Fine-grain Strengthening Research Articles

Effect of Co on microstructure and mechanical properties of precipitation hardening stainless steel

The effects of Co element on the microstructure of precipitation hardening stainless steel was investigated by metallographic microscope (OM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD), and the mechanical properties were measured by tensile, hardness and impact tests. The results show that with increasing Co content, the volume fraction of reversion austenite is increased. The precipitation of ε-Cu phase is remarkably decreased, leading to the improvement of ductility, while the strength and hardness are decreased. Co element improves the strength and toughness of stainless steel through fine-grain strengthening, solution strengthening and austenitic toughening.

Microstructure and Tensile Strength of Nano-TiCp/Mg–Zn–Ca Magnesium Matrix Nanocomposites Processed by Multidirectional Forging

In this work, a Mg–Zn–Ca magnesium matrix nanocomposite containing nano-sized TiC particles was firstly processed by multidirectional forging (MDF). With increasing the forging temperature from 230 to 310 °C for 1 MDF pass or the number of MDF passes at 270 °C, both volume fractions and average sizes of recrystallized grains increased. The average sizes of precipitated MgZn2 phases increased with the increase in either the initial forging temperature or MDF passes, and the volume fractions gradually decreased with increasing the initial forging temperature or decreasing the MDF passes. With decreasing the initial MDF temperature, thermal expansion mismatch strengthening slightly decreased while fine-grain strengthening and Orowan strengthening gradually increased, resulting in a gradual increased yield strength. The elongation of the present nanocomposite gradually increased from 8.2% after 1 MDF pass to 22.7% after 6 MDF passes, while both yield strength and ultimate tensile strength did not change significantly. This could be ascribed to that although the fine-grain strengthening gradually reduced, there was no significant change in the Orowan strengthening caused by MgZn2 phases with increasing the number of MDF passes.

Microstructure, Thermal Conductivity and Mechanical Properties of Mg-Zn-Mn-Y Quaternary Alloys

As-cast, as-extruded and T6 heat-treated Mg-2Zn-0.3Mn-[x] wt.% Y (x = 0, 0.4, 0.8) alloys were formed via a melting process, an extrusion process and T6 aging treatment, respectively. Microstructural analysis showed that the addition of yttrium (Y) resulted in grain refinement in the samples. The hot extrusion process and T6 heat treatment contributed to the precipitation of second phases and a reduction of the dislocation density as well as to a reduction of the lattice distortion. The thermal conductivity of the T6 heat-treated samples was the highest because it had the lowest lattice distortion. A higher Y content did not result in increased thermal conductivity. However, the yield strength and tensile strength of the alloys increased with increasing Y content. Fine-grain strengthening and dispersion strengthening were responsible for the higher yield strength and ultimate tensile strength of the as-extruded and T6 heat-treated samples.

Grain refinement mechanism and improved mechanical properties in Mg–Sn alloy with trace Y addition

Mg-0.5Sn-(0, 0.3 wt%) Y alloys were obtained by casting with a metal mould, then extruded to acquire the as-extruded sheets. The microstructures, and mechanical properties of the alloys were studied. The results indicated that Sn3Y5 particles generated after the addition of Y into the Mg–Sn alloys. This contributed to the presence of fine grains in the extruded Mg-0.5Sn-0.3Y sheets, the size of which decreased from ∼16 μm to ∼4 μm, accompanied by a significant improvement in the mechanical properties. Specifically, with Y addition, the ultimate tensile strength and elongation along the extrusion direction increased by 21% and 191%, respectively. The crystallographic matching relationships between Sn3Y5 and Mg were established via an edge-to-edge matching model. The calculated results illustrated that Sn3Y5 had small crystallographic misorientation with α-Mg, and served as the sites of heterogeneous nucleation for the Mg matrix, which led to grain refinement of Mg-0.5Sn-0.3Y sheets. Meanwhile, the diffraction patterns obtained by transmission electron microscopy along [0001]Mg // [112‾3‾]Sn3Y5 well agreed with the simulation results and validated the prediction. Furthermore, the macro-texture and work hardening behaviour of the extruded sheets implied that in the tensile test, the as-extruded Mg-0.5Sn-0.3Y sheets underwent both basal and prismatic <a> slips, while only the basal slip was observed in Mg-0.5Sn. As more slip systems were activated and more mobile dislocations could be formed, this in combination with the fine-grain strengthening effect, resulted in the large tensile elongation and high tensile strength of the Mg-0.5Sn-0.3Y sheets.

Interface characterization and scratch resistance of plasma sprayed TiO2-CNTs nanocomposite coating

TiO2 and TiO2-CNTs nanostructured coatings were fabricated by plasma spray with agglomerated nanocrystalline powders. The microstructure and interface between TiO2 and CNT of the nanocomposite coating were observed extensively using scanning and transmission electron microscopy. The scratch resistance of the two coatings was compared under a constant load scratch test. Results showed that both nanostructured coatings consisted of fully melted (FM) regions, partially melted (PM) with solid-state sintered regions, and pores. The addition of CNTs can effectively inhibit the growth of TiO2 grains and has a fine-grain strengthening effect. Besides, the interface bonding includes good chemical bonding by locally forming carbothermic reduction product in the FM regions and relatively weak physical bonding by van der Waals forces in the PM regions. In the scratch test, the lateral friction force and material removal volume of TiO2-CNTs coating were reduced remarkably compared to those of un-reinforced coating. The corresponding strengthening mechanisms induced by CNTs are scratched-out CNTs rolling on the scratch track and CNTs bridging inside the nanocomposite coating.

Microstructure and elastic constants of AlTiVMoNb refractory high-entropy alloy coating on Ti6Al4V by laser cladding

AlTiVMoNb refractory high-entropy alloy coatings (RHEACs) prepared by laser cladding have good metallurgical bonding with Ti6Al4V (TC4) substrates. X-ray diffraction data indicate that the coating structure is a body-centered cubic (BCC) phase, and the laser cladding process can refine the microstructure of the coating and reduce the value of the lattice constant a to achieve fine-grain strengthening. Ab initio model was established by the Exact Muffin-tin Orbitals combines Coherent Potential Approximation (EMTO-CPA) theory, the reflex power diffraction of X-ray and elastic constants of the coating was calculated. The experimental x-ray data and the calculation results are the same. The stress-strain method and Voigt-Reuss-Hill approximation obtained elastic constants and module. The elastic constant Cij of the RHEACs conforms to Pugh’s principle of mechanical stability and has suitable anisotropy and ductility. Notably, the calculated results of hardness and Young’s modulus are consistent with the trend of NanoBlitz 3D nanoindentation test data.

Abstract # 3213 Neurodegeneration in asthma

Mo-12Si-8.5B alloys with different Zr contents (0 at.%, 1 at.%, 2 at.%, 3 at.%, 4 at.%) were manufactured via a mechanical alloying process followed by hot pressing sintering technology. The microstructure of Mo-12Si-8.5B alloy exhibited a continuous submicro- and micro-scale α-Mo matrix in which the sub-micron Mo3Si/Mo5SiB2 particles were distributed. Addition of Zr to Mo-12Si-8.5B alloy promoted to form spherical nano-scale intermetallic Mo2Zr and ZrO2 particles, which were mainly located at the grain boundaries (GBs) as well as partially within the grains. The microstructure of Mo-12Si-8.5B-xZr alloys was remarkably refined by these Mo2Zr/ZrO2 nanoparticles. Additionally, results of mechanical properties indicated that the Zr addition improved the hardness, compression strength, yield strength and flexure strength of alloys. In particular, the Mo-12Si-8.5B-2Zr alloy exhibited extremely high compression strength (3.38 GPa), yield strength (3.17 GPa) and flexure strength (1.15 GPa). Quantitative analyses indicate that both fine-grained strengthening and Zr-rich particle strengthening mechanisms play a significant role in strengthening the Mo-Si-B-Zr alloys, the strengthening is dominantly governed by grain size reduction. Furthermore, Zr getters detrimental oxygen by synthesizing ZrO2 distributed at grain/phase boundaries, which contributes to increasing the GBs cohesion. Fracture surfaces revealed that the fracture mode transformed from intergranular to transgranular fracture owing to Zr addition.

Multicomponent synergistically affected mechanical properties, microstructure, and oxidation resistance of Zr–Al(Si)–C based composites

Herein, in-situ Zr3[Al(Si)]4C6-based composites with 10–40 vol% ZrB2–SiC (2-to-1 molar ratio) were prepared by hot-pressing sintering at 1850 °C. The simultaneously incorporated ZrB2–SiC constitute multicomponent reinforcements and has a synergistic effect on the matrix, which improves the sinterability, mechanical properties, and oxidation resistance of materials. It is found that both of the toughness and strength increase first and then decrease with the increasing content of ZrB2–SiC, while the hardness increases near linearly. Zr3[Al(Si)]4C6–ZrB2–SiC shows high strength (623 MPa), toughness (7.59 MPa m1/2), and hardness (18.6 GPa), which can be ascribed to the synergistic mechanisms of the binary ZrB2–SiC including fine-grained strengthening, particle reinforcement, intragranular microstructure, grain's pull-out and crack bridging, etc. In addition, the oxidation kinetics of as-prepared materials follow the parabolic law, and the composite shows a low oxidation rate of 0.87 × 10−5 kg2 m−4 s−1 when oxidized at 1400 °C.

Microstructure evolution and hot extrusion behavior of AZ31-Nd magnesium alloy adopting AZ31 chips and Mg-Nd chips

Hot extrusion is employed to fabricate an AZ31-Nd magnesium alloy adopting AZ31 chips and Mg-Nd intermediate alloy chips. The microstructure evolution and mechanical properties after different extrusion passes are studied, as well as the effect of solution and aging treatment on the mechanical properties of AZ31-Nd magnesium alloy. The results show that hot extrusion is fit for breaking up the intermediate alloy chips into fine and dispersive particles. These particles have an effect on a higher nucleation rate and a greater strength of the alloy together with fine oxide phases, which are subjected to fine-grain strengthening and dispersion strengthening. After four extrusion passes, the tensile strength reaches 313 MPa, and the elongation reaches 12.01%. The fracture is composed of cleavage planes, tear ridges and dimples. After solution and aging treatment, a large amount of the Al2Nd phases are precipitated. The precipitation of these phases imporves the tensile strength and elongation of AZ31-Nd alloy.

Evolutions of the Micro- and Macrostructure and Tensile Property of Cu-15Ni-8Sn Alloy During Electromagnetic Stirring-Assisted Horizontal Continuous Casting

The present study investigates the evolution of the micro- and macrostructure and tensile property of the Cu-15Ni-8Sn (weight percent) alloy prepared by horizontal continuous casting and electromagnetic stirring (EMS). The results show that the application of EMS is beneficial for grain refinement and for microstructure transformation from the dendrite to the rosette structure and that it leads to a significant improvement in the tensile property. The forced flow induced by EMS homogenizes the temperature field ahead of the solid-liquid interface, disturbing the heat flow direction and resulting in the columnar to equiaxed transition. The grain refinement under different electromagnetic stirring frequencies is mainly derived from the homogenization of the temperature and the remelting of dendritic arms. In addition, the evolution of the tensile property with and without EMS is discussed from the perspective of fracture mode and fine-grain strengthening.

Microstructure and Mechanical Property Evolution during Annealing of a Cold-Rolled Metastable Powder Metallurgy High Entropy Alloy

Precipitation strengthening is an effective approach to strengthen high-entropy alloys (HEAs) with a simple face-center-cubic (FCC) structure. In this work, CoCrFeNiMo0.2 HEAs were prepared by powder metallurgy, followed by cool rolling and subsequent heat-treatment at different temperatures. The effects of cold working and annealing on microstructure and mechanical properties have been investigated. Results show the fine and dispersed (Cr, Mo)-rich σ phase with a topologically close-packed structure precipitated in the FCC matrix after the prior cold deformation process, which enhanced the mechanical property of the CoCrFeNiMo0.2 alloy. The HEA annealed at 600 °C for 48 h had a tensile strength of 1.9 GPa but an elongation which decreased to 8%. The HEA annealed at 800 °C for 12 h exhibited a tensile strength of 1.2 GPa and an elongation of 31%. These outstanding mechanical properties can be attributed to precipitation strengthening and fine-grain strengthening.

Design of composition for Mg-Y-Zn-Al alloys using the Taguchi design and analysis of experiments

Taguchi design was employed herein to design a series of Mg–Y–Zn–Al alloys and to obtain the composition of the alloy with the best mechanical properties. The alloys were prepared according to the 3-level L9 Taguchi orthogonal table. The signal-to-noise ratios and analysis of variance were used to determine the optimal composition. The microstructures of the Mg–Y–Zn–Al alloys were investigated using an optical microscope, an x-ray diffractometer, a scanning electron microscope and energy dispersion spectra. The optimal composition of the Mg–Y–Zn–Al alloys is 1.2 at% Y content, 0.75 Y/Zn mole ratio and 0.9 wt% Al-5Ti-1B content which corresponds to Mg–4.1Y–4.1Zn–0.9(Al-5Ti-1B) (wt%) with the best mechanical properties. The ultimate tensile strength and elongation of Mg–4.1Y–4.1Zn–0.9(Al-5Ti-1B) (wt%) are 195.35 MPa and 9.45, respectively. The Y content had the largest impact on the ultimate tensile strength and the significant factor that influencing elongation is the Al-5Ti-1B content. Alloys were mainly composed of α-Mg, X phase, Mg4Y2ZnAl3, TiB2 and Al2Y. The TiB2 and Al2Y, which are formed by the addition of Al-5Ti-1B, can act as heterogeneous nucleation to refine α-Mg grains. The excellent properties of the alloy could be attributed to fine-grain strengthening and second phase strengthening caused by a large amount of X phase and Mg4Y2ZnAl3 discontinuously distributed at the grain boundaries.

Improved mechanical properties of AlSi7Mg/nano-SiCp composites fabricated by selective laser melting

AlSi7Mg is one of the most widely used aluminum alloys. However, its moderate strength limits its use as a structural material for critical parts. Most mechanisms used to strengthen the material sacrifice its ductility. In situ reactions during the selective laser melting (SLM) process rectify the trade-off by introducing dislocations around the subgrain boundaries and nano-twin planes in the by-product Si phase. This work was the first attempt to reinforce AlSi7Mg by nano-SiC particles fabricated by SLM. In addition, the rapid solidification rate caused solid solution strengthening, fine-grain strengthening and precipitation strengthening, which also contributed to the mechanical properties.

Effect of manganese on microstructure and properties of Mg-2Gd magnesium alloy

Effect of various Mn content (0, 0.5, 1.3, 1.5 2.0 wt%) on microstructure and mechanical properties were systematically investigated. The 0.5 wt% Mn addition results in complete recrystallization of as-extruded Mg-2Gd alloy, thus providing the best ductility comparable to other alloys. In the as-extruded Mg-2Gd-0.5Mn alloy, the majority of grains have a soft orientation, which is advantageous for starting basal slip and reducing the yield asymmetry. When the content of Mn exceeds 1.3 wt%, Mn takes effect in refining the grain size of as-extruded alloys but increases the proportion of unrecrystallization microstructure. With the addition of Mn, the strength of as-extruded alloys increases gradually while the ductility deteriorates. The main reasons for the strength increase are attributed to the combination of fine-grained strengthening, precipitation strengthening and texture strengthening. Besides, the addition of Mn enhances the corrosion resistance of as-extruded Mg–Gd alloys system because Mn removes harmful impurities.

Microstructure and high temperature wear behaviour of in-situ TiC reinforced AlCoCrFeNi-based high-entropy alloy composite coatings fabricated by laser cladding

AlCoCrFeNiTix (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) HEA coatings reinforced by in-situ TiC particles were successfully prepared on AISI1045 steel via laser cladding. The coatings are made up of two BCC phases identified as Fe-Cr and Al-Ni phases, and a few in-situ TiC phases with a shape of granular in micro-nano scale. Ti element prefers to dissolve into the Al-Ni phase and refines the equiaxed grains. The AlCoCrFeNiTi1.0 HEA composite coating with the highest volume fraction of TiC particles phase (2.6%) exhibits the highest average microhardness (860.1 HV0.3). It is found that the composite coating is strengthened by the effect of solid solution strengthening, dispersion strengthening, and fine-grain strengthening. The wear behaviour of the coatings was tested at 25 °C and 600 °C, respectively. The micromorphology and chemical composition analysis of the wear scars show that oxidation wear occurs in addition to adhesive wear during the wear process at 600 °C. The oxidation products are identified as Al2O3, CoO, Cr2O3, Fe2O3, Fe3O4, and NiO in terms of the calculation of flash temperature and Gibbs free energy. The calculation results of wear rates show that the AlCoCrFeNiTi1.0 composite coating exhibits the best wear resistance.

Strong and ductile steel via high dislocation density and heterogeneous nano/ultrafine grains

High strength and high ductility are critical to metallic materials and have always been pursued. A novel strategy to produce a medium Mn stainless steel with high dislocation density and heterogeneous nano/ultrafine-grains was designed and outstanding mechanical properties were achieved. The special heterogeneous microstructure was fabricated by cold rolling followed by rapid reverse annealing at intercritical temperature and a final step of low-temperature tempering. The results of this study demonstrated that the heterogeneous microstructure consisted of 74% reversed austenite, 16% deformed austenite and 10% tempered martensite. The dislocation density in austenite of the final microstructure had a high value of 1.12 × 1015 m−2 and the average grain size was 0.63 μm. The high yield strength (1324 MPa) and tensile strength (1401 MPa) of the steel were achieved and combined with high uniform elongation of 34.7%. The combination of high dislocation density (dislocation strengthening) and heterogeneous nano/ultrafine grains (fine-grain strengthening) resulted in such high yield strength. Simultaneously, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effect occurred within the heterogeneous nano/ultrafine grains leading to high strain hardening rate and good uniform elongation in the steel.

Effects of C and Mo on microstructures and mechanical properties of dual-phase high entropy alloys

A systematic study of the effects of interstitial element C and high melting point element Mo on the microstructural evolution and mechanical properties of dual-phase CoCrFeNiAl0.5 high entropy alloys (labeled as HEAs) is presented. In order to obtain a fine-grained microstructure, the HEAs were produced by powder metallurgy (P/M) method. Microstructures and mechanical properties were characterized using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA) and tensile tests. The results show that, with the addition of C and Mo, all the P/M HEAs exhibit a fine equiaxed microstructure. A small amount of carbides and σ phase are distributed evenly on the HEA matrix. The CoCrFeNiAl0.5 HEA containing 8 at. % of Mo and C shows combined high strength of 1280 MPa and reasonable ductility of 7.1%. The improvement of the tensile strength is caused by the solid solution strengthening, fine-grained strengthening and precipitation strengthening. The good ductility results from the fine-grained microstructure.

Microstructure and Mechanical Properties of Nanocrystalline Al-Zn-Mg-Cu Alloy Prepared by Mechanical Alloying and Spark Plasma Sintering.

In this study, Al, Zn, Mg and Cu elemental metal powders were chosen as the raw powders. The nanocrystalline Al-7Zn-2.5Mg-2.5Cu bulk alloy was prepared by mechanical alloying and spark plasma sintering. The effect of milling time on the morphology and crystal structure was investigated, as well as the microstructure and mechanical properties of the sintered samples. The results show that Zn, Mg and Cu alloy elements gradually dissolved in α-Al with the extension of ball milling time. The morphology of the ball-milled Al powder exhibited flaking, crushing and welding. When the ball milling time was 30 h, the powder particle size was 2–5 μm. The α-Al grain size was 23.2 nm. The lattice distortion was 0.156% causing by the solid solution of the metal atoms. The grain size of ball-milled powder grew during the spark plasma sintering process. The grain size of α-Al increased from 23.2 nm in the powder to 53.5 nm in the sintered sample during the sintering process after 30 h of ball milling. At the same time, the bulk alloy precipitated micron-sized Al2Cu and nano-sized MgZn2 in the α-Al crystal. With the extension of ball milling time, the compression strength, yield strength and Vickers hardness of spark plasma sintering (SPS) samples increased, while the engineering strain decreased. The compression strength, engineering strain and Vickers hardness of sintered samples prepared by 30 h milled powder were ~908 MPa, ~8.1% and ~235 HV, respectively. The high strength of the nanocrystalline Al-7Zn-2.5Mg-2.5Cu bulk alloy was attributed to fine-grained strengthening, dislocation strengthening and Orowan strengthening due to the precipitated second phase particles.

Microstructure, mechanical properties and strengthening mechanisms of in-situ prepared (Ti5Si3 + TiC0.67)/TC4 composites

Hybrid (Ti5Si3 + TiCx) (x = 0.67) particles reinforced TC4 matrix composites were in-situ prepared by hot-press sintering, basing on the reactions between Ti3SiC2 and Ti. To research the effects of Ti3SiC2 content on the microstructures and mechanical properties, 1–10 wt% Ti3SiC2 were selected. When Ti3SiC2 content was less than 5 wt%, Si element dissolved in the matrix, and TiCx distributed in the matrix homogeneously. While the content exceeded 5 wt%, Si was precipitating as Ti5Si3 attaching to TiCx particles, which resulted in the aggregations of reinforcements. The micro-hardness of matrix was improved with the solid solubility of Si increasing in matrix, and the compressive strength was improved with Ti3SiC2 content increasing. The hardness was mainly affected by solid solution strengthening. The compressive strength was affected by the solid solution strengthening, fine-grain strengthening and the improvement effect of reinforcements. It was determined that solid solution strengthening played the major role by theoretical calculation.

Study on microstructure and mechanical properties of Mg-xY-6xZn by squeeze casting**Foundation item: This work is supported by the National Natural Science Foundation of China (Nos. 51774254, 51774253, 51701187, U1610123, 51674226, 51574207, 51574206), The Science and Technology Major Project of Shanxi Province (No. MC2016-06).

The Mg-xY-6xZn (x = 0.5, 0.7, 1.0, at%) alloys were prepared by squeeze casting, the effects of different processing parameters on microstructure and mechanical properties of the alloys were studied. The results show that the microstructure of Mg–Y–Zn alloys with different content of Y and Zn composed of α-Mg matrix and quasicrystal I-phase under the applied pressure 100 MPa, and the mixing phase (α-Mg and I-phase) are mainly lamellar eutectic structure which network distributed in α-Mg matrix. The tensile strength increase and the elongation increase first and then decrease with the increase of Y and Zn content. Both the α-Mg matrix and the lamellar eutectic structure are obviously refined with the increase of the applied pressure in Mg95.1Y0.7Zn4.2 alloy, and both of the tensile strength and elongation increase first and then decrease. In this study, the tensile strength of Mg93YZn6 reach the best, which is 221.9 MPa and while the elongation is 5.2% under the applied pressure 100 MPa. The main reasons for the improvement of mechanical properties are fine-grained strengthening and second phase strengthening. The fracture morphology of Mg–Y–Zn alloys are mainly composed of small dimples and tearing ridges, presenting quasi-cleavage fracture characteristics.