Size-dependent thermomechanical vibration characteristics of rotating pre-twisted functionally graded shear deformable microbeams

A three-dimensional(3D)thermomechanical vibration model is developed for rotating pre-twisted functionally graded(FG)microbeams according to the refined shear deformation theory(RSDT)and the modified couple stress theory(MCST).The material properties are assumed to follow a power-law distribution along the chordwise direction.The model introduces one axial stretching variable and four transverse deflection variables including two pure bending components and two pure shear ones.The complex modal analysis and assumed mode methods are used to solve the governing equations of motion under different boundary conditions(BCs).Several examples are presented to verify the effectiveness of the developed model.By coupling the slenderness ratio,gradient index,rotation speed,and size effect with the pre-twisted angle,the effects of these factors on the thermomechanical vibration of the microbeam with different BCs are investigated.It is found that with the increase in the pre-twisted angle,the critical slenderness ratio and gradient index corresponding to the thermal instability of the microbeam increase,while the critical material length scale parameter(MLSP)and rotation speed decrease.The sensitivity of the fundamental frequency to temperature increases with the increasing slenderness ratio and gradient index,and decreases with the other increasing parameters.Moreover,the size effect can suppress the dynamic stiffening effect and enhance the Coriolis effect.Finally,the mode transition is quantitatively demonstrated by a modal assurance criterion(MAC).

Improving hydrogen storage thermodynamics and kinetics of Ce-Mg-Ni-based alloy by mechanical milling with TiF_(3)

Mg-based hydrides are too stable and the kinetics of hydrogen absorption and desorption is not satisfactory.An efficient way to improve these shortcomings is to employ reactive ball milling to synthesize the nanocomposite materials of Mg and additives.In this experiment,TiF_(3)was selected as an additive,and the mechanical milling method was employed to prepare the experimental alloys.The alloys used in this experiment were the as-cast Ce_(5)Mg_(85)Ni_(10),as-milled Ce_(5)Mg_(85)Ni_(10)and Ce_(5)Mg_(85)Ni_(10)+3 wt.%TiF3.The phase transformation,structural evolution,isothermal and non-isothermal hydrogenation and dehydrogenation performances of the alloys were inspected by XRD,SEM,TEM,Sievert apparatus,DSC and TGA.It revealed that nanocrystalline appeared in the as-milled samples.Compared with the as-cast alloy,ball milling made the particle dimension and grain size decrease dramatically and the defect density increase significantly.The addition of TiF_(3)made the surface of ball milling alloy particles markedly coarser and more irregular.Ball milling and adding TiF_(3)distinctly improved the activation and kinetics of the alloys.Moreover,ball milling along with TiF_(3)can decrease the onset dehydrogenation temperature of Mg-based hydrides and slightly ameliorate their thermodynamics.

Research status and prospects of the fractal analysis of metal material surfaces and interfaces

As a mathematical analysis method,fractal analysis can be used to quantitatively describe irregular shapes with self-similar or self-affine properties.Fractal analysis has been used to characterize the shapes of metal materials at various scales and dimensions.Conventional methods make it difficult to quantitatively describe the relationship between the regular characteristics and properties of metal material surfaces and interfaces.However,fractal analysis can be used to quantitatively describe the shape characteristics of metal materials and to establish the quantitative relationships between the shape characteristics and various properties of metal materials.From the perspective of two-dimensional planes and three-dimensional curved surfaces,this paper reviews the current research status of the fractal analysis of metal precipitate interfaces,metal grain boundary interfaces,metal-deposited film surfaces,metal fracture surfaces,metal machined surfaces,and metal wear surfaces.The relationship between the fractal dimensions and properties of metal material surfaces and interfaces is summarized.Starting from three perspectives of fractal analysis,namely,research scope,image acquisition methods,and calculation methods,this paper identifies the direction of research on fractal analysis of metal material surfaces and interfaces that need to be developed.It is believed that revealing the deep influence mechanism between the fractal dimensions and properties of metal material surfaces and interfaces will be the key research direction of the fractal analysis of metal materials in the future.

Evolution of second phases and mechanical properties of 7075 Al alloy processed by solution heat treatment

The effects of solution treatment on the evolution of the second phases and mechanical properties of7075Al alloy werestudied with scanning electron microscopy(SEM),energy dispersive X-ray spectrometry(EDS),differential scanning calorimetry(DSC),hardness and tensile tests.The results show that Mg(Zn,Cu,Al)2phases gradually dissolve into the matrix,yet the size andmorphology of Al7Cu2Fe phase exhibit no change with the increase of the solution treatment temperature and time due to its highmelting point.When the solution treatment temperature and time continue to increase,the formation of coarse black Mg2Si particlesoccurs.Compared to the as-cast alloy,the microhardness,tensile strength,and elongation of the sample under solution heat treatmentat460°C for5h are increased by55.1%,40.9%and109.1%,respectively.This is because the eutectic Mg(Zn,Cu,Al)2phases almostcompletely dissolve and basically no coarse black Mg2Si particles are formed.

Microstructure and mechanical properties of ADC12 composites reinforced with graphene nanoplates prepared by ultrasonic assisted casting

Microstructure and mechanical properties of ADC12 composites reinforced with graphene nanoplates(GNPs)prepared by high-intensity ultrasonic assisted casting were investigated.The results indicated that high-intensity ultrasound can promote the uniform distribution of GNPs in the melt,resulting in refining theα(Al)phase and Si phase.The optimal addition of GNPs was 0.9 wt.%,and the optimal ultrasonic time was 12 min.The tensile strength,the yield strength and the hardness of the composite produced under the optimal parameters were 256.8 MPa,210.6 MPa and HV 126.0,respectively,which increased by 30.5%,42.7%,and 34.8%compared with those of the matrix,respectively.After adding the GNPs,the fracture mechanism gradually turned from a brittle fracture to a ductile fracture.The good interface and distribution allowed GNPs to play the role in fine grain strengthening,dislocation strengthening and load transfer strengthening effectively.

Microstructure and mechanical properties of strontium-modified ADC12 alloy processed by heat treatment

The effects of heat treatment and strontium （SO addition on the microstructure and mechanical properties of ADC12 alloys were investigated, and two-stage solution treatment was introduced. The results indicated that the addition of Sr obviously refined the microstructure of ADC12 alloys. When 0.05 wt% Sr was added into the alloy, the eutectic Si phase was fully modified into fine fibrous structure; a-A1 and fl-A15FeSi phases were best refined; and the eutectic AlzCu phase was modified into block-like AlzCu phase that continuously distributed at the grain boundary. The ultimate tensile strength （UTS） （270.63 MPa） and elongation （3.19%） were increased by 51.2% and 73.4% respectively compared with unmodified alloys. After the two-stage solution treatment （500 ~C, 6 h＋520 ~C, 4 h）, for 0.05 wt% Sr modified ADC12 alloys, the Si phases transformed into fine particle structure and AlzCu phases were fully dissolved. The peak hardness value of the alloys processed by the two-stage solution treatment was increased by 8.3% and 6.8% respectively compared to solution treatment at 500 ~C and 520 ~C. After the aging treatment （175 ~C, 7 h）, the hardness and UTS were increased by 38.73% and 13.96% respectively when compared with the unmodified alloy.

Influence of cerium on microstructures and mechanical properties of Al-Zn-Mg-Cu alloys

Effect of element cerium （Ce） on microstructure and mechanical properties of A1-Zn-Mg-Cu alloys has been investigated by transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, differential scanning calorimetry （DSC） and hardness test. The results show that addition of Ce can remarkably refine the as-cast grains and eutectic microstructure. A transformation from Mg（Zn,Cu,A1）2 phase to A12CuMg phase is observed during homogenization. Furthermore, the Ce addition introduces changes in the precipitation process and consequently in the age-hardening behavior of the alloy. Microstructural measurements reveal that the addition of Ce promotes the precipitation of η＇ phase, but it also partly retards the precipitation of GP zones. The density of precipitates decreases in a certain degree and rod-like η＇ precipitates increase when Ce content is from 0.2% to 0.4% （mass fraction）.

Mechanical properties and wear resistance of medium entropy Fe40Mn40Cr10Co10/TiC composites

The Fe40Mn40Cr10Co10/TiC (volume fraction of TiC, 10%) composites were synthesized in combination of ball milling and spark plasma sintering (SPS) in the present work. Mechanical properties and wear resistance of the Fe40Mn40Cr10Co10/TiC composites were individually investigated. It was found that TiC particles homogenously distributed in the Fe40Mn40Cr10Co10/TiC composite after being sintered at 1373 K for 15 min. Meanwhile, grain refinement was observed in the as-sintered composite. Compared with the pure Fe40Mn40Cr10Co10 medium entropy alloy (MEA) matrix grain, addition of 10% TiC particles resulted in an increase in the compressive strength from 1.571 to 2.174 GPa, and the hardness from HV 320 to HV 872. Wear resistance results demonstrated that the friction coefficient, wear depth and width of the composite decreased in comparison with the Fe40Mn40Cr10Co10 MEA matrix. Excellent mechanical properties and wear resistance could offer the Fe40Mn40Cr10Co10/TiC composite a very promising candidate for engineering applications.

Microstructure and mechanical properties of FeCoCrNiMn high-entropy alloy produced by mechanical alloying and vacuum hot pressing sintering

Fe Co Cr Ni Mn high-entropy alloys were produced by mechanical alloying（MA） and vacuum hot pressing sintering（VHPS）. Results showed that the nano-crystalline alloy powders were obtained by MA and the corresponding phase structures were composed of FCC matrices and low amounts of BCC and amorphous phases. After VHPS, the BCC phases almost disappeared, simultaneously with the precipitation of σ phases and M23C6 carbides. An increase of sintering temperature resulted in grain growth of the precipitated phases. As the sintering temperature was increased from 700 to 1000℃, the strain-to-failure of the alloys rose from 4.4% to 38.2%, whereas the yield strength decreased from 1682 to 774 MPa. The bulk FeCoCrNiMn HEAs, consolidated by VHPS at 800℃ and 900℃ for 1 h, showed relatively good combination of strength and ductility.

Microstructure evolution and mechanical properties of Al−3.6Cu−1Li alloy via cryorolling and aging

An Al−3.6Cu−1Li alloy was subjected to room temperature rolling and cryorolling to investigate their effects on microstructure evolution and mechanical properties.The microstructure and aging characteristics of the room temperature-rolled and the cryorolled alloys with 70%and 90%of thickness reductions were studied by microstructure analysis and mechanical tests.The samples subjected to cryorolling with 90%of thickness reduction have high strength and good toughness.This is mainly due to the inhibition of dynamic recovery and the accumulation of high-density dislocations in cryorolled samples.In addition,the artificial aging reveals that the temperature at which peak hardness is attained is inversely proportional to the deformation amount and directly proportional to the rolling temperature.Moreover,bright field images of cryorolled samples after aging indicate the existence of T1(Al2CuLi)precipitates.This suggests that the high stored strain energy enhances the aging kinetics of the alloy,which further promotes the nucleation of T1 phases.

Effects of pre-rolling on mechanical properties and fatigue crack growth rate of 2195 Al-Li alloy

While pre-deformation is often conducted before aging treatment to increase the strength and microhardness of 2195 Al-Li alloy, it often increases the fatigue crack growth(FCG) rate and thus reduces the fatigue life of the alloy.To determine the effects and causes of pre-deformation and heat treatment on the mechanical properties and FCG rate of2195 Al-Li alloy, and to provide a suitable calculation model for the FCG rate under different pre-deformation conditions, 2195 Al-Li alloy specimens with different degrees of pre-rolling(0, 3%, 6%, and 9%) were investigated. The experimental results indicate that with the increase of pre-rolling, the density of the T1phase and the uniformity of the S′distribution and the microhardness, tensile strength, and yield strength of the alloy increase and at the same time the FCG rate increases, and thus the fatigue life is reduced. It was also found that the normalized stress intensity factor of elastic modulus(E) can be applied to correlate the FCG rate of pre-rolled 2195 Al-Li alloy with constant C and K parameters.

Comparisons of microstructure homogeneity,texture and mechanical properties of AZ80 magnesium alloy fabricated by annular channel angular extrusion and backward extrusion

This paper provided an efficient single pass severe plastic deformation(SPD)method,annular channel angular extrusion(ACAE),for fabricating AZ80 magnesium alloy shell part.The effect of ACAE process on the microstructure homogeneity,texture,and mechanical properties of extruded part was experimentally investigated.For comparison,conventional backward extrusion(BE)was also conducted on processing AZ80 part with same specification.The results showed that ACAE process has a better capacity to refine the microstructure and dramatic improve the deformation homogeneity of the extruded part than BE process.Due to two strong shear deformations were implemented,ACAE process could also concurrently modify the basal texture more notably than BE process.In particular,a bimodal texture was found in ACAE extruded part,which was greatly related to the enhanced synergetic action of basal slip and secondary<c+a>slip caused by the effective shear stress.More uniform and superior hardness along the thickness and height of part were achieved via ACAE process.Further surveying of tensile tests also showed that the part fabricated by ACAE process exhibited significantly higher and far more homogeneous tensile properties with an excellent balance of strength and ductility.The remarkable enhanced tensile properties of ACAE extruded part could be primarily attributed to the significant grain refinement,which provided a powerful grain boundary strengthening effect and meaningfully suppressed the development of twin-sized cracks during tensile deformation.It was established that ACAE process seemed to be a very promising single pass SPD method for manufacturing Mg-based alloy shell parts with more homogeneous microstructure and superior performance.

Corrosion damage evolution and mechanical properties of carbon fiber reinforced aluminum laminate

Fiber metal laminates(FMLs),a kind of lightweight material with excellent comprehensive performance,have been successfully applied in aerospace.FMLs reinforced with carbon fiber have better mechanical properties than those with glass or aramid fiber.However,carbon fiber binding metal may lead to galvanic corrosion which limits its application.In this paper,electrochemical methods,optical microscope and scanning electron microscope were used to analyze the corrosion evolution of carbon fiber reinforced aluminum laminate(CARALL)in corrosive environment and explore anti-corrosion ways to protect CARALL.The results show that the connection between carbon fiber and aluminum alloy changes electric potential,causing galvanic corrosion.The galvanic corrosion will obviously accelerate CARALL corroded in solution,leading to a 72.1%decrease in interlaminar shear strength,and the crevice corrosion has a greater impact on CARALL resulting in delamination.The reduction of interlaminar shear strength has a similar linear relationship with the corrosion time.In addition,the adhesive layers between carbon fiber and aluminum alloy cannot protect CARALL,while side edge protection can effectively slow down corrosion rate.Therefore,the exposed edges should be coated with anti-corrosion painting.CARALL has the potential to be used for aerospace components.

Simultaneously improving mechanical properties and corrosion resistance of as-cast AZ91 Mg alloy by ultrasonic surface rolling

Mg alloy casting parts commonly suffer from drawbacks of low surface properties,high susceptibility to corrosion,unsatisfactory absolute strength,and poor ductility,which seriously limit their wide application.Here,a surface nanocrystallization technique,i.e.,ultrasonic surface rolling(USR),was applied on an as-cast AZ91 Mg alloy sheet to improve its corrosion resistance and mechanical properties.The USR produces double smooth surfaces with Ra 0.036μm and gradient nanostructured surface layers on the sheet.Due to this special microstructure modification,the USR sheet exhibits 55%and 50%improvements in yield strength and ultimate tensile strength without visibly sacrificed ductility comparable to its untreated counterpart,as well as a 24%improvement in surface hardness.The USR sheet also shows good corrosion resistance in 3.5wt%NaCl aqueous solution.The corrosion current density of the USR sheet reduces by 63%after immersion for 1 h,and 25%after immersion for 24 h compared to that of the untreated counterpart.The enhanced strength and hardness are mainly related to the gradient nanostructure.The improved corrosion resistance is mainly ascribed to the decreased surface roughness,nanostructured surface,and residual compressive stress.The present results state that USR is an effective and attractive method to improve the multiple properties of Mg alloy cast-ing parts,and thus can be used as an additional and last working procedure to achieve high-performance Mg alloy casting parts.

Large nonlinear deflection behavior of IPMC actuators analyzed with an electromechanical model

This study presents an electromechanical engineering model for the analysis of the large deflection curves of ionic polymer-metal composite(IPMC)cantilever actuators under direct current(DC)voltages.In this paper,the longitudinal normal strain performance of the material was investigated using digital image correlation on a micro-scale.The deflection of the actuator is analytically obtained with the application of an elliptic integration method based on the relationship between strain gradient and excitation voltage,and the minimum excitation voltage is derived based on the assumption that the actuators have small deformations.The validity of the electromechanical model is then justified with the experimental results obtained from Pt-and Ag-IPMC actuators at various excitation voltages.The findings of this study confirm that the introduced electromechanical model can accurately describe the large nonlinear deflection behavior of IPMC actuators.

Novel Mg-Bi-Mn wrought alloys:The effects of extrusion temperature and Mn addition on their microstructures and mechanical properties

Designing and developing the Mg alloys with low cost and high performance is of the great significance.Novel Mg-1Bi-xMn(x=0,1and 2 wt.%)extruded alloys,in this work,were fabricated at different extrusion temperatures(220,250 and 300℃).The effects of extrusion temperature and Mn addition on the microstructures and mechanical properties of extruded alloys at room temperature were investigated.The results showed that decreasing the extrusion temperature could refine the average grain size,weaken the basal fiber texture intensity and improve the microstructural homogeneity of extruded alloys.When the Mn element was added to the Mg-1Bi alloy,the average grain size further reduced.Simultaneously,the number fraction of low angle grain boundaries(LAGBs)increased,along with the occurrence of regions without dynamic recrystallization(unDRX).The combined effects of grain refinement and coarse unDRXed structure made the textures of the extruded Mg-1Bi-xMn alloys never obviously change.Besides few large size un-dissolved second phases,fine Mg_(3)Bi_(2) and α-Mn phases were precipitated in the extruded Mg-1Bi-xMn alloys and partial nano-scaleα-Mn particles pined at grain boundaries(GBs)to effectively impede the migration of GBs for grain refinement.Microstructural variations determined the extruded Mg-1Bi-2Mn alloy to exhibit the highest yield strength of~319.2 MPa with the appropriate elongation-to-failure of~13%at the extrusion temperature of 220℃,and they enabled the extruded Mg-1Bi-1Mn alloy to show the highest elongation-to-failure of~26%without the obvious loss of yield strength of~252.1 MPa.

Analysis of isothermal forging process and mechanical properties of complex aluminum forging for aviation

Large complex 7A85 aluminum wing-body joint was forged employing isothermal forging process and its mechanical properties were studied.The tensile strength after forging is up to 587.5 MPa in longitudinal direction,15% higher than that using free forging.Moreover,the tensile strength of the forging is almost the same in three directions.Isothermal forging also performs well on overall fracture toughness,with a maximum value of 39.8 MPa·m1/2,and that of short transverse direction all reaches 36 MPa·m1/2 and above,with a maximum relative error of only 3.6%.The results indicate that the isothermal forging leads to better performance as well as higher uniformity in mechanical properties.

Precipitation behaviors and mechanical properties of a solution-treated Mg-Gd-Nd-Zn-Zr alloy during equal-channel angular pressing process

A magnesium alloy processed by equal-channel angular pressing (ECAP) exhibited excellent microstructure refinement and improved strength and hardness.The comprehensive mechanical properties of magnesium alloys have supported the expansion of their applications in the automotive,aerospace,and biomedical industries.Herein,pre-treatment of a solution-treated Mg-2.9Gd-1.5Nd-0.3Zn-0.3Zr alloy was conducted to investigate the precipitate behavior and microstructure evolution during the ECAP process.β;phase grains quickly precipitated from the solution-treated alloy,which accelerated grain refinement and enhanced the ductility after the ECAP process,as compared to the as-cast alloy reported in our previous study.Moreover,spherical precipitates (~200 nm) and fine phases (~100 nm) precipitated along the stripe-like Zn;Zr;phase,which formed a kabap-like structure dispersing homogeneously in the solution-treated alloy during the ECAP process.Owing to grain refinement,dislocations,sphericalβ;precipitates,and texture evolution,the solution-treated alloy after eight passes of ECAP exhibited good comprehensive mechanical properties,with the ultimate tensile strength,yield strength,and elongation values reaching210.9 MPa,263.9 MPa,and 27.9%,respectively.

Microstructure evolution and its effect on mechanical properties of cast Ti48Al6NbxSi alloys

In order to improve mechanical properties of TiAlNb alloys,different contents of silicon were added into Ti48Al6Nb alloy.The Ti48Al6NbxSi (x=0,0.1,0.2,0.3,0.4 and 0.5,at.%) alloys were prepared by vacuum arc melting.The phase constitution,microstructure evolution and mechanical properties of the alloys were studied.Results show that the Ti48Al6NbxSi alloys consist of γ-TiAl phase,α2-Ti3Al phase and B2 phase,and Ti5Si3 silicide phase is formed when the addition of silicon is higher than 0.3at.%.The addition of silicon leads to the decrease in γ phase and increase in α2 phase.The formation of silicide decreases the amount of Nb dissolved in the TiAl matrix,and therefore decreases B2 phase.Compressive tests show that the ultimate strength of the alloys increases from 2,063 MPa to 2,281 MPa with an increase in silicon from 0 to 0.5at.%,while the fracture strain decreases from 34.7% to 30.8%.The increase of compressive strength and decrease of fracture strain can be attributed to the decrease of B2 phase and the formation of Ti5Si3 phase by the addition of silicon.The strengthening mechanism is changed from solid solution strengthening when the addition of silicon is less than 0.3at.% to combination of solid solution strengthening and secondary phase strengthening when the addition of silicon is higher than 0.3at.%.

Enhanced mechanical properties of Mg-3Al-1Zn alloy sheets through slope extrusion

A novel extrusion approach,entitled slope extrusion(SE),was employed to manufacture AZ31(Mg-3Al-1 Zn,wt%)alloy sheets.The microstructures,textures,and mechanical properties were investigated,compared with those of the AZ31 sheet fabricated by conventional extrusion(CE).Through the combination of finite element simulation and actual experiment,the ultimate results indicated that significant grain refinement(from 9.1 to 7.7 and 5.6μm)and strong basal texture(from 12.6 to 17.6 and 19.5 mrd)were achieved by the SE process.The essence was associated with the additional introduced inclined interface in the process of SE,which could bring about more asymmetric deformation and stronger accumulated strain along the ND when compared with the process of CE.As a consequence,the SE sheets exhibited a higher yield strength(YS)and ultimate tensile strength(UTS)than the counterparts of the CE sheet,which was mainly assigned to the synergistic effects from grain refining and texture strengthening.