni3Si, β-NiAl, and γ′-Ni3Al were formed in the studied alloy, which was subjected to different annealing temperatures. A detailed diagram for isothermal decompositions has been established. The high strength of the studied alloy was primarily attributed to the Orowan precipitation strengthening, and secondary attributed to the solid solution strengthening, the Hall-Petch type grain boundary strengthening, and the work hardening. Multi-precipitation phases of Ni2Si, γ′-Ni3Al, β-Ni3Si played significantly into the strengthening effects, and the low electron diffraction influence of Mg and Cr contributed to both solution strengthening and maintaining a high electrical conductivity. Multi-phase precipitation strengthening and using trace amount alloying elements for solution strengthening will be a new strategy to develop high strength and high electrical conductivity copper alloys.

Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy.

Abstract: The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has led to worldwide attention by material scientists. Here we present a strong and ductile non-equiatomic HEA obtained after friction stir processing (FSP). A transformation-induced plasticity (TRIP) assisted HEA with composition Fe50Mn30Co10Cr10 (at.%) was severely deformed by FSP and evaluated for its microstructure-mechanical property relationship. The FSP-engineered microstructure of the TRIP HEA exhibited a substantially smaller grain size, and optimized fractions of face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., e) phases, as compared to the as-homogenized reference material. This results in synergistic strengthening via TRIP, grain boundary strengthening, and effective strain partitioning between the γ and e phases during deformation, thus leading to enhanced strength and ductility of the TRIP-assisted dual-phase HEA engineered via FSP.

Microstructure and properties of a CoCrFeNiMn high-entropy alloy processed by equal-channel angular pressing

Abstract: A CoCrFeNiMn high-entropy alloy (HEA) was processed by equal-channel angular pressing (ECAP) for up to four passes at 673 K and the results show that the strength increases gradually with increasing straining up to ~ 1 GPa with an elongation to failure of ~ 35% after four passes of ECAP. In this condition, the microstructure is a single-phase ultrafine-grained (UFG) CoCrFeNiMn HEA with an average grain size of ~ 100 nm and a high dislocation density. This UFG HEA was subjected to post-deformation annealing (PDA) at temperatures of 673–1073 K for 60 min and it is shown that the hardness increases slightly due to precipitation to 773 K and then decreases to 1073 K due to a combination of recrystallization, grain growth and a dissolution of precipitates. The formation of brittle σ -phase precipitates improves the strength significantly but with a minor decrease in ductility. Annealing at the peak temperature of 773 K produces a very high yield strength of ~ 1015 MPa and an ultimate strength of ~ 1080 MPa together with an excellent elongation to failure of ~ 30%. An analysis of the data shows that grain boundary strengthening is the most important strengthening mechanism in these ECAP samples both before and after PDA.

Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition

Abstract: Microstructural and mechanical properties of cold sprayed 6061 aluminum deposits on 6061-T6 aluminum alloy substrates are investigated under various heat treatment conditions, i.e. as-deposited, stress relieved and T6. The local mechanical property variation in the as-deposited material are explored using nanoindentation technique, and correlated with microstructural characterization conducted via electron back-scattered diffraction. It is found that the prior particle boundaries have ~ 0.4 GPa higher hardness than particle interiors, which is attributed to grain refinement in these regions promoted by local dynamic recrystallization. Also, the bulk-scale mechanical properties of the deposits are evaluated by microtensile testing in various post-heat treatment conditions and compared to those of conventionally processed 6061-T6 aluminum. The as-deposited material showed markedly higher ultimate strength (~ 460 MPa) and lower ductility (~ 3%) compared to conventionally processed material and this is attributed to significant cold working during the cold spray deposition process and associated grain boundary strengthening and dislocation strengthening mechanisms. Heat treated specimens showed a slight improvement in both ultimate strength and ductility compared to the as-deposited condition. These improvements are attributed to an improvement in metallurgical bonding at prior particle boundaries and a modest increase in the density of strengthening precipitates. Fractography of the specimens revealed that the heat treatment also changes the fracture characteristics of the cold sprayed 6061 aluminum deposit. The residual stress profiles and bond strength of the deposits are also studied using x-ray diffraction, tensile pull-off and three lug shear testing, respectively.

Influence of heat treatment on the microstructure and tensile properties of Ni-base superalloy Haynes 282

Abstract: The effect of heat treatment on the microstructure and mechanical properties of Ni-base superalloy Haynes 282 was investigated. Applying a standard two-step ageing (1010 °C/2 h +788 °C/8 h) to the as-received, mill annealed, material resulted in a the presence of discrete grain boundary carbides and finely dispersed intragranular γ′, with an average size of 43 nm. This condition showed excellent room temperature strength and ductility. The introduction of an additional solution treatment at 1120 °C resulted in grain growth, interconnected grain boundary carbides and coarse (100 nm) intragranular γ′. The coarser γ′ led to a significant reduction in the strength level, and the interconnected carbides resulted in quasi-brittle fracture with a 50% reduction in ductility. Reducing the temperature of the stabilization step to 996 °C during ageing of the mill annealed material produced a bi-modal γ′ distribution, and grain boundaries decorated by discrete carbides accompanied by γ′. This condition showed very similar strength and ductility levels as the standard ageing of mill-annealed material. This is promising since both grain boundary γ′ and a bi-modal intragranular γ′ distribution can be used to tailor the mechanical properties to suit specific applications. The yield strength of all three conditions could be accurately predicted by a unified precipitation strengthening model.

Precipitation structure and strengthening mechanisms in an Al-Cu-Mg-Ag alloy

Abstract: The evolution of the Ω-phase dispersion during aging in an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr-0.1Ge-0.08Ti-0.02Fe-0.01Si (wt%) alloy was examined at the temperatures, T , of 200 and 250 °C with dwelt times, τ , of 1.8–86.4 and 0.6–25.2 ks, respectively. The precipitation sequence during aging can be written as SSSS→Ag-Mg clusters→Ω-phase+θ′-phase→θ-phase+Ag,Mg,Cu-enriched phases. Only a minor portion of Cu is consumed for the precipitation of the Ω-phase. Approximately 3.3 wt% of Cu is retained in the solid solution after peak-aging. Therefore, the Ω-phase is a transition phase with a high free energy. The Ω-phase plates exhibit superior coarsening resistance. The kinetics of the coarsening is successfully predicted by the equation in a form of C tt × τ 0.2 , where C tt is a coefficient depending on aging temperature. Effect of aging on yield stress (YS) was analyzed in terms of additive sum of the contribution arising from the grain boundary strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening mechanisms. The contribution of the precipitate strengthening can be predicted satisfactorily by Nie and Muddle’s model of dislocation shearing of the {111} α plates. It was shown that the Ω-phase is a most efficient strengthening agent in the Al-Cu-Mg(-Ag) alloys and gives the major contribution to the overall YS. It is attributed to high efficiency of interfacial strengthening and high number density of the Ω-phase plates.

Achieving grain refinement and enhanced mechanical properties in Ti–6Al–4V alloy produced by multidirectional isothermal forging

Abstract: This study investigated the principle of multidirectional isothermal forging (MDIF) and determined the major microstructural evolution features and unique room-temperature mechanical properties of extra-low interstitial-grade Ti–6Al–4V alloy. The grain refinement mechanism, grain boundary characteristics, and phase transformation during MDIF were explored. After three-step MDIF, a homogeneous microstructure with a grain size of about 0.5 µm was produced. The ultrafine grained Ti–6Al–4V alloy exhibited high yield strength (1170 MPa), high ultimate tensile strength (1190 MPa), and good ductility (10.4%). The mechanism of grain refinement during MDIF included continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Grain subdivision resulted from CDRX or the transformation of cellular dislocation substructures into new ultrafine grains. The necklace of new DDRX grains formed along the initial grain boundaries of the Ti–6Al–4V alloy. The main strengthening mechanisms were grain boundary and dislocation strengthening. The strength and grain size followed the typical Hall–Petch relationship.

Grain size threshold for enhanced irradiation resistance in nanocrystalline and ultrafine tungsten

Abstract: Nanocrystalline metals are considered highly radiation-resistant materials due to their large grain boundary areas. Here, the existence of a grain size threshold for enhanced irradiation resistance in high-temperature helium-irradiated nanocrystalline and ultrafine tungsten is demonstrated. Average bubble density, projected bubble area and the corresponding change in volume were measured via transmission electron microscopy and plotted as a function of grain size for two ion fluences. Nanocrystalline grains of less than 35 nm size possess ∼10–20 times lower change in volume than ultrafine grains and this is discussed in terms of the grain boundaries defect sink efficiency.

Fabrication of a high strength ultra-fine grained Al-Mg-SiC nanocomposite by multi-step friction-stir processing

Abstract: In this study, multi-step friction-stir processing (FSP) was employed to fabricate an ultra-fine grained (UFG) Al-matrix nanocomposite with simultaneously enhanced indentation hardness and tensile properties For this aim, about 35 vol% of SiC nanoparticles were incorporated within an Al-Mg alloy matrix by applying up to five cumulative overlapping FSP passes Dispersion of nanoparticles at the stirred zone (SZ) and their interfaces with the aluminum matrix were studied by using scanning and electron backscattered electron microscopy The results showed that the grain and sub-grain structures of the SZ were refined down to about 14 µm and less than 1 µm respectively, as a result of dynamic recrystallization (DRX) during FSP The distribution of grains and their orientations was significantly affected by the presence of SiC nanoparticles during FSP SiC nanoparticles provided both direct and indirect influences on the strengthening of Al-matrix based on the Orowan looping and grain refinement mechanisms, respectively The morphology and distribution of precipitates were both broken down and partially dissolved during FSP as well The processed UFGed nanocomposite exhibited drastically improved hardness, yield stress (YS) and ultimate tensile strength (UTS) by up to ~140%, 75% and 60%, respectively, as compared to the annealed Al-Mg alloy Fractographic features revealed a combined ductile-brittle rupture behavior, while the ductile portion was more significant and preserved the elongation of nanocomposite up to about 30% Finally, the tensile flow behavior of the processed nanocomposite was described using a dislocation-based model which suggests that grain boundary strengthening is the dominant mechanism involved

Coarsening, densification, and grain growth during sintering of nano-sized powders—A perspective

Abstract: Sintering is one of the main approaches, among limited options, for building bulk nanomaterials from bottom up. A primary challenge of sintering nanosized powders is to control grain growth while achieving full densification. Considerable literature is now available in the search for unique mechanisms that could assist in achieving densification with minimum grain growth. It is critical to understand the detailed mechanistic steps, in order to design processes that might lead to bulk nanomaterials with maximum density and minimum grain size. Using experimental data of sintering nanosized tungsten carbide and tungsten powders, as well as selected data on other materials in the literature, this article examines the unique characteristics of nano-sintering, including the driving forces, kinetics, and intertwined processes of neck growth, coarsening, densification, and grain growth. When the density of the powder compact of nanosized particles are very low, coarsening of particles is responsible for most of the observed initial growth of grain sizes as well as densification. Because the mechanism of the initial coarsening depends on surface diffusion, surface diffusion thereby contributes to densification indirectly. Although the initial grain growth is only a small fraction of the total grain growth that occurs by the completion of sintering, it is the most critical part of grain growth, determining whether the grain size of the material can be controlled within the nanoscale regime.