Partial Recrystallization Research Articles

Morphology-Mechanical Performance Relationship at the Micrometrical Level within Molded Polypropylene Obtained with Non-Symmetric Mold Temperature Conditioning.

The control of the structural properties of a polymeric material at the micro and nano-metrical scale is strategic to obtaining parts with high performance, durability and free from sudden failures. The characteristic skin-core morphology of injection molded samples is intimately linked to the complex shear flow, pressure and temperature evolutions experienced by the polymer chains during processing. An accurate analysis of this morphology can allow for the assessment of the quality and confidence of the process. Non-symmetric mold temperature conditions are imposed to produce complex morphologies in polypropylene parts. Morphological and micromechanical characterizations of the samples are used to quantify the effects of the processing conditions on the part performance. Asymmetric distribution of temperatures determines asymmetric distribution of both morphology and mechanical properties. The inhomogeneity degree depends on the time that one side of the cavity experiences high temperatures. The spherulites, which cover the thickest of the parts obtained with high temperatures at one cavity side, show smaller values of elastic modulus than the fibrils. When the polymer molecules experience high temperatures for long periods, the solid-diffusion and the partial melting and recrystallization phenomena determine a better structuring of the molecules with a parallel increase of the elastic modulus.

Effects of Al addition on tensile properties of partially recrystallized austenitic TRIP/TWIP steels

Abstract Austenitic high-Mn steels are regarded as a promising candidate for high-strength cold-rolled steels because their combination of strength and ductility improves greatly by combined effects of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Although it is well known that Al plays key roles in tensile properties of austenitic TWIP steels, its effects in austenitic TRIP or TRIP/TWIP steel are still unclear yet. In this study, three austenitic steels (composition; 0.4C–15Mn–1Si-(0,0.5,1)Al-0.3Mo-0.5V (wt.%)) were fabricated, and the effect of Al alloying on microstructures and tensile properties were investigated in relation to the deformation behavior with TRIP and TWIP mechanisms. A partial recrystallization was conducted for enhancing the yield strength, which was characterized with electron backscatter diffraction (EBSD) grain orientation spread maps. The present steels showed 1 GPa of yield strength achieved by partial recrystallization with the precipitation of (V + Mo) complex carbides. Particularly in the non-Al-alloyed steel, the e-martensite formed in the early deformation stage, and the martensitic transformation continued until the failure, thereby resulting in the highest tensile strength (1.5 GPa) along with the highest strain hardening.

Piezoelectric hybrid scaffolds mineralized with calcium carbonate for tissue engineering: Analysis of local enzyme and small-molecule drug delivery, cell response and antibacterial performance

As the next generation of materials for bone reconstruction, we propose a multifunctional bioactive platform based on biodegradable piezoelectric polyhydroxybutyrate (PHB) fibrous scaffolds for tissue engineering with drug delivery capabilities. To use the entire surface area for local drug delivery, the scaffold surface was uniformly biomineralized with biocompatible calcium carbonate (CaCO3) microparticles in a vaterite–calcite polymorph mixture. CaCO3-coated PHB scaffolds demonstrated a similar elastic modulus compared to that of pristine one. However, reduced tensile strength and failure strain of 31% and 67% were observed, respectively. The biomimetic immobilization of enzyme alkaline phosphatase (ALP) and glycopeptide antibiotic vancomycin (VCM) preserved the CaCO3-mineralized PHB scaffold morphology and resulted in partial recrystallization of vaterite to calcite. In comparison to pristine scaffolds, the loading efficiency of CaCO3-mineralized PHB scaffolds was 4.6 and 3.5 times higher for VCM and ALP, respectively. Despite the increased number of cells incubated with ALP-immobilized scaffolds (up to 61% for non-mineralized and up to 36% for mineralized), the CaCO3-mineralized PHB scaffolds showed cell adhesion; those containing both VCM and ALP molecules had the highest cell density. Importantly, no toxicity for pre-osteoblastic cells was detected, even in the VCM-immobilized scaffolds. In contrast with antibiotic-free scaffolds, the VCM-immobilized ones had a pronounced antibacterial effect against gram-positive bacteria Staphylococcus aureus. Thus, piezoelectric hybrid PHB scaffolds modified with CaCO3 layers and immobilized VCM/ALP are promising materials in bone tissue engineering.

Influence of Thermal Processing on Resoloy Wire Microstructure and Properties

Magnesium fine wires are critical components for the fabrication of various absorbable implants. Herein, Resoloy fine wires with diameters of 200 and 500 μm are prepared with a multipass, cold‐drawing technique before undergoing two brief thermal anneals at 350 or 450 °C. The as‐drawn, cold‐worked (CW) wires show small but elongated α‐Mg grains, with their long axes and basal planes parallel to the drawing direction. Fine, continuous, and uniformly distributed Mg24Dy5 particles are observed within the wire. Although the brief anneals at 350 and 450 °C do not alter the texture of the Mg grains or the area fraction, aspect ratio, or distribution of the secondary phase, they do enable recovery and partial recrystallization at 350 °C and complete recrystallization at 450 °C, thereby providing a wide range of mechanical properties. The 200 μm wires display slightly higher strengths and slightly lower elongations. Modified simulated body fluid (m‐SBF) immersion results indicate that the Resoloy wires exhibit relatively uniform corrosion with rates as low as 1.72 ± 0.09, 1.44 ± 0.21, and 1.97 ± 0.10 mm y−1 for the cold‐worked, 350 °C, and 450 °C wires with 500 μm diameter, respectively.

Incoherent-light pulse annealing of nanoporous germanium layers formed by ion implantation

The study addresses the monocrystalline c-Ge substrates implanted by Ag+ ions with the energy of 30 keV, irradiation dose of 7.5·1016 ion/cm2 at current density of 8 μA/cm2 and annealed by incoherent-light pulse. By scanning electron microscopy and optical reflection spectroscopy measurements it was shown that after ion implantation an amorphous porous Ag:PGe layer of a spongy structure with nanowires on the c-Ge substrate were formed. Pulsed light annealing of the implanted samples leads to partial melting and recrystallization of the surface Ag:PGe layer. The spongy annealed structure of the Ag:PGe layer was not destroyed, however the diameters of nanowires increased by about 1,5 times.

Synthesis and characterization of nanoparticulate niobium- and zinc-doped bioglass-ceramic/chitosan hybrids for dental applications

Dental caries and periodontal disease are responsible for the most frequent set of chronic diseases in humans, for which no perfectly regenerative solutions are available yet. As a result, materials combining an intrinsic antibacterial activity with tissue regeneration properties for minimally invasive dental therapies are in high demand. Here we report on the fabrication and characterization of a novel nanocomposite material for such dental applications. The material is composed of narrowly disperse Na2O–CaO–P2O5–SiO2 bioglass-ceramic nanoparticles, 30–70 nm in diameter, doped with antibacterial and osteogenic zinc and niobium ions, and hybridized with chitosan. These systems were characterized for their particle size, morphology, atomic and phase composition, and glass-ceramic/polymer interface with the use of transmission electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and Fourier transform infrared spectroscopy. After annealing at 680 °C, amorphous silica in the bioglass-ceramic coexisted with silicalite-1 and combeite, the average crystallite size of which was 20–40 nm. In spite of the significantly better incorporation of zinc than of niobium inside the glass-ceramic network, zinc did not affect the particle size and shape distribution, while niobium lowered the average particle size. Chitosan increased the hydration capacity of the bioglass-ceramic and it formed a continuous interface around the bioglass-ceramic nanoparticles, devoid of micropores. This intimacy of the interface was confirmed by the downshift of the critical Si–O(–Si) vibration modes in the bioglass-ceramic upon hybridization with chitosan. The addition of zinc ions hampered the partial recrystallization during annealing by interfering with the Si–O network restructuring, in direct proportion with its concentration. Niobium ions produced a similar structure-breaking effect, which was evidenced, as in the case of zinc, by upshifting the antisymmetric Si–O–Si stretch of the bridging oxygen and increasing the full-width at half maxima for all the major Si–O(–Si) vibration modes. The effective electrostatic attraction between the aminated hydrocarbon chains of chitosan and the negatively charged silanol groups of silica may extend to the interaction with dentin collagen fibrils decalcified due to caries, making the material of potential interest for adhesive fillers of cariogenic lesions in teeth. Both the undoped and the doped bioactive glass-ceramics interacted favorably with odontoblast-like cells, accentuating their potential for further research for applications in minimally invasive reparative dentistry.

Laser Metal Fused Al Alloy Processed Using Different Heat Treatments

Tensile and dilatometric specimens were obtained with longitudinal axis parallel to the building direction. A first set of samples has been directly aged in the as- built state (AB) while a second batch underwent solubilization annealing (SA) at 540°C for 60min just before artificial ageing (AA) at 160°C for 8h. Solubilization caused the precipitation of Si particles, suppressed by the rapid solidification during SLM. Moreover, it caused partial recrystallization leading to a coarser microstructure. The ageing response resulted very different in the two cases, involving a higher starting hardness for the as-built material, showing a markedly higher hardness during the whole process. In the as built (AB) material, isochronal ageing experiments carried out by differential scanning calorimetry showed the precipitation of Mg2Si particles beneath Si, while the same transformations were much less intense in the solubilized sample, due to the lower Si supersaturation. The ageing process raised the yield strength of about 20% compared to the AB material, slightly reducing the fracture elongation.

Palaeoecology and palaeoenvironment of Mississippian coastal lakes and marshes during the early terrestrialisation of tetrapods

The Ballagan Formation of northern Britain provides an exceptional record of Early Mississippian ecosystems that developed as tetrapods emerged onto land. In this paper, we study two 500-metre sections of the formation near Berwick-upon-Tweed, which are characterised by abundant ferroan dolostone beds. Five lithofacies are identified: cemented siltstone and sandstone, homogeneous dolomicrite, mixed dolomite and siltstone, mixed calcite and dolomite, and dolomite with evaporite minerals. Cemented sediments have non-planar to planar subhedral dolomite crystals, up to 40 μm in size, whereas other facies predominantly comprise dolomicrite or planar euhedral dolomite rhombs 15 μm in size, with patches of larger rhombs indicating partial recrystallisation. The macro- and microfossil content of the dolostones is dominated by sarcopterygian (rhizodont) and actinopterygian fish, bivalves, Serpula, ostracods and Chondrites trace fossils; with rarer Spirorbis, chondrichthyans (Ageleodus, hybodonts and ?ctenacanths, xenacanths), non-gyracanth acanthodians, gastropods, eurypterids, brachiopods, plant debris, wood, lycopsid roots, charcoal, megaspores, phycosiphoniform burrows, Zoophycos? and Rhizocorallium. The oxygen and carbon isotope composition of dolomites range from –3.6‰ to –1.7‰ (for δ18O) and –2.6‰ to +1.6‰ (for δ13C) respectively indicating dolomite growth in mixed salinity waters. Frequent marine storm-surge events transported marine waters and animals into floodplain lakes, where evaporation, interstitial sulphate-reducing bacteria, iron reduction and methanogenesis allowed dolomite growth in the shallow sub-surface. Secondary pedogenic modification (by roots, brecciation, desiccation, and soil forming processes) is common and represents lake evaporation with, in some cases, saline marsh development. The dolostone facies are part of a complex environmental mosaic of sub-aerial dry floodplain, wet marshy floodplains, rivers, and lakes ranging in salinity from freshwater to hypersaline. Marine influence is strongest at the base of the formation and decreases over time, as the floodplain became drier, and forested areas became more established. Coastal lakes were an important habitat for animals recovering from the end-Devonian Hangenberg Crisis and may have acted as a pathway for euryhaline fishes, molluscs and arthropods to access freshwater environments.

Experimental and Numerical Study of the Effects of the Reversal Hot Rolling Conditions on the Recrystallization Behavior of Austenite Model Alloys

The experimental and numerical study of the effects of the recrystallization behavior of austenite model alloys during hot plate rolling on reverse rolling is the main goal of the paper. The computer models that are currently applied for simulation of reverse rolling are not strain-path-sensitive, thus leading to overestimation of the processing parameters outside the accepted process window (e.g., deformation in the partial austenite recrystallization region). Therefore, in this work, a particular focus is put on the investigation of strain path effects that occur during hot rolling and their influence on the microstructure evolution and mechanical properties of microalloyed austenite. Both experimental and numerical techniques are employed in this study, taking advantage of the integrated computational material engineering concept. The combined isotropic–kinematic hardening model is used for the macroscale predictions to take into account softening effects due to strain reversal. The macroscale model is additionally enriched with the full-field microstructure evolution model within the cellular automata framework. Examples of obtained results, highlighting the role of the strain reversal on the microstructural response, are presented within the paper. The combination of the physical simulation of austenitic model alloys and computer modeling provided new insights into optimization of the processing routes of advanced high-strength steels (AHSS).

On the compositional and structural redistribution during partial recrystallisation: a case of σ-phase precipitation in a Mo-doped NiCoCr medium-entropy alloy

In this work, we discussed the atomic-scale mass transport occurring in a Mo-doped NiCoCr medium-entropy alloy to form σ-precipitates during partial recrystallization at 700°C. We reveal the segregation of Cr/Mo to the fault planes of nano-twins/stacking faults in non-recrystallized zones. Upon annealing, the migration of recrystallized grain boundaries (GBs) redistributes the structure and composition, leading to the annihilation of faults and leaving behind the Mo-rich σ-precursors for precipitation. The segregated Cr/Mo solutes can also diffuse to the moving GBs and create additional σ-nucleation sites. The present results highlight the role of Mo in overcoming strength-ductility trade-off of the alloy.

Enhanced cryogenic tensile properties with multi-stage strain hardening through partial recrystallization in a ferrous medium-entropy alloy

We demonstrate the formation of partially recrystallized bimodal microstructures through single-step heat treatment after cold-rolling in Fe60Co15Ni15Cr10 (at%) ferrous medium-entropy alloy that leads to the enhanced yield-strength of ~1.1 GPa and ultimate-tensile-strength of 1.7 GPa at persistent ductility of 61%, evading strength-ductility trade-off. The retention of deformation substructures allows a yield-strength increase by ~79% compared to that in a fully-recrystallized state, together with the modification of mechanical stability without compositional variation, resulting in multi-stage strain hardening capability during tensile deformation at 77 K.

Лита сталь з регульованим аустенітним перетворенням під час експлуатації, одержана електрошлаковим переплавом

The ligature for finishing of base steel 3H3M3Ftype for obtaining steel 4H3N5M3Ftype with adjustable austenitic transformation is developed in the article. The phase-structural state of steel in the cast state is investigated. The uniform distribution of alloying components on the body of grains is shown. It was found that the investigated hardened steel is softening above the tempering temperature of 620 °C, because the heat resistance of steel decreases (below 40 HRC). It was developed a 3H3M3F base steel ligature for steel production with adjustable austenitic transformation of 4H3N5M3F brand. Ingots (ligature of the Fe―Ni―Mo―V―Mn system) weighing 25 kg were obtained. The ligature was obtained by means of an induction furnace in a casting mold. The temperature of the metal in the furnace before release was 1550 °C. The duration of refining did not exceed 20 minutes. The phase-structural state of cast steel is studied. The uniform distribution of alloying components on the body of grains is shown, as well as the absence of coarse carbide eutectic in metal. This allows to reduce energy-intensive technological operations (diffusion annealing, forging) for the die toolsmanufacture. The presence of martensitic structure in the steel in the cast state of the investigated ingot requires the main thermal operation — annealing. It is established that incomplete annealing at a temperature of 750 ± 20 °C under the condition of partial recrystallization of the investigated steel allows to improve the mechanical processing (cutting) for the production of the die tool. It was determined that the investigated hardened steel hardens above the tempering temperature of 620 °C, because the heat resistance of steel decreases (below 40 HRC). Thus, a die tool of the investigated steel for hot deformation, capable of operating up to the temperature of 620 °C. Keywords: steel, ligature, temperature, structure, hardness.

Штампова кована сталь 4Х4Н5М4Ф2 підвищеної стійкості

The results of researches on steel 4H4N5M4F2 modes thermo-deformation processing optimization are given. It is established that incomplete annealing (750 °С ± 20 °С in comparison with full annealing 860 °С) in cast and forged condition promotes to improve the machining of blanks for the manufacture of matrices. It is shown that the use of incomplete annealing, namely partial recrystallization promotes the formation of spheroidized rather than lamellar carbide phase, which leads to a decrease in the characteristics: strength threshold, yield strength, hardness 900 MPa, 800 MPa, 32 33 HRC in the cast state and 1200 MPa, 1050 MPa, 38 39 HRC in forged condition, respectively. This increases the fracture toughness: 180 J/cm2 in the cast state and 130 J/cm2 in the forged state. The optimized mode of forging at the temperature of 1170 ± 20 °С and heat treatment (hardening at 1100 ± 5 °С and tempering at 595 ± 5 °С) of steel 4H4N5M4F2 allowed to increase impact strength five times in comparison with cast experimental steel, and also to increase strength threshold of 100 MPa. Forged steel 4H4N5M4F2 has slightly lower heat resistance compared to cast, which hardens at temperatures above 630 °C during operation of the die steel tool. After operation of the extruder wheels made of investigated forged steel and forged steel 4H5MF1S, which was used at the enterprise in copper processing, the properties of both steels were determined. The investigated forged steel 4H4N5M4F2 is characterized by an increase in the strength threshold by 200 MPa and hardness by 6 HRC. After operation (production of 60 tons of copper products of M1 grade) the tool (wheel extruder) from H13 steel (analog 4H5MF1S) had micro and macrocracks on the side and inner parts, and in the investigated steel 4H4N5M4F2 such defects were absent. Thus, the studied steel is characterized by increased stability. Keywords: steel, heat treatment, forging, structure, physical and mechanical properties.

Heterogeneous Lamella Design to Tune the Mechanical Behaviour of a New Cost-Effective Compositionally Complicated Alloy

A novel heterogeneous lamella (HL) design strategy was proposed to improve mechanical properties of a new cost-effective Fe 35 Ni 35 Cr 25 Mo 5 compositionally complicated alloy (CCA). A HL structure was produced by single-step heat treatment (800 °C for 1h) after cold rolling. This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix (5∼20 μm), and regions containing ultra-fine grains or subgrains (200∼500 nm) together with nanoprecipitates (20-500 nm) and annealing twins. The 800 °C annealed sample with HL structure demonstrated a superior tensile property, with yield strength over 1.0 GPa and ductility of ~13%. Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation of σ phase at the shear bands with a high density of lattice defects (e.g. high-density dislocation walls and deformation twins). The latter restricted the growth of recrystallized grains, leading to the formation of ultrafine subgrains within the HL structure. The high yield strength resulted from the multistage hetero-deformation induced (HDI) strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates. The ductility was originated from the coexistence of multiple deformation mechanisms, which started with dislocation slip and formation of stacking faults at the initial stage, followed by nano-twinning at the higher strain level. This HL design strategy, comprising of composition and thermomechanical process designs, and the resultant microstructure tuning, open a broader window for development of cost-effective CCAs with enhanced performance.

Effect of annealing temperatures on microstructure and deformation behavior of Al0·1CrFeCoNi high-entropy alloy

Abstract Strength-ductility synergy of the Al0·1CrFeCoNi high-entropy alloy can be tuned largely by cold-rolling followed by annealing treatments. However, strain-hardening behaviors and the underlying deformation mechanisms of the low-to high-temperatures-annealed microstructures have not been well clarified. In this paper, the Al0·1CrFeCoNi high-entropy alloy was cold-rolled, followed by annealing at 673–1273 K to investigate the effect of annealing temperatures on the microstructure and deformation behavior. Annealing at 673, 873, 1,073, and 1273 K resulted in no recrystallization, partial recrystallization, full recrystallization, and grain-coarsening, which yielded a stretched-grained, heterostructured, fine-recrystallized, and coarse-grained microstructure, respectively. The heterostructured microstructure was comprised of stretched, partially recrystallized, and fine recrystallized grains. Uniaxial tension results show that the 873 K-annealed specimens possessed a good combination of strength (a yield strength of ~746 MPa) and ductility (a uniform elongation of ~28%), compared with the high-strength-yet-low-ductility cold-rolled and 673 K-annealed specimens, and the high-ductility-yet-low-strength 1073 and 1273 K-annealed specimens. The relationship between the yield strength and average grain sizes of the Al0·1CrFeCoNi high-entropy alloy follows the Hall–Petch equation with a friction stress of ~137 MPa, and a high Hall–Petch coefficient of ~664 MPa·μm1/2. Five and three different regimes existed in the strain-hardening rate versus true strain curves for the 873, and 1073 and 1273 K-annealed samples, respectively. The occurrence of the second and fourth regimes, and the second regime were detected essentially caused by nanotwinning. The correlation between microstructure, tension properties, deformation mechanism, and strain-hardening behavior is discussed.

Warm Tensile Deformation and Fracture Behavior of AZ31 Magnesium Alloy Sheets Processed by Constrained Groove Pressing

Constrained groove pressing (CGP), as a promising severe plastic deformation method suitable for processing sheet metals, was applied to AZ31 magnesium alloy sheets, and ultra-fine grained structure was achieved after two passes of CGP at 473 K. A bimodal structure was also developed due to partial dynamic recrystallization (DRX) and deformation homogeneity. Tensile tests at temperatures from room temperature to 523 K and strain rates from 1 × 10−4 to 1 × 10−1 s−1 were conducted to the processed alloy sheets, and their tensile deformation and fracture behavior were investigated in this work. The maximum elongation to failure of 38.7% is achieved at 1 × 10−4 s−1 and 473 K, and the yield strength and tensile strength are 37.0 and 43.0 MPa, respectively. The strain hardening ability increases gradually with increasing strain rate at elevated temperatures, and its dependence on strain rate is more significant at lower temperatures. The strain rate sensitivity coefficient gradually increases with increasing temperature, and the relatively high values of ~ 0.17 and ~ 0.14 are obtained at 473 and 523 K, respectively, indicating the absence of superplastic behavior. The fracture morphology shows that with increasing temperature, the fracture mode changes from brittle fracture to ductile fracture, which is closely related to the microstructural evolution during tensile deformation. The grain coarsening at 523 K may result in the slight decreases of elongation to failure and strain rate sensitivity coefficient as well as the recovery of strain hardening exponent. The apparent activation energy at 423-523 K is estimated to be 68.8-105.5 kJ/mol. Conclusively, DRX and grain growth should be the dominant mechanism accounting for the warm tensile deformation of AZ31 magnesium alloy sheets processed by CGP, while grain boundary sliding mechanism may contribute little, due to the developed bimodal structure during CGP and the relatively low temperatures for the tensile tests.

Heterogeneous structure-induced strength-ductility synergy by partial recrystallization during friction stir welding of a high-entropy alloy

To apply high-entropy alloys (HEAs) for extensive advanced structural uses, their welding properties should be well understood. In this study, Al0.3CoCrCu0.3FeNi HEA was butt welded by friction stir welding (FSW). The fine-grained partially recrystallized microstructure in the stir zone gave rise to a high tensile yield strength of 920 MPa with an elongation of 37%. By microstructural observation, the excellent mechanical properties of the stir zone material were attributed to the partially recrystallized heterogeneous structure, with which the synergetic strengthening improved the strength of the HEA with considerably less trade-off in its ductility. This unique phenomenon was unprecedented in any other friction-stir welded conventional alloys and was credited to the low stacking-fault energy and the high grain growth activation energy of the HEA. This work suggests that FSW can not only produce good HEA welds but serve as a special processing technique to enhance mechanical properties of HEAs.

A Raman spectroscopy and rheology study of the phase transitions of the ionic liquid choline acetate

Melting of the ionic liquid choline acetate, [Chol][Ac], at ~335 K is manifest in the differential scanning calorimetry (DSC) curve as a broad band covering ~30 K of temperature range. The events taking place in the pre-melting of [Chol][Ac] are revealed by Raman spectroscopy and rheological measurements. The low-frequency range of the Raman spectrum exhibits changes in the intermolecular vibrational modes and the higher frequency range indicates conformational changes of the choline cation. Oscillatory rheological measurements show that the sample experiences a stiffening of its rheological characteristics within the temperature interval encompassing the melting process. Taking together the spectroscopic and rheological data, it is concluded that the sample experiences partial recrystallization during the pre-melting of [Chol][Ac].

Annealing effects in commercial aluminium hot-rolled 7075(–Sc–Zr) alloys

In the present work, the effect of hot rolling (at temperature 300 °C/60 min) on mechanical, thermal properties, microstructure and recrystallization behaviour of the commercial AlZnMgCuFe(ScZr) (7075-Sc,Zr) alloys was investigated. Microhardness together with differential scanning calorimetry measurements was compared to microstructure development that was observed by transmission and scanning electron microscopy and electron backscatter diffraction. Microstructure observation proved two types of eutectic phase (T-phase and Mn,Fe,Si-containing phase) at (sub)grain boundaries in hot-rolled alloys. The grain size was ~ 20 μm in the AlZnMgCuFeScZr and ~ 1000 μm in the AlZnMgCuFe alloy. Sc,Zr-addition in hot-rolled AlZnMgCuFe(ScZr) alloys significantly refined the grains. Primary multilayer Al3(Sc,Zr) particles precipitated during casting and subsequent cooling in the alloy with Sc,Zr-addition. These particles were stable up to 450 °C. No changes of microhardness values (in accuracy of measurement) from room temperature up to 450 °C in the alloys were observed there. Thus, the probable presence of these secondary Al3(Sc,Zr) particles is most likely the reason of hardening (ΔHV ≈ 20) of the AlZnMgCuFeScZr alloy. A little higher electrochemical potential was measured for AlZnMgCuFeScZr alloy (− 1144 mV) than for AlZnMgCuFe alloy (− 1171 mV). One exothermic process was observed in hot-rolled alloys; apparent activation energy of this process was calculated as ~ 100 kJ mol−1. Additional precipitation and/or coarsening of S-phase together with precipitation of T-phase are probably connected with the exothermic process. Sc,Zr-addition has a significant antirecrystallization effect. No recrystallization was observed in the hot-rolled AlZnMgCuFeScZr alloy, neither after annealing at 350 °C/10 h nor after annealing at 450 °C/10 h. A partial recrystallization effect of the hot-rolled AlZnMgCuFe alloy is clearly registered after annealing at 350 °C/10 h and occurs very strongly after annealing at 450 °C/10 h.

Dynamic recrystallization analysis of reduction pretreatment process by multi-phase field method

The reduction pretreatment (RP) process is an effective method to improve billet quality, and the deformation recrystallization plays an important role in the process. Exploring the RP process parameters, a dynamic recrystallization model of GCr15 steel was established by the phase-field method and physical simulation. The recrystallization kinetics and flow stress curves during hot compression were simulated by using this mode. The effects of deformation parameters and initial grain size on the dynamic recrystallization were investigated. Moreover, by using the results obtained by Finite element method (FEM), dynamic recrystallization during the RP process was investigated though this model. It was found that increasing the deformation temperature, deformation rate and decreasing the initial grain size can promote the dynamic recrystallization kinetics. Large Zener-Hollomon parameters can enhance recrystallized grain refinement, while the recrystallized grain size was not affected by the initial grain size. During the RP process, when the reduction is insufficient (10%), partial recrystallization occurs in the billet. With the increase of reduction from 10% to 16%, the area of complete recrystallization increases gradually. When the reduction is the same, the recrystallization in the billet center increases with the decrease of casting speed. When the reduction is 10%, partial recrystallization occurs in the billet center at a casting speed of 0.7 m min−1, and fully recrystallization occurs in the billet center with a casting speed of 0.5 m min−1. Thus, when the reduction is difficult to increase further, the recrystallization in the billet center can be improved by decreasing the casting speed.