Stress-strain partitioning analysis of constituent phases in dual phase steel based on the modified law of mixture

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase （DP） steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio ofmartensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

Roles of pre-formed martensite in below-Ms bainite formation,microstructure,strain partitioning and impact absorption energies of low-carbon bainitic steel

The roles of pre-formed martensite(PM)in below-Ms bainite formation,microstructure,crystallography,strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction,transmission electron microscopy,micro digital image correlation technique and mechanical tests.It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms,indicative of its acceleration effect at the early stage of transformation.This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite,with initial bainite tending to form around the PM.However,the finishing time for below-Ms bainite transformation,especially at even lower temperatures,is retarded,owing to the dividing effect of PM on parent austenite grains,the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite.PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block.The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite.The specimens with PM show relatively uniform strain partitioning among various phases,contrasting with the specimens without PM,for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite(M/A)blocks or between adjacent M/A blocks.The impact absorption energies of the specimens with PM,when austempered at 30-60℃ below Ms,are more than twice higher than those of the specimens without PM,at no expense of tensile properties.

Hot deformation behavior of microstructural constituents in a duplex stainless steel during high-temperature straining

The hot deformation characteristics of 1.4462 duplex stainless steel （DSS） were analyzed by considering strain partitioning between austenite and ferrite constituents. The individual behavior of ferrite and austenite in microstructure was studied in an iso-stress condition. Hot compression tests were performed at temperatures of 800-1100~C and strain rates of 0.001-1 s-1. The flow stress was modeled by a hyperbolic sine constitutive equation, the corresponding constants and apparent activation energies were determined for the studied alloys. The constitutive equation and law of mixture were used to measure the contribution factor of each phase at any given strain. It is found that the contribution factor of ferrite exponentially declines as the Zener-HoUomon parameter （Z） increases. On the contrary, the austenite contribution polynomially increases with the increase of Z. At low Z values below 2.6. x 1015 （lnZ---35.5）, a negative contribution factor is determined for austenite that is attributed to dynamic recrystallization. At high Z values, the contribution factor of austenite is about two orders of magnitude greater than that of ferrite, and therefore, austenite can accommodate more strain. Microstructural characterization via electron back-scattered diffraction （EBSD） confirms the mechanical results and shows that austenite recrystallization is possible only at high temperature and low strain rate.

Growth of forearc highs and basins in the oblique Sumatra subduction system

Based on structural deformation analysis in the oblique Sumatra subduction system, we review uplift mechanisms of the forearc high and formation of the forearc basin. The development of the forearc high has been attributed to the flexural uplift, basin inversion, uplift of older accretion wedge, and backthrust in the landward margin of the accretion wedge. Observation of recently acquired seismic reflection data shows that the interplay between trenchward-vergent thrusts and arcward-vergent backthrusts has played a major role in the uplift of forearc high. The uplifted sediments on the forearc high were previously formed in a forearc basin environment. The present-day morphology of the forearc high and forearc basin is related to the uplift of the accretionary wedge and the overlying forearc basin sediments during Pliocene. Regardless of obliquity in the subduction system, the Sumatran forearc region is dominated by compression that plays an important role in forming Neogene basin depocenters that elongated parallel to the trench.

Effects of Temperature and Strain Rate on Flow Behavior and Microstructural Evolution of Super Duplex Stainless Steel under Hot Deformation

Hot compression tests were carried out in the temperature range of 1 223-1 473 Kand strain rate range of0.01-30s^-1 to investigate the flow behavior and microstructural evolution of super duplex stainless steel 2507（SDSS2507）.It is found that most of the flow curves exhibit a characteristic of dynamic recrystallization（DRX）and the flow stress increases with the decrease of temperature and the increase of strain rate.The apparent activation energy Qof SDSS2507 with varying true strain and strain rate is determined.As the strain increases,the value of Qdeclines in different ways with varying strain rate.The microstructural evolution characteristics and the strain partition between the two constituent phases are significantly affected by the Zener-Hollomon parameter（Z）.At a lower lnZ,dynamic recovery（DRV）and continuous dynamic recrystallization（CDRX）of the ferrite dominate the softening mechanism during the compression.At this time,steady state deformation takes place at the last stage of deformation.In contrast,a higher lnZ will facilitate the plastic deformation of the austenite and then activate the discontinuous dynamic recrystallization（DDRX）of the austenite,which leads to a continuous decline of the flow stress even at the last deformation stage together with CDRX of the ferrite.

A new strategy for fabrication of unique heterostructured titanium laminates and visually tracking their synchronous evolution of strain partitions versus microstructure

Heterostructured(HS)material with extraordinary mechanical properties has been regarded as one of the most promising structural materials.Here,we reported a new strategy for preparing heterostructured pure titanium laminates that possess a good combination of strength and ductility by combining gradient structure(GS)and heterogeneous lamella structure(HLS).The deformation characteristic versus microstructure evolution of GS/HLS titanium laminates,namely the strain partitions between different-sized grains(480–25μm)was visualized using a scanning electron microscope(SEM)equipped with electron backscattered diffraction(EBSD)mode combined with the digital image correlation(SEM-DIC)with an ultrahigh spatial resolution for the first time.As a result,the hetero-deformation of unique GS/HLS structure by the characteristic of strain partitions could be accurately captured.While the hetero-deformation could result in the hetero-deformation induced(HDI)stress strengthening and HDI hardening,which were regarded as the key reason that the resulting GS/HLS Ti laminates showed a superior combination of strength and ductility.This could promote a more in-depth understanding of the strengtheningtoughening mechanism of heterostructured material.

Reconfiguring dual-phase structure with equal micro-hardness to achieve double strength with slight ductility sacrifice

The application of duplex stainless steel is severely restricted owing to its inherently low yield strength.A high yield strength of duplex stainless steel is required to address the lightweight issues and re-duce the emission of polluting gasses.Herein,excellent mechanical properties of duplex stainless steel were achieved using a flash annealing treatment to obtain austenite and ferrite phases with equal hard-ness,and the yield strength doubled without a substantial ductility sacrifice.The improvement in yield strength was caused by the grain boundary strengthening(a contribution of 60%to the yield strength)and hetero-deformation-induced strengthening(remaining 40%contribution to the yield strength)owing to the significant difference in the strain partitioning between the two phases.The high strain-hardening capability ensured ductility without a significant decrease;this correlated to the multiple microstruc-ture deformation mechanisms.These mechanisms included the synergetic deformation of the ferrite with dislocation cell formation and austenite accompanied by the sequential appearance of dislocation accumulation,stacking faults,and deformation-induced nanoscale twins and martensite.Deformation-induced multilevel stacking fault networks and high-density Lomer-Cotterll locks further improved the strain hardening response by enhancing the hetero-deformation behavior of austenite and ferrite.

Composition design of a novel Ti-6Mo-3.5Cr-1Zr alloy with high-strength and ultrahigh-ductility

The strength-elongation to fracture(εf)trade-off and low strain hardening rate have been a longstanding dilemma in titanium alloys.In this work,we innovatively manufactured a novel Ti-6Mo-3.5Cr-1Zr alloy via the introduction of a stress-induced strengthening phase into the material.Stress-inducedω(SIω)phase transformation was expected to replace theα''martensitic transformation that resulted in the low yield strength of titanium alloys.The obtained alloy exhibited an extremely high strain hardening rate of up to^1820 MPa.The true peak tensile strength andεfreached^1242 MPa and^40%,respectively.The intrinsic mechanisms underlying the simultaneous improvement of strength and ductility of the material were systematically investigated via in-situ and ex-situ characterizations.In-situ electron backscatter diffraction(EBSD)/digital image correlation(DIC)results showed that SIωphase transformation dominated the early stage of plastic deformation(1.5%–3%)and promoted the strain partitioning between the stress-induced bands andβmatrix.Subsequently,the formation of{332}<113>βtwins andωtwins was observed via ex-situ EBSD.In-situ transmission electron microscopy results revealed that dislocation pile-up(DPU)occurred at the SIω/βinterface.The coupling effects associated with the transformation induced plasticity(TRIP),twinning induced plasticity(TWIP),and DPU mechanisms contributed to the enhanced strength andεfof the designed titanium alloy.