STRAIN INDUCED STRUCTURAL TRANSITION OF INTERFACES AND TWINS IN A HOT－DEFORMED DUAL－PHASE TIAL ALLOY

The structure characteristics of a2/γinterfaces and the features of deformation twins in a quasi-isothermal forged Ti-45Al-10Nb alloy were studied by highresolution transmission electron microscopy. Three types of strain induced a2/γinterfaces and two types of strain induced twin boundaries were identified The most,important features are high density of ledges and the existence of I/3[111] Frank partial dislocation. Mechanisms for the formation these interfaces were proposed Two types of deformation twins were observed These deformation twins always start from the ledges it seems that ledges at interfaces are important features of interfacial structure for the mechanical behavior of alloys.

Microstructural Features During Strain Induced Ferrite Transformation in 08 and 20Mn Steels

The microstructure evolution during strain induced ferrite transformation was followed in thermal-simulation tests of clean 08 and 20Mn steels. The influences of carbon equivalence and initial austenite grain size on ferrite grain refinement and the volume fraction of ferrite during straining were inspected. The results revealed that the accelerating effect of ferrite transformation by strain was increased as the carbon equivalence decreased. However, finer ferrite grains were obtained at higher carbon content. At strain of similar to1.5 ferrite grains less than 3 mum and 2 mum can be obtained in 08 and 20Mn steels respectively. Whereas the ferrite grain refinement in 08 steel was due to both effects of strain induced transformation and ferrite dynamic recrystallization, that in 20Mn was mainly due to strain induced transformation. Heavy strain can produce fine ferrite grains in coarse austenite grained 08 steel, but it would lead to band microstructure in coarse austenite grained 20Mn.

Effects of pre-deformation on microstructure and tensile properties of Al-Zn-Mg-Cu alloy produced by modified strain induced melt activation

The effects of Al-8B grain refiner on microstructure and tensile properties of an Al-12Zn-3Mg-2.5Cu alloy produced by modified strain induced melt activation process were investigated. Pre-deformation of 60% was used by hot working at 300 ℃. After pre-deformation, the samples were heated to a temperature above the solidus and below the liquidus point and maintained in the isothermal conditions at three different temperatures（500, 550 and 590 ℃） for varying time（10, 20 and 40 min）. It was observed that strain induced melt activation has caused the globular morphology of α（Al） grains. Microstructural study was carried out on the alloy by using optical microscope and scanning electron microscope in both unrefined and B-refined conditions. The results showed that for the desired microstructures of the alloy during SIMA process, the optimum temperature and time are 550 ℃ and 10 min, respectively. After the T6 heat treatment, the average tensile strengths increased from 278 to 585 MPa and 252 to 560 MPa for samples refined with 3.75% Al-8B before and after SIMA process, respectively. The ultimate strength of SIMA specimens is lower than that of B-refined specimens.

Strain-induced Microstructural Changes and Effects of Alloying Elements for Fe_3Al-based Alloys

The strain-induced microstructural changes of Fe3Al-based alloys during room temperature deformation and high temperature creep were investigated. The results illustrated the strain-induced disor dering occured during room temperature deformation. Creep strain could induced two opposite processes, which are strain-induced disordering and creep recovery-induced reordering. These two opposite creep induced processes during creep result in reducing the influence of primary microstructure on the rupture life.

Strain-induced ferroelectric phase transitions in incipient ferroelectric rutile TiO_2

Uniaxial strain induced ferroelectric phase transitions in rutile TiO2 are investigated by first-principles calculations. The calculated results show that the in-plane tensile strain induces rutile TiO2, paraelectric phase with P4-2/mnm （D4h） space group, to a ferroelectric phase with Pm（Cs） space group, driven by the softening behaviour of the Eul mode. In addition, the out-of-plane tensile strain, vertical to the ab plane, leads to a ferroelectric phase with P42nm （C4v） space group, driven by the softening behaviour of the A2u mode. The critical tensile strains are 3.7% in-plane and 4.0% out-of-plane, respectively. In addition, the in-plane compression strain, which has the same structure variation as out- of-plane tensile strain due to Poisson effect, leads the paraelectric rutile TiO2 to a paraelectric phase with Pnnm （D2h） space group driven by the softening behaviour of the B1g mode. These results indicate that the sequence ferroelectric （or paraelectric） phase depends on the strain applied. The origin of ferroelectric stabilization in rutile TiO2 is also discussed briefly in terms of strain induced Born effective charge transfer.

Effect of V and V-N Microalloying on Deformation-Induced Ferrite Transformation in Low Carbon Steels

Deformation-induced ferrite transformation （DIFT） has been proved to be an effective approach to refine ferrite grains. This paper shows that the ferrite grains can further be refined through combination of DIFT and V or V-N microalloying. Vanadium dissolved in γ matrix restrains DIFT. During deformation, vanadium carbonitrides rapidly precipitate due to strain-induced precipitation, which causes decrease in vanadium dissolved in matrix and indirectly accelerates DIFT. Under heavy deformation, deformation induced ferrite （DIF） grains in V microalloyed steel were finer than those in V free steel. The more V added to steel, the finer DIF grains obtained. Moreover, the addition of N to V microalloyed steels can remarkably accelerate precipitation of V, and then promote DIFT. However, DIF grains in V-N microalloyed steel easily coarsen.

Effect of Strain Rate on the Ferrite Grain Refinement in a Low Carbon Nb-Ti Microalloyed Steel during Low Temperature Deformation

Grain refinement is one of the effective methods to develop new generation low carbon microalloyed steels possessing excellent combination of mechanical properties. The microstructural evolution and ferrite grain refinement at the deformation temperature of 865℃, above Ar3, with different strain rates were investigated using single pass isothermal hot compression experiments for a low carbon Nb-Ti microalloyed steel. The physical processes that occurred during deformation were discussed by observing the optical microstructure and analyzing the true stress-true strain responses. At strain rates of 0.001 and 0.01s^-1, there is no evidence of work hardening behavior during hot deformation and strain-induced transformation （SIT） leads to dynamic flow softening in flow curves. Optical microscopy observation shows that ultrafine and equiaxed ferrite with grain sizes of 2μm can be obtained by applying deformation with strain rate of 0.1 s^-1 due to SIT just after deformation. Furthermore, increasing the strain rate from 0.001 to 0.1 s^-1 reduces both the grain size of the equiaxed ferrite and the amount of deformed ferrite.

Applied Strain Field on Microstructure Optimization of Ti-Al-Nb Alloy Computer Simulated by Phase Field Approach

The effects of applied tensile strain on the coherent α_2→O-phase transformation in Ti-Al-Nb alloys are explored bycomputer simulation using a phase-field method. The focus is on the influence of the applied strain direction onthe microstructure and volume fraction of the O-phase precipitates. It is found that altering applied strain directioncan modify microstructure of Ti-25Al-10～12Nb (at. pct) alloy during α_2→O-phase transformation effectively andfull laminate microstructure in the Ti-25Al-10Nb (at. pct) alloy can be realized by an applied strain only along thedirection 30°away from the α_2 phase <1010> in magnitude equivalent to the stress-free transformation strain. Thesimulation also shows that not only the magnitude of applied strain but also the applied strain direction influencesthe O-phase volume fraction and the effect of strain direction on the volume fraction is up to 25%.

Understanding poromechanical response of a biogenic coalbed methane reservoir

Biogenic coalbed methane(BCBM)reservoirs aim to produce methane from in situ coal deposits following microbial conversion of coal.Success of BCBM reservoirs requires economic methane production within an acceptable timeframe.The work reported here quantifies the findings of previously published qualitative work,where it was found that bioconversion induces strains in the pore,matrix and bulk scales.Using imaging and dynamic strain monitoring techniques,the bioconversion induced strain is quantified here.To understand the effect of these strains from a reservoir geomechanics perspective,a corresponding poromechanical model is developed.Furthermore,findings of imaging experiments are validated using core-flooding flow experiments.Finally,expected field-scale behavior of the permeability response of a BCBM operation is modeled and analyzed.The results of the study indicated that,for Illinois coals,bioconversion induced strains result in a decrease in fracture porosity,resulting in a detrimental permeability drop in excess of 60%during bioconversion,which festers itself exponentially throughout its producing life.Results indicate that reservoirs with high initial permeability that will support higher Darcian flowrates,would be better suited for coal bioconversion,thereby providing a site-selection criteria for BCBM operations.

Microstructure and mechanical properties of AZ61 magnesium alloy parts achieved by thixo-extruding semisolid billets prepared by new SIMA

New strain induced melt activation（new SIMA） method was employed to prepare high-quality semisolid billet of AZ61 magnesium alloy.Optical microscopy and tensile test were used to study the microstructure and mechanical properties of the thixo-extruded component.The results showed that the optimal process parameters for achieving the complete filling status involved the applied pressure of 784 MPa,the pressure holding time of 90 s and the die temperature of 450 ℃.Compared to semisolid isothermal treatment,high mechanical properties such as the tensile strength of 300.5 MPa and elongation of 22% and fine microstructure were obtained in the thixo-extruded parts.With increasing the isothermal temperature and holding time,the tensile strength and elongation were increased firstly and then decreased.When the press pass was increased from 1 to 4,the tensile strength and elongation of the thixo-extruded parts were greatly enhanced and microstructure was refined obviously.

Ferroelectric and piezoelectric properties of SrZrO_3-modified Bi_(0.5)Na_(0.5)TiO_3 lead-free ceramics

The lead-free SrZrO3-modified Bi0.5Na0.5TiO3(BNT-SZ100 x, with x=0-0.15) ceramics were fabricated by a conventional solid-state reaction method. The effects of SZ addition on BNT ceramics were investigated through X-ray diffraction(XRD), scanning electron microscopy(SEM), ferroelectric and electric field-induced strain characterizations. XRD analysis revealed a pure perovskite phase without any traces of secondary phases. Ferroelectric and bipolar field induced-strain curves indicated a disruption of ferroelectric order upon SZ addition into BNT ceramics. A maximum value of remnant polarization(32 μC/cm2) and piezoelectric constant(102 pC/N) was observed at 5%(mole fraction) of SZ. Maximum value of the electric field-induced strain(Smax=0.24%) corresponding to normalized strain(Smax/Emax= d*33= 340 pm/V) was obtained at BNT-SZ9.

Relaxation of Deformed Austenite and Refinement of Bainite in a Nb-Containing Microalloyed Steel

By means of stress relaxation and strain-induced precipitation in deformed austenite, bainite with micron sheaves size was obtained in Nb-containing steel. Microstructures of deformed samples isothermally relaxed for various time followed by cooling in water were examined. Stress relaxation test and transmission electron microscopy (TEM) were employed to detect precipitation of microalloy elements, such as Nb, Ti, during isothermal holding after deformation. All the samples were constituted by lath-like bainite along with acicular ferrite, but the size of bainitic sheaves and the amount of acicular ferrite were changed with relaxation time. To achieve optimum refinement, relaxation should be confined in the stage when the precipitates have sufficiently grown and started to coarsen. The sample having not undergone relaxation does not exhibit obvious refinement despite of its higher dislocation density. These results indicate that relaxation promotes bainite to refine, which is because deformed austenitic grains are divided by dislocation walls formed during relaxation and acicular ferrite formed before bainitic transformation.

Effects of compressing and remelting in SIMA processing on semi-solid structure evolution of an Al-Zn wrought alloy

Structure evolution of an Al-Zn wrought alloy in remelting processing in thestrain induced melt activated (SIMA) serai-solid procedure was observed, and effects of factors, theremelting temperature, the holding time, and the compression strain, on structures and grain sizesof the alloy were investigated. The results show that (1) the proper temperature of remelting is inthe range of 610 to 615℃; (2) the grain size in specimen with greater compression strain is smallerthan that with smaller compression strain in condition of the same remelting temperature andholding time, and the grain size in local area with great local equivalent strain is smaller thanthat with small one; (3) liquid occurs in form of cluster in matrix during remelting and itsquantity increases with remelting time increasing; liquid in specimen with great compression strainoccurs earlier than that with small one, and quantity of liquid in the center of specimen withgreater local equivalent strain is greater than that in the two ends of it; (4) distortion energyafter deforming in matrix of the alloy is the significant factor to activate melting of matrix atlocal area with great local equivalent strain.

Change in dislocation configuration of deformed Fe-Ni-Nb-Ti-C-B alloy during stress relaxation

Transmission electron microscopy (TEM) was applied to investigate theevolution of dislocation configuration and strain induced precipitation behavior during relaxationprocess after deformation in Fe-Ni-Nb-Ti-C-B alloy. Experimental results indicate that thedislocation density is very high and distribute randornly before relaxation. As the relaxation timeincreasing, dislocation cells will form gradually by polygonization. The strain inducedprecipitation retards the progress. In the final relaxation stage, most dislocations get rid ofpinning of precipitates and the cells have developed into subgrains with large size.

Preparation and thixoforging of semisolid billet of AZ80 magnesium alloy

Semisolid billet of AZ80 magnesium alloy was prepared by new strain induced melt activated (new SIMA) process and thixoforging experiment was performed.The results show that after as-cast AZ80 magnesium alloy is processed by equal channel angular extrusion, microstructure is refined well due to heavy dynamic recrystallization occurring in severe plastic deformation.Compared with semisolid isothermal treatment and conventional SIMA, semisolid billet with fine and spheroidal grains are achieved in new SIMA.Thixoforging process of semisolid billet prepared by new SIMA has many advantages such as good surface quality of final component, high ability to fill cavity and net-shape.The fine and spheroidal grains and high mechanical properties such as tensile strength of 298 MPa and elongation of 28% can be developed in final part thixoforged.

THERMAL STABILITY OF NON-EQUILIBRIUS MICROSTRUCTURE IN MICROALLOYED STEEL DURING REHEATING

Cooled in water after isothermal relaxation of deformed austenite for different times, an Nb-bearing microalloyed steel always exhibits synthetic microstructures, in which bainitic ferrite ,dominates. Dislocation configurations and distributions of strain induced precipitates inside bainitic ferrite of samples relaxed for different times were distinct. When compared with the austenite model steel, which maintained fcc structure even at room temperature, the strain induced precipitates were not found in the sample without relaxation whereas these were distributed outside dislocations in sample relaxed for 1000s. Most of the strain induced precipitates distribute along dislocations and pin dislocations in sample relaxed for appropriate time. After bainitic transformation, the dislocations formed in deformed austenite remain to be pinned by the precipitates. When these samples were reheated to and held at 650 or 700℃, the non-equilibrloas microstructures tended to evolve into equilibrioas ones. The sample relaxed for 60s displayed the highest thermo-stability, whereas microstructure evolution was the quickest in the sample relaxed for 1000s even though it was the softest prior to reheating. Dislocations inside laths got rid of pinning of precipitates, and their polygonization became the precursor to the evolution of microstructures during reheated and held, followed by gradual disappearance of lath boundaries caused by dislocation climbing. Finally, recrystallization occurred and polygonal ferrite appeared. By hardness measurement, it was found that softening is not a single process occurring during reheated, in which hardness fluctuates with time. There were two peaks in the hardness-time curve of each sample having undergone relaxation, while single peak occured in the curve of the sample not being relaxed. These results indicated that the thermo-stability of microstructures was determined by their history of formation to a considerable degree.

Study on Microstructure and Performance of Advanced Bush Strip

The strain induced ferrite formed under different conditions was observed with SEM and optical microscope. The nucleation sites of strain induced ferrite include grain boundary, grain inside, deformed band and annealing twin boundary. The shapes of the ferrite accordingly are equiaxed irregular polygonal, strip shaped and acicular.

Effect of compression ratio on microstructure evolution of Mg−10%Al−1%Zn−1%Si alloy prepared by SIMA process

A new Mg−10%Al−1%Zn−1%Si alloy with non-dendritic microstructure was prepared by strain induced melt activation(SIMA)process.The effect of compression ratio on the evolution of semisolid microstructure of the experimental alloy was investigated.The results indicate that the average size ofα-Mg grains decreases and spheroidizing tendency becomes more obvious with the compression ratios increasing from 0 to 40%.In addition,the eutectic Mg2Si phase in the Mg−10%Al−1%Zn−1%Si alloy transforms completely from the initial fishbone shape to globular shape by SIMA process.With the increasing of compression ratio,the morphology and average size of Mg2Si phases do not change obviously.The morphology modification mechanism of Mg2Si phase in Mg−10%Al−1%Zn−1%Si alloy by SIMA process was also studied.

Comparison of Ferrite Refinement Mechanisms by Different Processing Schedules in 08 and 20Mn Steels

The influence of deforming temperature on ferrite refinement was analyzed by comparing the microstructures obtained by deformation at above A(r3), in two-phase region of (alpha + gamma) and at below A(t) in clean 08 and 20Mn steels. The results indicate that ferrite refinement through strain induced transformation by deformation at above A(r3) is more effective than that by deformation simply through ferrite dynamic recrystallization. The main problem of ferrite refinement by deformation at below A(r3) is the inhomogeneity of microstructure which is controlled by the orientations and sizes of ferrite grains and the distribution of second phases. Ferrite dynamic recrystallization after strain induced transformation can further effectively refine ferrite.

Effect of Strain Path Reversal on Austenite Grain Subdivision and Deformation Induced Ferrite Transformation Studied by Hot Torsion

The effects of large strain and strain path reversal on the deformation microstructure evolution in austenite below the recrystallisation temperature were studied by hot torsion using a non-transforming Fe-30wt%Ni model austenitic alloy.Results show that the high angle boundaries (HABs) can be generated by both microstructure mechanism through dislocation accumulation and texture mechanism via subgrain rotation.However,multiple strain path reversals lead to less well-developed HABs in the original grains compared to single reversal deformed to the same amount of total accumulative strain.This effect is attributed to the subgrain rotation mechanism being less effective at small strains.In comparison,the same hot torsion tests were conducted using a microalloyed steel at a temperature between Ae 3 and Ar 3.After single strain path reversal,substantial deformation-induced austenite-to-ferrite phase transformation was observed.Meanwhile,a test with multiple strain path reversals but with the same total strain produces much lower levels of strain-induced ferrite formation.This difference is correlated to the observations made in the Fe-30wt%Ni model alloy.It is believed that the different amount of strain-induced ferrite originated from the different levels of strain-induced HABs within the austenite which act as ferrite nucleation sites.