SRX Grains Research Articles

Interaction between phase transformation and static recrystallization during annealing of rolled TC18 titanium alloy

TC18 titanium alloy was subjected to hot rolling at 840 °C, followed by annealing experiments at 830, 860, and 890 °C for different time lengths. The effects of annealing temperature and time on phase transformation and static recrystallization (SRX) behavior, as well as the interaction between phase transformation and SRX, were studied. The results showed that at the low annealing temperature of 830 °C, the microstructure underwent β → α phase transformation and α grain growth with the increase of annealing time, and the SRX of β phase proceeded through subgrain nucleation mechanism induced by α particles. At the middle annealing temperature (860 °C), the α → β phase transformation occurred before the SRX of β phase, and the SRX of β phase was dominated by the grain boundary (GB) bulge mechanism. Meanwhile, the reduction of α phase content accelerated the SRX rate and grain growth. As the annealing temperature (890 °C) exceeded the β transus temperature, the α phase completely transformed to the β phase, and the SRX of the β phase proceeded with a very fast grain growth rate. In addition, the interaction between phase transformation and SRX affected the distribution of α particles. The large-sized α particles (830 °C) were located at the triple GB junction and hindered the SRX β grain growth, while smaller α particles (860 °C) were bypassed by β GB and dispersed within the SRX β grains.

Evolution of static recrystallization microstructure caused by residual strain of an Al-richen Ni-based single crystal superalloy

Recrystallization has a devastating effect on the service process of single crystal superalloys (SX). This study mainly investigated the static recrystallization (SRX) behavior of an as-cast high Al content nickel-based SX under local or global deformation at room temperature, as well as heat treatment at various temperatures. The microstructural evolution was observed under different conditions using electron back scatter diffraction (EBSD), which was employed to determine the SRX threshold. The results indicated that the critical SRX temperature exceeds 1260 °C, and the critical plastic strain is greater than 5 %. In particular, the characteristic of the higher SRX tendency in the interdendritic region (IDR) of the as-cast SX was discovered, which was attributed to the phenomenon that the IDR with lower deformation resistance was prone to significant plastic accumulation and the bulk γ' could be the nucleation substrate of the SRX. Furthermore, with the increase in heat treatment temperature, there are certain changes in the morphology characteristics of re-precipitated γ' in the vicinity of the SRX grain boundary with a higher migration rate. This is mainly attributed to the decrease in the internal subcooling of the bulk γ' phase and the increase in the concentration of γ' forming elements at the grain boundary front.

A computational approach to restoration phenomena during annealing of rolled Cu-2Be: recrystallization, grain growth and abnormal grain growth

In this study, an integrated paradigm was developed for the simulation of static recrystallization (SRX) after cold rolling of a Cu-2Be alloy. The proposed framework also models with the normal and abnormal grain growth (AGG) of the after transient recrystallization through post-rolling annealing when stored and interfacial energies are varying. The simulation of AGG was done to study the influence of precipitates and sub-boundaries. The initial stored energy as a result of deformation was calculated. The finite element analysis was used to compute the heterogeneous distribution of stored energy due to heterogeneous deformation and is then incorporated to Monte Carlo (MC) algorithm to attain SRX grain size and kinetics. The stored energy assigned to the as-rolled grain micrograph is introduced to the MC simulation as the initial condition considering the size-scale of the calculation domain. The effects of the Zener drag pressure stemming from the presence of second phase particles on the recrystallization progress was also incorporated into the MC model. To evaluate the simulation predictions, the numerical results of the SRX fractions and grain sizes after cold rolling was studied and compared to the experimental ones, and a reasonable accordance is achieved. The numerical results presented that precipitates and sub-boundaries in cooperation take a significant role in persuading AGG by wetting alongside triple junctions. To validate the quantitative results, SRX transformation kinetics has been investigated using the differential scanning calorimetry.

Texture evolution related to static recrystallization during annealing of TA15 alloy tube formed by flow forming

In this work, the static recrystallization (SRX) responsible for texture development during the annealing of the TA15 flow-formed part was investigated by comparatively analyzing the SRX microstructure and texture under different pre-strains and annealing temperatures. The results indicated that SRX grains are produced by discontinuous (DSRX) and continuous (CSRX) mechanisms, while the dominant mechanism depends on the pre-deformation and annealing conditions. At a smaller pre-strain, DSRX is prevalent, and some DSRX grains whose {11–20} axes are parallel to the normal direction preferentially grow via the long-range migration of 60–90° grain boundaries. With increased pre-strain, the dominant mechanism gradually changes from DSRX to CSRX. At a larger pre-strain, the CSRX always prevails. Most CSRX grains have an orientation similar to deformed grains which exhibit non-standard basal texture. However, CSRX grains related to deformation-induced intense and localized shearing (CSRX-DILS) show different orientations. The occurrence of DSRX and CSRX-DILS has a minor effect on global annealing texture type owing to the lower fraction but only weakens its concentration. Most CSRX grains primarily affect the texture intensity based on their growth rate. At a pre-strain of 3.36, a higher misorientation angle induced by flow forming can accelerate the growth of CSRX grains, thereby forming an intense CSRX texture and consequently strengthening the global texture. However, the CSRX grain growth at 6.10 pre-strain is limited, resulting in a weaker global texture. With the above effects by SRX, the global annealing texture inherits the features of deformation texture, i.e., non-standard basal texture, but its intensity decreases.

Practical approach for determining material parameters when predicting grain size after static recrystallization

The static recrystallization (SRX) behavior of 42CrMo steel was investigated by a non-isothermal hot compression test. The conventional empirical equation for SRX was applied to a finite element (FE) simulator using a nonconventional manner of parameter characterization. Parameter characterization was conducted by applying the FE-coupled optimization method to cylinder compression, which is an axisymmetric upsetting process. An accurate and practical flow stress model was used to precisely determine the critical strain value and define the instant when SRX occurred. The results revealed that the microstructural evolution during SRX was substantially influenced by temperature, strain rate, degree of deformation, and initial austenite grain size. The predicted recrystallized SRX grain sizes exhibited good agreement with the measured values when the model's parameters for the calculation of grain size were obtained using a FE-coupled optimization scheme.

Unveiling the self-annealing phenomenon and texture evolution in room-temperature-rolled Cu-Fe-P alloy sheets

The present study addresses the microstructural and texture evolution of Cu-Fe-P alloy sheets rolled at room temperature (RT). Cu-Fe-P specimens were subjected to 20, 40, 60, and 80% reduction ratios (RR) through room-temperature rolling (RTR). Nano-sized Fe2P precipitates were observed in the initial and deformed specimens. Continuous-rolling deformation causes strain hardening, whereas exposing severely deformed specimens to RT causes the self-annealing phenomenon. Deformed specimens to 60 and 80% RRs exhibited strain localizations (SLs) that formed only in Copper-type grains. In this study, a visco-plastic self-consistent (VPSC) polycrystal model was used to calculate the relative slip activity for major texture orientations. Copper-type grains showed the occurrence of predominant single-slip system which was the primary reason for the formation of SLs. Static recovery (SRV) and static recrystallization (SRX) were the main softening phenomena found in the RTR specimens. Both discontinuous SRX (DSRX) and continuous SRX (CSRX) were observed as the SRX phenomenon in RTR specimens. Major SRX grains were elongated in shape due to the combined effects of a higher orientation gradient (CSRX), to the larger SE differences between the matrix grains (restricted growth), and to a pinning effect by Fe2P particles. The as-received specimen showed weak plane-strain texture components at maxima around the S component ((123)<634>). As the RR increased, the volume fractions of Copper ((112)<111>), Brass ((110)<112>), and S components were likewise increased (except at 60% RR). The highest fraction of plane-strain components was observed for the RTR-80 specimen. The large number of SLs and the higher orientation gradient decreased the intensity of the plane-strain texture in the RTR-60 specimen, whereas a higher fraction of SRX in the RTR-80 specimen enhanced the plane-strain texture components. Time-dependent decay in the hardness values in the RTR specimens were attributed to SRV and SRX.

Investigation of isothermal continuous hot deformation behavior of the new as-cast alumina-forming austenitic alloy

Multi-pass isothermal continuous hot deformation experiments were designed for homogenization and recrystallization in the as-cast alumina-forming austenite (AFA) heat-resistant alloy. Flow curves and microstructure characterization were used to study the grain evolution with different deformation parameters in each pass. In particular, attention was paid to the dendrites and segregated phase. Under different hot deformation conditions, the alloys show significant work-hardening characteristics, which result from the strengthening effect of the primary segregated MC-type carbide. The increase in deformation temperature contributes significantly to dynamic recrystallization (DRX), and the main mechanism of DRX is discontinuous DRX (DDRX). The continuous DRX (CDRX) is less occurring. Static recrystallization (SRX) occurs during the isothermal holding time between deformation passes. SRX becomes more and more apparent with temperature and the holding time. The increase in the deformation passes leads to a gradual disappearance of the alloy dendrites and an increase in the recrystallization volume fraction. Meanwhile, the segregated phase gradually decomposes into particles and partially dissolves. However, the segregated phase particles eventually show band-like distribution. SRX grains around the segregated phase will exhibit similar orientations due to particle-stimulated nucleation (PSN) mechanisms. Until the grain size increases to the point that it can escape from particle restraint, it will transform to random orientation. The above study confirms that multi-pass isothermal continuous hot deformation can improve the as-cast microstructure and guide practical industrial production.

Unveiling the room-temperature softening phenomenon and texture evolution in room-temperature- and cryogenic-rolled ETP copper

The present study addresses the evolution of microstructural and crystallographic texture in room-temperature-rolled (RTR) and cryogenic-rolled (CR) electrolytic tough-pitch (ETP) copper. Copper specimens were subjected to 20, 40, 60, and 80% reductions via RTR and CR processing. The microstructure evolution of the severely deformed RTR and CR specimens revealed deformed and recrystallized grains. Static recrystallization (SRX) at room temperature (RT) was observed in the severely deformed RTR and CR specimens. Both discontinuous SRX (DSRX) and continuous SRX (CSRX) were responsible for the nucleation of small grains at RT. Particle-stimulated nucleation (PSN) was also responsible for small-grain formations around the Cu2O particles. The initial ETP copper specimen showed a weak texture whose intensity increased after the RTR and CR deformation. The texture intensity of the severely deformed specimens was affected by both deformation and the SRX phenomena. The CR specimens showed texture that was stronger than that of the RTR specimens up to the point of an intermediate reduction (60%), whereas at higher deformation (80%), the RTR specimens showed stronger texture intensity compared with that of the CR specimens. The texture weakening in the CR80 specimen was due to the formation of strain localizations (SLs) and to the nucleation at SLs. Large amounts of stored energy and SL formation in severely deformed CR specimens led to a different recrystallization texture compared with that of RTR specimens. A Cube component was dominant in the RTR80, whereas Goss, Copper and S components were observed in the CR80 specimen for SRX grains. The CR specimen showed a relatively high hardness value compared with the RTR specimen. Time-dependent decay in the hardness values for both the RTR and the CR specimens were attributed to room-temperature recovery (SRV) and recrystallization (SRX).

Improving the isotropy and formability of extruded Mg-2Gd-1Zn (wt.%) alloy sheet by introducing an ellipse texture

In this work, a novel texture modification, termed the ellipse texture, is introduced by the pre-rolling and subsequent annealing (PRA) process in as-extruded (AE) Mg-Gd-Zn (GZ21) alloy. The static recrystallization (SRX) mechanisms and the evolution of microstructure during the PRA process are investigated by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. The results show that the introduction of the ellipse texture distribution leads to the improved planar isotropy of the mechanical properties compared with the AE sheets, when tensile along different directions. Accordingly, the Erichsen value (IE value) increases to 5.7 ± 0.1 mm. Furthermore, the activation of non-basal dislocations (prismatic <a> and pyramidal <c+a> dislocations) during the pre-rolled process induces preferred nucleation sites for the SRX grains with the c-axis oriented to TD. The co-segregation of Gd and Zn atoms at grain boundaries leads to the preferred growth of these TD texture component grains. Both the preferred nucleation and growth of the SRX grains contribute to the formation of ellipse texture distribution in the PRA GZ21 alloys.

Evolution of grain boundary character distribution in near-surface regions of a cold-rolled nickel-based superalloy during induction heating process

The influence of induction heating on the grain boundary character distribution (GBCD) in near-surface regions of a cold-rolled Nickel-based superalloy was researched. After induction heating, most of low-Σ coincidence site lattice (CSL) boundaries were Σ3 boundaries, which were mainly formed via the growth accident model. Moreover, the grain structures evolution during induction heating had a great influence on the evolution of GBCD. At the low strain of 0.1, both the fraction of Σ3 boundaries and grain size increased with the increasing temperatures, while the former was closely related to the better development of grain-clusters at the higher temperature. In addition, the coherent Σ3 boundaries were easier to be formed at the higher temperature during induction heating, owing to their low interface energy and mobility. At the large strain of 0.5, the fraction of Σ3 boundaries also increased with the increasing temperatures, but the grain size exhibited the opposite trend, which was closely related to the well development of static recrystallization (SRX) behaviors. Meanwhile, there was a “symbiotic relationship” between the SRX grains and Σ3 boundaries during induction heating. Through the electrochemical corrosion tests, it was proved that induction heating can contribute to the improvement of corrosion properties of superalloys via increasing the fraction of Σ3 boundaries, while the best corrosion resistance appeared in the samples treated at 800 °C with the strain of 0.5. Moreover, the evolution of both Σ9 and Σ27 boundaries was also closely related to strains and induction heating temperatures, but their fractions were less than 4%.

Static Recrystallization Microstructure Evolution in a Cold-Deformed Ni-Based Superalloy during Electropulsing Treatment

The influence of electropulsing treatment (EPT) parameters on the static recrystallization (SRX) microstructure in a cold-deformed Ni-based superalloy was investigated. During EPT, both the volume fraction of SRX grains and the average grain size increased with the increasing EPT temperature, which was attributed to the thermal effect and athermal effect induced by EPT. The mobility of SRX grain boundaries was promoted at the higher temperature due to the thermal effect, while the nucleation rate would be increased by EPT through decreasing the thermodynamic barrier. The formation of parallel dislocations caused by electron wind force could also play an indirect role in promoting SRX process. Moreover, the volume fraction of SRX grains increased significantly with the extension of EPT time at 700 °C, while the EPT time had a trivial effect on the average grain size. In addition, the sufficient deformation was essential to the occurrence of SRX behavior during EPT, and the localized Joule heating effect could promote the SRX behavior in the samples with the larger strains. Besides that, the influence of twining and carbides on the SRX behaviors was also investigated.

Microstructure and texture optimization by static recrystallization originating from {10-12} extension twins in a Mg-Gd-Y alloy

During the deformation of Mg alloys, {10–12} extension twin often contributes to the formation of basal texture but rarely assists the nucleation of recrystallization, i.e., effective grain refinement, therefore it seems to make against the improvement of formability and mechanical properties. In this work, {10–12} extension twin has been creatively utilized as a preference nucleation site for static recrystallization (SRX), achieving grain refinement and orientation randomization in a Mg-Gd-Y alloy using multi-directional impact forging (MDIF) and subsequent annealing treatment. Effect of {10–12} extension twin on SRX behavior has been investigated by annealing treatment at 450 °C using quasi-in-situ optical microscopy (OM) and quasi-in-situ electron back-scattering diffraction (EBSD). The microstructural evolution during annealing shows that several SRX gains can nucleate from the grain boundary of untwinned grains, but they only have few influences on the final microstructure due to their limited volume faction and sluggish growth. In contrast, a large number of SRX gains can initiate from {10–12} extension twin and grow up without the confine of twin boundaries. Finally, they consume their parent grains and make the main contribution to grain refinement. This should be attributed to those pinned {10–12} twin boundary, by interacting with various dislocation slips during the MDIF process, which can operate like grain boundary, store enough strain energy, and promote the nucleation of SRX during annealing. On the other hand, SRX grains usually keep initial random orientation and further randomize the forging texture during annealing treatment.

A modified kinetics model and softening behavior for static recrystallization of 12Cr ultra-super-critical rotor steel

In this paper, the static recrystallization (SRX) of 12Cr ultra-super-critical (USC) rotor steel was investigated by a series of hot compression tests on a Gleeble1500D thermal simulator. Double-hit hot deformation tests were conducted at temperatures of 1223K–1323 K and strain rates of 0.001 s−1–0.1 s−1 with inter-pass times of 10s–180 s. A conventional kinetics model of SRX was established based on flow curves obtained by regression analysis under different deformation conditions. However, a significant deviation resulted between predicted and experimental values for the SRX fraction, and therefore a modified kinetics equation was proposed by analysing the SRX characteristics and possession of the capability to accurately predict SRX softening behaviour was confirmed. Effects of hot deformation parameters on SRX softening behaviour and SRX grain size were discussed. Furthermore, microstructure deforming was observed at different inter-pass times with an optical microscope (OM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). Analysis showed that recovery was the main softening mechanism and the fundamental nucleation mechanism for SRX was bulging at the grain boundary.

Microstructure evolution of hot work tool steel 5CrNiMoV throughout heating, deformation and quenching

The microstructure evolutions of hot work tool steel 5CrNiMoV throughout the heating, deformation and quenching procedures are investigated. Microstructures of the heated, compressed, and cooled specimens are observed by high-temperature confocal laser scanning microscope (HT-CLSM), optical microscope, and electron backscatter diffraction (EBSD). The measurement shows that the austenite transformation starts at 730 °C and ends at 765 °C. After the temperature exceeds 850 °C during the heating, the in-situ observed grains are first refined by static recrystallization (SRX) and then grow with the holding time, accompanied by the grain boundary gradually become straight. The microstructures of hot compressed specimens indicate that the dynamic recrystallization (DRX) fraction increases with growing temperature and strain, in which discontinuous dynamic recrystallization (DDRX) is the main mechanism of DRX. The grain boundaries formed by DDRX are serrated and have a larger curvature, which are different from those formed by SRX and subsequent grain growth. The lath martensite generated from the water quenching following the compression tends to align parallel to each other in the prior austenite grain (PAG). It comprises the packets containing the elongated blocks with the same habit planes.

EBSD study of the effect of electropulsing treatment on the microstructure evolution in a typical cold-deformed Ni-based superalloy

Evolution of grain structure and grain boundary character distribution in a cold-deformed Ni-based superalloy during the electropulsing treatment (EPT) was investigated by EBSD technique. Compared with the conventional heat treatments (CHT), the nucleation and growth of SRX grains were significantly accelerated by EPT, which was mainly attributed to the high mobility of dislocations and solute atoms, and the efficient energy transfer under the effect of EPT. Especially, with the introduction of electric current, the activation energy of dislocation motion could be reduced by generating additional energy near the dislocation line, which made the motion of dislocation easy to start. The dislocations would also be easy to move in the form of slipping and climbing, under the effect of electron wind force. On the other hand, most of Σ3 boundaries were newly formed through growth accidents in the form of coherent Σ3 boundaries during EPT, and the mutual interactions between different Σ3n (n = 1, 2, 3) boundaries were rather limited. In addition, EPT could indirectly promote the formation of Σ3 boundaries by accelerating the SRX process, but the formation of Σ9 and Σ27 boundaries was inhibited at the temperatures higher than 700 °C. Moreover, the connectivity of random boundaries in the cold-deformed alloy treated by EPT was also discussed.

Static recrystallization kinetics and microstructure evolution of 7055 aluminum alloy

In order to acquire flow characteristics in multi-pass hot plastic deformation and the optimized processing parameters for 7055 aluminum alloy, the double-pass hot compressive flow stress behavior was studied at the temperatures from 300 to 420 °C, the strain from 0.2 to 0.4 and the pass interval time from 10 to 100s on Gleeble-3180 thermo-simulation machine. The static recrystallization (SRX) kinetics and grain size model of 7055 aluminum alloy were acquired by regression analysis. The metallographic tests were conducted and the effects of pass interval time, temperature and strain on microstructure evolution of SRX were analyzed. The results show that 7055 aluminum alloy is temperature sensitive material and the yield flow stress decreases with ascending temperature and pass interval time. Large strain, high temperature and long pass interval time are beneficial to larger SRX fraction and grain refinement. The investigation of flow characteristic and microstructure evolution can be a guidance to acquire optimized processing parameters for multi-pass hot deformation of 7055 aluminum alloy.

Characterization of Static Recrystallization of a Ni-Cr-Mo-Based C276 Superalloy in Two-Stage Isothermal Compression

In order to clarify the microstructure evolution characteristics during holding period after slight degree of deformation for a Ni-Cr-Mo-based C276 superalloy, two-stage isothermal compression tests have been performed at the temperature range of 1000-1150 °C and the strain rate range of 0.1-5 s−1. The stress–strain curves and microstructure observation infer that both static recovery (SRV) and static recrystallization (SRX) took place during the holding period when the specimens were deformed at 1100-1150 °C and only SRV occurred in the specimens that deformed at the temperatures lower than 1100 °C. The softening effects increase with the increasing temperature and deformation degree, while the influence of strain rate seems less pronounced because of the limited stored energy resulted from the small deformation. The grain boundary characteristics, analyzed using electron backscatter diffraction technique, indicate that high densities of twins and $$\varSigma 3$$ grain boundaries form associated with the nucleation and growth of SRX grains. Finally, the recrystallization during and after deformation, i.e., dynamic recrystallization, SRX and metadynamic recrystallization, was discussed concurrently.

Annealing-induced microstructural evolution and mechanical anisotropy improvement of the Mg-Gd-Y-Zr alloy processed by hot ring rolling

Hot ring rolling (HRR) is widely used to fabricate large-size seamless ring-shape components. However, this process produces a severe mechanical anisotropy, which is not acceptable in practical industrial application and should be avoided. This paper reports the annealing-induced microstructural evolution and mechanical anisotropy improvement of HRRed Mg-8.12Gd-1.82Y-0.35Zr alloy ring. Results show that annealing treatment could improve the microstructural homogeneity of the ring owing to the occurrence of static recrystallization (SRX). SRX grains are preferentially formed at twin boundaries and initial grain boundaries due to high dislocation density, high level of stored energy, and stress concentration. Twin boundaries, to some extent, can suppress the migration of SRX grain boundaries. In addition, occurrence of SRX weakens the texture intensity and basal pole inclination, leading to a decrease in the mechanical anisotropy of the ring. Considering the weakened anisotropy, comparable strength, and improved ductility, annealing + aging (T6), compared with direct aging (T5), is a more suitable heat treatment schedule for the HRRed Mg-8.12Gd-1.82Y-0.35Zr alloy ring.

Microstructure evolution and static recrystallization during hot rolling and annealing of an equiaxed-structure TC21 titanium alloy

An equiaxed-structure TC21 titanium alloy, consisting of predominant equiaxed α grains and remained β phase, was hot rolled at 750 °C, and subsequently annealed at different temperatures. Based on electron back-scattered diffraction (EBSD) observation, the microstructure evolution and static recrystallization (SRX) during rolling and annealing were investigated. An apparent evolution of grain morphology occurred during rolling and annealing. The equiaxed α grain was elongated along the transverse direction (TD) during hot rolling, and then got an equiaxed morphology due to the SRX and boundary splitting during annealing. In contrast, subsequent annealing had a limited effect on the textures. The Kernel Average Misorientation (KAM) calculation showed that the β phase endured a larger deformation amount than α phase, which induced a more preferential SRX in β phase. Meanwhile, the different SRX mechanism between α and β phases caused a different evolution of SRX and grain morphology with increasing annealing temperature. The continuous SRX (cSRX) in α phase made the SRX fraction increase and grain size decrease with increasing annealing temperature, with an exception of a grain growth at 940 °C. However, SRX occurred in β phase by subgrain coalescence, which induced the increase of grain size and decrease of SRX fraction with increasing annealing temperature. Conclusively, 880 °C was an appropriate annealing temperature to get a uniform and refined microstructure for the hot-rolled TC21 titanium alloy.

Study of static recrystallization behavior in hot deformed Ni-based superalloy using cellular automaton model

A cellular automaton model (CA) with probabilistic state switches is developed to simulate the microstructural evolution in a Ni-based superalloy during static recrystallization (SRX). The kinetics of SRX is formulated on a mesoscale level. SRX nuclei grow under the driving pressure resulting from the reduction of stored energy and grain boundary energy. It is found that SRX grain size can be well predicted by the developed CA model. Solute elements of the studied superalloy have a drag effects on grain boundary motion. But, this solute drag effect is not obvious at high deformation temperatures. Nucleation rate of the SRX grain increases with the increase of strain, strain rate, and deformation temperature. Besides, the inhomogeneity of grain size first increases, then decreases with the increase of SRX fraction. The changes of inhomogeneity mainly result from the evolutions of the primary and SRX grain sizes.