A strategy for creating ultrafine-grained microstructure in magnesium alloy sheets

An Mg-3Al-1Zn alloy with fully recrystallized microstructure and a mean grain size of 1 μm has been produced by high-ratio differential speed rolling under the condition that the cold sheet is subjected to rolling with hot rolls preheated to 473 K, resulting in a total thickness reduction of 68% after two-step rolling. No surface or internal cracks were developed. The microstructure was homogenous along the thickness direction. A bimodal grain size distribution was obtained in which approximately 40% of the grains were ultrafine with submicron size coexisting with other grains with a size of several microns. The proposed processing method holds great potential for continuous production of ultrafine-grained magnesium alloy sheets.     

Enhanced grain refinement and mechanical properties in a severely plastic deformed Ni-30Cr alloy

The present study was carried out to evaluate the microstructures and mechanical properties of severely deformed Ni-30Cr alloys. Cross-roll rolling (CRR) was introduced as a severe plastic deformation (SPD) process. Ni-30Cr alloy sheets were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the recrystallized microstructure. Electron back-scattered diffraction (EBSD) was introduced to analyze grain boundary character distributions (GBCDs). The application of CRR to the Ni-30Cr alloy effectively enhanced grain refinement through heat treatment; consequently, the average grain size was significantly refined from 33 μm in the initial material to 0.6 μm. This grain refinement directly improved the mechanical properties, in which yield and tensile strengths significantly increased relative to those of the initial material. We systematically discuss the grain refinement and accompanying improvement in mechanical properties in terms of the effective strain imposed by CRR relative to conventional rolling (CR).     

Synthesis of ultra high strength Al–Mg–Si alloy sheets by differential speed rolling

The ultrafine-grained Al–Mg–Si alloy sheets, which were fabricated by severe plastic deformation (SPD) using a high-speed-ratio differential speed rolling (HRDSR) and subsequent low temperature aging, exhibited an ultra high strength (yield stress: 455 MPa, ultimate tensile strength: 489 MPa). The strengthening effect was impressive compared with the results obtained by using other SPD techniques. The achievement could be attributed to formation of very fine grains due to significantly increased dislocation density in solute supersaturated matrix, high Hall-Petch constant and particle strengthening gained by formation nano-scale precipitates during the low temperature aging after the HRDSR process.     

Nano grained AZ31 alloy achieved by equal channel angular rolling process

Equal channel angular rolling (ECAR) is a severe plastic deformation process which is carried out on large, thin sheets. The grain size could be significantly decreased by this process. The main purpose of this study is to investigate the possibility of grain refinement of AZ31 magnesium alloy sheet by this process to nanometer. The effect of the number of ECAR passes on texture evolution of AZ31 magnesium alloy was investigated. ECAR temperature was controlled to maximize the grain refinement efficiency along with preventing cracking. The initial microstructure of as-received AZ31 sheet showed an average grain size of about 21 μm. The amount of grain refinement increased with increasing the pass number. After 10 passes of the process, significant grain refinement occurred and the field emission scanning electron microscopic (FESEM) micrographs showed that the size of grains were decreased significantly to about 14-70 nm. These grains were formed at the grain boundaries and inside some of the previous larger micrometer grains. Observation of optical microstructures and X-ray diffraction patterns (XRD) showed the formation of twins after ECAR process. Micro-hardness of material was studied at room temperature. There was a continuous enhancement of hardness by increasing the pass number of ECAR process. At the 8th pass, hardness values increased by 53%. At final passes hardness reduced slightly, which was attributed to saturation of strain in high number of passes.     

Novel severe plastic deformation technique—accumulated extrusion (AccumEx)

Accumulated extrusion, a novel severe plastic deformation technique based on conventional extrusion process, is proposed and has been validated on commercial pure aluminium sheets. Four sheets were extruded together at 75% reduction, and this product was recut into four pieces and reextruded up to eight passes to a strain of 13.2. The tensile strength increased up to 200?MPa after six passes. The elongation to failure was 21% after one pass and 6% after six passes. Ultrafine grains with average grain size of 600?nm were observed after eight passes. The refinement process was monitored along all three directions. Texture evolution played an influential role on the misorientation profile and high angle grain boundary fraction.     

Ion implantation effect on atomic structure of deformed Si, Ir, W, Ni and Cu

It has been revealed, that in Ir subjected to severe plastic deformation, an ultrafine grained structure (UFG) is formed (the grain size of 20-30 nm). Practically no defects have been detected within the grains, while, in the case of Ar⁺ implantation, the subgrain structure with characteristic sizes of about 3-5 nm is formed; defects have been detected within subgrains.The subgrain structure was also revealed in UFG Ni and Cu after severe plastic deformation (SPD) (subgrain size of 3-15 nm), but in the latter case the observed boundary region is broader and subgrain is highly disoriented.     

锻态TB6钛合金β相区压缩变形行为和动态再结晶

使用圆柱形试样在Thermecmaster-Z型热模拟试验机上进行锻态TB6钛合金β相区的热压缩实验(变形温度950~1100℃,应变速率0.001~1 s^-1),研究了合金的高温压缩变形和动态再结晶行为。结果表明,这种合金在β相区的变形激活能为246.7 kJ/mol,其热变形机制是动态再结晶,动态再结晶新晶粒的主要形核机制是弓弯形核。当应变速率为0.01~0.1 s^-1、变形温度为<1000℃时动态再结晶的发展比较充分,变形组织明显细化;当变形温度高于1000℃、应变速率低于0.001 s^-1时,动态再结晶的晶粒明显粗化。在动态再结晶的晶粒尺寸D与Z参数之间存在着相关性,其函数关系为D=6.44×10²·Z^-0.1628。     

Multimodal ultrafine grain size distributions from severe plastic deformation at high strain rates

Severe plastic deformation at high strain rates in machining is explored as a vehicle for engineering grain size distributions in copper. Typical strengthening in unimodal, ultrafine grained chips is found to accompany poor ductility when generated at large strains but low strain rates. However, at higher rates, in situ heating in the deformation zone engenders dynamic recovery that results in material with a multimodal grain size distribution that is comparable in strength to the unimodal chips, but additionally possesses improved ductility.     

Ultrafine-grained steel fabricated using warm multiaxial forging: Microstructure and mechanical properties

Grain refinement of bulk metals using severe plastic deformation (SPD) is a popular approach to improve both strength and toughness. In this paper, grain refinement of steel processed by warm multiaxial forging (MAF) and its mechanical behavior has been investigated. Coarse-grained, plain low carbon steel was deformed using MAF at 500 °C. Microstructural evolution is characterized using electron backscattered diffraction and mechanical behavior has been studied. Fraction of low angle grain boundaries (LAB) is observed to increase with strain up to total engineering strain of 1.3 thereafter it starts decreasing whereas, high angle grain boundaries showed just the opposite trend. It appears that initially grain subdivision takes place with imposition of strain thereby increasing the fraction of LAB. After a critical strain these LAB transforms into the high angle boundaries (HAB). The initial coarser grains of average 30 μm size subdivided into grains of the size finer than 0.5 μm. This has been confirmed by TEM micrographs. Improved tensile strengths and hardness values are obtained after warm MAF.     

Change in grain size and flow strength in P/M Rene 95 under isothermal forging conditions

The changes in microstructure induced by plastic deformation in hot isostatically pressed (HIPed) P/M Rene 95 under isothermal conditions are discussed. Results of the constant true strain rate compression tests are presented for initially fine (7 μm) and coarse (50 μm) grained compacts deformed at temperatures of 1050 °C, 1075 °C and 1100 °C and at strain rates in the range from 10⁻⁴ s⁻¹ to 1 s⁻¹. Under these test conditions, both the fine and coarse-grained compacts recrystallize and their grain size are refined during flow. This grain refinement gives rise to softening in both materials. Ultimately, their microstructures transform into the same equiaxed fine-grained microduplex structure at which point their flow strength becomes identical. Continued deformation at that point produces no further change in grain size or flow strength. Under this steady state regime of deformation, the microduplex grain size and flow strength are independent of the original microstructure but are conditioned by the strain rate at a given temperature. The steady state grain size increases whereas the steady flow strength decreases with a decrease in strain rate and/or an increase in temperature.     

搅拌铸造SiCp/2024复合材料热挤压-轧制变形组织及性能

利用搅拌铸造-热挤压-轧制工艺制备SiCp/2024复合材料薄板。通过金相观察(OM)、扫描电镜(SEM)及力学测试等手段研究了该复合材料在铸态、热挤压态及轧制态下的显微组织及力学性能,分析了材料在塑性变形过程中显微组织及力学性能的演变。结果表明,该复合材料铸坯主要由80～100μm的等轴晶组成,粗大的晶界第二相呈非连续状分布,SiC颗粒较均匀地分布于合金基体中;热挤压变形后,晶粒沿挤压方向被拉长,SiC颗粒及破碎的第二相呈流线分布特征;轧制变形后,基体合金组织进一步细化,晶粒尺寸为30～40μm,SiC颗粒破碎明显,颗粒分布趋于均匀,轧制变形对挤压过程中形成的SiC颗粒层带状不均匀组织有显著的改善作用。数学概率统计指出,塑性变形有利于提高颗粒分布的均匀性。力学测试表明,塑性变形后,复合材料的抗拉强度、屈服强度和延伸率显著提高。SiCp/2024铝基复合材料主要的断裂方式为:合金基体的延性断裂、SiC颗粒断裂及SiC/Al界面脱粘。     

Size effect on deformation behavior and ductile fracture in microforming of pure copper sheets considering free surface roughening

In meso/micro-scaled plastic deformation, material deformation and ductile fracture are quite different from those in macro-scale. The roughness of the free surfaces of workpiece increases with deformation and the decrease of grain number in the sample thickness direction, leading to the nonuniformity of specimen thickness. The so-called size effect and free surface roughening may in turn affect the deformation behavior, ductility and fracture morphology of the samples. To explore the coupled effect of workpiece geometry and grain size on material flow behavior in meso/micro-scaled plastic deformation, uniaxial tensile test of pure copper sheets with different thicknesses and comparable microstructure was performed. The experimental results reveal that the material flow stress, fracture stress and strain, and the number of microvoids on fracture surface are getting smaller with the decreasing ratio of specimen thickness to grain size. In addition, the modified Swift’s equation and the corrected uniform strain are closer to the experimental ones considering the thickness nonuniform coefficient induced by the free surface roughening. Furthermore, the observation of fracture morphologies confirms that the local deformation caused by the free surface roughening leads to strain localization and a decreased fracture strain when there are only a few grains involved in plastic deformation.     

Surface plastic deformation distribution and microstructural evolution in the compound rolling of Ti–50Al billet

Compound rolling, which uses two different roller profiles to create plastic strain variation in the surface of a material, is described in this study. Based on the local load theory, equipment for the plastic deformation on the surface of the rectangular billet has been produced. The compound rolling behavior of Ti–50Al billet has been studied using this equipment. In order to study the deformation distribution of compound rolling, the flow net method for strain measurement has been employed. The deformation differences between compound rolling and flat rolling have been investigated with the commercial finite element method (FEM) code DEFORM-3D. The microstructure and the hardness from the surface to the center of the Ti–50Al billet developed through compound rolling has been characterized. These results indicate that the compound rolling technique results in severe plastic deformation near the surface with limited strain towards the center of the billet. This can result in compound microstructures, with fine recrystallized grains in the near surface region and the original directionally solidified microstructures in the center, and improve the hardness on the surface of the billet significantly.     

高Nb-TiAl合金的高温变形行为及其板材的性能

对高Nb-TiAl合金进行多步热压缩,研究其高温变形行为及其板材的性能。结果表明,热压缩变形后高Nb-TiAl合金的组织中等轴γ晶粒和α晶粒的增多、层片晶团的体积分数和尺寸降低,使其变形能力提高。根据这些结果确定了最优轧制工艺为应变速率低于0.5 s-1、道次变形量前期应不高于25%、变形温度高于1150℃。选用上述工艺对其其进行5道次大变形量轧制,制备出表面质量良好、无缺陷的高Nb-TiAl合金板材,其尺寸为600 mm×85 mm×3 mm。这种板材具有双态组织,平均晶粒尺寸小于5μm,其室温屈服强度、抗拉强度和塑性分别为948 MPa、1084 MPa和0.94%,800℃下抗拉强度为758 MPa。     

Evolution of ultrafine microstructures in commercial purity aluminum heavily deformed by torsion

Commercial purity aluminum (1100Al) bars were severely plastic deformed by torsion deformation at room temperature. The specimens were deformed to ultrahigh equivalent strain of 5.85 in maximum. Microstructure evolution during the torsion deformation was characterized using electron back scatter diffraction analysis on two different sections: the longitudinal section parallel to the torsion axis and transverse section perpendicular to the torsion axis. The grain size decreased and the fraction of high angle grain boundary increased with increasing equivalent strain. Elongated ultrafine grained structure was obtained after an equivalent strain of 3.27. We have found that the microstructure evolution in the specimen deformed by torsion exhibited similar behavior to those in the same material heavily deformed by accumulative roll bonding. The average grain size of 0.32 μm with the high angle boundary fraction of 0.76 was achieved in the specimen deformed to an equivalent strain of 5.27. Though the microstructure and hardness on the transverse section varied depending on the radial positions, they could be arranged as a simple function of equivalent strain. The present work confirmed that the torsion deformation worked as a kind of severe plastic deformation.     

镁合金的塑性变形机制和孪生变形研究

概述了镁合金的塑性变形机制,介绍了镁合金的主要孪生系及其表征技术,详细分析了变形温度、变形速率、受力方向和晶粒尺寸等对镁合金孪生变形的影响,讨论了孪生变形对镁合金塑性变形、动态再结晶、力学性能与断裂的影响。孪生通常发生在粗大晶粒中,晶粒细化可以激活镁合金中的非基面滑移,抑制孪生变形和降低镁合金的各向异性,指出细晶镁合金的研制和工业化生产是变形镁合金发展的重要方向。     

Achieving a Superplastic Forming Capability through Severe Plastic Deformation

Processing by severe plastic deformation (SPD) leads to very significant grain refinement in metallic alloys. Furthermore, if these ultrafine grains are reasonably stable at elevated temperatures, there is a potential for achieving high tensile ductilities, and superplastic elongations, in alloys that are generally not superplastic. In addition, the production of ultrafine grains leads to the occurrence of superplastic flow at strain rates that are significantly faster than in conventional alloys so that processing by SPD introduces the possibility of using these alloys for the rapid fabrication of complex parts through superplastic forming operations. This paper examines the development of superplasticity in various aluminum alloys processed by equal‐channel angular pressing (ECAP).     

Microstructure tailoring of Al0.5CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation (SPD) have high strength but often poor uniform ductility.SPD via high-ratio differential speed rolling (HRDSR) followed by an annealing treatment was applied to Al0.5CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved.The optimized microstructure was composed of an ultrafine-grained FCC matrix (1.7-2 μm) with a high fraction of high-angle grain boundaries (61 ％-66 ％) and ultrafine BCC particles (with a size of 0.6-1 μm and a volume fraction of 8 ％-9.3 ％) distributed uniformly at the grain boundaries of the FCC matrix.In the severely plastically deformed microstructure,the nucleation kinetics of the BCC phase was accelerated.Continuous static recrystallization (CSRX) took place during the annealing process at 1273 K.Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains.The precipitation of the hard and brittle σ phase was,however,suppressed.The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14 ％-20 ％.These tensile properties are comparable to those of transformation-induced plasticity steel.Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified,and each strengthening mechanism contribution was estimated.The calculated results matched well with the experimental results.     

Mechanical and microstructural characterizations of ultrafine grained Zircaloy-2 produced by room temperature rolling

The effect of deformation strain at room temperature on the microstructural and mechanical properties of Zircaloy-2 was investigated in the present work. The sample was initially heat treated at 800 °C in argon environment and quenched in mercury prior to rolling. The deformed alloys were characterized by using EBSD and TEM. It reveals the misorientation of incidental grain boundaries (IDBs) due to large plastic strain induced in the sample. The recovery of deformed alloy upon annealing leads to the formation of ultrafine and nanostructured grains in the alloy. The hardness achieved after 85% room temperature rolling (RTR) is found to be 269 HV, while the tensile strength is 679 MPa and 697 MPa in the rolling and transverse direction, respectively. The improvement in strength is due to generation of high dislocation density and ultrafine grains in the deformed alloy with 85% thickness reduction, during rolling. The deformed alloy subjected to annealing at 400 °C for 30 min sample shows increase in ductility (6% and 7.2%) in rolling and transverse direction, respectively, due to the annihilation of dislocations as evident from the TEM study.     

Effects of Strain Rate and Deformation Temperature on Microstructures and Hardness in Plastically Deformed Pure Aluminum

The microstructures and hardness of pure Al samples subjected to plastic deformation with different tem- peratures and strain rates were investigated. The results showed that the strain-induced grain refinement is significantly benefited by increasing strain rate and reducing deformation temperature. The saturated size of refined subgrains in Al can be as small as about 240 nm in cryogenic dynamic plastic deformation (DPD). Grain boundaries of the DPD Al samples are low-angle boundaries due to suppression of dynamic recovery during deformation. Agreement of the measured hardness with the empirical Hall-Petch relation extrapolated from the coarse-grained Al implies that the low-angle boundaries can contribute to strengthening as effective as the conventional grain boundaries.