Microstructure and texture evolution in hot rolled AA 3004 aluminium alloy

Abstract

The evolution of microstructure and texture in industrially hot rolled AA 3004 aluminium alloy has been studied. Specimens were subjected to isothermal annealing treatments and the recrystallisation kinetics was determined via measurement of changes in hardness at surface and centre locations. Metallographic examination of specimens was performed using electron channelling contrast. With the aid of an image processor, the Mg₂Si precipitate count and size distributions were obtained. A significant increase in the number of precipitates was observed to occur after the completion of recrystallisation. Quantitative texture analysis was performed on the tapered specimens to characterise the through thickness effect. The quantity of cube texture component was shown to be higher at the surface than at the centre location and decreased with increasing exit temperature of the hotband.

MST/1564     

TiC/Fe-Al原位复合材料制备及其显微结构

本文研究了用废弃的钢和球墨铸铁以及钛铝添加剂为原料,经熔炼和热处理制备TiC/Fe3Al复合材料的工艺过程.用光学显微镜、XRD等方法观察了复合材料的相组成、显微结构(TiC颗粒的大小和形状),进而分析了原料成分、热处理时间和温度对原位反应、TiC颗粒的生成及显微结构的影响规律.     

Fe3Al基摩擦材料的结构与摩擦磨损性能

采用热压烧结方法制备了一种适用于重载高速工况下的新型Fe3Al金属间化合物基摩擦材料,并对其微观结构、力学性能以及干滑动摩擦磨损性能进行了试验研究.结果表明,Fe3Al金属间化合物基摩擦材料密度低,强度高,摩擦系数高而稳定,高温耐磨性好.不同条件下摩擦磨损机制不同,轻载荷下主要是磨粒磨损和疲劳磨损,表现为微犁沟和凹坑;重载荷下摩擦初中期表现为严重塑性变形、裂纹扩展和疲劳断裂,摩擦后期以氧化磨损为主.     

Microstructure and mechanical properties of Fe3Al alloys with chromium

Alloys based on Fe₃Al have an equilibrium DO₃ structure at low temperatures and transform to a B2 structure above about 550°C. The influence of different rates of quenching from the B2 region to room temperature and of subsequent heat treatments on the microstructure and mechanical properties of powder metallurgy (P/M) alloys with two different chromium contents have been examined. Optimizing the processing to maximize the amount of B2 order, without eliminating dislocations that enhance both strength and ductility, yields room-temperature ductility approaching 20%, although the fracture mode is primarily brittle cleavage. The B2 structure generally has lower flow stress than the DO₃ structure because of its lower strain-hardening rate, although B2 order actually has higher yield strength when the structure is free of dislocations. Increasing the chromium content from 2% to 5% has little effect on ductility, although the 2% Cr alloys generally have higher yield strengths and larger order parameters.     

Microstructure and texture evolution in continuously cooled AA 3004 hot rolled sheet

Abstract

A quantitative study of variations in microstructure and texture evolution in the through thickness direction of industrially hot rolled AA 3004 aluminium alloy has been carried out. The microstructural features of the specimens were examined with the aid of the electron channelling contrast technique in conjunction with an image analysis system. The number of recrystallised grains and the size distributions of coarse and fine intermetallics were measured to evaluate the variation between surface and centre. Significant differences in the number of recrystallised grains and the average size of coarse intermetallics in the through thickness direction of the hotband were observed. After isothermal annealing of the hotband for various times the fine intermetallic area fraction increased and was higher at the centre than at the surface. Quantitative texture analysis was carried out on the specimens and various texture components estimated to characterise the through thickness texture evolution. The proportion of cube texture component was higher near the surface than at the centre and this difference increased after isothermal annealing. Since negligible change occurred in the cube content of the centre specimen, growth of cube grains was deduced to have taken place primarily near the surface region. These observations illustrate that mechanisms of cube texture formation, heterogeneous nucleation of precipitates causing retarded recrystallisation, and Zener drag are evidently applicable even to complex commercial alloys.

MST/1849     

Fe/Al2O3陶瓷梯度涂层的微观结构与性能

采用喷涂法和溶胶-凝胶法相结合的工艺,以FeAlNi混合粉体为过渡层材料在钢基体表面制备了Fe/Al2O3梯度涂层,并对其微观结构与性能进行分析.结果表明:当烧结温度为1220℃时,梯度涂层与钢基体的界面结合强度达到25.3MPa,涂层主要由α-Al2O3,AlFeO3和NiFe2O4等物相组成.Fe/Al2O3梯度涂层与钢基体的结合主要通过吸附与扩散化合两种方式共同起作用.涂层中没有明显的孔洞和平整的界面,且有树枝状组织生成,涂层与钢基体实现良好的结合,这表明涂层成分的梯度化设计能够有效地缓和界面处的应力集中,改善涂层与钢基体的界面结合状态,提高涂层材料的使用性能.     

Microstructure and texture of rolled and annealed β titanium alloy Ti–10V–4.5Fe–1.5Al

This work describes the evolution of texture during cold rolling and annealing of a hot rolled and solution treated sheet of a low cost β titanium alloy Ti–10V–4.5Fe–1.5Al. The alloy was cold rolled up to 60% reductions and then annealed in β phase field at different temperatures to study the re-crystallisation textures. The rolling and re-crystallisation textures obtained in this study are compared with those of other β titanium alloys and bcc metals and alloys such as tantalum and low carbon steel.     

等离子喷涂Fe/Al2O3涂层的微观结构及介电性能

以往将等离子喷涂层用于介电性能的研究较少。采用大气等离子喷涂在石墨表面制备了一系列Fe/Al2O3陶瓷涂层。采用表面分析技术研究了不同Fe/Al2O3含量涂层的微观结构和在8.2～12.4GHz频率范围的介电性能,探讨了Fe含量对涂层相组成和微观结构以及介电性能的影响。结果表明:Fe/Al2O3陶瓷涂层中Fe粉和气孔分布均匀;随着Fe粉量的增加,颗粒热导率变大,涂层中未熔融颗粒减少,γ-Al2O3随之增加,气孔率逐渐下降;涂层的复介电常数随着Fe粉含量的增加而增大。     

Microstructure and texture of Al coating during kinetic spraying and heat treatment

Microstructure and texture formation of an Al coating during kinetic spraying (or cold gas dynamic spraying) and heat treatment were investigated. Coating formation by kinetic spraying is based on super-sonic collision of in-flight micron-sized particles and their severe interfacial plastic deformation under ultra-high strain rates (1.0 × 10⁶–0.5 × 10⁹ s⁻¹), which induces adiabatic shear instability. Shear texture, 45°-rotated Cube {001} <110>, and static recovered microstructure were formed at the interface of Al during kinetic spraying because Al has the equivalent slip system {111} <110> of a face-centered cubic having high stacking fault energy (SFE). During heat treatment, discontinuous recrystallization and grain growth led to transformation of the shear texture into a strong Cube texture, {001} <100>, and misorientation angle transition. Given the mechanical and physical properties of the Al, metallurgical mechanisms of microstructure and texture formation of kinetic-sprayed and heat-treated Al coatings were suggested based on transmission electron microscopy and electron backscatter diffraction analysis.     

Deciphering the potentiodynamic polarization curves of iron aluminides Fe3Al and Fe3Al + Cr

The potentiodynamic polarization curves of Fe₃Al and Fe₃Al + Cr intermetallics obtained in aerated pH 4 H₂SO₄ acidic solution have been theoretically analyzed. The role of chromium in minimizing the hydrogen embrittlement (HE) of the intermetallic Fe₃Al (resulting in its poor ductility) has been addressed based on the analysis. In the case of the chromium-alloyed iron aluminide, calculations indicate that hydrogen liberation does not occur on the surface due to the shift of the corrosion mixed potential to a value nobler than the electrode potential for the hydrogen evolution reaction. This shift occurs due to the induction of passivity on alloying with Cr resulting in the formation of a passive film. The minimization of HE of iron aluminides on alloying with Cr can thus be understood.     

Hot corrosion of Fe3Al

The oxidation and hot corrosion behavior of the binary iron aluminide, Fe-25Al (at.%), has been studied at 1100 K, 1225 K and 1330 K. Hot corrosion studies were conducted by coating the specimen surfaces with 2.5 ± 0.2 mg/cm² of Na₂SO₄ prior to exposure in pure oxygen. Parabolic rate constants were obtained from weight gain data. The faster kinetics in the initial stages of oxidation have been related to the formation of -Al₂O₃ and the slower kinetics in the later stages of oxidation, to the formation of -Al₂O₃. The overall rate of hot corrosion was higher than that of oxidation at all the temperatures. The presence of -Fe₂O₃ in addition to alumina was indicated by XRD analysis of the scales present on the surface of the samples after hot corrosion. FTIR spectra from the spalled scales in hot corrosion divulged the presence of -Al₂O₃, -Fe₂O₃ and sulfate. Cross-sectional microscopy revealed that the metal-scale interfaces were pitted in hot corrosion conditions and the pits contained aluminum sulfide. Sulfides were also identified along the grain boundaries in the intermetallic near the scale-metal interface. The hot corrosion process has been explained based on sulfide formation and its subsequent oxidation.     

Tensile failure of unflawed polycrystalline Al2O3

Scanning electron microscopy and acoustic emission are used to investigate the initial stages of tensile failure in unflawed polycrystalline alumina. It is found that deformation twinning plays an important role in crack initiation even at low homologous temperatures, and that the temperature-dependent strength behaviour between 23 to 410° C is controlled by twinning.     

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al₂O₃ were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 m. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.     

Hall-Petch behaviour of stoichiometric Fe3Al

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Pulsed neutron spectroscopic imaging for crystallographic texture and microstructure

A time-of-flight (TOF) spectroscopic neutron imaging at a pulsed neutron source is expected to be a new material analysis tool because this method can non-destructively investigate the spatial dependence of the crystallographic and metallographic information in a bulk material. For quantitative evaluation of such information, a spectral analysis code for the transmission data is necessary. Therefore, we have developed a Rietveld-like analysis code, RITS. Furthermore, we have applied the RITS code to evaluation of the position dependence of the crystal orientation anisotropy, the preferred orientation and the crystallite size of a welded α-iron plate, and we successfully obtained the information on the texture and the microstructure. However, the reliability of the values given by the RITS code has not been evaluated yet in detail. For this reason, we compared the parameters provided by the RITS code with the parameters obtained by the neutron TOF powder diffractometry and its Rietveld analysis. Both the RITS code and the Rietveld analysis software indicated values close to each other, but there were systematic differences on the preferred orientation and the crystallite size.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.