Fine-grain Strengthening Research Articles

Microstructural and mechanical behavior of Al-Mg-Mn-Sc-Zr alloy subjected to multi-axial forging at room temperature

The as-homogenized Al-5.5 Mg-0.3Mn-0.2Sc-0.1Zr alloy with average grain size of 60 μm and coherent Al3(Sc,Zr) precipitates of 5 nm in diameter was deformed to an accumulative equivalent strain (∑ε) of 7.02 by multi-axial forging (MAF) in channel die at room temperature. The mechanical property and microstructure of the as-homogenized (∑ε = 0), 3rd (∑ε = 2.34), 6th (∑ε = 4.68) and 9th pass (∑ε = 7.02) samples were investigated. Results show that dynamic recrystallization was inhibited due to the pinning effect of AlxMn and Al3(Sc,Zr) precipitates. High density dislocation structure produced a remarkable strain hardening resulting a significant improvement of mechanical property at low equivalent strain (∑ε ≤ 2.34), while, the grain size changed little and the fine-grain strengthening effect was not obvious. Low-angle grain boundaries (LAGBs) formed gradually with increasing of equivalent strain as dislocation recombining, and developed into high-angle grain boundaries (HAGBs), resulting an obvious structure refinement at high equivalent strain. As a combined strengthening effect of strain hardening and grain boundary strengthening, the yield strength (YS) and ultimate tensile strength (UTS) increased to as high as 509 MPa and 597 MPa respectively when equivalent strain reached to 7.02.

Effect of Si addition on microstructure and wear behavior of AlCoCrFeNi high-entropy alloy coatings prepared by laser cladding

AlCoCrFeNiSix (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) high-entropy alloy (HEA) coatings were deposited on the surface of AISI 304 stainless steel via laser cladding technology with different content of Si element. The evolution in microstructure, microhardness, and wear behavior of the coatings was investigated comprehensively to explore the effect of Si element. The results showed that the HEA coatings consisted of disordered Fe-Cr and ordered Al-Ni solid solution phases with a body-centered cubic (BCC) structure. Si element dissolved into the solid solutions, leading to lattice distortion. As Si content increased, the microstructure was refined, and a small quantity of Cr23C6 phase was precipitated along the grain boundaries. The microhardness of the coatings was linearly proportionate to Si content. The quantitative analysis revealed that the improvement of microhardness was dominated by the effect of dislocation strengthening, rather than solution strengthening and fine-grain strengthening. With the increase in Si content, the average friction coefficient and wear rate of the HEA coatings were all reduced remarkably, and the main wear mechanism evolved from adhesive wear, abrasive wear and delamination wear to oxidation wear. These phenomena were believed to be associated with the formation of the oxide film on the worn surface, which was identified as Fe2O3, Fe3O4, Cr2O3, SiO2, and SiO.

Nano-in-situ-composite ultrafine-grained W–Y2O3 materials: Microstructure, mechanical properties and high heat load performances

Tungsten has been confirmed as an idea plasma-facing material in future nuclear fusion reactor, however, existing commercial tungsten material can not meet the extremely high heat load environmental because its coarse grain and low toughness. In this work, fine-grained W–Y2O3 materials were prepared by nano-in-situ composite method. The microstructure, mechanical properties and strengthening mechanism were analyzed. Meanwhile, the effect of adding Y2O3 content on the properties was systematically investigated. Besides, the damage behavior and failure mechanism of the W–Y2O3 composites were studied exposed to the transient high thermal load. Results show that a very fine and homogenized microstructure was obtained, the average W grain size is about 1.7 μm, which tends to increase with rising of the content of Y2O3, while the Y2O3 particle with size of 80–150 nm evenly dispersed in the W matrix. It is worth noting that a coherent relationship was observed between Y2O3 the W matrix by the forming of a (Y, W) O composite, improving the interface compatibility between W matrix and Y2O3 ceramic phase. Accordingly, a synergistic effect of fine-grained strengthening, dispersion strengthening and coherent strengthening has been brought by nano-in-situ composite, which plays an effective role in enhancing W mechanical properties. The maximum tensile strength and elongation at RT is about 520 MPa and 1.5%, while they are 415 MPa and 8.3% at 400 °C, respectively. Moreover, high heat load tests show that nano-in-situ-composite W–Y2O3 material has excellent high thermal-shock resistance, it can endure 600 MW/m2 thermal shock without any cracks, which is demonstrably superior to the traditional commercial tungsten value.

Comparison of microstructures, mechanical and tribological properties of arc-deposited AlCrN, AlCrBN and CrBN coatings on Ti-6Al-4V alloy

The AlCrN, CrBN and AlCrBN coatings were deposited on Ti6Al4V alloy by cathodic vacuum arc with powder-metallurgical targets. The phase structures, surface and cross-sectional microstructures, mechanical and tribological properties of these coated samples were comparatively studied. The results show that the B-containing coatings consist of fcc solid solution embedded into a-BN/CrB amorphous phases, whereas the AlCrN coating is composed of single-phase fcc-AlCrN solid solution. The preferred orientation is changed from (111) plane in CrBN coating to the (200) plane in the AlCrBN coating. As a result of complex effects of B-induced fine-grained strengthening and the Al/B solution strengthening, the AlCrBN coating shows the highest hardness and the best plastic deformation resistance. The results of wear tests reveal that all PVD coated samples show the better tribological properties compared to the uncoated Ti6Al4V alloy, and the highest wear resistance is observed in the AlCrBN coated sample. The main wear mechanism for coated samples is oxidation wear, whereas the one for the uncoated Ti6Al4V alloy is abrasive wear.

Microstructure Evolution and Mechanical Properties of AQ80 Alloy During Forward Extrusion and Twist Deformation

A new severe plastic deformation technique, forward extrusion and twist deformation, was adopted to manufacture magnesium (Mg) alloys at different temperatures. The microstructure evolution and mechanical properties of extruded AQ80 alloy were investigated. The strain distribution in FETD extrusion was simulated with finite element simulation software. The extruded samples exhibit finer grain sizes, weaker basal textures, higher recrystallized volume fractions and higher strengths than the traditional forward extrusion samples. This technology can refine the microstructure and improve the mechanical properties of Mg alloys. At lower temperatures, the grain size is smaller with a finer second phase, which is detected as Mg17AL12. Mg17AL12 has a particle-stimulated nucleation effect and a retarding effect on grain growth, resulting in much finer DRXed grains. Due to the effects of fine-grained strengthening and second phase strengthening, the strength and elongation of AQ80 alloy can be improved significantly.

Interface microstructure and strengthening mechanisms of multilayer graphene reinforced titanium alloy matrix nanocomposites with network architectures

Discontinuously reinforced 3D network structured Ti6Al4V (TC4) matrix composites with multilayer graphene (MLG) were fabricated via 3D dynamic mixing and spark plasma sintering (SPS) at high pressures (250–500 MPa). The interface microstructure, mechanical properties and strengthening mechanisms were systematically studied with various MLG contents. Experimental results exhibited that MLG can be relative uniformly dispersed onto the surface of TC4 powders by the 3D dynamic mixing method, SPS parameter of 700 °C-500 MPa caused weak interface bonding between MLG and Ti matrix, and 900 °C-250 MPa was determined as the optimal sintering condition. Appropriate ratio of in-situ generated TiC phase with approximately 30 vol% retained MLG at the interface was beneficial to the interface bonding. The compressive strength of the composites was remarkably enhanced with excellent compressive ductility. Superior mechanical properties with the highest strengthening efficiency (65.5%) and tensile strength, acceptable tensile ductility (9.0%) and higher Vickers microhardness were achieved in the 0.15 wt% MLG composites due to its better interface microstructure. The network interface strengthening mechanism by the TiC phase and residue MLG is proposed to be the dominant mechanism with a few contributions from C solid solution and fine-grain strengthening.

Influence of Coiling Temperature on Microstructure, Precipitation Behaviors and Mechanical Properties of a Low Carbon Ti Micro-Alloyed Steel

The microstructural evolution, nanosized precipitation behaviors and mechanical properties of a Ti-bearing micro-alloyed steel at different coiling temperatures were studied using optical microstructure (OM), scanning electron micrograph (SEM), transmission electron microscopy (TEM), Vickers hardness and tensile tests. When the coiling temperature was 500 °C, the specimen showed mainly bainitic structure, whereas polygonal ferrite was visible as the coiling temperature increased to 650 °C and 700 °C. The Vickers hardness of tested steel reached the maximum, which can be attributed to the largest number of nanosized precipitates in ferrite at the coiling temperature of 650 °C. A coiling temperature of 650 °C was optimal for the formation of TiC because of the high diffusion rate of alloying elements and kinetics of precipitation. In the laboratory rolling experiment, when the coiling temperature was 630 °C, the steel with yield strength of 682 ± 2.1 MPa and tensile strength of 742 ± 4.9 MPa was produced. The fine-grain strengthening and precipitation strengthening were 262 MPa and 268 MPa, respectively.

Effects of Zirconium and Yttrium Oxide on Mechanical and Oxidation Properties of Mo–3Si–1B–1Zr–1Y2O3 (wt.%) Alloy

Mo–3Si–1B alloys with zirconium (1 wt.%) and yttrium oxide (1 wt.%) additives were fabricated by vibrating sintering techniques. The doped Mo–3Si–1B alloys consisted mainly of α-Mo, Mo3Si, and Mo5SiB2 (T2) phases. It was found that the grains were reduced, and the intermetallics particles were dispersed more homogeneously after the addition of Zr and Y2O3. The optimization in microstructure induced corresponding improvements in both fracture toughness and oxidation resistance. The predominant strengthening mechanisms were fine-grain strengthening and particle dispersion strengthening. In addition, fracture toughness test showed that the additions could improve the toughness of Mo–3Si–1B alloys, for which the toughening mechanism involved a crack trapping by α-Mo phases and extensive small second phase particles in the alloys. What should be paid attention to is the satisfactory oxidation resistance, both at medium-low temperature (800 °C) and high temperature (1200 °C) with doped additives.

Effect of tungsten micro-scale dispersed particles on the microstructure and mechanical properties of Ti–6Al–4V alloy

Ti–6Al–4V (TC4) alloy and Ti–6Al–4V-5 wt.%W (TC4-5%W) alloy were fabricated by spark plasma sintering (SPS). The crystalline structure and microstructure of both TC4 alloy and TC4-5%W alloy were confirmed using X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The XRD patterns suggested that a similar crystalline structure was formed in both of TC4 alloy with and without the W addition. SEM and TEM indicated that the dispersed W particles can refine the β grains, α colonies and coarsen the α laths by acting as the supernumerary nucleation sites for α laths during the β→α+β transformation process. Moreover, a comparative analysis of the mechanical properties of both TC4 and TC4-5%W alloys was obtained by the tensile tests, which is found that the W addition can increase the strength and ductility of TC4 alloy simultaneously. It is found that fine-grain strengthening is the dominant strengthening mechanism, which contributed 72% of the total strength increment. Meanwhile, the load-bearing effect also contributes to some of the total strength increments in this study. This study concluded that TC4 with W dispersed particles had a great potential to use as a novel Ti alloy with high strength and good ductility.

Effects of On-Line Vortex Cooling on the Microstructure and Mechanical Properties of Wire Arc Additively Manufactured Al-Mg Alloy

A novel on-line vortex cooling powered by low-cost compressed air was proposed to reduce common defects such as low forming precision, coarse grains, and pores caused by heat accumulation in the Wire Arc Additive Manufacturing (WAAM) of aluminum alloy. The impacts of interlayer cooling (IC), substrate cooling (SC), on-line cooling (OL), and natural cooling (NC) processes were compared on the morphology, microstructure, and mechanical properties of as-deposited walls, revealing that the OL process significantly lowers the interlayer temperature and improves forming precision. The high cooling rate produced by the OL process reduced the absorption of hydrogen in the molten pool, lowering porosity. Furthermore, the grains are refined due to the developed undercooling. However, the high cooling rate enhanced the segregation potential of Mg element and raised the content of the β phase. Conclusively, the maximum tensile strength, elongation, and microhardness of the as-deposited wall are achieved via the OL process, and the fine-grain strengthening mechanism plays an important role in improving mechanical properties. The OL process is cheaper and poses a significant effect; it is highly suitable for the additive manufacturing of complex components compared with other forced cooling processes.

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

Abstract The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al−Zn−Mg−Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al3(Sc,Zr) and Al3Zr dispersoids. The volume of Al3Zr particles is less than that of Al3(Sc,Zr) particles, but the distribution of Al3(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.

Effect of laser scanning speed on microstructure and properties of Fe based amorphous/ nanocrystalline cladding coatings

Laser cladding of (Fe0.497Cr0.243Mo0.084B0.093C0.084)98Nb2 on 45 steel substrate was fabricated by using a coaxial powder feeding method with the maximum thickness of 1 mm coating. The effect of the laser scanning speed on macromorphology, microstructure and microhardness of cladding layer and corrosion resistance was investigated. From the substrate to the upper layer, the growth structures of the coating are plane crystal, cellular crystal, columnar crystal, equiaxed crystal and amorphous, respectively. The amorphous content in the coatings is dependent upon laser scanning speed which will influence the dilution rate and actual cooling rate by changing the heat input. As scanning speed increasing, the coating revealed the property of higher hardness and better corrosion resistance, which can be attributed to fine-grain strengthening, dispersion strengthening and growth of the amorphous phase. Moreover, the amorphous phase (HV 1024) showed a much higher value of microhardness than that of the crystalline phase (HV 706), which indicate that the amorphous phase is the main contributor to the average hardness of laser deposited metallic glass.

Microstructure and mechanical properties of CoCrFeNiMo high-entropy alloy coatings

The CoCrFeNiMo coatings were prepared by laser cladding on 45 steel surface. The microstructure, hardness and wear resistance of CoCrFeNiMo high-entropy alloy were investigated by means of scanning electron microscopy, X-ray diffractometer, hardness tester and wear tester. The effect of laser scanning speed on the microstructure and mechanical properties of the coating was studied. Experimental results show that the coating and matrix combines well, the microstructure of the coatings is mainly composed of equiaxed grains. The phase structure of the coating is mainly composed of face-centered cubic (FCC) structure, body-centered cubic (BCC) structure and Laves phase. There is a slight segregation of Mo element. The CoCrFeNiMo coating has high hardness, the highest surface hardness is 741HV. Fine-grained strengthening, BCC structure and Laves phase provided a guarantee for the excellent wear resistance of the coating. With the increase of scanning speed, the structure is refined, the hardness is improved and the wear resistance is enhanced.

Strengthening of an Al0.45CoCrFeNi high-entropy alloy via in situ fabricated duplex-structured composites

Phase precipitation and recrystallization process of Al0.45CoCrFeNi high-entropy alloys (HEAs) were investigated by calculation of phase diagram and electron backscattered diffraction. To reveal the influences of microstructures on the response of mechanical behaviors, the tensile properties, hardness, elastic modulus, and fracture characteristics were thoroughly studied here. The as-processed duplex microstructures demonstrate that intercritical annealing treatments in the dual-phase filed could greatly slow down the recrystallization kinetics, attributing to the in situ precipitation of Ni, Al-rich phase formed initiatively along grain boundary. A series of fine-grained dual-phase HEAs, accompanying with yield strength widely ranging from 558 to 1223 MPa, were well fabricated via annealing the cold-rolled HEAs. Surprisingly, present HEAs exhibit excellent resistance to anneal softening, together with a relatively high recrystallization temperature of 900 °C (~ 0.63 Tm). Additionally, the coupled effect of fine-grain strengthening and precipitation strengthening is successful in manipulating the properties of face-centered-cubic HEA system.

Effect of fiber laser welding on solute segregation and proprieties of CoCrCuFeNi high entropy alloy

The effect of fiber laser welding on solute segregation and proprieties of CoCrCuFeNi high entropy alloy was investigated. The microstructure and mechanical properties of the parent metal and the fusion zone were comparably studied. The parent metal was dendrite, with elemental Cu segregated to interdendrites owing to small bonding energies with Fe, Co, Ni, and Cr atoms. After laser welding, the microstructure in the center of the fusion zone was predominantly equiaxial grains, whereas that in the edge region was mainly columnar crystals oriented perpendicular to the fusion line. The segregation of Cu in the fusion zone is alleviated by grain refinement and molten pool agitation. Because of the fine-grain strengthening and precipitation hardening effect, the hardness and yield stress of the fusion zone are 12.84% and 26.87% greater than those of the parent metal, respectively.

Study on high temperature strengthening mechanism of ZrO2/Mo alloys

ZrO2/Mo alloy bars are fabricated by hydrothermal method and powder metallurgy process, with their microstructures, density, hardness, strength, reduction of area and elongation under ambient temperature investigated experimentally. The high temperature deformation stress of these alloy bars are tested through thermomechanical simulator. Results show that the second phase (ZrO2) in the microstructure was fine. As addition amount of the second phase increases, the microstructures of the matrix are refined, the density decreased while the hardness and strength are enhanced. Under high temperature, the second phase in ZrO2/Mo alloy bars can improve compressive stress of the alloy effectively by fine-grain strengthening and intra-granular and grain-boundary strengthening. At 1400 °C, the deformation stress of Mo alloy bars with 1.5% ZrO2 can be 20% higher than that of pure molybdenum. The strengthening effect of the second phase at high temperature are analyzed. The increase of strengthening effect is actually the manifestation of the pinning effect on hindering recrystallization behavior. It is shown that the improvement of mechanical properties of alloys at elevated temperatures should be mainly attributed to the retardation of recrystallization by the second phase.

Grain refinement induced friction reduction and anti-wear performances of electrodeposited graphene/Ni composites with low content reduced graphene oxide

Graphene and its derivatives have attained a considerable amount of popularity as effective reinforcements in the electrodeposited nickel (Ni) matrix composites for lubrication. However, the composites suffer from certain challenges, such as the agglomeration of graphene nanosheets in the electrodeposition process and the uncertain role of graphene on the friction reduction and wear resistance during sliding. By using a polyvinylpyrrolidone assisted reduction method to improve the dispersity of reduced graphene oxide (RGO) in the electrolyte, we prepare the bulk RGO/Ni composites with different RGO adding amount and perform a complete research on the tribological behaviours. Our work reveals that although an extremely low amount of RGO nanosheets (carbon content below 0.018%) have been incorporated into the Ni matrix, it induces significant grain refinement and friction reduction effects. Compared with the RGO-free Ni deposit, the friction coefficient and wear rate of the composite is reduced by 25.6% and 27.5%, respectively. The improved tribological properties are ascribed to the fine-grain strengthening effect and the formation of a continuous easy-shear nickel oxide film on the contact surface. This finding offers a new view on the wear mechanism of graphene/Ni composites with low graphene content, for which the fine-grain strengthening rather than the formation of carbon-rich transfer layer will dominate the lubricating and anti-wear performances.

Microstructures and properties of novel nanocomposite WNx coatings deposited by reactive magnetron sputtering

Novel nanocomposite tungsten nitrides (WNx) coatings were deposited on cemented carbides by direct current (dc) reactive magnetron sputtering under different deposition powers. The microstructures and related mechanical properties of the coatings were carefully investigated. At deposition power of 2 kW, the WNx coating is entirely composed of well-developed columnar fcc-W2N grains, showing the strongest (2 0 0)W2N preferred orientation, whereas the coatings deposited at other conditions are composed of nanocomposite structures consisting of nanocrystalline bcc-W and fcc-W2N embedded into amorphous-phase matrix, showing weak (2 0 0)W2N and (1 1 1)W2N preferred orientations. Moreover, there is an orientation relationship between bcc-W and fcc-W2N: (−1 0 −1)w//(1 1 −1)W2N + 2.1° and [−1 1 1]w//[0 1 1]W2N. The amorphous films with thickness of several nanometers always occur in a special periodic multilayer structure or in the large angle grain boundaries of bcc-W. Such amorphous films are resulted from the intensive bombardment of sputtered W atoms, which leads to the densification of the coating and further restrains the diffusion of W atoms for crystallization growth. Moreover, strong ion bombardment can interrupt the W-W metallic bond, and further restrain crystallization growth. Therefore, the nanocomposite structure shows the higher hardness and fracture toughness when compared with columnar W2N coating mainly due to the fine-grained strengthening and solution strengthening.

High specific strength Mg-Li-Zn-Er alloy processed by multi deformation processes

Under the industrial demand of energy saving, weight loss has become one of the current concerns. By increasing the strength of Mg-Li alloy with high lithium content, the industrial demand can be met by higher specific strength. Mg-16Li-2.5Zn-2.5Er (LZ162-2.5Er) alloy is extruded at 100 °C and cold rolled subsequently. The microstructures and mechanical properties of as-cast, as-extruded and cold-rolled LZ162-2.5Er alloys were investigated. A highest specific strength of 178 kNm/kg was obtained. During extrusion, fine-grain strengthening is stimulated by dynamic recrystallization. The subsequent cold rolling increases the dislocation density and uniformizes the distribution of the second phases further, causing work hardening and dispersion strengthening. In this paper, a process of high specific strength Mg-Li alloys for lightweight industry is provided as a reference.

Microstructure and Mechanical Properties of Mg–8Y–2Ho–2Zn Alloy with Long Period Stacking Ordered Phase

In this study, microstructures, phase compositions, phase morphologies of the as-cast and as-extruded Mg–8Y–2Ho–2Zn (wt%) alloys were investigated by optical microscope, scanning electron microscope, energy-dispersive spectrometer, transmission electron microscope and X-ray diffraction. The mechanical properties at room temperature and elevated temperatures were evaluated by tensile test. The primary secondary phases in the as-cast alloy are 18R-type long period stacking ordered (LPSO) phase. After extrusion, the microstructure mainly consists of considerable fine dynamically recrystallized (DRX) grains, the long strip 18R-type LPSO-phase at grain boundary along the extrusion direction and the fine lamellar 14H-type LPSO-phase in the interior of DRX grains. When the temperature is increased, the strength of the alloy is reduced, but the elongation progressively rises. The as-extruded alloy shows higher strength and larger elongation than the as-cast alloy at each test temperature. The superior mechanical properties of the as-extruded alloy are ascribed to various strengthening mechanisms induced by co-addition of Y and Ho element, e.g., solid solution strengthening, fine-grain strengthening and strengthening of fine LPSO-phase.