Recrystallized Coarse Grains Research Articles

Enhancing strength-ductility synergy of Cu alloys with heterogeneous microstructure via rotary swaging and annealing

The trade-off of strength and ductility in the Cu alloys seriously restricts its wide application. In this study, the paradox of strength and ductility of the Cu alloy is ameliorated by rotary swaging (RS) and subsequent annealing, and reveals its microstructural evolution. It was found that the grain size was obviously refined after the RS, resulting in yield strength as high as 760 MPa but with a ductility of only 1.5 %. After annealing, the RS-350-20 sample (RS sample annealed at 350°C for 20 min) exhibited partial recrystallization at the edge and middle positions, while full recrystallization occurred at the center position, which formed a gradient grain size distribution. Tensile results showed that RS-350-20 sample exhibited an excellent combination of the yield strength of 495 MPa and ductility of 27%. High strength stemmed from residual deformation grains, recrystallized ultrafine/fine grains and heterogeneous deformation-induced (HDI) strengthening. The good ductility originated from recrystallized coarse grain, pronounced HDI hardening and activated deformation twins. In addition, microstructural characterization further revealed that dislocations were accumulated near the recrystallized grains to maintain strain continuity at the interface of deformed grains and recrystallized grains during tensile strain. With increasing strain, deformation twins were activated near the recrystallized grains, which facilitates high strain hardening capability meanwhile maintaining high strength. These findings provide insight for optimizing the strength and ductility of Cu alloys by a feasible processing.

Development of bimodal grain-structured Al-Zn-Mg-Cu-Zr alloys for strength-ductility synergy via microalloying with Hf and Sc

The effect of the combined addition of Sc and Hf on the microstructure and properties of high-strength Al-Zn-Mg-Cu-Zr alloys were systematically investigated. The multiscale particles are acquired after homogenization, with the network-structured coarse AlZnMgCu(Hf) and W-AlCuSc phases distributed along grain boundaries and fine W, Al3M and Al-Sc-Zr-rich particles located within the grains. After simple rolling and solute annealing, bimodal grain structures can be tailored via the collaborative effect from the linearly distributed coarse phases resulting in recrystallized coarse grain (CG) bands and the submicron particles inhibiting grain growth to form fine grain (FG) zones. As Sc gradually replaces Hf, the proportion of submicron particles increases, and the area fraction of CG bands decreases significantly from 88% to 28%. The alloy containing ∼50% CG bands shows an increase in elongation of > 23% with almost no loss in strength compared to the others. The similar strength is attributed to balance tradeoffs among the nanoprecipitates, secondary Al3M dispersoids, and grain structure. The coupling of the intergranular fracture in CG bands and the shear fracture in FG zones contributes to excellent ductility. This finding offers meaningful insights into the collaboration between multiscale particles and grain structure, and provides a simple way to obtain bimodal grain-structured Al-Zn-Mg-Cu alloys with excellent strength-ductility synergy.

Improved strength-ductility combination of pure Zr by multi-scale heterostructured effects via rotary swaging and annealing

In the present study, pure Zr with a heterostructure containing nano/ultrafine grains and recrystallized coarse grains was processed by rotary swaging at room temperature followed by partial recrystallization. The as-swaged Zr shows a significantly increased strength compared to its coarse-grained counterparts, due to abundant nano-grains and ultrafine grains refined by rotary swaging and activation of pyramidal slips. In addition, a strong <10 1‾ 0> fiber texture parallel to the SD is formed in the as-swaged Zr. A similar texture with weaker intensity is found in the as-annealed Zr. Moreover, a basal-type HCP-FCC phase transformation was detected during rotary swaging and maintained after annealing. In this case, the as-annealed Zr with the heterostructure exhibits a superior combination of high strength and enhanced ductility, which is ascribed to the multi-scale heterostructured effects: a) the nano/ultrafine grains and hetero-deformation induced strengthening contributed to the observed high strength, b) the work hardening from recrystallized coarse grains and hetero-deformation induced strain hardening improved the ductility, c) the dislocation motion could be effectively impeded at the FCC/HCP coherent interfaces to enhance the strength, d) the soft FCC phase can accommodate c-axis strain and provide more slip systems, which is believed to obtain good ductility. Furthermore, pyramidal slips are activated at the interface between coarse and ultrafine grains, which helps to improve the tensile properties of heterostructured Zr.

Superior mechanical properties of pure Zr with heterogeneous structure

Grain refinement always provides a high strength of metals and alloys but this is accompanied by a remarkable deterioration of ductility. Here we report a pure Zr with a heterogeneous structure, characterized by recrystallized coarse grains embedded in the ultrafine-grained matrix, which is produced by rolling at room temperature and subsequent partial recrystallization. The results show that the heterostructured Zr exhibits a superior combination of high strength and improved ductility and higher strain hardening than its coarse-grained counterparts. In terms of the high strength of heterostructured Zr, it is ascribed to the abundant ultrafine grains and significant hetero-deformation induced strengthening. Whereas good ductility is derived from the dislocation hardening of recrystallized coarse grains, the hetero-deformation induced hardening, and the sharp (0002) basal texture. The present study can contribute to understanding the essence of a desirable combination of strength and ductility of heterostructured materials. • Heterostructured Zr was obtained by cold rolling and partial recrystallization. • Heterostructured Zr showed high strength and good ductility simultaneously. • Hetero-deformation induced strengthening contributes to high strength and ductility. • The coarse grains and sharp (0002) basal texture aids of keeping ductility.

Microstructure evolution, mechanical properties and deformation characteristics of ultrafine-grained annealed pure aluminum

In this paper, ultrafine-grained pure aluminum was first fabricated by heavy extrusion. Subsequently, the prepared samples were annealed at 350 oC for different times to tailor heterostructures with the aim of achieving strength-ductility balance. The microstructure characterization revealed that the annealed samples formed heterostructures composed of recrystallized coarse grains, retained ultrafine grains, and coarsening banded structure. Typical rolling texture components prevailed as annealing time was below 8 min. When annealing time exceeded 8 min, these texture components were gradually replaced by the Cube-oriented and random-oriented grains, and at the same time, abnormal grain growth occurred with a faster kinetic (growth exponent ñ3.4). Tensile tests indicated that 5–8 min annealed samples obtained a reasonable strength-ductility balance, which was higher than their counterparts predicted by the rule-of-mixture based on the recrystallization model. In addition, the macro-deformation characteristics of the heterostructured sample were comparatively studied using digital image correlation, while the micro-deformation behaviors were revealed by in-situ tension. The results showed that the heterostructured sample caused inhomogeneous deformation and significant geometrically necessary dislocations gradient at the hetero-boundaries. The loading-unloading-reloading tests found that the interaction of heterostructures induced a noteworthy hetero-deformation-induced (HDI) stress, thereby contributing to HDI strengthening.

Abnormal selective dissolution by the partial recrystallization in a plastically deformed austenitic stainless steel

The influence of microstructure evolution during ageing on the corrosion behavior of deformed Super304H stainless steel was studied. For deformed specimen after ageing, an abnormal dissolution current peak existed in the potentiodynamic polarization curve at early passivation when surface passive film was thin and sensitive to galvanic corrosion, which terminated as passivation proceeded to a complete and compact film. Partial recrystallization during ageing formed uneven microstructure where the recrystallized coarse grains were selectively dissolved due to their inferior corrosion resistance compared to the residual nano-grains. Increased recrystallization region in the over-deformed state intensified the selective dissolution to a higher current peak.

Effect of recovery and recrystallization on microstructure and magnetic properties of Fe-0.4P rolled sheets

Powder metallurgical processing of the soft magnetic Fe-P alloys is well known. In recent times, the wrought metallurgical processing of these alloys is gaining importance due to the possibility of achieving a good combination of soft magnetic properties by optimising the microstructure through carefully designed heat treatment. The Fe-P alloys are mainly focussed towards realizing an alternative and cost effective alloy to conventional electrical steels. Since not much work on bulk Fe-P alloy system has been reported, we have put efforts on processing Fe-P alloy by wrought metallurgy route to bring out comprehensively the dependence of the microstructure on magnetic properties. Fe-0.4 wt.% P alloy sheets (~0.5 mm) were prepared by adopting the conventional melting, forging and rolling, and heat treated at various temperatures (500 – 1000 °C). Recovery and recrystallization behaviour was investigated using electron back scattered diffraction technique and correlated with the microstructural parameters and mechanical/magnetic properties. Transmission electron microscopy studies revealed the formation of Fe3P nanoprecipitates (2-4 nm) leading to a reduction in lattice strain as confirmed from X-Ray diffraction data analysis. Heat treatment at 1000 °C/1h resulted in the formation of fully recrystallized coarse grain structure with coercivity of 80 A/m. Upon further aging at 300 - 500 °C/0.5 h, the coercivity reduced to 43 A/m with a core loss of 347 W/kg. The attractive combination of soft magnetic properties was attributed to the coarse grain and nanoprecipitate microstructure with reduction in lattice strain. The results were also compared with commercial non-oriented Si-steels (M700-50A and M400-50A).

Laminated Fe-34.5 Mn-0.04C composite with high strength and ductility

To obtain a good combination of strength and ductility, a laminated composite structure composed of recovered hard lamellae and soft recrystallized lamellae has been produced in a single phase austenitic Fe-34.5 Mn-0.04C steel by cold rolling and partial recrystallization. Enhanced mechanical properties in both strength and ductility have been obtained in the composite structure compared to a fully recrystallized coarse grain structure. A further increase in strength with only minor loss in total elongation has been achieved by a slight cold rolling of the composite structure, which also removes the small yield drop and Lüders elongation observed in the composite structure.

Improvement of tensile properties of pure Cu and CuCrZr alloy by cryo-rolling process

The present study investigates the effect of cryo-rolling process, i.e. cold-rolling at liquid-nitrogen temperature followed by heat treatment, on tensile properties of pure copper and precipitation-hardened CuCrZr alloy. The cryo-rolling process resulted in a simultaneous improvement of strength and ductility of pure copper. On the other hand, a cryo-rolled CuCrZr alloy showed higher tensile strength but comparable ductility with a conventional cold-rolled CuCrZr alloy. Microstructural analysis indicates that the drastically-beneficial effect of cryo-rolling on pure copper may be due to its heterogeneous size distribution of grains which consist of cryo-rolled fine grains, residual cryo-rolled grains and recrystallized coarse grains. The modest but certain benefit of cryo-rolling on CuCrZr alloy can be explained by different texture formation compared with conventional cold-rolling. Effect of neutron irradiation on tensile properties of cryo-rolled CuCrZr alloy is also examined.

Simulation of ballistic performance of coarse-grained metals strengthened by nanotwinned regions

Coarse-grained (CG) metals strengthened by nanotwinned (NT) regions have both ultrahigh strength and good ductility. The presence of the NT regions contributes to their ultrahigh strength, while their good ductility is attributed to the recrystallized coarse grains. These characteristics make them a potential candidate for bullet-proof material. In this paper, numerical simulations based on the mechanism-based strain gradient plasticity and the Johnson–Cook failure criterion are carried out to investigate the effects of twin spacing and microstructural attributes on the ballistic performance of CG copper strengthened by NT regions. We investigate the performance of fourteen idealized microstructures, and find that smaller twin spacing and regular distribution of NT regions are more conducive to the promotion of the ballistic performance. We also uncover that the role of the shape of NT regions is significantly affected by twin spacing. Furthermore, we make a comparison with its CG counterpart without NTs, and find that microstructures with array arrangement of NT regions have higher limit velocities and smaller relative displacements than the single phase CG structure. This makes them a strong candidate for helmets and other personal protective equipments. It is believed that the simulated results could provide useful insights into the development of this advanced class of metals for ballistic protection.

Enhanced ductility in a bimodal ultrafine-grained Ti–6Al–4V alloy fabricated by high energy ball milling and spark plasma sintering

Ultrafine-grained and nanocrystalline alloys have strengths several times higher than those of their coarse-grained counterparts, but they often exhibit limited ductility at room temperature. Enhancement of ductility is critical to the practical application of those kinds of materials. In the present study an ultrafine-grained Ti–6Al–4V alloy with a bimodal microstructure were fabricated by means of spark plasma sintering of a mixture of ball milled and unmilled powders. The sintered sample showed a microstructure consisting of equiaxed ultrafine grains (74%), lamellar-structured coarse grains (20%) and recrystallized coarse grains (6%). The bimodal-structured alloy exhibited high compressive strengths (σy=1368MPa, σmax=2306MPa) and a large plastic strain to failure (εp=24%) simultaneously. The high strength primarily results from the contribution of equiaxed ultrafine grains, while the enhanced ductility may be attributed to (1) the improved strain hardening capability by the presence of coarse grains, (2) the α/β interfaces in lamellar structure that allow for easy slip transmission, (3) the high fraction of high angle grain boundaries in the ultrafine-grained matrix and (4) occurrence of crack blunting and deflecting due to the introduction of lamellar-structured regions.

Experimental Study of Squeeze Casting of ZK60 Magnesium Alloy

Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10-2 s-1 and 10-3 s-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Enhanced Tensile Ductility of AZ80 Magnesium Alloy

Tensile behaviors of extruded and rolled AZ80 Mg alloy were investigated with elongation-to-failure tensile tests at constant temperatures of 300 °C, 350 °C, 400 °C, and 450 °C, and constant strain rates of 10-2 s-1 and 10-3 s-1. Experimental data show that the material exhibits tensile ductilities of over 100% at 400 °C and 450 °C, featured by long steady state deformation. Microstructure studies show that annealed coarse grains were remained in the gauge region during the tensile tests, and the enhanced tensile ductilities resulted from dislocation creep, other than dynamic recrystallization or grain boundary sliding. Cavity evolution and recrystallized coarse grains near fracture end caused premature failure of the material.

Investigation on Corrosion Behaviors of Ultra-Fine Grain Copper in 3.5% NaCl Solution

The corrosion behavior of ultra-fine grain (UFG) copper bulk prepared by equal channel angular pressing (ECAP) was studied in 3.5% NaCl solution. The effect of ECAP deformation on the copper corrosion is controversial in the literature, and worth to verify by means of various experimental techniques. Corrosion performances of UFG copper were investigated in comparison with that in recrystallized coarse grain (CG) copper by polarization curves, Tafel extrapolation method, electrochemical impedance spectroscopy(EIS). The shape of polarization curves and type of corrosive attack remains the same in the UFG and the coarse-grain state. UFG copper exhibited a lower corrosion current and high self-corrosion potential in comparison with CG copper. Electrochemical experimental results showed that UFG copper increased in resistance to corrosion compared with CG copper. This decrease in corrosion resistance was mainly attributed to the more compact corrosion film of UFG copper. The compact passive film led to decrease of the diffusion capability of ions within the corrosion film and corrosion rate.

Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing

Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that in recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which is sensitive to dislocations and grain boundaries. UFG copper exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on the surface of UFG copper is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.

Intergranular Corrosion of Ultrafine Grain Copper Fabricated by ECAP

Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that of recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which attack dislocations and grain boundaries preferentially. UFG copper have exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on its surface is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.

Microstructure characterization of bulk nanocrystalline Fe–0.8C alloy produced by mechanical milling and spark plasma sintering

The microstructure of bulk nanocrystalline Fe–C alloy produced by mechanical milling (MM) and spark plasma sintering (SPS) has been characterized using transmission electron microscopy (TEM) and a three-dimensional atom probe (3DAP) to understand the origin of the unusually high yield strength and plastic strain observed from the sample. A bimodal grain structure consisting of fine (∼250 nm) and coarse (∼900 nm) grains was observed as a result of partial recrystallization. The fine grained region was found to be a duplex phase structure comprised of ferrite and cementite grains. From TEM and 3DAP analyses, the presence of fine oxide particles containing chromium was confirmed. The presence of the recrystallized coarse grains is attributed to the large plastic strain in compression.

Hot rolling workability, texture and grain boundary character distribution of B2-type FeAl, NiAl and CoTi intermetallic compounds

Hot-rolling workability, texture and grain boundary character distribution of B2-type FeAl, NiAl and CoTi intermetallic compounds were investigated as a function of alloy stoichiometry. All the FeAl (i.e., Fe-38Al, -43Al and -48Al, denoted by at.%), and stoichiometric NiAl and CoTi were successfully hot-rolled at 1273 K but off-stoichiometric NiAl (Ni-48Al and -52Al) and CoTi (Co-48Ti and -49Ti) failed. After hot-rolling, all the FeAl showed microstructures with recrystallized coarse grains while (stoichiometric) CoTi retained a deformed microstructure. Hot-rolled (stoichiometric) NiAl showed an intermediate microstructure between FeAl and CoTi. The hot-rolling and annealing textures of FeAl essentially consisted of {111}〈u v w〉. For NiAl and CoTi, {111}〈110〉 and {111}〈112〉 were prominent in the hot-rolling texture, respectively. Both the hot-rolled NiAl and CoTi showed fully-recrystallized microstructures by subsequent annealing, but the resultant recrystallization textures were similar to their hot-rolling textures. On the other hand, grain boundary character distributions of FeAl, NiAl and CoTi with fully-recrystallized microstructures were similar to one another and characterized by a high frequency of low angle boundaries (i.e. Σ 1 boundaries). Based on these results, recrystallization and grain boundary structure of the B2-type ordered intermetallic compounds were briefly discussed.

６０８２アルミニウム合金溶接材の各部の破壊靭性

The effects of Mn addition on toughness of welded Al–Mg–Si alloys have been investigated. In-situ SEM observations of fracture toughness tests have also been used to characterize crack initiation and propagation behavior through the weldment. Recrystallization of HAZ is completely suppressed by adding Mn, while a sample without Mn exhibits recrystallized coarse grain structure. Recrystallization in HAZ can lead to drastic decrease of its fracture toughness comparing with the samples containing Mn. Many microcracks are distributed at grain boundaries within several mm ahead of a crack-tip where crack-tip stresses are relatively weak. Fracture-mechanical analysis reveals that the toughness degradation is attributed to such premature damage evolution before the onset of crack extension. However, the microcracking is not attributed to so-called liquation cracks, but degradation of the grain boundaries due to the formation of film-like Al–Mg intermetallic compounds. The microcracks ahead of the crack-tip aligned at an angle of approximately 60° from an initial notch direction. Tension obliquely applied to the HAZ next to welding pool appears to give rise to such inclination of the microcracks. It is suggested that the toughness degradation can be suppressed by utilizing such anisotropy even if the GB films are formed during welding.

An alternative approach to processing dispersion-stabilized superalloys

A possible simpler, more direct, more reproducible and less expensive method for processing nickel-base oxide-dispersion-stabilized alloys is described. The process calls for melting or remelting of fully alloyed master alloys and then atomizing to produce a high yield of fully alloyed, rapidly solidified fine powders, ideally finer than about 44–60 μm. The fine powders are attrited to flakes nearly a micron thick in a protective fluid, followed by ball milling in an inert atmosphere to blend in very fine refractory oxide particles. Excellent dispersion of the oxides is obtained. After consolidation by hot extrusion, direcitonally recrystallized coarse grains with a 〈100〉 fiber texture and high grain aspect ratio are produced using a solid state crystal-growing zone-aligned polycrystal (ZAP) process. The stress rupture strength of the ZAP superalloy product is twice that of the same alloy given a conventional recrystallization heat treatment and about four times that of the as-extruded material. The application of this technique to both solid solution and precipitation-strengthened oxide-dispersed alloys is discussed.