Main Strengthening Research Articles

Effect of Cu addition and heat treatment on the microstructure and microwear of selective laser melted Mg–Al–Zn alloy

In this study, the microstructure, intermetallic phase, microhardness and wear resistance of unheated and heat-treated Mg-6Al-1Zn-xCu (x = 0, 0.5, 1, 2, and 4) alloys were investigated. The selective laser melted Mg-6Al-1Zn alloy that contained Cu had a typical continuous network structure that consisted of fine α-Mg grains and a eutectic phase. Uniformly distributed Mg2Cu was the main strengthening phase before heat treatment, and the analysis showed that the Mg2Cu was mainly present in the form of particles in the alloy. Most of the eutectic dissolved into the α-Mg matrix, and a granular Al2Cu phase was formed in the matrix after solution treatment. Al2Cu played a significant role in improving the high-temperature properties and wear resistance of the alloy. During the aging treatment, a helical phase (H phase) was formed that was rich in Cu and Al. Compared with those for the other Cu contents, the Mg-6Al-1Zn-2Cu alloy had the best wear resistance and antifriction properties.

Study on axial compressive properties of RC square columns strengthened with prestressed CFRP sheets

Carbon-fiber reinforced polymer (CFRP) has the advantages of high strength, high elasticity, light weight, corrosion resistance, high fatigue resistance, and ease of construction and has become the main seismic strengthening strategy for civil engineering structures. This paper presents a new technique to quickly strengthen reinforced concrete square columns using prestressed CFRP. The axial compression performance of three groups of 10 prestressed CFRP reinforced concrete square columns was tested. By observing the effects of different prestressed CFRP wrapping methods and the range and degree of prestressing on the axial compression, it was possible to analyze the mechanism by which CFRP actively restrains the reinforced columns. Changes in the ductility of the reinforced components were examined and their axial bearing capacity was studied theoretically. The test results and analytical studies show that, the larger the degree of prestressing and range of wrapping, the stronger the lateral deformation constraint placed on the core concrete, and the greater the bearing capacity and ductility. The increase in bearing capacity can reach 65.91%. Increasing the degree of prestressing of the CFRP is also more effective than increasing its reinforcement range. The axial bearing capacity of the reinforced columns was also calculated theoretically by establishing strength calculation models for the confined concrete for different methods and ranges of reinforcement. The predicted results were in good agreement with the test results, so, the models can provide a firm theoretical basis for practical applications.

Effect of Nitrogen‐Substituted Carbon on Thermal Stability of Cr‐Mo‐V Hot‐Working Die Steel

To extend service life of Cr‐Mo‐V hot‐working die steel, a method of alloying with nitrogen‐substituted carbon is proposed. The mechanical properties of the test steels in the long‐time heating and holding process are measured. The effect of nitrogen‐substituted carbon on thermal stability is characterized and analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Thermo‐Calc. The results show that the ratio of N in V(C,N) can be increased by nitrogen‐substituted carbon at high temperature, which significantly improves its stability. As a result, it can not only refine the grain but also broaden the austenitizing temperature range and promote the complete solution of alloy elements such as Cr, Mo. In the process of long‐term heating and insulation, nitrogen‐substituted carbon can effectively reduce the precipitation of harmful carbides and the coarsening rate by raising the activation energy of tempering transformation with 20.0 kJ mol−1, and increase the precipitation of the main strengthening phase V(C,N). Nitrogen‐substituted carbon, as an effective alloying method, innovatively improves thermal stability and service life of Cr‐Mo‐V hot‐working die steel.

Ultrafine-grained Al–Zn–Mg–Cu alloy processed via cross accumulative extrusion bonding and subsequent aging: Microstructure and mechanical properties

Ultrafine-grained (UFG) materials produced by severe plastic deformation methods generally exhibit a higher strength, but a lower uniform ductility. In this paper, an UFG Al–Zn–Mg–Cu alloy was fabricated via cross accumulative extrusion bonding (CAEB) and subsequent aging to achieve high strength and moderate uniform elongation. The influences of CAEB and subsequent aging on microstructure and tensile properties of the alloy were investigated. After CAEB, TEM characterization revealed that the microstructure exhibited typically ED-elongated ultrafine grains (740 ± 20 nm). Post-CAEB aging led to the formation of many nano-sized particles with different morphologies. These particles precipitated in the grain interiors and at grain boundaries, among which the main strengthening phase was η′ phase. EBSD characterization showed that the fraction of high angle grain boundaries increased to 61.0% for the post-CAEB aged alloy. At the same time, microtexture was significantly weakened as the CAEB process was applied to the alloy. The main texture components were mainly composed of Cu and S as well as a small amount of Brass and Cube. In addition, the tensile test at room temperature demonstrated that the post-CAEB aged sample possessed a high ultimate tensile strength of 562 MPa while simultaneously maintaining a moderate uniform elongation of 21.5%.

Preferentially oriented SiC/graphene composites for enhanced mechanical and thermal properties

Dense SiC ceramics with a high alignment of graphene nanoplatelets (GNPs) were fabricated via a liquid-phase hot pressing (HP) process. The effects of GNPs on the phase composition, microstructure evolutions, mechanical and thermal properties of SiC ceramics were analyzed in detail. As the GNPs content increased from 0 to 3 wt %, the density and hardness of the SiC/GNPs composites decreased monotonically, whereas the flexural strength, fracture toughness and thermal conductivity increased initially and then decreased. The decrease in density and hardness is due to the low density and low hardness of GNPs themselves. The increase in thermal conductivity is mainly due to electron thermal transmission and the widening effect of phonon thermal transmission channel by the textured GNPs. The main strengthening and toughening mechanisms include fine grain strengthening, dislocation strengthening, crack bridging, crack deflection and crack termination. The SiC/GNPs composites sintered under uniaxial pressure condition (HP) display anisotropic thermal and mechanical properties. The anisotropic thermal conductivities of the SiC/GNPs composites are likely due to in plane GNPs parallel to the direction of heat flow, while through plane GNPs are perpendicular to the direction of heat flow. The anisotropy in mechanical properties of the SiC/GNPs composites are due to the through plane GNPs preferentially oriented to intersect the cracks, while the in plane GNPs preferentially oriented to parallel the cracks.

Microstructural features of novel corrosion-resistant maraging steel manufactured by laser powder bed fusion

A new iron-based alloy, M789 steel, has been recently introduced to the additive manufacturing (AM) industry, designed to be utilized in the laser powder bed fusion (LPBF). The effect of microstructural evolution on the hardness and tensile strength of the alloy was evaluated using optical and electron microscopy techniques. The crystallographic texture analysis suggests that elongated grains with significant low-angle grain boundaries (LAGBs) were formed, which transformed into large needle-like martensitic structures (with a lower volume fraction of LAGBs) after solutionizing and aging heat treatment. It appears that the main strengthening of the alloy is the formation of Ni3Ti precipitates.

Mechanical and tribological properties of graphene nanoplatelets-reinforced titanium composites fabricated by powder metallurgy

Titanium matrix composite reinforced by graphene nanoplatelets (GNPs) was fabricated via powder metallurgy route. Hot isostatic pressing and hot extrusion were used to consolidate the mixed powder of GNPs and TC4 titanium (Ti) alloy. The microstructures, mechanical properties and sliding wear performance of Ti/GNPs composite had been researched to evaluate the reinforcing effect of GNPs on titanium matrix. Microstructure observation indicates that GNPs could restrain grain growth slightly in titanium matrix. Titanium matrix and graphene exhibit a clean and firm interface formed by means of metallurgical bonding on atomic scale. Compared with the monolithic titanium alloy, the composite with 1.2 vol.% GNPs exhibits significantly improved elastic modulus and strength. The sliding wear test shows that there is an obvious enhancement in the tribological performance of Ti/GNPs composite with 1.2 vol.% GNPs. The results of this work indicate that GNP is an efficient reinforcement material in titanium matrix. The strengthening mechanism including precipitates strengthening, load transfer and grain refinement mechanism of GNPs in titanium matrix was discussed. A modified shear-lag model was used to analyze the reinforcement contribution of the stress transfer mechanism. The calculation shows that the stress load mechanism constitutes the main strengthening mechanism in Ti/GNPs composite.

Microstructure and Mechanical Properties of Squeeze Cast Al-5 Mg-3Zn-1Cu-1Si Alloy Along Cross Section

The variation of microstructure and mechanical properties along the cross section of squeeze cast Al-5Mg-3Zn-1Cu-1Si alloy is studied under both squeeze-cast and peak-aged condition (470 °C × 24 h + 125 °C × 24 h). For squeeze-cast alloy, along the depth direction, second dendrite arm space and the percentage of $${\alpha }$$-Al increase significantly, while the volume fraction of T-Mg32(AlZnCu)49 and Mg2Si phase showed a dramatic decrease. The ultimate tensile strength of squeeze-cast alloy slightly decreased from 357.04 MPa for the surface layer to 344.21 MPa for the central layer, yield strength dropped from 296.86 MPa to 241.03 MPa, but the elongation almost tripled from 1.42% to 3.78%. After peak aging, the ultimate tensile strength of surface layer and central layer reached 538.45 MPa and 504.03 MPa, yield strength reached 447.46 MPa and 442.65 MPa, and the elongation reached 5.23% and 3.97% respectively. Meanwhile the difference of ultimate tensile strength and elongation was increased, but that of yield strength reduced after aging treatment. Precipitation strengthening is the main strengthening mechanism while the improvement in ductility is due to the dissolution of T-Mg32(AlZnCu)49 and the morphological change of eutectic Mg2Si.

A first-principles study on the site occupancy behavior of transition metals in L12-Al3Li phase

The main strengthening mechanism in Al-Li alloys is generally achieved by the existence of a high volume fraction of the L12-Al3Li precipitation from Al matrix. In this work, we do density functional theory (DFT) total energy calculations on the site occupancy behaviour of transition metals in L12-Al3Li compound. The ground state properties of L12-Al3Li, fcc-Al, and bcc-Li are determined and compared to the available literature data. The formation energies of intrinsic point defects in L12-Al3Li phase as well as Al3LiX alloy formation energy are calculated and identified. We show that the Li antisite is preferential to from in the L12-Al3Li phase. Finally, the site occupancy behaviour of transition metals in L12-Al3Li as well as the solute atomic volume effect on the solute site preference and lattice expansion of the L12-Al3Li have been studied, the later results can be used for an indirect estimation of the site substitution behaviour of the alloying elements in L12-Al3Li phase.

Creep behavior and mechanical properties of Al-Li-S4 alloy at different aging temperatures

Creep age forming techniques have been widely used in aerospace industries. In this study, we investigated the effect of aging temperature (143 °C–163 °C) on the creep behavior of Al-Li-S4 aluminum alloy and their mechanical properties at room temperature. The mechanical properties were tested by tensile testing, and the microstructural evolution at different aging temperatures was examined by transmission electron microscopy. Results show that the creep strains and the room-temperature mechanical properties after creep aging increase with the aging temperature. As the aging temperature increases, the creep strain increases from 0.018% at 143 °C to 0.058% at 153 °C, and then to 0.094% at 163 °C. Within 25 h aging, the number of creep steps increases and the duration time of the same steps is shortened with the growth of aging temperatures. Therefore, the increase in aging temperatures accelerates the progress of the entire creep. Two main strengthening precipitates θ′ (Al2Cu) and T1 (Al2CuLi) phases were characterized. This work indicates that the creep strain and mechanical properties of Al-Li-S4 alloys can be improved by controlling aging temperatures.

LEGAL FUNDAMENTALS OF ORGANIZATIONAL SUPPORT OF ACTIVITIES OF ECONOMIC COURTS

Legal bases of organizational, financial and logistical support of commercial courts, which meet the standards of the modern rule of law and contribute to the effective functioning of the judicial system as a whole, are the most important guarantee of the status of judges and quality administration of justice, especially in commercial courts. Subject to full compliance with the principles of organizational support for the functioning of commercial courts, the procedure of commercial litigation is effectively and comprehensively implemented. Given the next round of judicial reform in Ukraine, we can see the relevance of the study primarily as a study of the legal framework for organizational support of commercial courts to establish areas for updating legislation in this area. The purpose of the article is to study and systematize the legal basis of organizational support of commercial courts in Ukraine. The article is devoted to the study of the legal basis of organizational support of commercial courts in Ukraine. There are determined that the components of the mechanism of organizational support of the court are: 1) subjects of management of commercial courts - the President of Ukraine, the Verkhovna Rada of Ukraine, the Supreme Court, the Commercial Court of Cassation, chairmen of commercial courts, the High Qualification Commission, the High Council of Justice municipality; 2) subjects of organizational support of the activity of commercial courts - the State Judicial Administration and its territorial administration, the staff of commercial courts. It is proposed to disclose legal acts of the state in this area regarding two groups: general and special. The main legal bodies dealing with the organizational support of commercial courts, individual entities, studying their main tasks and strengthening, which is enshrined in the relevant legislation. The constant process of reforming the judicial system and the adoption of relevant bylaws in the framework of organizational support of commercial courts in Ukraine is emphasized.

Mechanical properties and yield asymmetry of Mg-Y-Zn alloys: Competitive behavior of second phases

Microstructure and mechanical properties evolution of as-cast Mg-Y-Zn alloys have been systematically investigated with various phase types via adjusting Y/Zn ratio. The results show that the investigated alloys have typical α-Mg dendritic microstructure with different intermetallic compounds including I phase, I + W phases, W phase, LPSO + W phases and LPSO phase. Phase evolution in Mg-Y-Zn alloys is analyzed in detail combining microstructure observations and differential scanning calorimetry results. Strengthening effects of mechanical properties with five different compositions indicate that the second phase strengthening is the main strengthening mechanism, and that the contribution follows the order of W > I + W > LPSO + W > I > LPSO. Based on transmission electron microscopy analysis, the lamellar W phase effectively hinders dislocation movement compared to other phases, and alloys with W phase exhibit high strength. LPSO phase shows fracture and kink bands during tension and compression, respectively, which leads to evident yield asymmetry (>1.1) in alloys containing LPSO. Other alloys with I or W phase have a yield anisotropy around 1.0. The tensile property of Mg-Y-Zn alloys is enough for mechanical design.

Site occupancy of alloying elements in γ′ phase of nickel-base single crystal superalloys

L12 structured γ′ phase is the main strengthening phase of Ni-base superalloys, which contain multiple alloying elements. Locations of these elements in the lattice are crucial for understanding the mechanical/physical properties of γ′ phases. Here, we reveal the site occupancy of various elements in γ′ phases of two multi-component Ni-base single crystal superalloys by using advanced scanning transmission electron microscopy and energy disperse spectroscopy. Cr, Mo, Re, Ta, W and Ti atoms preferentially locate at Al sublattices, and Co and Ru atoms prefer Ni sublattices. Moreover, the addition of Ru does not affect the site occupancy of other alloying elements.

(Al, Zn)3Zr dispersoids assisted η′ precipitation in anAl-Zn-Mg-Cu-Zr alloy

The influence of (Al,Zn)3Zr dispersoids on the precipitation of the main strengthening (Mg,Zn)-rich phases was investigated during isothermal aging of a model Al-Zn-Mg-Cu-Zr alloy. Upon homogenization of the alloy, dispersoids of (Al,Zn)3Zr with a L12 structure are present. Isothermal aging at 120 °C for 0.5 h leads to the homogeneous formation of spherical GP zones in the α-Al matrix and heterogeneous nucleation on (Al,Zn)3Zr dispersoids. After 2 h of aging, GP zones remain present in the α-Al matrix while the accelerated transformation of GP zones to plate-shaped (Mg,Zn)-rich ηʹ precipitates is shown on the {111} planes at the interface of the L12 dispersoids. Even at grain boundaries, the similar composite structure comprising ηʹ precipitates on the coarser Zr-dispersoid is observed, along with 10-nm wide precipitate-free zones around them. The composition and structure of pre-existing dispersoids, their role in the formation of the composite structure are discussed.

Effect of Heat Treatment on Microstructure of Al4Cu-SiC Composites Consolidated by Powder Metallurgy Technique

Silicon carbide particles-reinforced Al4Cu composites containing 2.5, 5 and 7.5 wt.% of reinforcement were fabricated using powder metallurgy (PM) technique. The sintered Al4Cu-SiC composites were solution treated for 6 h at two different temperatures (495 and 530 °C) and then aged at 180 °C for various aging periods (4, 12 and 24 h). Effects of heat treatment on the microstructural changes and microhardness were investigated by scanning electron microscope, transmission electron microscope, x-ray diffraction and microhardness tests. The results indicate the ceramic particles-reinforced Al4Cu matrix requires different heat treatment parameters compared to the unreinforced alloy. The applied solution temperature did not allow for thorough dissolution of alloy phases in the matrix material produced by PM route. However, in Al4Cu-SiC composites the main strengthening phase (Al2Cu) was uniformly distributed in the matrix. The solid-solution temperature increase affects the reduction in time to reach peak hardness. The highest increment in microhardness by aging treatment was observed for composite with the addition of 5 wt.% of SiC solution treated at 530 °C for 12 h (110 HV0.05).

Enhanced mechanical properties of Al–Si–Cu–Mn–Fe alloys at elevated temperatures through grain refinement and dispersoid strengthening

The effect of Ti content on the microstructure and mechanical properties of heat-treated Al–Si–Cu–Mn–Fe alloys was investigated. It was found that the mechanical properties increased with the increase of Ti content. This was attributed to the refinement of grain size, the increased amount of T (Al20Cu2Mn3), the α-Fe (Al15(FeMn)3(CuSi)2) precipitated particles, and the decrease in Al2Cu. At an elevated temperature of 300°C, the heat-treated Al–Si–Cu–Mn–Fe alloy with 0.5% Ti demonstrated the best mechanical properties, which are superior to those of commercial aluminium alloys. The yield strength contribution at 300°C was quantitatively evaluated based on the dispersoid, solid solution, and matrix contributions. It was confirmed that the main strengthening mechanism in the experimental alloys was the dispersoid strengthening.

A new 50Cr6Ni2Y alloy steel prepared by Direct laser Deposition: Its design, microstructure and properties

To meet the requirements for directed laser deposition (DLD) key metal wear parts with hardness gradient from 300 to 900 HV, a novel 50Cr6Ni2Y alloy steel was designed as the sub-surface working layer (whose hardness is 500–600 HV) by simulating the chemical composition and optimizing its Cr/C ratio. Spherical 50Cr6Ni2Y alloy powder was prepared using the vacuum induction gas atomization (VIGA) method. The microstructure and properties of the DLD 50Cr6Ni2Y alloy steel sample were studied. The results show that the designed 50Cr6Ni2Y alloy powder has good powder characteristics. With the process parameters of laser power 2000 W, scanning speed 5 mm/s and overlap ratio 35%, the 50Cr6Ni2Y alloy steel sample without crack and pore defects was fabricated. The microstructure of the sample was mainly composed of bainite and (Fe,Cr)23C6, and the mean microhardness reached 534 HV, The ultimate strength and the yield strength was 1139 MPa and 989 MPa, respectively. The average elongation was 2% and the specific wear rate was 1.03 × 10−4 mm3/(N·m). The microhardness and wear resistance of DLD 50Cr6Ni2Y alloy steel are reinforced by dislocation, nano-sized (Fe,Cr)23C6 and bainite slabs. The Orowan mechanism was the main precipitate strengthening mechanism in the DLD sample. This novel 50Cr6Ni2Y alloy steel powder could meet the needs of sub-surface working layer for DLD key wear parts.

Effects of solution treatment on the microstructure and mechanical properties of naturally aged EN AW 2024 Al alloy sheet

The solution experiments on cold rolled 2024 Al sheet with a 1070 Al coating were carried out at various temperatures and holding time. The effects of solution parameters on the microstructure evolution and mechanical properties were studied. The results showed that the alloying elements diffused from the 2024 Al matrix to the 1070 Al coating, following the peak-valley pattern. The cold rolled 2024 Al sheet mainly have the second phases of Al2Cu, S (Al2CuMg), Si, Mg2Si, and the impurities of AlFeMnSi, AlCuFeMnSi, AlCuFeMn, and the amount of them was reduced with increasing solution temperature or holding time. T (Al20Cu2Mn3) had precipitated before aging, and only the Cu–Mg clusters were formed during the natural aging, which should be the main strengthening mechanism. The grain structure was elongated during cold rolling, while the static recrystallization and grain growth occurred during the solution treatment. The ultimate tensile strength, yield strength and elongation were gradually enhanced with increasing the solution temperature to 510 °C. When the solution temperature is further increased to 530 °C, the elongation was obviously decreased due to the over-burning. Moreover, it was found that the solution temperature had much stronger effects on the mechanical properties than that of the holding time.

Analysis of abnormal texture formation and strengthening mechanism in an extruded Mg-Gd-Y-Zn-Zr alloy

A Mg-Gd-Y-Zn-Zr magnesium alloy with different initial states was extruded under different extrusion parameters. The effect of solution treatment and extrusion parameters on the microstructure, texture and mechanical properties were analyzed in detail, and the abnormal texture formation and strengthening mechanism was revealed. When extruded at low temperature and small extrusion ratio, the bimodal microstructure consisting of fine dynamically recrystallized grains and coarse deformed grains occurred both in the as-cast alloy and solution-treated alloy. When the extrusion temperature and extrusion ratio were increased, the amount and size of dynamically recrystallized grains increased and the grain size of the solution-treated alloy showed higher growth rate. Furthermore, an abnormal texture with <0001> parallel with extrusion direction developed due to the occurrence of non-basal slip and continuous dynamic recrystallization. This could be enhanced by solution treatment, high temperature, and large extrusion ratio. Both the as-cast alloy and solution-treated alloy exhibited the highest tensile strength after extrusion at 300 °C with an extrusion ratio of 9. Grain refinement was the main strengthening mechanism utilized in both the as-cast alloy and the solution-treated alloy. Work hardening played an important role in the sample extruded at low temperature and small extrusion ratio, with the highest contribution of about 33 MPa after extrusion at 300 °C with an extrusion ratio of 9. Texture strengthening contributed more in the sample extruded at high temperature and large extrusion ratio, but no more than 24.1 MPa. Solution strengthening was another strengthening mechanism in the extruded as-cast alloy, especially at high temperature and large extrusion ratio (no more than 9 MPa).

The effect of pre-deformation on the precipitation behavior of AlCuMg(Si) alloys with low Cu/Mg ratios

The addition of a trace amount of Si significantly affects the precipitation sequence of the Al–Cu–Mg alloy with a low Cu/Mg ratio, such as forming Si-modified Guinier-Preston-Bagaryatsky (GPB) zones to suppress the precipitation of the S phase. The introduction of pre-deformation also affects the precipitation behavior during the subsequent aging process. In this paper, the atomic-resolution high-angle-annular-dark-field (HAADF) imaging technique and first-principles calculation are used to study the effect of pre-deformation on the precipitation behavior of an Al-3.0Cu-1.8Mg-0.5Si alloy during aging at 180 °C. In order to learn the effect of minor Si-addition, a Si-free alloy with similar chemical composition is also studied. It is found that for Si-free alloy, the 6% pre-deformation promotes the precipitation of the S phase dispersedly during the subsequent aging, leading to an increase of the peak hardness. For Si-containing alloy, the 6% pre-deformation prior to aging suppresses the precipitation of Si-modified GPB zone acting as the original main strengthening precipitates and promotes the formation of successive composite precipitates including the S phase, various GPB zones, C phase and sub-units of C/Qʹ phase. The successive composite precipitates are easy to grow up and coarsen, which weakens the contribution of precipitation strengthening compared to the small Si-modified GPB zone. As such, the 6% pre-deformation prior to aging at 180 °C provides a positive strengthening effect for Si-free alloy, but a negative strengthening effect for the Si-containing alloy.