Influence of thermomechanical processing on superplastic forming of Mg–Al alloys

Abstract

The aim of this paper is to study the influence of the initial microstructure of several Mg–Al alloys on their superplastic formability and on their post-forming microstructure and mechanical properties. Various thermomechanical processing routes, such as annealing, conventional rolling, severe rolling and cross rolling, were used in order to fabricate AZ31 and AZ61 alloys with different grain sizes. These materials were then blow formed into a hat shaped die. It was found that the processing route has only a small effect in the formability of Mg–Al alloys or on the post-forming microstructures and properties due to rapid dynamic grain growth taking place at the forming temperatures. Nevertheless, good formability is achieved as a result of the simultaneous operation of grain boundary sliding and crystallographic slip during forming.     

Nanoscale Grain Growth Behaviour of CoAl Intermetallic Synthesized by Mechanical Alloying

Grain growth behaviour of the nanocrystalline CoAl intermetallic compound synthesized by mechanical alloying has been studied by isothermal annealing at different temperatures and durations. X-ray diffraction method was employed to investigate structural evolutions during mechanical alloying and annealing processes. The disordered CoAl phase with the grain size of about 6 nm was formed via a gradual reaction during mechanical alloying. The results of isothermal annealing showed that the grain growth behaviour can be explained by the parabolic grain growth law. The grains were at nanometric scale after isothermal annealing up to 0·7 T _m. The grain growth exponent remained constant above 873 K indicating that grain growth mechanism does not change at high temperatures. The calculated activation energy indicated that the grain growth mechanism in the disordered CoAl phase at high temperatures was diffusing Co and Al atoms in two separate sublattices. Furthermore, an equation has been suggested to describe the grain growth kinetics of nanocrystalline CoAl under isothermal annealing at temperatures above 873 K (T/T _m ≥ 0·5).     

Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

Recovery and recrystallization processes in binary Fe3Al based alloys

Three binary alloys of composition Fe-24Al, Fe-28Al and Fe-34Al were prepared to study the recovery, recrystallization and grain growth processes in Fe₃Al based alloys. These alloys were rolled and annealed at temperatures in the range 873 K to 1273 K for two hours. Grain size measurements were performed as a function of composition, annealing temperature and time. Transmission electron microscopy showed recovered and recrystallized grains after annealing at 873 K. The mechanisms of recovery processes was found to be by the migration of single dislocations towards each other to form linear arrays which can subsequently form square or hexagonal dislocation networks. Recrystallization can take place either by the enclosure of dislocation free regions by dislocation networks or by the preferential growth of subgrains. The composition dependence of the recovery and recrystallization processes is weak.     

Effect of degree of cold work and annealing temperature on the microstructure and properties of cold drawn copper wires and tubes

This work gives the results of influence of temperature and deformation degree on changes in the metal grain growth of drawn copper products, because this mutual dependence was observed. Wire samples of O.F.H.C. copper and copper tube samples, chemical content of 99·97% Cu and 0·024% P, were exposed to recrystallized annealing after drawing. The annealing was carried out at temperatures of 400, 450, 500, 550 and 600°C, for 30 min, in laboratory conditions. Investigation results show that after drawing with high cold deformation degree (96–99%), the annealing leads to the changes in the continuous grain growth with increased temperature. The smaller grain size appears at 550°C in comparison with the lower annealing temperature. Annealing has influence on mechanical characteristics of tested samples and during drawing of copper products these characteristics are adequately changed: with regard to the annealing at the 500°C, the characteristics of strength and plasticity increase as a result of decreased grain size. The increase of annealing temperature to 600°C leads to the increased grain size and decreased values of strength and plasticity characteristics.     

7B04包铝复合板热变形行为及其对组织演变的影响

利用热模拟实验研究7B04包铝复合板在变形温度为380~450℃和应变速率为0.1~30 s^-1的热压缩性能,结果表明:随着真应变的增加,热加工图失稳区逐渐向高应变速率区域扩展。最适宜的热加工区域为:温度380~410℃,应变速率5~30 s^-1。采用EBSD技术对变形后的组织进行表征,结果表明:随着温度的增加和应变速率的降低,再结晶晶粒趋向于晶界平直化及晶界取向差逐渐增加的方向演变。包铝层在变形过程中主要发生连续动态再结晶,而7B04基体中同时存在不连续动态再结晶、连续动态再结晶(含几何动态再结晶)。材料最佳的热变形温度为410℃和应变速率10 s^-1,此时7B04基体和包铝层的晶粒尺寸均保持在较小的范围内。     

陶瓷材料的超塑性

虽然陶瓷材料在本质上是一种脆性材料；然而研究已表明细晶陶瓷材料具有超塑性，在高温下能产生很大的拉伸形变．本文综述了超塑性的特征和Y2O3稳定四方ZrO2多晶体这种典型的超塑性陶瓷材料的形变机理，形变特征以及动态晶粒生长、玻璃相和产生孔穴对其超塑性形变的影响，此外，还总结了其他陶瓷材料，包括Al2O3、Al2O3－Y2O3稳定四方ZrO2、纳米陶瓷和玻璃陶瓷的超塑性行为和特征．     

Grain growth in ultrafine grained aluminium processed by hydrostatic extrusion

Ultrafine grained materials can be produced by a number of techniques among which one can distinguish hydrostatic extrusion. In aluminium, this method can be used to obtain a structure with the grain size of 300 nm and high fraction of HAGBs (more than 70%). During annealing this structure undergoes significant changes which were evaluated quantitatively. Annealing for 1 h at temperatures up to 200 °C results in normal grain growth whereas at higher temperatures or for longer annealing times a transition from normal to abnormal growth is observed. The activation energy for low temperature regime is 43 kJ/mol whereas for high temperature annealing—128 kJ/mol. The former corresponds to grain boundary diffusion whereas the latter is close to activation energy of self diffusion in aluminium. The change in activation energy well corresponds to the transition in grain growth mechanism from normal to abnormal.     

The effect of cryogenic deformation on the limiting grain size in an SMG Al-alloy

The minimum grain size obtainable in an Al–0.1%Mg submicron grained (SMG) alloy, subjected to cryogenic plane strain deformation, and its subsequent stability during room temperature deformation have been investigated. A decreasing steady state grain size was obtained with reducing deformation temperature. However, a true nanocrystalline grain structure was not obtained even at 77 K with the high angle boundary spacing only approaching the nanoscale in the sample normal direction. The cryogenically deformed material was unstable on subsequent deformation at room temperature and underwent rapid dynamic grain growth. Dynamic grain coarsening is shown to limit the minimum grain size achievable in an SPD process, even under cryogenic conditions.     

Thermal stability of Al/nanocrystalline-Si bilayers investigated by in situ heating energy-filtered transmission electron microscopy

In situ heating energy-filtered transmission electron microscopy was employed to investigate the interfacial intermixing/reactions during thermal annealing of Al/nanocrystalline-Si (nc-Si) bilayers in the temperature range of 150–500 °C. In comparison with the Al/amorphous-Si (a-Si) bilayer, the Al/nc-Si bilayers were found to be much more stable against thermal annealing. Wetting and c-Si growth processes along Al grain boundaries, which take place during annealing of Al/a-Si bilayers, do not occur in Al/nc-Si bilayers, because of the lack of thermodynamic driving forces in the latter case. As a consequence, also in contrast with Al/a-Si bilayers, no layer exchange occurs in Al/nc-Si bilayers, not even after annealing at 500 °C. Instead, intermixing of Al/nc-Si is realized at the Al/nc-Si interface by the formation of Al spikes growing into the nc-Si sublayers at temperatures higher than 300 °C. The relatively low Al-spike formation temperature in Al/nc-Si systems, as compared with that for Al/single-crystalline Si systems, is ascribed to the higher Gibbs energy of nanocrystalline Si as compared to single-crystalline Si.     

Enhanced tensile ductility in a nanostructured Al–7·5%Mg alloy

Abstract

This paper reports work on the enhanced tensile ductility in a nanostructured Al–7·5%Mg alloy with a mean grain size of 90 nm processed via consolidation of cryomilled Al–Mg powders. An annealing treatment at a temperature of 773 K for 2·5 h modified the extruded microstructure slightly without causing significant grain growth, as revealed by TEM and XRD patterns. The annealing treatment significantly improved the ductility, with a remarkably small loss in strength. The observed high thermal stability of the cryomilled Al alloy was attributed to the existence of impurity elements introduced during cryomilling and the presence of a supersaturated solid solution. The reported phenomenon of enhanced tensile ductility was attributed to a mechanism involving dislocation activity in submicron grains during plastic deformation.     

Dynamic deformation behavior of ultrafine grained aluminum produced by ARB and subsequent annealing

Commercial purity aluminum (1100-Al) sheets with various grain sizes, ranging from 0.2 to 10 μm, were fabricated through accumulative roll bonding (ARB) and subsequent annealing at various temperatures. Mechanical properties of these materials were examined at various strain rates ranging from 10^?2 to 10³ s^?1 (from quasi-static deformation to dynamic deformation). Yield strength of the UFG specimens did not change so much when the strain rate changed. Yielding behavior of the UFG Al with grain size of 1.4 µm was characterized by yield-drop phenomenon, which appeared at higher strain rate. It was found that strain-hardening of the Al matrix was significantly enhanced at high strain rates, which was independent of the grain size. Uniform elongation increased with increasing strain rate in the specimens with the grain size larger than 1 µm, while post-uniform elongation increased with increasing strain rate in the submicrometer grain-sized specimens. Consequently, total elongation of all specimens was improved as the strain rate increased.     

Intergranular fracture of Al–Li–Cu–Mg alloy resulting from non-equilibrium eutectic melting during solution treatment

Abstract

This investigation has examined intergranular fracture during heat treatment and deformation of an Al–Li–Cu–Mg alloy and of an Al–Li–Cu alloy. When solution treatment of the Al–Li–Cu–Mg alloy was initiated by rapid heating to temperatures ≥ 545°C, non-equilibrium eutectic melting of a grain boundary precipitate phase occurred and the liquid spread along grain boundaries as a thin film. On quenching, intergranular cracks were observed at grain boundaries into which a liquid film had penetrated during solution treatment. For less rapid heating rates, non-equilibrium eutectic melting did not occur and no intergranular cracks were observed after quenching. No evidence of non-equilibrium eutectic melting was observed in the Al–Li–Cu alloy irrespective of the rate of heating to 550°C. During tensile testing of as quenched and quenched and aged specimens of the two alloys, intergranular fracture occurred in most specimens, whether or not non-equilibrium eutectic melting had taken place during solution treatment, indicating that at least one additional mechanism of intergranular fracture was initiated by deformation.

MST/947     

Local melting and tool slippage during friction stir spot welding of Al-alloys

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.     

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.     

Effect of composition on critical (cold working) strain and recrystallised grain size of dilute Al–Fe alloys

Abstract

Low strain cold deformation and annealing techniques have been applied to establish the effects of composition and heat treatment variables on the recrystallisation behaviour of Al–Fe alloys. The parameters controlling the critical strain required to initiate recrystallisation and the grain size produced by subsequent recrystallisation annealing have been established to be the volume fraction of secondary phases, the eutectic colony size in both as cast and processed material, matrix composition, the initial grain size preceding final cold deformation, the amount of deformation before annealing, and the final annealing temperature. The results derived from the application of the strain annealing test showed that increasing the volume fraction of secondary precipitate phases, the homogenisation and final annealing temperatures, or the eutectic colony size, decreases the critical strain and increases the maximum grain size produced after annealing, but increasing the initial grain size has opposite effects on these parameters.

MST/1348     

Hot ductility of wrought 90Cu–10Ni alloy

Abstract

The ductility of a wrought 90Cu–10Ni alloy was investigated by tensile testing over a range of temperatures and for materials of different initial grain sizes. Ductilities were found to vary in a complex manner with temperature and grain size, but at all temperatures above 650 K failure occurred by grain boundary cavitation; good ductilities were associated with either grain boundary migration or recrystallisation during deformation. The microstructures of the deformed specimens were studied using optical microscopy and scanning electron microscopy, and the orientation of cracked boundaries, with respect to the tensile axis, was studied. Analysis of the results indicates that over the temperature range studied more than one cavity growth mechanism was operating. The ductility is discussed in terms of the relationship between the cavity growth mechanisms and restoration processes.

MST/722     

退火工艺对Al/Mg/Al复合板界面结合与阻尼性能的影响

研究了Al/Mg/Al三明治结构复合板的退火热处理工艺,探讨了退火温度、时间对复合界面和阻尼性能的影响。结果表明:退火使得Mg层中的孪晶及变形组织消失,晶粒明显长大,且可以促进Al-Mg界面原子的相互扩散。随着退火温度的升高,界面效应对复合板的阻尼性能影响由不利转变为有利,在250℃下随着退火时间的延长,复合板的阻尼性能有一定的提高。综合复合板的组织与性能要求,得到Al/Mg/Al复合板的最佳退火工艺为250℃×2h,在应变振幅为5×10~(-4)下复合板的阻尼值Q~(-1)达0.045。     

Grain refinement of Cu–Zn–Al and Cu–Al–Ni by Ti addition

Abstract

To obtain fine grained Cu based shape memory alloys after thermomechanical processing, Ti is added to β-Cu–Zn–Al or β-Cu–Al–Ni as a particle forming element. This work consists of a study of the mechanism that controls the grain growth limiting effect during the final annealing treatment. A critical evaluation of the grain growth models in particle containing materials and comparison with the experimental results lead to the conclusion that the grain growth inhibition is mainly attributable to the effect of the second phase particles but also to the influence of Ti atoms in solid solution.

MST/678     

Microstructure,grain growth,and hardness during annealing of nanocrystalline Cu powders synthesized via high energy mechanical milling

In this paper, the microstructure and hardness evolutions of commercially pure Cu subjected to high energy mechanical milling and subsequent annealing treatments in the temperature range of 400–700 °C are investigated. The results demonstrated the simultaneous occurrence of recovery, recrystallization, and grain growth during annealing of the nanocrystalline Cu. The volume fraction of the recrystallized grains estimated using the grain orientation spread exhibits lower values as a result of its dynamic recovery at higher temperatures. The normal grain growth in the range of 400–600 °C and significant abnormal grain growth at higher temperatures are observed during annealing. As a result of the abnormal grain growth, the microhardness value rapidly decreases for the sample annealed at 700 °C. An analysis of the grain growth kinetics using the parabolic equation in the temperature range of 400–600 °C reveals a time exponent of n ≈ 2.7 and an activation energy of 72.93 kJ/mol. The calculated activation energy for the grain growth in the nanocrystalline Cu is slightly less than the activation energy required for the lattice diffusion. This low activation energy results from the high microstrain as well as the Zener-pinning mechanism that arises from the finely dispersed impurities drag effect.