Effects of Temperature and Disperse Phase of Disordered γ on Strain Hardening Rate of Ni_3Al

Investigations have been made on the effects of temperature and fine disperse phase of disordered γ on strain hardening rate of Ni3Al based alloy. The result is found that there exists a peak temperature for the strain hardening rate of Ni3Al based alloy below the peak temperature of its yield strength. Analysis shows that the appearance of the peak temperature of strain hardening rate is caused by both the decreasing of the movability of <101> superdislocations on {111}slip plane and the increasing of the dynamic recovery in Ni3Al with increasing temperature. A Ni3Al based alloy hardened by disperse phase of disordered γ has been obtained by controlling chemical composition and treating processes. The peak temperature of strain hardening rate of this alloy is increased due to the fine disperse phase of disordered γ, which causes the reductions of the movability of the superdislocations and the dynamic recovery in Ni3Al

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys(BCC-HEAs).In this study,we report a novel dynamic strengthening mechanism,martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA(TiZrHf)Ta(at.%)that can profoundly enhance the work hardening capability,leading to a large uniform ductility and high strength simultaneously.Different from conventional transformation induced plasticity(TRIP)and twinning induced plasticity(TWIP)strengthening mechanisms,the martensitic twinning transformation strengthening mechanism combines the best characteristics of both TRIP and TWIP strengthening mechanisms,which greatly alleviates the strengthductility trade-off that ubiquitously observed in BCC structural alloys.Microstructure characterization,carried out using X-ray diffraction(XRD)and electron back-scatter diffraction(EBSD)shows that,upon straining,α”(orthorhombic)martensite transformation,self-accommodation(SA)α”twinning and mechanicalα”twinning were activated sequentially.Transmission electron microscopy(TEM)analyses reveal that continuous twinning activation is inherited from nucleating mechanical{351}type I twins within SA“{351}”<■11>typeⅡtwinnedα”variants on{351}twinning plane by twinning transformation through simple shear,thereby accommodating the excessive plastic strain through the twinning shear while concurrently refining the grain structure.Consequently,consistent high work hardening rates of 2–12.5 GPa were achieved during the entire plastic deformation,leading to a high tensile strength of 1.3 GPa and uniform elongation of 24%.Alloy development guidelines for activating such martensitic twinning transformation strengthening mechanism were proposed,which could be important in developing new BCC-HEAs with optimal mechanical performance.

Flow characteristics and hot workability of a typical low-alloy high-strength steel during multi-pass deformation

Heavy components of low-alloy high-strength(LAHS) steels are generally formed by multi-pass forging. It is necessary to explore the flow characteristics and hot workability of LAHS steels during the multi-pass forging process, which is beneficial to the formulation of actual processing parameters. In the study, the multi-pass hot compression experiments of a typical LAHS steel are carried out at a wide range of deformation temperatures and strain rates. It is found that the work hardening rate of the experimental material depends on deformation parameters and deformation passes, which is ascribed to the impacts of static and dynamic softening behaviors. A new model is established to describe the flow characteristics at various deformation passes. Compared to the classical Arrhenius model and modified Zerilli and Armstrong model, the newly proposed model shows higher prediction accuracy with a confidence level of 0.98565. Furthermore, the connection between power dissipation efficiency(PDE) and deformation parameters is revealed by analyzing the microstructures. The PDE cannot be utilized to reflect the efficiency of energy dissipation for microstructure evolution during the entire deformation process, but only to assess the efficiency of energy dissipation for microstructure evolution in a specific deformation parameter state.As a result, an integrated processing map is proposed to better study the hot workability of the LAHS steel, which considers the effects of instability factor(IF), PDE, and distribution and size of grains. The optimized processing parameters for the multi-pass deformation process are the deformation parameters of 1223–1318 K and 0.01–0.08 s^(-1). Complete dynamic recrystallization occurs within the optimized processing parameters with an average grain size of 18.36–42.3 μm. This study will guide the optimization of the forging process of heavy components.

Characterization of Tensile Strain Hardening Behaviors for 32Mn-7Cr-1Mo-0.3N Cryogenic Austenitic Steel

The strain hardening behaviors of 32Mn-TCr-1Mo-0.3N austenitic steel were characterized by a simple and effective method. The results show that Hollomon relationship is not applicable during total uniform deformation stage. The flow equation was proposed, Inσ=αexp（lnε/b）＋c. The variation rates of strain hardening exponents with true strain at 77 K are obviously higher than that at other temperatures and the value of d^2σ/dε^2 becomes positive during high strain stage. The characters of this variation are principal reasons for increasing elongation at 77 K. The forming of mechanical twin as well as ε-martensite leads to a high elongation at 77 K.

Energy absorption characteristics of novel high-strength and hightoughness steels used for rock support

Nowadays,the development of novel metallic materials for rock support have attracted research interests since they can significantly improve the deformation and energy absorption capacities of rock bolts.Although previous studies proved the importance and mechanical advantages of utilizing high-strength and high-toughness(HSHT)steels in rock support,there is no systematic analysis to reveal the essential energy absorption parameter and the guidelines for further development of metallic rock support materials.This paper analyzes the energy absorption characteristics of novel HSHT steels(negative Poisson’s ratio(NPR)and twinning-induced plasticity(TWIP)steels)in comparison with conventional rock support materials.A physically based crystal plasticity(CP)model was set up and calibrated to study the effect of strain hardening rate(SHR).Meanwhile,the roles of underlying physical mechanisms,i.e.the dislocation density and twin volume fraction,were studied.The results show that the improvement of energy absorption density(EAD)is essential for further development of rock support materials,besides the increase of energy absorption rate(EAR)for previous development of conventional rock support materials.The increase of EAD requires increases of both strength and deformation capacity of materials.For HSHT steels,the decrease of SHR has a positive effect on the improvement of EAD.In addition,the increase of EAD is followed by the increase of twin volume fraction and the decrease of plastic Poisson’s ratio which can promote deformation plasticity of materials.Meanwhile,the increase of EAR is correlated with the accumulation of dislocation density,which can increase the strength of materials.This paper provides the theoretical basis and guidelines for developing rock support materials in deep underground engineering and other related fields.

Grain refinement and mechanical properties of pure aluminum processed by accumulative extrusion bonding

Ultrafine-grained aluminum processed by a new severe plastic deformation technique, accumulative extrusion bonding (AEB), was investigated. Microstructural characterization indicated good interfacial bonding and an average grain size of ~440 nm was obtained after six passes. Tensile testing revealed that the strength reached the maximum value of 195 MPa and the total elongation exceeded 16% after five passes. The hardness was also significantly improved and almost reached saturation after the first pass. SEM fractography of AEB-processed specimens after tensile test showed that failure mode was shear ductile fracture with elongated shallow dimples. Comparison with conventional accumulative roll bonding indicates that this new AEB technique is more effective in refining grain and improving mechanical properties of the specimens.

Influence of grain size on the tensile ductility and deformation modes of rolled Mg–1.02 wt.%Zn alloy

The influence of grain size on the tensile deformation and ductility for Mg–1.02%Zn(wt.%)alloy was investigated.The uniform elongation is nearly insensitive to the increase of grain size,but the post-uniform elongation is significantly decreased with increasing grain size.The high ductility in the fine-grained samples is due to the lower frequency of twins and increased dynamic recovery from the enhanced activation of prismatic<a>slip.

Comparative Study on Constitutive Modeling of Tantalum and Tantalum Tungsten Alloy

As a model bee metal, tantalum and its alloys have wide applications in defense-related fields. The KHL （Khan, Huang, Liang, 1999） model and the constitutive model proposed by Nemat-Nasser et al （Nemat-Nasser and Kapoor, 2001） for tantalum and its alloys were analyzed and compared with each other. A set of published data recorded during elastic-plastic deformations of tantalum, tantalum alloy containing tungsten of 2.5% （Ta-2.5W）, over a wide range of strains, strain rates, and temperatures were used to correlate the two models. Overall, it can be concluded that KHL model correlates much better with the data than the model used by Nemat-Nasser et al.

Comparison between the Methods of Determining the Critical Stress for Initiation of Dynamic Recrystallization in 316 Stainless Steel

Several methods have been proposed to calculate the critical stress for initiation of dynamic recrystallization （σc） on the basis of mathematical methods. One＇ of them is proposed by Stewart et al. in which this critical point appears as a distinct minimum in the （-dθ/dσ vs σ） through differentiating from θ vs σ. Another one is presented by Najafizadeh and Jonas by modifying the Poliak and Jonas method. According to this method, the strain hardening rate was plotted against flow stress, and the value of σc was attained numerically from the coefficients of the third-order equation that was the best fit from the experimental θ-σ data. Hot compression tests were used in the range of 1000 to 1100℃ with strain rates of 0.01^-1 s^-1 and strain of I on 316 stainless steel. The result shows that Najafizadeh and Jonas method is simpler than the previous one, and has a good agreement with microstructures. Furthermore, the value of normalized critical stress for this steel was obtained uc=σc/σp=0.92.

DEFORMATION AND MICROFRACTURE OF GRANULAR BAINITE

Relationship among deformation history,fracture process and stress distribution of granular bainite has been investigated.The main process of fracture of granular bainite is the forma- tion.growth and coalescence of the microvoids.Even though the microcracks have formed at the earlier stage of deformation,they are not fateful for fracture because in the successive deformation stage the microcracks change their orientation toward the tensile axis.The strain hardening rate of granular bainite has a minimal value during the deformation process. Before and after the strain of the minimal value,the material shows different stress distribu- tion and microfracture mechanism.

Effects of Grain Size and Cryogenic Temperature on the Strain Hardening Behavior of VCoNi Medium‑Entropy Alloys

The mechanical behavior of VCoNi medium-entropy alloys with five different grain sizes at three different temperatures was investigated.The VCoNi alloys with different grain sizes exhibit a traditional strength–ductility trade-off at 77 K,194 K and 293 K.Both the yield strength and the uniform elongation of the VCoNi alloys with similar grain size increase with decreasing the deformation temperature from 293 to 77 K.Obvious strain hardening rate recovery characterized by an evident up-turn behavior at stage II is observed in VCoNi alloys with the grain size above 11.1μm.It is found that the extent of the strain hardening rate recovery increases with increasing grain size or decreasing deformation temperature.This may mainly result from the faster increase in the dislocation multiplication rate caused by the decrease in the dislocation mean free path,the decrease in the absorption of dislocations by grain boundaries and the dynamic recovery from the cross-slip with increasing grain size,as well as the suppressed dynamic recovery at cryogenic temperatures.The critical grain sizes for the occurrence of the recovery of strain hardening rate are determined to be around 9.5μm,8.3μm and 3μm for alloys deformed at 293 K,194 K and 77 K,respectively.The basic mechanism for the strain hardening behavior of the VCoNi alloys associated with grain size and deformation temperature is analyzed.

Comparison on Mathematical Models for Description of Flow Curves of Stable Austenitic Steels

The flow curves were measured for the stable austenitic steels 304L and 304LN by means of tensile test at room temperature,which are described by the models σ=K1εn1 + exp(K2 + n2ε), σ=Kεn1+n2lnε and σ=σ0+Kεn (where, K1, K2, n1 andn2; K, n1 and n2; σ0, K and n are constant). The comparison of the maximum deviations and the consideration of thevariation of the work hardening rate with true strain show that the flow curves for the austenitic steels 304L and 304LN canbe described by the model σ=Kεn1+n2 lnε at higher precision.The derivatives of the models σ=K1εn1 + exp(K2 + n2ε) and σ=Kεn1+n2lnε with respect to true strain, exhibit theextreme at low true strain. This inherent character indicates that both models are unsuitable to describe the part of the workhardening rate curve at low true strain.

EFFECT OF TENSION CONDITIONS ON SUPERPLASTIC BEHAVIOUR OF AN Al-Li-Cu-Mg-Zr ALLOY

A much higher elongation of a warm rolled superplastic Al-Li-Cu-Mg-Zr alloy was made under two-stage strain rate tests comparing with the single ones.During initial stage of deformation a deformation-induced continuous recrystallization which converted a subgrain structure into a recrystallized grain structure by a continuous increase in boundary misorientations had occurred.The higher the strain rate,the faster the continuous recrystallization and the finer the recrystallized grains.The fine recrystallized grain structure formed during the first stage deformation is the essential condition for the material to have high strain rate hardening and strain hardening during the low second stage superplastic deformation.The combination of strain rate hardening and strain hardening is the reason why the higher elongation may be obtained during two-stage superplastic deformation of the alloy.

Compressive Mechanical Properties of Normalized 10^#Low Carbon Steel

Compressive mechanical properties of 10^# lowcarbon steel with normalizing heat treatment are studied. A Gleeble system is adopted to analyze the quasi-static properties and thermal softening effects of heat treated 10^# steel,while a Hopkinson bar apparatus is used to investigate its dynamic characteristics under different strain rates. The results showthat yield stress of heat treated 10^# steel is more than that of untreated one at room temperature. When the specimens are tested at different temperatures,yield stresses decrease with increasing temperature except 573 K. Moreover,the influence of strain rate on yield stress are verified,which shows that the yield stress increases sharply from 500 s^-1 to 1 890 s^-1,while it changes a little from 1 890 s^-1 to 4 850 s^-1. The results indicate that yield stress is mainly influenced by hardening effect at lowstrain rate and controlled by both thermal softening effect and strain rate hardening effect at high strain rate.

A Novel Approach for Rapid Evaluating Cavitation Erosion Resistance of Metallic Materials

For the evaluation of cavitation erosion resistance of metallic materials,it has always been a time-consuming technical problem.This paper innovatively proposed a parameter named cavitation hardening rate to characterize the plastic deformation and work hardening behavior of metallic materials during the cavitation process and then found a positive correlation between cavitation hardening rate and strain hardening index.Finally,the relationship between cavitation volume loss and strain hardening index was obtained,and a simple and rapid evaluation method for cavitation erosion resistance was established.

Behavior of steel fiber-reinforced high-strength concrete at medium strain rate

Impact compression experiments for the steel fiber-reinforced high-strength concrete(SFRHSC)at medium strain rate were conducted using the split Hopkinson press bar(SHPB)testing method.The volume fractions of steel fibers of SFRHSC were between 0 and 3%.The experimental results showed that,when the strain rate increased from threshold value to 90 s^(-1),the maximum stress of SFRHSC increased about 30%,the elastic modulus of SFRHSC increased about 50%,and the increase in the peak strain of SFRHSC was 2-3 times of that in the matrix specimen.The strength and toughness of the matrix were improved remarkably because of the superposition effect of the aggregate high-strength matrix and steel fiber high-strength matrix.As a result,under impact loading,cracks developed in the SFRHSC specimen,but the overall shape of the specimen remained virtually unchanged.However,under similar impact loading,the matrix specimens were almost broken into small pieces.

Effect of low-temperature ausforming on bainitic transformation and mechanical properties in ultrahigh-strength bainitic steel

Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.

Stress-strain behavior of multi-phase high performance structural steel

A series of ferrite/bainite(F/B) multi-phase steels containing different volume fractions of ferrite were obtained.The effect of soft phase(ferrite) content on the work-hardening behavior of the steel was studied by the finite element simulation with V-BCC model and the modified Crussard-Jaoul(C-J) analysis.It is shown that the multi-phase steels have an excellent anti-deformation ability,such as higher stress ratio(R t1.5 /R t0.5),higher uniform elongation and lower yield to tensile strength ratio.For the F/B multi-phase steels,increasing the proportion of ferrite would help to increase the uniform elongation.However,introducing much more fraction of ferrite would not be helpful to improve the stress ratio of multi-phase steel.The ferrite plastic strain constrained by bainite would be beneficial to increasing the work hardening rate.The optimum proportion of ferrite will result both higher stress ratio and uniform elongation in multi-phase steel.

Characterization of the Hot Deformation Behavior of Cu–Cr–Zr Alloy by Processing Maps

Hot deformation behavior of the Cu-Cr-Zr alloy was investigated using hot compressive tests in the tem- perature range of 650-850℃ and strain rate range of 0.001-10 s-1. The constitutive equation of the alloy based on the hyperbolic-sine equation was established to characterize the flow stress as a function of strain rate and deformation temperature. The critical conditions for the occurrence of dynamic recrystallization were determined based on the alloy strain hardening rate curves. Based on the dynamic material model, the processing maps at the strains of 0.3, 0.4 and 0.5 were obtained. When the true strain was 0.5, greater power dissipation efficiency was observed at 800-850 ℃ and under 0.001-0.1 s-1, with the peak efficiency of 47%. The evolution of DRX microstructure strongly depends on the deformation temperature and the strain rate. Based on the processing maps and microstructure evolution, the optimal hot working conditions for the Cu-Cr-Zr alloy are in the temperature range of 800-850 ℃ and the strain rate range of 0.001-0.1 s-1.

Dependence of tensile properties on microstructural features of bimodal-sized ferrite/cementite steels

A medium-carbon steel was processed through different warm rolling techniques,and the microstructural features with bimodal grain size distribution were found to be different.The combination of strength and ductility was ameliorated in the steel processed through warm rolling characterized by biaxial reduction.The enhanced strength is attributed to the densely distributed fine intragranular cementite particles and the small grain size in the coarse grain regions.The enhanced uniform elongation is due to the improved work hardening behavior at the large-strain stage.This work hardening behavior is predominantly ascribed to the finely dispersed intragranular particles.The relatively small grain size with nearly equiaxed shape in the coarse grain regions helps stabilize the uniform deformation to a large strain.