Synergistic enhancement on mechanical properties and corrosion resistance of biodegradable Mg-Zn-Y alloy via V-microalloying

For the sake of improving the mechanical properties and corrosion resistance of biodegradable Mg alloy synergistically,various content of element V(0,0.05,0.10,0.15,0.20 wt.%)are introduced into an Mg-Zn-Y alloy with long-period stacking ordered(LPSO)structure,and the effects of V on its microstructure,mechanical properties and corrosion resistance are investigated systematically.The results indicate that the grains are effectively refined by V addition,and the primaryα-Mg in Mg-Zn-Y-V0.1 alloy is most significantly refined,with grain size being decreased by 62%.The amount of 18R LPSO structure is increased owing to the V addition.The growth mode of the second phase(W-phase and 18R LPSO structure)is transformed to divorced growth pattern,which ascribes to the thermodynamic drive force of V to promote the nucleation of LPSO phase.Thus,18R LPSO structure presents a continuous distribution.Due to grains refinement and modification of second phase,the tensile strength and strain of alloys are both enhanced effectively.Especially,the ultimate tensile strength and the elongation of V0.1 alloy are 254 MPa and 15.26%,which are 41%and 61%higher than those of V-free alloy,respectively.Owing to the continuously distributed 18R LPSO structure with refined grains and stable product film,the weight loss and hydrogen evolution corrosion rates of V0.1 alloy are 7.1 and 6.2 mmy^(-1),respectively,which are 42.6%and 45.4%lower than those of V-free alloy.

Enhancing the mechanical properties of casting eutectic high -entropy alloys via W addition

The effect of W element on the microstructure evolution and mechanical properties of Al_(1.25)CoCrFeNi3 eutectic high-entropy alloy and Al_(1.25)CoCrFeNi_(3-x)W_(x)(x=0,0.05,0.1,0.3,and 0.5;atomic ratio)high-entropy alloys(HEAs)were explored.Results show that the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs are composed of face-centered cubic and body-centered cubic(BCC)phases.As W content increases,the microstructure changes from eutectic to dendritic.The addition of W lowers the nucleation barrier of the BCC phase,decreases the valence electron concentration of the HEAs,and replaces Al in the BCC phase,thus facilitating the nucleation of the BCC phase.Tensile results show that the addition of W greatly improves the mechanical properties,and solid-solution,heterogeneous-interface,and second-phase strengthening are the main strengthening mechanisms.The yield strength,tensile strength,and elongation of the Al_(1.25)CoCrFeNi2.95W0.05 HEA are 601.44 MPa,1132.26 MPa,and 15.94%,respectively,realizing a balance between strength and plasti-city.The fracture mode of the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs is ductile–brittle mixed fracture,and the crack propagates and initiates in the BCC phase.The eutectic lamellar structure impedes crack propagation and maintains plasticity.

A novel method towards improving the hydrogen storage properties of hypoeutectic Mg-Ni alloy via ultrasonic treatment

Ultrasonic treatment has great contributions on modifying the morphology,dimension and distribution of constituent phases during solidification,which serve as dominate factors influencing the hydrogen storage performance of Mg-based alloys.In this research,ultrasonic treatment is utilized as a novel method to enhance the de-/hydriding properties of Mg-2Ni(at.%)alloy.Due to ultrasonic treatment,the microstructure of as-cast alloy is significantly refined and homogenized.Ascribing to the increased eutectic boundaries and shortened distance insideα-Mg for hydrogen atoms diffusion,the hydrogen uptake capacities and isothermal de-/hydriding rates improve effectively,especially at lower temperature.The peak desorption temperature reduces from 392.99°C to 345.56°C,and the dehydriding activation energy decreases from 101.93 k J mol^(-1)to 88.65 k J mol^(-1).Weakened hysteresis of plateau pressures and slightly optimized thermodynamics are determined from the pressure-composition isotherms.Owing to the refined primary Mg,a larger amount of hydrogen with the higher hydriding proportion is absorbed in the first stage when hydrides nucleate in eutectic region and grow on primary Mg periphery subsequently before MgH2colonies impinging,resulting in the enhancement of hydrogenation rates and capacities.

The mechanism of hot crack formation in Ti-6Al-4V during cold crucible continuous casting

Hot crack is one of common defects in castings, which often results in failure of castings. This work studies the formation of hot crack during cold crucible continuous casting by means of experiments and theoretical analysis. The results show that hot crack occurs on the surface and in the circumference of ingots, where the solidified shell and the solidification front meet each other. The tendency of hot cracking decreases with the increase of withdrawal velocities in some extent. The hot crack is caused mainly by the friction force between the shell and the crucible inner wall, and it takes place when the stress resulting from friction exceeds the tensile strength of the shell. The factors of μ_m, η_t, η_s and η_m, affecting hot cracks are analyzed and verified. In order to decrease the tendency of hot cracks, technical parameters should be optimized by decreasing μ_m, η_t, η_s and η_m.

Investigations about the Atomic Structure and Mechanical Behavior of Metallic Glasses after Melt Hydrogenation

“Hydrogen in metallic glasses”has become a popular topic for material scientists,yet few studies focus on the atomic⁃scale details.Herein,by utilizing molecular dynamic simulations,the changes on the atomic structure of Cu50Zr50 metallic glasses after melt hydrogenation were systematically analyzed,with the aim of understanding the differences of mechanical behavior between these amorphous alloys.The simulated analyses indicate that the hydrogenated samples become more compact than the H⁃free one,but the fraction of the dominant coordination polyhedra with higher degree of local fivefold symmetry significantly decreases accompanied by the addition of H atoms.Accordingly,melt hydrogenation can induce much more local“soft spots”in metallic glasses to alleviate the degree of strain localization during deformation,i.e.,it has a positive influence on the deformability of glassy alloys in agreement with experimental results.

Effects of Rapid Cooling Rate on Microstructure Formation and Microhardness of Binary Ti⁃44Al Alloy

In order to refine microstructure grains and improve mechanical properties of TiAl alloys,Ti44Al(at.%)alloy was rapidly solidified by melt spinning under different cooling rates.Microstructure and microhardness of the alloy before and after rapid solidification were investigated.XRD results show that the ratio ofα2 phase in binary alloy increased with the cooling rates,which is caused by moreαphases directly transforming toα2 phases.Grain morphology changed from long dendrite to the mixture of equiaxed and dendrite to equiaxed with the increase of cooling rates.The grain size was refined from 200-600μm of as⁃cast to 18μm of the alloy cooled at 4.9×10^5 K/s,which is caused by the undercooling induced from rapid solidification.Lamellar spacing was decreased from 4.5μm of as⁃cast to 1.1μm by rapid solidification.With the increase of cooling rate,the content ofα2 phase increased andγphase decreased gradually.Rapid solidification can reduce the segregation of elements.The microhardness was improved from 247 HV to 556 HV,which results from grain refinement strengthening,reduction of lamellar spacing,and more content ofα2 phase.

Microstructure Recrystallization and Mechanical Properties of a Cold-Rolled TiNbZrTaHf Refractory High-Entropy Alloy

The equiatomic TiNbZrTaHf alloy was successfully rolled at room temperature with the reduction of ~ 85%. The microstructure and tensile properties were investigated after cold working and annealing. It was determined that the recrystallization temperature of the TiNbZrTaHf alloy between 850 and 900 ℃. Complete recrystallization and normal grain growth occurred, the high stability of single phase was maintained after annealing at 1000, 1200, and 1400 ℃. But the precipitated phase appeared after long term annealing, as seen after 500 h at 1000 ℃. After cold working, the tensile strength and the elongation of TiNbZrTaHf were 1137 MPa and 25.1%, respectively. The annealed alloy has a high tensile strength (σ_(b )= 863 MPa) and ductility (ε_(e )= 28.5%). Moreover, the oxidation of TiNbZrTaHf alloy at elevated temperatures has a significant impact on its mechanical properties. The poor oxidation resistance of TiNbZrTaHf can accelerate tensile failure by inducing fractures at grain boundaries.

Morphological modification of Mg_(2)Si phase and strengthening mechanism in Mg_(2)Si/Al composites by Eu addition and T6 heat treatment

In order to improve mechanical properties of Mg_(2)Si/Al composites,Eu element was added to mod-ify Mg_(2)Si morphology,and T6 heat treatment was used to control Mg_(2)Si morphology and precipitate strengthening phase.Microstructure and morphology of Mg_(2)Si were observed by synchrotron X-ray to-mography and TEM,and first-principles calculations were also performed to testify the effect of Eu mod-ification.Results show that the size of primary Mg_(2)Si particles decreases and eutectic Mg_(2)Si phase trans-forms from lamellar to fibrous by Eu addition.After heat treatment,sharp angles of primary Mg_(2)Si par-ticles passivate,and eutectic Mg_(2)Si dissolves and appears to be short dot-like.Meanwhile,nano-sizedβ"phase precipitates in the matrix.For morphology of Mg_(2)Si with Eu modification,TEM results show that Eu impedes the growing of Mg_(2)Si,which is verified by first-principles calculations that Eu atom preferentially adsorbs on Mg_(2)Si{100}facet.The adsorption and suppression growing of Mg_(2)Si transform the morphology of Mg_(2)Si and thus improve the elongation.UTS and elongation of the heat-treated Eu modified composites are 281 MPa and 8.4%,which improved 81%and 200%compared to the as-cast Al-15%Mg_(2)Si composite.The strengthening mechanism mainly results from precipitation strengthening of nano-sizedβ"precipitates in the heat-treated composite.

Strengthening FCC-CoCrFeMnNi high entropy alloys by Mo addition

In order to strengthen the face-centered-cubic(FCC) type CoCrFeMnNi high entropy alloys(HEAs), different contents of Mo(0–16 at.%, similarly hereinafter) were alloyed. Phase evolution, microstructure,mechanical properties and related mechanism of these HEAs were systematically studied. The results show that sigma phase is appeared with addition of Mo, and the volume fraction of it increases gradually from 0 to 66% with increasing Mo content. It is found that Mo is enriched in sigma phase, which indicates that Mo element is beneficial to form sigma phase. Compressive testing shows that the yield strength of the alloys increases gradually from 216 to 765 MPa, while the fracture strain decreases from 50%(no fracture) to 19% with increasing of Mo. The alloy exhibits the best compressive performance when Mo content reaches 11%, the yield strength, fracture strength and fracture strain are 547 MPa, 2672 MPa and44% respectively. The increased volume fraction of sigma phase plays an important role in improving the compressive strength of(CoCrFeMnNi)_(100-x)Mo_xHEAs.

NbMoTiVSi_(x) refractory high entropy alloys strengthened by forming BCC phase and silicide eutectic structure

In order to improve mechanical properties of refractory high entropy alloys,silicide was introduced and NbMoTiVSi_(x)(x=0,0.1,0.2,0.3,and 0.4,molar ratio) refractory high entropy alloys are prepared by vacuum arc melting.Phase composition,micro structure evolution and mechanical properties were systematically studied.Results show that the silicide phase is formed in the alloys with addition of silicon,and the volume fraction of silicide increases from 0 to 8.3 % with increasing of silicon.Microstructure observation shows that the morphology of dendrite changes from columnar to near equiaxed,eutectic structure is formed at grain boundaries and composed of secondary BCC phase and silicide phase.The average length of the primary and second dendrites decreases with the increasing of silicon.Whereas,the ratio of eutectic structure increases from 0 to 19.8 % with the increment of silicon.The refinement of microstructure is caused by heterogeneous nucleation from the silicide.Compressive tests show that the yield and ultimate strength of the alloys increases from 1141.5 MPa to 2093.1 MPa and from 1700.1 MPa to 2374.7 MPa with increasing silicon content.The fracture strain decreases from 24.7 %-11.0 %.Fracture mechanism is changed from ductile fracture to ductile and brittle mixed fracture.The improvement of the strength is caused by grain bounda ry strengthening,which includes more boundaries around primary BCC phase and eutectic structure in grain boundary,both of them is resulted from the formation of silicide.

Microstructure evolution, Cu segregation and tensile properties of CoCrFeNiCu high entropy alloy during directional solidification

CoCrFeNiCu(equiatomic ratio)samples(?8 mm)were directionally solidified at different velocities(10,30,60 and 100μm/s)to investigate the relationship between solidification velocity and microstructure formation,Cu micro-segregation as well as tensile properties.The results indicate that the morphology of the solid-liquid(S-L)interface evolves from convex to planar and then to concave with the increase of solidification velocity.Meanwhile,the primary and the secondary dendritic arm spacings decrease from100μm to 10μm and from 20μm to 5μm,respectively.They are mainly influenced by the axial heat transfer and grain competition growth.During directional solidification,element Cu is repelled from the FCC phase and accumulates in the liquid owe to its positive mixing enthalpy with other elements.Tensile testing results show that the ultimate tensile strength(UTS)gradually increases from 400 MPa to 450 MPa,and the strain of the specimen prepared at the velocity of 60μm/s is higher than those of others.The fracture mode of all specimens is the mixed fracture containing both ductile fracture and brittle fracture,in which ductile fracture plays a fundamental role.In addition,the brittle fracture is induced by Cu segregation.The improvement of UTS is resulted from columnar grain boundary strengthening.

An innovative method for the microstructural modification of TiAl alloy solidified via direct electric current application

Ti-48Al-2Cr-2Nb alloy solidified with the application of direct electric current has a refined and homogeneous microstructure without segregation. We observed an initial decrease followed by a subsequent increase in grain size and lamellar spacing, with the increase in current density. Similar trend can also be obtained by varying the amount of α2-phase(Ti_3Al). Using a directional solidification processing method,the columnar crystal microstructure transforms into an equiaxed crystal microstructure at a current density of 32–64 m A/mm^2. High dislocation density is also introduced with a minimum cross-sectional grain size of 460 μm at a current density of 64 mA/mm^2. The application of electric current alters the free energy of the critical nucleus and temperature via joule heating, causing a transformation from a columnar grain microstructure into an equiaxed grain microstructure. The increase in current density leads to a rise of the nucleation rate, and a resulting undercooling combined with temperature gradient contribute to growth of the primary phase, which finally results in grain coarsening at a critical current density of 96 mA/mm^2.The climb and cross-slip of dislocation and the migration of grain boundary ultimately create variable lamellae spacing of TiAl alloy.

Rolling reduction-dependent deformation mechanisms and tensile properties in aβtitanium alloy

This work investigated the dependence of deformation mechanisms and mechanical properties on cold rolling reductions of the metastable TB8 titanium alloy.Results shown that the crystal orientation of the matrix changes with the increasing level of reduction,leading to the activation of complex deformation mechanisms in the matrix.When the rolling reduction is 10%,the deformation mechanisms are dominated by dislocations and{332}<113>deformation twins.As the reductions increase to 20%-50%,the secondary deformation twinning(SDT)is triggered in primary deformation twins besides the primary kink band is activated.Meanwhile,the secondary kink bands and{332}<113>twins have be observed in the primary kink bands.When the reduction reaches to 60%,the deformation mechanisms are dominated by dislocations and deformation twins.Furthermore,the matrix refined by crisscrossing among the twins,kink bands and other deformation mechanisms during cold rolling,which shortens the dislocation mean free path and then affects the strength and shape of the alloy.The dynamic Hall-Petch effect and the interaction between multi-scale deformed structures control the work hardening behavior of the alloy.

Using multiple regression analysis to predict directionally solidified TiAl mechanical property

The mechanical properties of TiAl alloy prepared by directional solidification were predicted through a machine learning algorithm model.The composition,input power,and pulling speed were designated as input variables as representative factors influencing mechanical properties,and multiple linear regression analysis was conducted by collecting data obtained from the literature.In this study,the R^(2)value of the tensile strength prediction result was 0.7159,elongation was 0.8459,nanoindentation hardness was 0.7573,and interlamellar spacing was 0.9674.As the R^(2)value of the elongation obtained through the analysis was higher than the R^(2)value of the tensile strength,it was confirmed that the elongation had a closer relationship with the input variables(composition,input power,pulling speed)than the tensile strength.By adding the elongation to the tensile strength as an input variable,it was observed that the R^(2)value was further increased.The tensile test prediction results were divided into four groups:The group with the lowest residual value(predicted value-actual value)was designated as group A,and the group with the largest residual value was designated as group D.When comparing the values of group A and group D,more overpredictions occurred in group A,while more under predictions occurred in group D.Using the residuals and R^(2)values,the cause of the well-prediction was studied,and through this,the relationship between the mechanical properties and the microstructure was quantitatively investigated.

Twin and twin intersection phenomena in a creep deformed microalloyed directionally solidified high Nb containing TiAl alloy

The creep behavior of a directionally solidified TiAl alloy with a high Nb content after microalloying by the addition of small amounts of W,Cr,and B elements was investigated by scanning electron microscopy and transmission electron microscopy.By means of directional solidification and microalloying,a TiAl alloy with a fine and uniform microstructure and continuous columnar crystals was obtained.High-density dislocations,deformation nanotwins,and twin intersections were observed inγlamellae.The results show that,in comparison with the ternary TiAl alloy,the microalloyed high Nb containing TiAl alloy exhibited better creep properties at 760℃and 275 MPa.The decrease in stacking fault energy can promote dislocation dissociation and the formation of deformation twins,and the twin intersections can hinder the movement of dislocation to enhance the creep performance of the TiAl alloy.

Reducing porosity and optimizing performance for Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Porosity is a major casting defect in alloys with large solidification intervals due to the disordered microstructure and broad mushy zones,which decreases badly the mechanical performance.Hence,finding ways to effectively reduce the porosity,further to optimize microstructure and mechanical performance is of great significance.In this regard,the Al-Cu-based alloys with large solidification intervals are continuously processed by coupling the travelling magnetic fields(TMF)with sequential solidification.Additionally,experiments combined with simulations are utilized to comprehensively analyze the mechanism of TMF on the reduction in porosity,including shrinkage porosity and gas porosity,from different perspectives.Current findings determine that downward TMF can effectually optimize together the porosity,microstructure and performance,by inducing the strong long-range directional melt flows,stabilizing the mushy zones,and optimizing the feeding channels and exhaust paths,as well as increasing the driving force of degassing process.Eventually,downward TMF can increase the ultimate tensile strength,yield strength,elongation and hardness from 175.2 MPa,87.5 MPa,13.3%and 80.2 kg mm^(-2) without TMF to 218.6 MPa,109.3 MPa,15.6%and 95.5 kg mm^(-2),while reduce the total porosity from0.95%to 0.18%.However,Up-TMF exerts negative effects on the optimization of porosity,microstructure and performance due to the opposite strong directional magnetic force and melt flows.Overall,our study provides an effective way to optimize together the porosity,microstructure and mechanical performance,and reveals their relationship,as well as details the relevant mechanisms of TMF on the porosity reduction from different perspectives.

Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Alloys with large solidification intervals are prone to issues from the disordered growth and defect formation;accordingly, finding ways to effectively optimize the microstructure, further to improve the mechanical properties is of great importance. To this end, we couple travelling magnetic fields with sequential solidification to continuously regulate the mushy zones of Al-Cu-based alloys with large solidification intervals. Moreover, we combine experiments with simulations to comprehensively analyze the mechanisms on the optimization of microstructure and properties. Our results indicate that only downward travelling magnetic fields coupled with sequential solidification can obtain the refined and uniform microstructure, and promote the growth of matrix phase -Al along the direction of temperature gradient.Additionally, the secondary dendrites and precipitates are reduced, while the solute partition coefficient and solute solid-solubility are raised. Ultimately, downward travelling magnetic fields can increase the ultimate tensile strength, yield strength, elongation and hardness from 196.2 MPa, 101.2 MPa, 14.5 % and85.1 kg mm-2 without travelling magnetic fields to 224.1 MPa, 114.5 MPa, 17.1 % and 102.1 kg mm-2,and improve the ductility of alloys. However, upward travelling magnetic fields have the adverse effects on microstructural evolution, and lead to a reduction in the performance and ductility. Our findings demonstrate that long-range directional circular flows generated by travelling magnetic fields directionally alter the transformation and redistribution of solutes and temperature, which finally influences the solidification behavior and performance. Overall, our research present not only an innovative method to optimize the microstructures and mechanical properties for alloys with large solidification intervals,but also a detailed mechanism of travelling magnetic fields on this optimization during the sequential solidification.

Enhanced strength and ductility in Ti46Al4Nb1Mo alloys via boron addition

To improve the strength and ductility of Ti Al alloys by second phase, Ti46 Al4 Nb1 Mo alloys doped with different B content(0.4%, 0.8%, 1.2%, 1.6% and 2.0%, atomic percent, hereafter in at.%, referred to as TNMx B) were prepared. Macro/microstructure evolution, mechanical properties and deformation mechanisms of the alloys were studied systematically. Results showed that the microstructure of TNM-0.4 B and TNM-0.8 B alloy remained columnar dendrites, and the secondary dendritic arms of columnar grains were more obvious. When the content of B is 1.2%, the columnar dendrites transformed to equiaxed grains, and theα2/γ lamellae colony size was further refined in TNM-1.6 B and TNM-2.0 B alloy. The morphologies and kinds of the borides were changed with increasing B content, XRD results showed that Ti B phase appeared with 1.6%B content, and both Ti B and Ti B2 phase formed in TNM-2.0 B alloy. There were straight and curved Ti B phases located around grain boundaries in TNM-0.4 B and TNM-0.8 B alloy, and when the content of B increased to 1.2%, the curved Ti B phases were reduced, while the tiny and straight Ti B phases increased. With further increasing B content to 1.6% and 2.0%, the tiny and straight Ti B phases were coarser. Compressive testing results showed that the mechanical properties of the TNM alloy were enhanced with increasing B content. The maximum strength and strain of TNM alloy were 2339 MPa and33.7% with 1.6% B addition. The compressive strength and strain were mainly enhanced via refinement of lamellar colony and formation of Ti B, and it is found that pile-up of dislocations and deformed twins promoted by Ti B are predominant in improving the mechanical properties of TNM alloys with higher strength and strain.

Microstructures and mechanical properties of Nb-alloyed CoCrCuFeNi high-entropy alloys

Nb has a positive effect on improving the mechanical properties of metal materials, and it is expected to strengthen CoCrCuFeNi high-entropy alloys （HEAs） with outstanding ductility and relatively weak strength. In this paper, the alloying effects of Nb on the microstructural evolution and the mechanical properties of the （CoCrCuFeNi）100-xNbx HEA were investigated systematically. The result shows that Nb promotes the phase transition from FCC （face-centered cubic） to Laves phase, and the volume fractions of Laves phase increase from 0% to 58.2% as the Nb content increases, Compressive testing shows that the addition of Nb has a positive effect on improving the strength of CoCrCuFeNi HEA. The compressive yield strength of （CoCrCuFeNi）100-xNbx HEAs increases from 338 MPa to 1322 MPa and the fracture strain gradually reduces from 60.0% （no fracture） to 8.1% as the Nb content increases from 0 to 16 at.%. The volume fraction increase of hard Laves phase is the key factor for the strength increase, and the reduction of the VEC （valence electron concentration） value induced by the addition of Nb is beneficial for the increase of the Laves phase content in these alloys.

Effects of Tantalum on Microstructure Evolution and Mechanical Properties of High-Nb TiAl Alloys Reinforced by Ti2AlC

Experiments have been carried out to study the relationship between the addition of tantalum and microstructure,especially the formation of the B2 phase in lamellar colonies.The mechanical properties,with different contents of Ta,were also measured.Ti46Al8Nb2.6CxTa alloys were prepared by casting with the content of Ta varying from zero to 1.0 at.%.Experimental results show that the B2 phase forms in lamellar colonies with the addition of Ta,and its content increases when the content of Ta increases.Meanwhile,theγphase decreases and the lattice parameter of theα2 phase increases.The size of the lamellar colony decreased from 29.9 to 21.6μm.Ta dissolves into Ti_(2)AlC by substitution,and its solubility is more than 1.1%tested by EDS.Nb,which is necessary for the formation of the B2 phase,comes from two aspects.The first is that Ta dissolves into the Ti_(2)AlC and partly replaces the Nb atom and the second is the decrease in theγphase because it has higher solid solubility for Nb.The increase in Nb in the liquid phase increases the composition supercooling and heteronucleation at the solidification front,which accounts for refining the lamellar colony.Room temperature compressive testing showed that the compressive strength and the strain increased when the Ta content increased up to 0.8%and then decreased.Improvement of the compressive properties resulted from the grain boundary strengthening and their decrease induced by more content of the B2 phase.Tensile properties,at elevated temperature,were improved with testing temperature increasing from 750 to 950℃,because solid solution strengthening is a major influence factor.