Conventional Dynamic Recrystallization Research Articles

Laser shock peening strengthens additively manufactured high-entropy alloy through novel surface grain rotation

Additive manufacturing has flourished as an advanced technique to process metals and alloys. However, this strategy usually introduces undesired defects that deteriorates the mechanical performance of structural materials. Herein laser shock peening (LSP) is proposed as an efficient strengthening approach to reshape the surface morphology of a prototypical dual-phase AlCoCrFeNi high-entropy alloy (HEA) after additive manufacturing, in which remarkable strengthening is achieved. Combined electron back scatter diffraction and transmission electron microscope characterizations reveal that the mechanical enhancement is attributed to the grain refinement and accumulation of dislocations at the impact surface. In extreme condition of LSP, the grain refinement is not accommodated by the conventional dynamic recrystallization anymore, but a novel mechanism of parental columnar grain rotation which can be rationalized by a continuum-level theory from a geometrical perspective. The new mechanism is verified by large-scale atomistic simulations which further recognizes the critical role of multiple unstable dislocation slip and amorphization in formation of smaller grains under shock. Our strategy offers a promising pathway toward polishing morphology of HEAs and thus, prohibiting the potential intrinsic defected induced-mechanical degradation of the additively manufactured metals and alloys via novel microscopic mechanism.

Microstructure Evolution and Deformation Mechanism of Tantalum-Tungsten Alloy Liner under Ultra-High Strain Rate by Explosive Detonation.

The microstructure evolution and plastic deformation mechanism of a Ta-2.5W liner under the ultra-high-strain-rate conditions generated by the explosive detonation were investigated in this study. For this purpose, a modular soft-recovery apparatus was designed to non-destructively recover the Ta-2.5W explosively formed projectile (EFP) in the ballistic endpoint. The electron backscattered diffraction (EBSD) method was employed to examine the microstructure of the Ta-2.5W liner before and after deformation. The microstructure of the recovered EFP exhibited significant grain refinement with preferred fiber texture. The theoretical computation results showed that the temperature of the EFP was in the range of 0.27–0.65 Tm. The deformation mechanism of the Ta-2.5W liner forming EFP driven by the detonation is the continuous dynamic recrystallization (CDRX) induced by high strain deformation, rather than the conventional dynamic recrystallization of nucleation and growth. The new grain structures evolve when the low-angle grain boundaries are transformed into the high-angle grain boundaries, and the specific grain refinement mechanism is the progressive rotation of subgrains near pre-existing grain boundaries.

Flow behavior and dynamic recrystallization mechanism of A5083 aluminum alloys with different initial microstructures during hot compression

Hot compression tests of A5083 aluminum alloys with extruded and homogenized initial microstructures were carried out at varying temperatures (350 to 450 °C) and strain rates (0.1 to 10/s). The effects owing to nonuniform temperature and strain distributions caused by deformation were compensated by inverse analysis to determine the flow curves. The obtained flow curves with a homogenized initial microstructure were lower than those with an extruded initial microstructure. Constitutive descriptions were obtained to study the dynamic kinetics during compression, and comparisons between the predicted results from constitutive equations and the experimental results from associate microstructures verified their accuracy. Electron backscatter diffraction (EBSD) was utilized for observing microstructures. The characteristics of continuous dynamic recrystallization, particle-stimulated dynamic recrystallization and conventional dynamic recrystallization were confirmed. The dynamic recrystallization process with a homogenized initial microstructure is slower than that with an extruded initial microstructure, owing to the different amounts of subgrain structures during the initial deformation stage, which can affect the speed of the continuous dynamic recrystallization process. Microhardness tests were conducted to study the connection between microstructure and mechanical property. The microhardness after deformation with an extruded initial microstructure are higher than those with a homogenized initial microstructure.

Hot Deformation Behavior and Microstructural Evolution of SA508‐IV Steel

The hot deformation behavior and microstructural evolution of SA508‐ІV steel are studied through hot compressive tests in the temperature range of 950–1250 °C, and in the strain rate range of 0.001–1 s−1. The hot deformation activation energy and the stress exponent are calculated to be 328.73 KJ mol−1 and 3.29 by the regression analysis of sine hyperbolic function. On this basis, the constitutive Equations are developed. Moreover, the relationship between work hardening and dynamic softening is explained, and a two‐stage constitutive model is established to predict the flow stress of SA508‐ІV steel. The predicted and experimental values show a good consistency. Based on the conventional dynamic recrystallization kinetics model, the volume fraction of the dynamic recrystallization is also estimated, and the results show that the maximum fraction is close to 100% under the higher temperature and lower strain rate. Furthermore, the microstructural evolutions at different deformation conditions are also analyzed, and the DRX grain size has an exponential increase with increasing deformation temperature and decreasing strain rate.

Dynamic Recrystallization Behavior of Ti22Al25Nb Alloy during Hot Isothermal Deformation

AbstractDynamic recrystallization (DRX) of Ti22Al25Nb alloy has been investigated based on the analysis of hot compression experimental data in this paper. The true stress–strain curves of Ti22Al25Nb can be divided into two types: the single-peak conventional dynamic recrystallization (CDRX) curves and the multi-peak discontinuous dynamic recrystallization (DDRX) curves. The multi-peak curves indicate the alternate dominance of hardening mechanism and softening mechanism. A critical Zener–Hollomon parameter ln ZC has been proposed to divide CDRX from DDRX. The hardening rate and DRX volume fraction of CDRX and DDRX have been analyzed. In addition, the effect of the second-phase particle on the DRX has been studied. DRX process was enhanced by the second-phase particles, but the growth process of recrystallized grains was restrained.

Hot deformation behavior and process parameter optimization of Ti22Al25Nb using processing map

The hot deformation behavior of Ti22Al25Nb was investigated by hot compression test. The flow stress–strain curves can be divided into two types: conventional dynamic recrystallization (DRX) and discontinuous DRX. The different softening mechanism and microstructure observation of conventional DRX and discontinuous DRX were analyzed. The processing map (PM) of Ti22Al25Nb was built to predict the safe deformation region. The optimal low strain rate domain (DOM I) with high power dissipation efficiency indicates the complete DRX. Additionally, in the high strain rate and low-temperature domain (DOM III), the power dissipation efficiency is low and some adiabatic shear bands and glide bands are observed, which are unsafe and should be avoided. Finally, the DRX map was established. In DOM I, it reveals low dislocation density and high DRX content, which is in agreement with PM.

Effect of chemical composition and processing variables on the hot flow behavior of leaded brass alloys

Hot compression tests were carried out on CuZn39Pb2 and CuZn39Pb3 leaded brasses in the temperature range of 600–800°C at strain rates of 0.001–1s−1. The stress–strain curves at low temperatures were characterized by a single peak, while at high temperatures, the flow curves were characterized by a long plateau associated with dynamic recovery. The phase diagram of Cu–Zn–Pb system drawn by the Thermocalc software showed that increasing Pb could shift the phase border between α+β and β towards the Zn corner, thereby increasing the volume fraction of α at the deformation temperatures. Hot deformation at low temperatures, e.g., 600–700°C, changed the strings of cornered α islands to more globularized discrete ones. However, the conventional dynamic recrystallization could not be observed. At 800°C, fine α particles formed through dynamic recrystallization were coexistent with acicular ones formed through quenching of β from high temperatures. It was found that at low temperatures, e.g., below 700°C–750°C, Pb could contribute to avoiding flow localization in both alloys. At 800°C, more Pb could dissolve into β leading to more tendency to flow localization. The results showed that at low temperatures, i.e., below 700°C, both materials exhibited higher strength than that predicted by the law of mixture. At high temperatures, particularly beyond 700°C, the predicted values of the law of mixture lay below the experimental flow stresses. This was attributed to the decrease in the volume fraction of α and more dissolution of Pb into β.

Hot Deformation Behavior of as-Cast AZ61 Magnesium Alloy

Thermal compression tests of AZ61 magnesium alloy were performed at deformation temperature 25-400°C with strain rates ranged from 0.02 to 20 s-1, and the microstructure was studied. The conventional dynamic recrystallization (DRX) and shear zones were observed on grain boundaries at temperatures above 200°C. The shear zone and the temperature rise are considered to be responsible for the fracture of specimens. For the large internal stress concentration in the shear zones and the deformation incompatibility between matrix and shear zones, the fracture grew along with the shear zones and the failure occurred in the large deformation field of the matrix at the high strain rate 20 s-1 even at 400°C.

Development of Ultrafine-Grained Dual-Phase Steels: Mechanism of Grain Refinement During Intercritical Deformation

Heavy deformation of metastable austenite (below Ae3) or both austenite and ferrite in the two-phase region (between Ar3 and Ar1) is known to develop an ultrafine ferrite grain structure with an average grain size of less than 3 μm. Different dynamic softening mechanisms, such as dynamic recovery, dynamic recrystallization, and dynamic strain-induced austenite→ferrite transformation (DSIT), are responsible for such grain refinement. However, the sequence of those metallurgical events and the temperature range over which any particular mechanism dominates is not yet well understood. The current study throws some light on this aspect by applying heavy, single-pass compressive deformation (with true strain of 1.0) on the microalloyed steel samples over a temperature range of 1173 K to 873 K (900 °C to 600 °C) using a Gleeble simulator (Dynamic Systems Inc., Poestenkill, NY) and water quenching the samples immediately after deformation. The current study showed the dominating effect of the following mechanisms with respect to the deformation temperature: (1) DSIT followed by conventional dynamic recrystallization (Conv-DRX) of ferrite at higher deformation temperatures (≥1073 K [800 °C]), (2) extended recovery and continuous dynamic recrystallization (Cont-DRX) of ferrite at intermediate deformation temperatures (~1023 K [750 °C]), and (3) simple dynamic recovery of ferrite at lower deformation temperatures (≤923 K [650 °C]).

Influence of asymmetric hot rolling on microstructure and rolling force with austenitic steel

In order to establish a new processing technique of producing steels with fine grain, asymmetric hot rolling and symmetric rolling were performed. The asymmetric condition was introduced by applying mismatched roll diameters. The diameter ratios between big and small rolls were 1.00, 1.05 and 1.11. The rolling temperatures were between 900 °C and 1100 °C. The thickness reduction of workpiece was set at 15%, 30% and 60%. The results showed that asymmetric rolling produced higher volume fraction of recrystallization grain and smaller average grain size at the center layer of rolled sample than symmetric rolling. The diameter ratio of 1.05 tended to generate the highest recrystallization level and the smallest average grain size. With high temperature and low thickness reduction value, the grain growth was obvious. The conventional dynamic recrystallization mechanism was prevailing, while accompanied by twinning. At low temperature of 900 °C, even at thickness reduction of 60%, symmetric rolling cannot initiate dynamic recrystallization as asymmetric rolling did. The asymmetric rolling force was smaller than that of symmetric rolling, except at 900 °C and thickness reduction of 60%. The rolling force also increased with descending temperature and climbing thickness reduction.

Grain Size in Mg Alloys: Recrystallization and Mechanical Consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying addition on both of these reactions is briefly considered.

A Criterion for Conventional Dynamic Recrystallization: Application to Magnesium Alloys

An analytical approximation for the steady state dynamic recrystallized grain size is combined with a simple nucleation criterion to assess the propensity for dynamic recrystallization. In line with observation, the criterion predicts dynamic recrystallization in 99.9995% pure Al but not in material 99.5% pure. It also agrees with the observation that zone refined ferrite can display dynamic recrystallization at high temperatures and low strain rates but not at lower hot working temperatures. The criterion is applied here to common wrought magnesium alloys to argue that conventional dynamic recrystallization is expected under "normal" hot working conditions.

Dynamic Recrystallization of Austenitic Stainless Steel under Multiple Peak Flow Behaviours

The hot working behaviour of a Fe-0.02C-8.2Ni-18.5Cr (mass%) austenitic stainless steel was studied at high temperatures and low strain rates. These conditions led to a multiple peak stress-strain curve, where the second peak had a much larger strain and stress than the first peak. The first peak showed evidence of some grain refinement, indicative of conventional dynamic recrystallization. However, after this there was significant grain coarsening, similar to abnormal grain growth, with the formation of substructure within the grains. The strain to the first peak in the multi peak tests was much lower strain than expected from extrapolation of the single peak data.

Inferring dynamic recrystallization in ferrite using the kinetics of static recrystallization

A general relationship between the kinetics of dynamic and static recrystallization is developed. It is predicted that conventional dynamic recrystallization will occur whenever the deformation time exceeds the adjusted start time for static recrystallization. This approach is verified using data for austenite and lead. It is then applied to current and previous work on ferrite. The model provides support for the contention that conventional dynamic recrystallization occurs in low carbon ferrite if deformation is carried out at high temperatures and low strain rates. In the present work, which was carried out at 700 °C, evidence for dynamic recrystallization was observed for strain rates less than around 0.01 s−1. At higher strain rates, the model predicts a critical strain for the onset of dynamic recrystallization that exceeds the critical strain for the beginning of the recovery steady-state region. While the model allows dynamic recrystallization to begin in this region, the critical strain for its onset is expected to increase rapidly with increasing strain rate and decreasing temperature once steady state has been reached.

Predicting the critical strain for dynamic recrystallization using the kinetics of static recrystallization

School of Engineering and Technology, Deakin University, Pigdons Rd, Geelong, VIC 3217,Australia(Received November 12, 1999)(Accepted March 9, 2000)Keywords: Steels-austenite; Dynamic recrystallization; Recrystallization1. IntroductionOne view of dynamic recrystallization is that it comprises static recrystallization occurring in the timescale of deformation. While this is clearly not true for geometric dynamic recrystallization [1] orcontinuous dynamic recrystallization [1], there is some metallographic support for it with respect to thebeginning of conventional dynamic recrystallization. The microstructure after a few volume percent ofrecrystallization by this mechanism appears very similar to that seen after a small degree of staticrecrystallization [2,3].A number of workers have constructed mathematical equations for the beginning of conventionaldynamic recrystallization (DRX) using a formulation based on the nucleation mechanisms of staticrecrystallization (SRX) [2,4,5]. These models fit the observed trends in DRX quite well. However, theydo not allow any firm conclusions to be drawn with respect to the relative kinetics of the two processes.In the present work, conventional equations for the kinetics of SRX are modified to allow “SRX” tobegin prior to the end of deformation. In this manner a critical strain for the beginning of “SRX” duringdeformation is derived. This value is then compared with observations of the critical strain required forthe initiation of DRX in steel.2. BackgroundThe static recrystallization kinetics following the hot deformation of steel are often expressed in termsof the time taken after deformation for 50% recrystallization (t

Grain Refinement under Multiple Warm Deformation in 304 Type Austenitic Stainless Steel.

The dynamic process of fine grain evolution as well as deformation behaviour under warm working conditions was studied in compression of a 304 type austenitic stainless steel. Multiple compression tests were carried out at a strain rate of 10-3 s-1 to produce high cumulative strains, with changing of the loading direction in 90o and decreasing temperature from 1223 to 873 K (0.7-0.5Tm) in each pass. The steel exhibits two types of deformation behaviours with different mechanical and structural characteristics. In the deformation region where flow stresses are below about 400 MPa, conventional dynamic recrystallization takes place accompanied mainly by bulging of serrated grain boundaries. The dynamic grain size evolved can be related to the high temperature flow stress through a power law function with a grain size exponent of -0.72. On the other hand, in the region of higher stresses above 400 MPa the flow stresses show small strain rate and temperature dependence, and so it is suggested to be in an athermal deformation region. The stress-strain curves show a steady state like flow without any strain softening, while the multiple deformation to high cumulative strains brings about the evolution of fine grained structures with grain sizes less than one micron. The relationship between the warm temperature flow stresses and the grain sizes evolved also can be expressed by a unique power law function of grain size with an exponent of -0.42. The interrelations between the mechanisms of plastic deformation and microstructure evolution at warm and high temperatures are analysed in detail and also the multiple compression method for obtaining ultra fine grained structure is discussed as a simple thermomechanical processing.