Microstructural evolution and dynamic recrystallization mechanisms of additively manufactured TiAl alloy with heterogeneous microstructure during hot compression

Microstructural evolution and dynamic recrystallization(DRX)mechanisms of a Ti-48Al-2Cr-2Nb(at.%)alloy prepared by selective electron beam melting(SEBM)during hot deformation at 1150℃and 0.1 s^(-1)were investigated by hot compression tests,optical microscope(OM),scanning electron microscope(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results show that the initial microstructure of the as-SEBMed alloy exhibits layers of coarseγgrains and fineγ+α_(2)+(α_(2)/γ)lamellar mixture grains alternately along the building direction.During the early stage of hot deformation,deformation twins tend to form within the coarse grains,facilitating subsequent deformation,and a small number of DRX grains appear in the fine-grained regions.With the increase of strain,extensive DRX grains are formed through different DRX mechanisms in both coarse and fine-grained regions,involving discontinuous dynamic recrystallization mechanism(DDRX)in the fine-grained regions and a coexistence of DDRX and continuous dynamic recrystallization(CDRX)in the coarsegrained regions.

Continuous Dynamic Rotation Measurements Using a Compact Cold Atom Gyroscope

We present an experimental demonstration of the rotation measurement using a compact cold atom gyroscope. Atom interference fringes are observed in the stationary frame and the rotating frame, respectively. The phase shift and contrast of the interference fringe are experimentally investigated. The results show that the contrast of the interference fringe is well held when the platform is rotated, and the phase shift of the interference fringe is linearly proportional to the rotation rate of the platform. The long-term stability, which is evaluated by the overlapped Allan deviation, is 8.5 × 10^-6 rad/s over the integrating time of 1000s.

Grain refinement and weak-textured structures based on the dynamic recrystallization of Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr alloy

We utilized electron backscatter diffraction to investigate the microstructure evolutions of a newly developed magnesium-rare earth alloy(Mg–9.80 Gd–3.78 Y–1.12 Sm–0.48 Zr)during instantaneous hot indirect extrusion.An equiaxed fine-grained(average grain size of 3.4±0.2μm)microstructure with a weak texture was obtained.The grain refinement was mainly attributed to the discontinuous dynamic recrystallization(DDRX)and continuous DRX(CDRX)processes during the hot indirect extrusion process.The twin boundaries formed during the initial deformation stage effectively increased the number of high angle grain boundaries(HAGBs),which provided sites for new grain nuclei,and hence,resulted in an improved DDRX process.Along with DDRX,CDRX processes characterized by low angle grain boundary(LAGB)networks were also observed in the grain interior due to effective dynamic recovery(DRV)at a relatively high temperature of 773 K and high strain rates.Thereafter,LAGB networks were transformed into HAGB networks by the progressive rotation of subgrains during the CDRX process.

Hot deformation behaviors and dynamic recrystallization mechanism of Ti-35421 alloy inβsingle field

Hot deformation behaviors and microstructure evolution of Ti-3Al-5Mo-4Cr-2Zr-1Fe(Ti-35421)alloy in theβsingle field are investigated by isothermal compression tests on a Gleeble-3500 simulator at temperatures of 820-900°C and strain rates of 0.001-1 s^(-1).The research results show that discontinuous yield phenomenon and rheological softening are affected by the strain rates and deformation temperatures.The critical conditions for dynamic recrystallization and kinetic model of Ti-35421 alloy are determined,and the Arrhenius constitutive model is constructed.The rheological behaviors of Ti-35421 alloys aboveβphase transformation temperature are predicted by the constitutive model accurately.The EBSD analysis proves that the deformation softening is controlled by dynamic recovery and dynamic recrystallization.In addition,continuous dynamic recrystallization is determined during hot deformation,and the calculation model for recrystallization grain sizes is established.Good linear dependency between the experimental and simulated values of recrystallized grain sizes indicates that the present model can be used for the prediction of recrystallized grain size with high accuracy.

Unveiling the planar deformation mechanisms for improved formability in pre-twinned AZ31 Mg alloy sheet at warm temperature

To investigate the role of pre-twins in Mg alloy sheets during warm planar deformation,the stretch forming is conducted at 200℃.Results suggest the formability of the pre-twinned AZ31 Mg alloy sheet is enhanced to 11.30 mm.The mechanisms for the improved formability and the deformation behaviors during the planar stretch forming are systematically investigated based on the planar stress states.The Schmid factor for deformation mechanisms are calculated,the results reveal that planar stress states extremely affect the Schmid factor for{10-12}twinning.The detwinning is activated and the prismatic slip is enhanced in the pre-twinned sheet,especially under the planar extension stress state in the outer region.Consequently,the thickness-direction strain is accommodated better.The dynamic recrystallization(DRX)type is continuous DRX(CDRX)regardless of the planar stress state.However,the CDRX degree is greater under the planar extension stress state.Some twin lattices deviate from the perfect{10-12}twinning relation due to the planar compression stress state and the CDRX.The basal texture is weakened when the planar stress state tends to change the texture components.

Grain refinement of magnesium alloys processed by severe plastic deformation

Grain refinement of AZ31 Mg alloy during cyclic extrusion compression （CEC） at 225-400 ℃ was investigated quantitatively by electron backscattering diffraction （EBSD）. Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 ℃. The mean misorientation and the fraction of high angle grain boundaries （HAGBs） increase gradually by lowering extrusion temperature. Only a small fraction of {101^-2} twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip {101^-1}〈1120〉and basal slip {0001}〈1120〉 at 225-350 ℃ and 400 ℃, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization （CDRX） mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Influence of welding speed on microstructure formation in friction-stir-welded 304 austenitic stainless steels

The influence of welding speed on the joint microstructures of an austenitic stainless steel(ASS)produced by friction stir welding(FSW)was investigated.The FSW process was conducted using a rotational speed of 400 r/min and welding speeds of 50 and 150 mm/min.The study was carried out using electron backscattered diffraction(EBSD)technique in different regions of the resultant stir zones(SZs).The results show that the texture of the advancing side(AS)was mainly composed of C{001}〈110〉and cube{001}〈100〉texture components along with partial B/B{112}〈110〉component.Moving from the AS toward the center and the retreating side(RS),the cube texture component disappeared and the A;/A*{111}(112)component developed and predominated the other components.Higher welding speed greatly affected and decreased the intensity of the textures in the resultant SZs.Moreover,higher welding speed(lower heat input)resulted in lower frequency of cube texture in the AS.

Microstructure Evolution of AZ31 Mg Alloy during Change Channel Angular Extrusion

Microstructure evolution of AZ31 Mg alloy during change-channel angular extrusion （CCAE） was investigated. The grains of AZ31 Mg alloy were refined significantly from 500 mm to 15 mm after CCAE deformed at 523 K. Dislocations were induced at the initial stage of extrusion and they rearranged themselves to form dislocation boundaries and sub-grain boundaries during deformation. When the specimen through the horizontal change channel with the strain increased, the sub-boundaries evolved to high angle grain boundaries （HAGB）. The process of grain refinement can be described as continuous dynamic recovery and recrystallization （CDRR）.

Continuous dynamic monitoring of a centenary iron bridge for structural modification assessment

A multi-channel continuous dynamic monitoring system has been installed in a centenary iron arch bridge on late November 2011. The historic infrastructure, completed in 1889 and crossing the Adda river about 50 km far from Milan, is the most important monument of XIX century iron architecture in Italy and is still used as roadway and railway bridge. The monitoring project follows a series of preliminary ambient vibration tests carried out on the bridge since June 2009. The paper describes the bridge structure and its dynamic characteristics identified from the experimental studies developed since 2009, the installed monitoring system and the software developed in LabVIEW for automatically processing the collected data. Subsequently, the tracking of automatically identified natural frequencies over a period of about 18 months is presented and discussed, highlighting the effects of environmental and operational conditions on the bridge dynamic characteristics as well as the detection of structural changes, mainly based on natural frequencies shifts.

Control Model of Bloom Dynamic Soft Reduction

There are significant effects of process parameters on internal qualities of bloom, and these process parameters are as follows. position and reduction amount, reduction distribution, reduction rate, and so on. Developing a control model is the key to apply soft reduction technology successfully. As the research object, 360 mm ×450 mm bloom caster in PISCO （Panzhihua Iron and Steel Co. ） has been studied, and the research method for control model of dynamic soft reduction has been proposed. On the basis of solidification and heat transfer model, the position of soft reduction and reduction distribution of each frame are determined according to the bloom temperature distribution and solid fraction in bloom center calculated. Production practice shows that the ratio of center porosity which is less than or equal to 1.0, increased to 97.27%, ratio of central segregation which is less than or equal to 0.5, increased to 80.91%, and ratio of central carbon segregation index which is more than or equal to 1.10, decreased to 4% with the applying model of dynamic soft reduction.

Analysis of SIRVI model with time dependent coefficients and the effect of vaccination on the transmission rate and COVID-19 epidemic waves

COVID-19 epidemic models with constant transmission rate cannot capture the patterns of the infection data in the presence of pharmaceutical and non-pharmaceutical interventions during a pandemic.Because of this,a new modification of SIR model that contain the vaccination compartment with time dependent coefficients and weak/lossimmunity is explored.Literature review confirms that the effect of vaccination on the time dependent transmission rate is still an open problem.This study answers this open problem.In this study,we first prove the well-posedness and investigate the model dynamics to show their continuous dependence on the model parameters.We then provide an algorithm to derive the time-dependent transmission function for the epidemiologic model and the data of the infected cases.The derived coupled nonlinear differential equations show the effect of vaccination on the transmission rate.Unlike previous studies,we first filter the published data and solve the nonlinear coupled differential equations using the finite difference technique,where the coefficient of the coupled nonlinear differential equations is a function of given data.We then show that time-dependent transmission function can be represented by linear combinations of Gaussian radial base function.We then validate the prediction of our models using numerical simulations,where we used the published data of COVID-19 confirmed cases by the Ministries of Health in Saudi Arabia and Poland.Finally,the numerical solutions of a SIRVI model with time dependent transmission rate show that the waves for currently active cases are in good agreement with the data of Saudi Arabia and Poland.

Analysis on hot deformation and following cooling technology of 25Cr2Ni4MoVA steel

The true stress–true strain curves of 25Cr2Ni4MoVA steel were obtained by uniaxial compression experiments at 850–1200℃ in the strain rate range of 0.001–10.0 s^(−1).And the dynamic continuous cooling transformation curves were obtained at the cooling rate range of 0.5–15.0℃ s^(−1) from the austenitization temperature of 1000℃ to the room temperature by pre-strain of 0.2 as well.The power dissipation map and the dynamic continuous cooling transformation diagram were constructed based on the data provided by these curves.Compared with the optical micrographs of the compressed samples,the full dynamic recrystallization region is located between 1000 and 1200℃ and at the strain rate range from 0.01 to 10.0 s^(−1) with the power dissipation efficiency not less than 0.33.In the full dynamic recrystallization region,the power dissipation efficiency increases and the dynamic recrystallization activation energy decreases with the temperature increasing.With the strain rate decreasing,the power dissipation efficiency increases firstly and then starts to decrease as the strain rate is less than 0.1 s^(−1),and dynamic recrystallization activation energy changes on the contrary.According to the dynamic continuous cooling transformation diagram,slow cooling is a better way for the hot-deformed piece with large size or complex shape to avoid cracking as the temperature of the piece is lower than 400℃,and different cooling ways can be used for the hot-deformed piece with small size and simple shapes to obtain certain microstructure and meet good compressive properties.

Effect of Pre-deformation on Grain Ultrafining by Intercritical Deformation in Low-Carbon Microalloyed Steels

In this study, the effect of pre-deformation at recrystallization and non-recrystallization zone on the grain ultrafining by the subsequent intercritical deformation （ID） was investigated on low-carbon microalloyed steel. The results showed that ultrafine grain microstructure with an average size of - 1.0 μm was fabricated through pre-deformation in the recrys- tallization zone followed by ID. When pre-deformed at the non-recrystallization zone prior to ID, the grain size increased to 1.6 μm with a heterogeneous distribution along with the well-developed dynamic recovery of ferrite. The grain ultrafining mechanism was attributed to the combined action of the deformation-induced ferrite transformation and the continuous dynamic recrystallization. In particular, the continuous dynamic recrystallization process during ID occurred on the pro-eutectoid ferrite as a result of the subgrain rotation mechanism and the absorbing dislocations mechanism.