Main Strengthening Mechanisms Research Articles

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.

Microstructures and strengthening mechanisms of Mg-8.2Gd-4.6Y-1.5Zn-0.4Zr alloy containing LPSO, β′ and γ type phases

The microstructures and strengthening mechanisms of the Mg-8.2Gd-4.6Y-1.5Zn-0.4Zr (wt%) alloy with long-period stacking ordered (LPSO), β′ and γ type phases were systematically studied. The results show that the LPSO with lamellar and block structures forms near the grain boundaries. The grains are clearly refined, and the 18R LPSO phase is oriented along the extrusion direction after extrusion. Some particles also precipitate from the Mg matrix dynamically. The extruded alloy exhibits a remarkable age-hardening response, and mechanical properties, with a tensile strength (TS) of 449 MPa, yield strength (YS) of 362 MPa, and elongation of 7.9% obtained in the peak-aged alloy. The strengthening mechanisms of the alloy in different states are discussed. Grain boundary and precipitation strengthening are the main strengthening mechanisms for the peak-aged alloy.

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.

Comparison of the strengthening effects of Nb, V, and Ti on the mechanical properties of 20MnSi low-alloy steel

Abstract Three alloys, containing niobium, vanadium and titanium, respectively, were refined and the strengthening effect attained after adding them individually in a 20MnSi low-alloy rebar steel was investigated. The results show that the strengthening effect attained due to the addition of niobium is the best, whereas that due to the addition of titanium is the poorest. Grain refinement and precipitation strengthening are the main strengthening mechanisms observed in niobium-steel and vanadium-steel, whereas only precipitation strengthening is observed in titanium-steel. Moreover, the average grain size of niobium-steel is the smallest among the four types of steels, while the size of ferrite and pearlite microstructures show almost no obvious change as compared to the base steel in the case of titanium-steel. Furthermore, the volume fractions of ferrite and pearlite in the four tested steels have no noticeable change.

Microstructure and mechanical properties of zinc matrix composites reinforced with copper coated multiwall carbon nanotubes

Zinc alloy has been considered as a promising candidate for orthopedic implants due to its suitable degradation rate and good biocompatibility. However, its mechanical properties still have to be optimized. Metal matrix composites (MMC) are an effective way to improve the mechanical properties of zinc alloy. In this study, pure zinc as matrix, Cu-coated multiwall carbon nanotubes (MWCNTs) were used as reinforcement, and MWCNTs/Zn composites were successfully prepared by combining the use of electroless deposition (ED), spark plasma sintering (SPS), and hot-rolling techniques. The results show that MWCNTs are uniformly dispersed in the zinc matrix and a tight interface is formed between the MWCNTs and the zinc matrix. As the content of the MWCNTs increases, the mechanical properties of the MWCNTs/Zn composites gradually increase. The Cu-coated MWCNTs/Zn composites have higher mechanical properties than uncoated Cu. 3 vol% Cu-coated MWCNTs/Zn composite has a highest ultimate tensile strength of 281 MPa, but the elongation is only 4%. Load transfer effect and fine grain strengthening are two main strengthening mechanisms of MWCNTs/Zn composites. This research provides a new method for the design and preparation of MWCNTs/Zn composites.

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.

Tuning composition and microstructure by doping Ti and C for enhancing mechanical property and wear resistance of Al0.2Co1.5CrFeNi1.5Ti0.5 high entropy alloy matrix composites

Abstract Spark plasma sintering was used to prepare in-situ synthesized TiC reinforced Al0.2Co1.5CrFeNi1.5Ti0.5 high entropy alloy matrix composites with adding Ti and C elements by the molar ratio of 1:1. Face-centered cubic (FCC) solid solution and in-situ synthesized TiC were the main components of the composite. The content of nanocrystals which induced by grain boundary pinning effect of TiC increased with increasing of Ti and C content. The micro Vickers hardness increased from 410 HV0.3 to 850 HV0.3 and the yield strength increased from 1080 MPa to 2164 MPa when the C content increase to the molar ratio of 2.0. Interstitial solution strengthening, dispersion strengthening, grain-boundary strengthening and fine grain strengthening were the main strengthening mechanisms for the in-situ TiC enhanced Al0.2Co1.5CrFeNi1.5Ti0.5 high entropy alloy matrix composites. The wear resistance of Al0.2Co1.5CrFeNi1.5Ti2.5C2.0 composite was about 40 times higher than that of Al0.2Co1.5CrFeNi1.5Ti0.5 high entropy alloy. The reinforcing effect of TiC and the protective effect of TiO2 oxide glaze layer which formed by the in-situ tribo-chemical reaction during the friction process were the main reasons for the improvement of wear resistance.

On the influence of nanoparticles as addition to the A356 aluminum‐alloy: Is it acting as a refining or strengthening mechanism?

AbstractThe effect induced by adding nanoparticles to cast hypo‐eutectic aluminum 7 % silicon alloys (A356) is continually reported to show enhanced mechanical properties. The experimental results show a strengthening effect associated with an increase in ductility and/or toughness, induced by adding nanoparticles. The aim of this work is to add a contribution towards understanding the strengthening effect observed in the nanoparticles‐reinforced A356 alloy. This is done by calculating the conventional incremental summation of several strengthening mechanisms contributing to strengthening and reinforcement in composites. The results obtained by applying the mathematical calculations are compared with the experimental results reported in literature. Based on the mathematical approach, this work shows that the main strengthening mechanisms acting for metal matrix nano composites (MMNCs) are those contributing to the strengthening of the matrix. These result from contributions by Orowan strengthening and coefficient of thermal expansion (CTE) and elastic modulus (EM) mismatch.

Creep behavior and a deformation mechanism based creep rate model under high temperature and low stress condition for single crystal superalloy DD5

This work reports the investigation on creep behavior under high temperature and low stress condition for single crystal superalloy DD5 and the subsequent establishment of a deformation mechanism based creep rate model. The γ/γ′ microstructure evolution and the precipitate-dislocation interaction mode were investigated systematically during creep test at 1100 °C and 120 MPa, for a better understanding of underlying creep deformation characteristics. The γ/γ′ microstructure becomes rafted and topological inverted eventually as creep deformation continues. The formation of dense interfacial dislocation network that impedes dislocations from cutting into γ′ precipitates is assumed to be one of the main strengthening mechanisms during creep. The climbing over rafted γ′ precipitates is seen as a recovery/softening process which annihilates dislocations, receding the strengthening effect of interfacial dislocation network. The cutting through rafted γ′ precipitates is proposed to account for the acceleration of creep, a strong dependence of creep properties on rafting (the effect of widening horizontal γ channel on the degraded back stress of cutting through rafted γ′ precipitates) then arises. The established model involving these characteristics was found to predict the creep behavior (below 1% creep strain) under different temperatures and stresses for single crystal superalloy DD5 accurately. Several microstructure characteristics and physical properties emerge from the model, which is promising for alloy design and compositional optimization.

Microstructure, Mechanical Properties, and Sliding Wear Behavior of Oxide-Dispersion-Strengthened FeMnNi Alloy Fabricated by Spark Plasma Sintering

The face-centered-cubic (fcc) CoCrFeMnNi high-entropy alloy suffers from low strength and wear resistance at ambient temperature. Herein, we developed a strategy to overcome the strength/ductility trade-off and simultaneously increase the wear resistance via the in situ formation of uniformly dispersed Mn3O4 nanoparticles in an ultrafine-grained fcc FeMnNi matrix. The obtained equiatomic FeMnNi alloy exhibited a high yield strength of up to 912 MPa and an elongation of 19 pct. Grain boundary and oxide-dispersion-strengthened were found to be the main strengthening mechanisms. Ball-on-disk wear tests showed that the FeMnNi alloy had low wear rates in the order of 10−4-10−5 mm3/(N m) upon sliding against an alumina ball, and the wear mechanism changed from abrasive wear to oxidation and fatigue wear at high loads and sliding velocities. The presence of Mn3O4 nanoparticles hindered the severe plastic flow of the fcc matrix during sliding. The excellent combination of strength, ductility, and tribological performance of the present alloy renders it as a promising candidate for structural applications.

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.

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.

Effect of short carbon fiber content on SCFs/AZ31 composite microstructure and mechanical properties

The magnesium matrix composites containing various content of short carbon fibers reinforcement (SCFs/AZ31) were fabricated by ball milling and solid state synthesis. Morphologies of composite, distribution of SCFs, and interface of matrix and reinforcement were analyzed. The effect of SCFs content on the mechanical properties of the composite was studied. When the content of SCFs is less than 3%, SCFs uniformly distribute in the composite and promote dynamic recrystallization, the refining effect of grain is obvious, and meanwhile SCFs can play the effect of load transfer to the matrix. With the increase of SCFs content, the degree of aggregation increases, the grain size of composite increases and mechanical properties decrease. For mechanical performance, the strengthen effects was firstly enhanced and then deteriorated with increasing SCFs content. The main strengthening mechanisms of composite are grain refinement strengthening, thermal residual stress strengthening, and load transfer strengthening.

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.

The Preparation and Mechanical Properties of a Pure Titanium-Based Matrix Composite Reinforced with Graphene Nanoplatelets

A lightweight titanium matrix composite material was fabricated by vacuum sintering using semi-powder metallurgy. The graphene nanoplatelets (GNPs) were used as a reinforcement for the titanium matrix composites. Fabricating the composite materials used three steps: dispersion, formation, and sintering. In particular, GNPs were dispersed by ionic liquid through a centrifugal testing machine instead of ball milling in the process. The better pressure for composite forming was 600 MPa. At the same time, the better sintering temperature and holding time were 1200 °C and 3 h. The influences of the GNP addition on the density, microstructure, and microhardness of the Ti/GNP composites were investigated. For the mechanical properties of the composites, we focused on the tensile strength with different GNP contents. The Ti 0.075 wt% and Ti 0.15 wt% GNP composites exhibited yield strengths of 850 and 948 MPa, which demonstrated 66% and 85% increase compared to those of extruded titanium materials with no GNP additive (512 MPa yield strength). The main strengthening mechanisms of Ti/GNP composites are grain refinement strengthening, thermal mismatch strengthening, and dispersion strengthening.

Annealing-dependent microstructure, magnetic and mechanical properties of high-entropy FeCoNiAl0.5 alloy

High-entropy FeCoNiAl0.5 alloy was produced and subjected to heat treatment at different temperatures (400, 600, and 800 °C). The microstructure evolution, magnetic properties and mechanical properties of the alloy were found to be highly dependent on the annealing temperature. The as-cast alloy was composed of FeCoNi-riched matrix (face-centered cubic structure) and the Al-riched region (body-centered cubic structure). After annealed at 400 °C, Al–Ni-based ferromagnetic rod-like precipitates with body-centered cubic structure were abundantly produced within the matrix, improving saturation magnetization (Ms), coercivity (Hc) and hardness. Increasing the annealing temperature to 600 °C, not only gave rise to a higher number density of the rod-like precipitates, but also to the formation of some nanosized spherical precipitates. This microstructure feature is more effective in strengthening the material (achieving a peak hardness of ~304 HV) but at the same time results in slight declination of Ms and Hc. When the annealing temperature was up to 800 °C, the most strengthening nanoscale precipitates dissolved, and the hardness of the alloy was reduced accordingly. The main strengthening mechanisms are discussed, and a good combination of mechanical and magnetic properties under optimized heat treatment may be available.

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.

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.

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).