Strengthening Effect Of Grain Boundaries Research Articles

Preparing gradient microstructure to achieve enhanced performance in GH4586 superalloy: Experiments and models

In this present study, a combined method of controlling strain deformation (CSD) and gradient-temperature heat treatment (GTHT) was adopted to prepare gradient microstructure in GH4586 superalloy. Consequently, it brought enhanced tensile strength from 1313.1±18.7 MPa at intermediate transition (IT) region to 1506.3±15.7 MPa at high-strain and low-temperature (HS-LT) region, presenting an improvement of 11.6%. And it also enhanced fracture toughness from 77.7±0.3 MPa⋅m1/2 at IT region to 86.7±3.7 MPa⋅m1/2 at low-strain and high-temperature (LS-HT) region, presenting an improvement of 14.7%. The formation of gradient microstructure along the longitudinal direction of CSD+GTHT-treated sample was thoroughly investigated from perspectives of precipitates distribution, activated slip systems and dislocations evolution. During CSD, the size and volume fraction of γ′ precipitates decreased under the effect of strain-induced dissolution behavior. The activation of (1‾ 11)[101] and (11‾ 1)[011] slip systems dominated dislocation movement at each region and there retained increasing dislocation density due to more extensively activated slip systems at HS region. During GTHT, the high-density dislocations promoted recrystallization while the concentratedly distributed γ′ precipitates obstructed movement of recrystallized front, leading to formation of heterogeneous structure with high-density grain boundaries and dislocations at HS-LT region. Finally, a model reflecting the effect of grain boundary strengthening, precipitation strengthening and dislocation strengthening on mechanical property of gradient microstructure was established and successfully applied to property prediction of GH4586 superalloy.

Impact of in-process cooling and tool rotation speed on the mechanical and microstructural characteristics of friction stir processed AA2024

Despite significant grain size refinement, friction stir processing (FSP) of heat-treatable aluminum alloys is found to degrade the alloys of strength and hardness qualities. The drop in strength brought on by coarsening or the dissolution of the strengthening precipitate(s) could not be made up for by the improvement in strength brought about by the refinement of grain size following FSP. In order to get around this, the current work looks into a practical method of heat rejection from the FSP zone in addition to grain size refinement during FSP of AA2024 to regulate the precipitate evolution. Under natural cooling, air cooling, and CO2 cooling conditions, friction stir processing (FSP) was used to change the microstructure of Solutionized AA2024 at three distinct rotation speeds of 750, 1180, and 1500 rpm (fixed traverse speed of 50 mm/min). The microstructural evolution (through optical microscopy and SEM) and mechanical properties of FSP samples under different conditions of rotation speed and cooling and their differences were mainly discussed. All the FSP samples have a higher Vickers hardness than the base metal (minimum variation of 16.79% and maximum variation of 41.05%). The maximum hardness, yield strength, tensile strength, and elongation have been obtained under air cooling conditions and a rotation speed of 1180 rpm, which is a result of the simultaneous and effective effect of grain boundary strengthening and precipitation hardening.

Effect of equal-channel angular pressing on microstructure, aging kinetics and impact behavior in a 7075 aluminum alloy

Among the precipitation hardenable 7xxx series aluminum alloys, AA7075 alloys (one of the most important engineering alloys) due to their low density (lightweight), high toughness and strength, and enhanced fatigue behavior have been used over the years in aerospace, aircraft, automotive, construction and marine applications. In the present study, the influence of the artificial aging through conventional heat treatment (i.e., precipitation hardening) and the thermomechanical treatment (equal-channel angular pressing (ECAP) + post-ECAP aging) on the quasi-static tensile, impact toughness and precipitation behavior as well as on the microstructure of an AA7075 alloy is investigated systematically. The results indicate that the ECAP process as well as post-ECAP aging result in considerably increased strength and hardness of the AA7075 alloys because of the superimposed effects of grain boundary strengthening (Hall-Petch relation), strain hardening (by means of the increased dislocation density) and precipitation hardening (due to the fine precipitates formed in Al matrix). Multi-pass ECAP processing has been found to result in a considerable decrease of the impact toughness (from 15.3 J·cm−2 to 7.6 J·cm−2) associated with the failure mode (i.e., ductile-to-brittle transition) of the AA7075 alloys, whereas the artificial peak aging leads to a marked improvement (∼67 %) in the impact toughness in ECAPed conditions. Moreover, the ECAP process noticeably accelerates the precipitation kinetics of AA7075 alloys: The peak aging time in the ECAPed alloy is reduced significantly – from 30 h to 4 h. The results of the present study provide a deeper understanding of the combination of ECAP and heat treatment, and their effect on the fracture mode and toughness under impact loading, the aging kinetics and the microstructural evolution of an AA7075 alloy.

Grain boundary strengthening in nanocrystalline Mo0.2CoNi medium-entropy alloys produced via high-pressure torsion

In this study, we produced nanocrystalline (NC) MoCoNi-based medium-entropy alloys (MEAs) with a single face-centered cubic (FCC) phase, which exhibit exceptional hardness through combining compositional and microstructural features of alloys. The alloys were prepared via powder metallurgy and high-pressure torsion, and the processing condition was optimized to achieve full densification and prevent grain growth in order to maximize the effect of grain boundary strengthening. Consequently, NC Mo0.2CoNi MEAs revealed the refined microstructures with a grain size of approximately 50 nm and outstanding hardness of 857 HV (corresponding to approximately 2.86 GPa in yield strength). This remarkable property, even compared to the single FCC phase alloys with similar grain size, is attributed to the existence of alloying elemental atoms of Mo with large atomic radius, which leads to severe lattice distortion allows to improve friction stress as well as the Hall-Petch coefficient. This study aims to discuss the potential of developing hard metallic materials by using grain boundary strengthening from alloys with large atomic size difference.

The coupled effects of grain boundary strengthening and Orowan strengthening examined by dislocation dynamics simulations

DD simulations are performed to unravel the coupled effects between grain boundary strengthening and Orowan strengthening. With the simulations of a plastically-deformed grain embedded in elastic matrix, the internal stresses associated with inter- and intragranular obstacles are quantified. First, plastic yielding is controlled by the length of dislocation sources. Then, in plastic deformation, the stress contribution of bypassing mechanism increases significantly with the size and number of precipitates, while the stress contribution of geometrically necessary dislocations at grain boundaries slightly decreases. Compared with regular spatial distribution of precipitates, random distribution induces a higher strengthening effect by reducing the dislocation mobility. Simulations of four-grain aggregates involving grain refinement and precipitation are systematically investigated. The volume fraction of precipitates is the key factor controlling the Orowan strengthening in addition to the grain size effect. A generalized Hall–Petch equation based on the mean free path of dislocations is proposed. At low strain, a relatively uniform internal stress field is found inside the aggregates with weak stress concentrations around grain boundaries and precipitates. The underlying mechanisms identified in this work are essential for understanding the plastic behavior of precipitate-strengthened polycrystals.

Overcoming strength-ductility trade-off in CoCrNi medium entropy alloy by forming fine crescent grains in laser powder-bed fusion

This study investigated the influence of microstructure on the strengthening mechanism in CoCrNi medium entropy alloys (MEAs) fabricated by laser powder-bed fusion (L-PBF). An excellent combination of high strength and elongation was achieved in L-PBFed CoCrNi MEA. The yield strength of L-PBFed CoCrNi MEA was significantly improved because of the combined effect of grain boundary strengthening and dislocation strengthening. Due to the temperature gradient difference during L-PBF process, the crescent shape grain boundaries were evident in L-PBFed samples, which increased the mechanical properties.

Microstructure and mechanical properties of carbon nano-onions reinforced Mg-based metal matrix composite prepared by friction-stir processing

In consideration of the increasing demand for lightweight structural metal materials, it is essential to develop Mg-based metal matrix composite (MMC) with well-matched strength and ductility. In this study, carbon nano-onion (CNO) reinforced AZ31B Mg alloy MMC was successfully fabricated via friction stir processing (FSP). The microstructure and mechanical properties of the FSP CNO/AZ31B MMC were investigated. The obtained results showed that a homogeneous fine grain structure was achieved in the stir zone without gaps and microcracks. The grain refinement mechanisms were attributed to continuous and discontinuous dynamic recrystallization. The CNOs also played a critical role in the activation of {10−12} twinning behaviour. {10−12} twins can not only further refine the recrystallized grains but also reduce the basal texture intensity. The FSP CNO/AZ31B MMC exhibited enhanced yield strength due to the combined effects of grain boundary strengthening, dislocation strengthening, Orowan strengthening, and load bearing mechanism of reinforcements. Owing to the {10−12} twins and CNO induced back-stress strengthening, the FSP CNO/AZ31B MMC preformed improved strain hardening capacity. The yield strength and fracture elongation were enhanced by 14 % and 11.8 % compared with those of the matrix material, respectively, indicating that the strength and ductility were simultaneously improved due to the addition of CNO reinforcement.

Heterogeneous grain size and enhanced hardness by precipitation of the BCC particles in medium entropy Fe–Ni–Cr alloys

We report the chemical-composition-dependent precipitation of Cr-rich BCC particles and their role in the hardening of Fe–Cr–Ni medium entropy alloys (MEAs). Three alloys with different chemical compositions: 33Fe–32Cr–35Ni MEA (33Fe), 40Fe–30Cr–30Ni MEA (40Fe), and 45Fe–30Cr–25Ni MEA (45Fe) (at%), were investigated. After annealing at 800 and 950 °C, all three alloys had a heterogeneous grain size distribution, except for 40Fe annealed at 950 °C. The samples annealed at 800 °C showed a more heterogeneous grain size distribution, smaller average grain size, and lower degree of recrystallization than the samples annealed at 950 °C. This difference is ascribed mainly to the contents and precipitation kinetics of Cr-rich BCC particles at the two temperatures. The hardness decreased with the increase in annealing temperature for all the samples. The 33Fe sample exhibited higher hardness than 40Fe and 45Fe because of the joint effect of grain boundary strengthening and precipitation strengthening by the Cr-rich BCC particles. Different peak-load-dependent hardness behaviors were observed in 33Fe-A800 with relatively coarse BCC particles and 40Fe-A800 with fine BCC particles. The fine particles led to higher local hardness, whereas coarse particles resulted in higher microhardness owing to the hetero-deformation-induced strengthening effect. These results provide new insights into the strength optimization of medium- and high-entropy alloys through the dispersion of coarse Cr-rich BCC particles.

Development of a high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy using Laser Powder Bed Fusion: Design of a heterogeneous microstructure incorporating synergistic multiple strengthening mechanisms

Laser Powder Bed Fusion (L-PBF) provides great advantages in creating supersaturated solid solutions due to its intrinsic ultrafast cooling and high solidification rate, which is particularly desired for enhanced solid solution strengthening and precipitation strengthening. In the present work, a novel high strength Zr/Sc/Hf-modified Al-Mn-Mg alloy with a good L-PBF processability is investigated. The as-built alloy exhibits a heterogeneous microstructure featuring a bi-modal grain structure with coarse columnar and fine equiaxed grains and heterogeneously distributed nanometer and submicrometer-sized cuboid-shaped primary Al3(Sc,Zr,Hf) particles. The Al3(Sc,Zr,Hf) phase, exhibiting a homogeneous elemental distribution of Zr, Sc, and Hf, adopts a cubic L12 structure with a lattice parameter of 0.4044 ± 0.009 nm, showing a good coherency with α-Al. Additionally, the rapid L-PBF solidification enables the formation of submicrometer-sized elongated and globular primary Al6Mn and a supersaturated Al matrix with a high dislocation density. This results in a good combination of strength (yield strength of 438 ± 3 MPa and ultimate tensile strength of 504 ± 2 MPa) and ductility (elongation at fracture of 10.9 ± 1.4 %), with a moderate work hardening strength of 113 ± 12 MPa. Direct-ageing at 325 °C for 10 h promotes the formation of a large amount of rod-shaped Al6Mn precipitates and a few spherical Al3(Sc,Zr,Hf) nanoprecipitates. The heat treatment increases the hardness from 166 ± 2 HV in as-built condition to 173 ± 4 HV and enhances the tensile strength (yield strength of 487 ± 2 MPa and ultimate tensile strength of 542 ± 3 MPa) but slightly reduces the ductility to 7.4 ± 0.7 %. The high strength was achieved by the synergistic effect of grain boundary strengthening, solid solution strengthening, and Orowan strengthening mechanisms.

A New Extruded Mg-6Bi-3Al-1Zn Alloy with Excellent Tensile Properties

Aiming to develop low-cost Mg alloys with good mechanical properties, a new Mg-6Bi-3Al-1Zn (wt.%, BAZ631) alloy having a high ultimate tensile strength of ~402 MPa and an elongation of ~15% was fabricated by single pass extrusion. The as-extruded BAZ631 alloy demonstrates an almost fully dynamic recrystallized microstructure with an average grain size of ~2.9 μm. Moreover, both nano-scale Mg3Bi2 and Mg17Al12 precipitates were found dispersed in the matrix. These excellent mechanical properties are attributed to the combined effects of grain boundary strengthening, dispersion and precipitation strengthening induced by both micro and nanoscale second phase particles, and solid-solution strengthening. This high strength and good ductile Mg-Bi based alloy without rare-earth elements addition is expected to inspire a new alloy design strategy for fabricating high performance Mg products for larger-scale industrial applications.

A newly developed Mg-Zn-Gd-Mn-Sr alloy for degradable implant applications: Influence of extrusion temperature on microstructure, mechanical properties and in vitro corrosion behavior

The object of this study is to investigate the influence of extrusion temperature on the microstructure, mechanical properties and in vitro corrosion behavior of Mg–2Zn–1Gd–0.4Mn–0.1Sr (ZGMS) alloy. The results show that as–extruded ZMGS alloy mainly contains α–Mg and Mg3Zn3Gd2 (W) phases, and the W phase is uniformly distributed in the Mg matrix. With decreasing extrusion temperature, the grain size is refined, and numerous nano–sized W particles dynamically precipitate during extrusion process. The mechanical properties of as–extruded experimental alloy is significantly improved with decreased extrusion temperature.This is owing to the effects of grain boundary strengthening, precipitation strengthening and dislocation strengthening. The experimental alloy exhibits the lowest corrosion rate and relatively uniform corrosion after extrusion at 360 °C. The corrosion products layer are mainly composed of Ca10(PO4)6(OH)2, Ca3(PO4)2 Mg(OH)2 and MgHPO4. The as–extruded alloy owns excellent mechanical properties (yield strength of 206.3 MPa ultimate tensile strength of 261.1 MPa and elongation of 27.3%) and corrosion rate (0.17 mg/cm2/day) after extruded at 360 °C. The alloy presents tremendous potential in degradable implant applications. Additionally, the relationship between microstructure and corrosion behavior was systematically discussed.

Microstructure evolution and hardness distribution of linear friction welded AA5052-H34 joint and AA5083-O joint

The microstructure evolution of the AA5052-H34 joint and AA5083-O joint fabricated by linear friction welding (LFW) was evaluated in a wide range of interfacial temperatures, i.e., applied pressures. The grain refinement of Al alloys with high stacking fault energy was found to be mainly dominated by discontinuous dynamic recrystallization (DDRX) at low temperatures. While, it was clear that grain refinement at high temperatures was dominated by continuous dynamic recrystallization (CDRX). In the joint of work hardened AA5052-H34 fabricated at low applied pressure of 50 MPa (above the 0.5 Tm), the annealing effect was more significant than the grain-boundary strengthening effect at the WC, leading to the softening. However, the increased applied pressure to 100 MPa resulted in the flat hardness distribution due to the increased strengthening by grain refinement. On the other hand, in the joint of annealed AA5083-O, the flat hardness was obtained at the lower pressure of 50 MPa, because both the low angle grain boundary length per unit area and dislocation density at the WC was almost equivalent to that of base material.

Microstructural characteristic and mechanical properties of titanium-copper alloys in-situ fabricated by selective laser melting

In this paper, Ti-Cu alloys were in-situ fabricated by selective laser melting (SLM) with 1.25, 2.5 and 5 wt% Cu contents, namely Ti-1.25Cu, Ti-2.5Cu and Ti-5Cu, respectively. The microstructural characteristic and mechanical properties of Ti-Cu alloys were systematically investigated. The results showed that the SLM-fabricated Ti-Cu alloys have a fine microstructure, and the morphology of martensite exhibited a transition from parallel-sided plates or laths to acicular plates with increasing Cu content. Due to the accommodation effect of the martensitic transformation strain, the 14–20% of the martensite partitioning interfaces was constituted of {101̅1} twin boundaries. The microhardness, yield strength and ultimate tensile strength were enhanced synergistically with increasing Cu content, which can be attributed to the combined effects of grain-boundary strengthening, solid-solution strengthening, dislocation strengthening and precipitation strengthening.

Exploring stress equivalence for solid solution strengthened Mg alloy polycrystals

The principle of “stress equivalence” proposed by Basinski et al. offers insight regarding the nature of thermally activated plasticity of solid solution alloys of FCC and HCP (Mg) crystal structures at low temperatures. More recently, Leyson and Curtin have developed a theoretical framework which builds upon the Labusch solute strengthening model. The current work analyzes some recent results from the literature (for single crystal Mg–Zn, Mg–Y, and Mg–Dy) which permit a further examination of the concept of “stress equivalence.” Then, using an effective Taylor factor obtained from polycrystal plasticity modeling of textured Mg, the results of cryogenic mechanical tests of polycrystalline Mg–Sc and Mg–Y alloys are analyzed to test their adherence to the principle of “stress equivalence.” The analysis accounts for the truly long-range strengthening effect of grain boundaries through established, temperature dependent Hall-Petch relationships, but makes no other assumption of an “athermal stress” that is frequently mentioned in the literature. The results emphasize the broad applicability of Basinski's principle of stress equivalence, by showing that the low temperature thermally activated tensile response of textured, polycrystalline Mg alloys can be predicted once the flow stress at a single rate and temperature is known.

Hot Deformation Behavior and Microstructure Evolution of Fe-5Mn-3Al-0.1C High-Strength Lightweight Steel for Automobiles.

The hot deformation behavior of a newly designed Fe–5Mn–3Al–0.1C (wt.%) medium manganese steel was investigated using hot compression tests in the temperature range of 900 to 1150 °C, at constant strain rates of 0.1, 1, 2.5, 5, 10, and 20 s−1. A detailed analysis of the hot deformation parameters, focusing on the flow behavior, hot processing map, dynamic recrystallization (DRX) critical stress, and nucleation mechanism, was undertaken to understand the hot rolling process of the newly designed steel. The flow behavior is sensitive to deformation parameters, and the Zener–Hollomon parameter was coupled with the temperature and strain rate. Three-dimensional processing maps were developed considering the effect of strain and were used to determine safe and unsafe deformation conditions in association with the microstructural evolution. In the deformation condition, the microstructure of the steel consisted of δ-ferrite and austenite; in addition, there was a formation of DRX grains within the δ-ferrite grains and austenite grains during the hot compression test. The microstructure evolution and two types of DRX nucleation mechanisms were identified; it was observed that discontinuous dynamic recrystallization (DDRX) is the primary nucleation mechanism of austenite, while continuous dynamic recrystallization (CDRX) is the primary nucleation mechanism of δ-ferrite. The steel possesses unfavorable toughness at the deformation temperature of 900 °C, which is mainly due to the presence of coarse κ-carbides along grain boundaries, as well as the lower strengthening effect of grain boundaries. This study identified a relatively ideal hot processing region for the steel. Further exploration of hot roll tests will follow in the future.

Hierarchical grain size and nanotwin gradient microstructure for improved mechanical properties of a non-equiatomic CoCrFeMnNi high-entropy alloy

This study explored a multi-mechanism approach to improving the mechanical properties of a CoCrFeMnNi high-entropy alloy through non-equiatomic alloy design and processing. The alloy design ensures a single-phase face-centered cubic structure while lowering the stacking fault energy to encourage the formation of deformation twins and stacking faults by altering the equiatomic composition of the alloy. The processing strategy applied helped create a hierarchical grain size gradient microstructure with a high nanotwins population. This was achieved by means of rotationally accelerated shot peening (RASP). The non-equiatomic CoCrFeMnNi high-entropy alloy achieved a yield strength of 750 MPa, a tensile strength of 1050 MPa, and tensile uniform elongation of 27.5%. The toughness of the alloy was 2.53 × 1010 kJ/m3, which is about 2 times that of the same alloy without the RASP treatment. The strength increase is attributed to the effects of grain boundary strengthening, dislocation strengthening, twin strengthening, and hetero-deformation strengthening associated with the heterogeneous microstructure of the alloy. The concurrent occurrence of the multiple deformation mechanisms, i.e., dislocation deformation, twining deformation and microband deformation, contributes to achieving a suitable strain hardening of the alloy that helps to prevent early necking and to assure steady plastic deformation for high toughness.

Ultrafine grained metals and metal matrix nanocomposites fabricated by powder processing and thermomechanical powder consolidation

Ultrafine grained metals and metal matrix nanocomposites with enhanced strength and good tensile ductility for structural applications have been successfully fabricated by processing and thermomechanically consolidating powders and, in some cases, post-consolidation heat and thermomechanical treatments. This paper provides an overview of the substantial amount of research work mainly published by the author’s research groups rather than a comprehensive review of the vast amount of very active research work published in this area. The microstructures and tensile properties of the samples fabricated and the correlations between them strongly suggest that the synergistic effects of grain boundary strengthening and intragranular ceramic (and other hard) nanoparticles are highly desirable for significantly enhancing the tensile yield strength and maintaining generally good tensile ductility. The extra boundaries between hard regions and soft regions introduced by heterogeneous microstructure can further enhance the strength of the material without sacrificing tensile ductility by inducing back stress in the soft regions. However, these boundaries also induce forward stress in the hard regions which has a weakening effect on them and needs to be managed by dispersing nanoparticles in the hard regions or other means.

Ultra-fine grain TixVNbMoTa refractory high-entropy alloys with superior mechanical properties fabricated by powder metallurgy

TixVNbMoTa refractory high-entropy alloys (RHEAs) were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) based on classic VNbMoTaW RHEA. The effect of Ti content on the microstructures and mechanical properties of the TixVNbMoTa RHEAs were systematically investigated. The results showed that the milling powders with more Ti content were agglomerated severely in the early stage of the MA process, leading to the extension of mechanical alloying time. The mechanically alloyed powders in all groups exhibited single body-centered-cubic (BCC) phase after the MA process. The sintered alloys were consisted of ultra-fine matrix and precipitation phase. The grain sizes of the matrix and precipitation phases, as well as the volume fraction of the precipitation phases were increased with the increase of Ti content. The MA and SPS processes and the replacement of Ti not only greatly reduce the densities, but also significantly improved the ductility, strength and specific strength of the alloys. The yield strengths were decreased first and then increased with the increase of Ti content in the TixVNbMoTa RHEAs, which is mainly attributed to the weakened effect of grain boundary strengthening, and the enhanced effect of the substitution solid solution strengthening and the interstitial solid solution strengthening. The rapidly increased sizes and volume fractions of the precipitated phases in Ti1.5 and Ti2 destroyed the continuity of matrix and the coordination of deformation, resulting in the decrease of ductility.

Investigation on the grain boundary strengthening effect of a nickel‐based superalloy

AbstractThe grain boundary strengthening effect (GBSE) was investigated for nickel‐based superalloy bicrystals with various misorientations under different temperatures and loading directions. The results show that the GBSE is enhanced with the increase of misorientation and insensitive to the temperature and the loading direction relative to the GB also has a strong effect on the GBSE. On the other hand, the GBSE is often accompanied by GB cracking. The trend of grain boundary (GB) cracking is heightened with the increase of the misorientation and temperature, and the transformation of the loading direction from vertical to parallel.

Strengthening effect of grain and twin boundaries in zirconium bi-crystal micropillars

ABSTRACT In this work, the effect of crystallographic orientations and the strengthening due to grain/twin boundaries were investigated by uniaxial compression of single- and bi-crystal micropillars of pure zirconium (Zr) with ≥99.9% purity. Micropillars fabricated using focused ion beam (FIB) milling were compressed using a flat punch tip in a nanoindenter to obtain stress–strain curves for individual orientations of Zr. Only near-basal-oriented samples were deformed by multiple slip, while samples with other orientations were deformed by a single slip. Bi-crystal samples had significantly higher flow stress than single-crystal samples – the increment in flow stress arising from grain boundaries being higher than that arising from twin boundaries. Our analysis shows that while twin-boundary strengthening can be explained based on a size effect according to the Hall-Petch equation, the higher strengthening effect of grain boundaries appears to arise from a combination of a sample-size effect as well as dislocation accumulation at boundaries.