Structure stability of AA3003 alloy with ultra-fine grain size

The study on the structure stability of AA3003 alloy produced by an intense plastic straining process named accumulative roll bonding (ARB) has been conducted. The results show that continuous recrystallization took place in the ARBed 3003 alloy with increasing the annealing time at 250°C and increasing the annealing temperature to 275°C. While, discontinuous recrystallization began in some regions after 300°C annealing, and nearly finished after 400°C annealing. Furthermore, an unusual tensile behavior was observed in this alloy after annealing at 250–275°C. The Hall-Petch dependence was observed in the plot of microhardness versus d ^–1/2 of the ARBed 3003 alloy, but its dependence slope was changed. The ultra-fine grains (<1 m) formed in the ARBed 3003 alloy can be stable until annealing at 250°C for 1 h, and the fine grains (<2 m) can be stable until annealing at 275°C for 1 h. Therefore, grain structure formed in the ARBed 3003 alloys after intense plastic strain is reasonably stable. In addition, the mechanism of structure stability and mechanical behavior were also discussed.     

Possibility of grain refinement for superplasticity of a Mg-AI-Zn alloy by pre-deformation

For a commercial Mg-Al-Zn alloy sheet, tensile tests are carried out under various strains, strain rates and temperatures to investigate the possibility of grain refinement by dynamic recrystallization during pre-deformation. It is found from the microstructural observations that relatively fine grains of 10 m or so, in diameter are attained under the conditions of 250°C and 8.3 × 10^–4 s^–1. The specimen pre-strained under this condition also exhibits a fairly good superplasticity of total elongation beyond 300%.     

Influence of different deformation processing on the AZ31 magnesium alloy sheets

AZ31 magnesium alloy sheets were processed by normal rolling (NR), one-pass equal channel angular rolling (1P-ECAR), and cross equal channel angular rolling (C-ECAR) at 400 °C on a die with 105 ° channel angle. The microstructure, texture, and tensile properties of sheets were measured. The results show that ECAR processing can weaken the basal plane texture, thus obviously improve the mechanical properties. The yield ratio σ_s/σ_b decreases and strain hardening exponent n increases along rolling direction (RD) during ECAR, which means that the uniform plastic formability is enhanced. After C-ECAR, the mechanical properties along both the RD and transverse direction (TD) are improved. Different twinning types, fine contraction twinning in the NRed sheets and coarse extension twinning in the ECARed sheets, were observed. The easier activation of twinning and basal 〈a〉 slip leads to the lower yield strength of the ECARed sheets. Dynamic recrystallization (DRX) during the rolling process has great effect on the microstructure of the as-deformed and annealed sheets. The annealed C-ECARed sheets have significant finer and homogenous grains than the annealed NRed sheets, which is attributed to the rarely DRX process during ECAR. The average grain sizes of the annealed C-ECARed samples and NRed samples are 14 and 24 μm, respectively.     

A new grain-refinement process for superplasticity of high-strength 7075 aluminium alloy

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10^–4s^–1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

Topographic study of superplastically formed AA7475 hemispherical domes

Sliding of grain groups occurs during superplastic stretching of 7475 aluminium alloy with a spherical punch. Such co-operative grain boundary sliding (CGBS) is accompanied by cavity formation in intersection sites of CGBS surfaces, formation of striated bands and migration of sliding grain boundaries. Fibres were observed evolving from the striated bands between grains separated due to CGBS. This fibring process anticipates extreme ductility of the material and could be considered as microsuperplasticity originating from operation of a diffusion-like process or incipient melting.     

初始晶粒尺寸对高温交叉轧制镁合金板材组织和力学性能的影响

目的 揭示晶粒尺寸对多道次高温交叉轧制AZ31镁合金板材组织和力学性能的影响规律及机制.方法 通过对不同初始晶粒尺寸的镁合金板材进行高温交叉轧制变形及热处理,获得不同状态的镁合金板材,采用金相显微分析、X射线衍射(XRD)分析及室温拉伸实验等手段研究镁合金板材的晶粒组织(形态、尺寸、取向)及力学性能.结果 经过多道次交...     

Effect of strain-modified particles on the formation of fined grains and the properties of AA7050 alloy

With a new two-pass deformation, a fine-grained AA7050 alloy was obtained owing to small particles which can affect the grain refinement. The banded structures were produced in the elongated grain interiors after the 1st-pass deformation at 300 °C. And deformation bands containing dislocation arrays and small spherical particles were obtained. A few new fined grains appeared along the elongated grain boundaries. After the 2nd-pass deformation at 430 °C, isolated chains of new fine grains were developed in the elongated grain interiors. The boundary glide and the increase of grain boundary misorientation due to cumulative strain could refine the elongated grains. The pinning effect of the particles accelerated the formation of deformation bands. The increase of deformation temperature promoted the rapid evolution of grain refinement during the deformation. The strength of the fine-grained AA7050 alloy was enhanced while the ductility was decreased.     

Nucleation control of diamond synthesized by microwave plasma CVD on cemented carbide substrate

Diamond was coated on to cemented carbide substrate by microwave plasma CVD, in which nucleation control of diamond crystals was investigated under constant deposition conditions; total pressure 30 torr, CH₄ flow rate 1 ml min^–1, H₂ flow rate 199 ml min^–1 and microwave power 550 W. Nucleation tends to occur selectively on the edge part of WC grains of the cemented carbide substrate with coarse WC grain size of about 1 m, where the nucleation density was 9×10⁶ cm^–2. The density increased to about 5×10⁷ cm^–2 when using a finegrained substrate (WC grain size 0.5 m). A considerably enhanced nucleation was observed by introducing a number of fine microflaws on to the substrate surface. Microflawing treatment with diamond fine powder (grain size 0–1/4 m) suspended in an ultrasonic cleaner bath was effective for increasing the diamond nucleation density up to 5×10⁸ cm^–2. The grain size of grown diamond crystals decreased with increasing microflawing time.     

Role of pre-vertical compression in deformation behavior of Mg alloy AZ31B during super-high reduction hot rolling process

Mg alloy AZ31B plates were processed by hot rolling with different thickness reductions per pass and pre-vertical compression followed by super-high reduction hot rolling (PVCR), respectively. Microstructure evolution, rolling formability variation and mechanical responses were investigated. As reduction per pass increased, the number of shear bands deflecting toward rolling direction increased, resulting in easy crack initiation in and around the bands. With increasing reduction per pass up to 80%, twinning and twinning-induced dynamic recrystallization (DRX) dominated the deformation of the edge material at 350?°C, resulting in local recrystallization with coarse grains and further largest edge-crack degree. Pre-induced {10$\overline{1}$2} tensile twins by pre-vertical compression (PVC) increased number density of nucleation sites for dynamic recrystallization during the subsequent severe rolling, which enhanced the dominant role of continuous dynamic recrystallization. Designed PVCR-b was proved to be a relatively effective method to improve rolling formability of rolled Mg alloy AZ31B plates. With this method, mean grain size of AZ31B plate was significantly refined from ～600?mm to ～14.1?mm and more homogeneous grain size distribution along transverse direction (TD) was achieved. In addition, basal texture intensity was greatly weakened. As a result, tensile anisotropy was distinctly decreased and fracture elongation increased dramatically.     

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.     

Microstructure and mechanical properties of Mg–4.5Al–1.0Zn alloy sheets produced by twin roll casting and sequential warm rolling

Microstructure and mechanical properties of Mg–4.5Al–1.0Zn (designated as AZ41M in short) alloy sheets produced by twin roll casting, sequential warm rolling and post annealing at 350 °C were studied in this paper. Microstructure of twin roll casting strip consisted of dendrite structure, eutectics and intermetallic compounds located in the interdendritic region. AZ41M alloy sheets showed higher strength and lower elongation after sequential warm rolling, while post annealing after warm rolling induced the decrease of strength and increase of elongation. This results in the balance of strength and elongation in AZ41M alloy sheets. The grain refinement during manufacturing processes was attributed to the formation of heavy shear bands, high dislocation density, twinning, and precipitates of Al₂Ca/Mg₂Ca or Al₈Mn₅ and the Ca dissolution into Mg₁₇Al₁₂ phase.     

The effects of aging treatment and strain rates on damage evolution in AA 6061 aluminum alloy in compression

AA 6061 aluminum alloy in T4, T6 and T8 temper were subjected to quasi-static compressive loading at a strain-rate of 3.2 × 10⁻³ s⁻¹ and dynamic compressive loading at strain-rates between 7.0 × 10³ and 8.5 × 10³ s⁻¹. The effects of strain rates and temper condition on the deformation behavior of the alloy are discussed. Under the quasi-static loading, deformation was relatively homogeneous and controlled by strain hardening, which is more pronounced in the naturally aged than the artificially aged alloys. Thermal softening played a dominant role under impact loading leading to strain localization along narrow bands called adiabatic shear bands (ASBs). Both deformed bands consisting of aligned second phase particles and transformed bands consisting of fine recrystallized grains were observed. The average size of the recrystallized grains in the transformed bands is about 600 nm and varies slightly depending on the temper condition. The fine grains are suggested to form by dynamic recrystallization. The T4 alloy showed the highest propensity for thermal softening, strain localization and cracking under impact loading while the T8 alloy showed the least tendency. The degree of recrystallization in the transformed band is influenced by temper condition with T8 alloy having the highest fraction of unrecrystallized grains inside the transformed bands. This is related to the temperature rise in the transformed bands that was estimated to be highest in the T4 alloy and lowest in the T8 alloy. The combined effects of high temperature and severe strain inside the transformed bands caused dissolution of second phase particles and induced microstructural changes that resulted in less silicon inside the transformed bands than in the adjacent region.     

On the solidification microstructure of copper-rich niobium alloys

The microstructure of copper-rich niobium alloys has been studied as a function of cooling rate. The main alloy composition investigated was Cu-7 wt% Nb. The cooling rate was varied from 40° C sec^–1 to approximately 7×10^4° C sec^–1. The microstructure as observed in the scanning electron microscope changes from well-dispersed niobium spheres for fast-cooling to an heterogeneous distribution of niobium flowers for slow-cooling.     

Microstructure development in hot deformed AA6082

Age hardenable aluminium alloys are suitable for hot deformation at temperatures between 300 and 600 °C, depending on the alloy and on the process. During thermo mechanical processes, dynamic and/or static restoration mechanisms take place. In this work, hot compression tests of hot rolled AA6082 and of a hot extruded AA6082 were carried out by means of a Gleeble^® 1500 thermo-mechanical system, between 450 and 550 °C and at 0.1-10 s⁻¹ of strain rate followed by fast cooling. The materials were thermo-mechanically processed before hot deformation to transform the complex initial microstructure into a fully recrystallized microstructure. Some compression tests of non recrystallized samples were carried out for comparison. All the samples were examined using electron backscatter diffraction (EBSD), followed by a determination of grain size and subgrain size distributions as well as of the local misorientation distribution. The as received samples showed a duplex microstructure consisting of elongated grains oriented in the rolling and extrusion directions, and some small recrystallized grains. The main restoration mechanism for all the materials is shown to be dynamic recovery, and the Zener-Hollomon parameter, as well as the flow stress was correlated to the subgrain size. The hot rolled material flow behaviour was sensitive to the initial microstructure, and geometric dynamic recrystallization was found.     

Microscopy investigation on fracture mechanisms in hot-isostatically pressed Si3N4/SiC-platelet composites

Fracture mechanisms in hot-isostatically pressed (HIP) Si₃N₄/SiC-platelet composites have been investigated by transmission electron (TEM) and scanning electron (SEM) microscopy followed by profilometric analyses. Two composites containing 25 vol% platelets were compared. They were fabricated from the same raw materials and by the same procedure except for the cooling rate from the sintering temperature. The study consists of experimental observations as well as measurements of fractographic parameters which dictates the level of toughening, such as the percentage of intergranular fracture, lengths and angles associated with the debonding process at the matrix/platelet interface. The presence of microcracking in the neighbourhood of the main crack, a higher fraction of intergranular fracture, as well as substantial debonding at the nitride/carbide interface up to high orientation angles were found in the composite cooled at low rates ( 100°Ch^–1) which, despite the unchanged microstructure, was substantially tougher than that cooled at 650°Ch^–1. These trends were not observed in the composite subjected to fast cooling. The stronger interfacial bonding found after fast cooling under high pressure was attributed to an apparent compressive stress remaining stored at the grain boundary, rather than to a weakening of the platelets or the matrix grains. Calculations based on the mechanics analysis of crack/interface interactions and on quantitative profilometric data, indicated a difference of about one order of magnitude in the apparent interface fracture energy of the two composites.     

Texture and microstructure evolution in cold rolled AZ31 magnesium alloy

Microstructure and texture evolution of an AZ31 magnesium alloy during cold roll was investigated. Shear bands formed and fine recrystallized grains appeared in the shear bands at the reduction of 22% during cold roll process. Texture of hot-extruded AZ31 magnesium alloy can be expressed by (0002) texture, while those of cold roll sheet were characterized by (0002) texture with a double-peak distribution, showing that basal texture tilted about ± 10°away from the normal direction toward the rolling direction.     

Microstructure and mechanical properties of Mg–Zn–Ca alloy processed by equal channel angular pressing

An ultrafine-grained (UFG) Mg–5.12 wt.% Zn–0.32 wt.% Ca alloy with an average grain size of 0.7 μm was produced by subjecting the as-extruded alloy to equal channel angular pressing (ECAP) for 4 passes at 250 °C. The fine secondary phase restricted the dynamic recrystallized (DRXed) grain growth during the ECAP processing, resulting in a remarkable grain refinement. A new texture was formed in the ECAPed Mg alloy with the {0 0 0 2} plane inclined at an angle of 58° relative to the extrusion direction. The yield stress (YS) was decreased in the as-ECAPed alloy with finer grains, indicating that the texture softening effect was dominant over the strengthening from grain refinement. The ductility of the as-ECAPed alloy was increased to 18.2%. The grain refinement caused an obvious decrease in work hardening rate in the as-ECAPed alloy during tensile deformation at room temperature.     

Tensile properties of a few Mg-Li-Zn alloy thin sheets

The tensile properties of experimentally produced Mg-6Li-1Zn, Mg-9.5Li-1Zn and Mg-12Li-1Zn alloy thin sheets at room temperature are investigated in this study. Uniaxial tension tests are carried out for various strain rates between 1.4 × 10^–5 and 8.3 × 10^–2 s^–1, and the microstructural and textural changes during the tests are examined. The Mg-6Li-1Zn sheet is composed mainly of the (hcp) phase and inferior to the other sheets in ductility. The (bcc) phase is dominant in the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets, and they have a considerable sensitivity to strain rate. It is observed that the grains are elongated with textural change mainly in the phase at low strain rates, and the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets have sufficiently high ductility at low strain rates. The Mg-9.5Li-1Zn sheet composed of ( + ) two phase is superior to the Mg-12Li-1Zn sheet of single phase in the tensile strength.     

Rapid quenching of a commercial dental alloy

A commercial silver-tin dental alloy (Ag₃Sn) has been rapidly quenched from the molten state in a controlled atmosphere gun splat quencher in order to obtain a chemically homogeneous structure and a fine grain size. The grain size obtained varies between 0.1 and 10 m. Microstructural features have been examined by scanning and transmission electron microscopy. In very thin regions of as-quenched foils there is considerable faulting, but in slightly thicker regions, quenched-in dislocation loops are observed. The thickest areas of splats show dendritic structures and a cooling rate of 10⁷ Ks^–1 can be estimated from the secondary dendrite arm spacings. Both X-ray and electron diffraction confirm that the crystal structure remains orthohombic after very rapid quenching. Preliminary results on the effect of small grain size on amalgamation properties are reported and compared with powders that have been produced commercially.