Mechanical properties at room temperature of an Al-Zn-Mg-Cu alloy processed by equal channel angular pressing

Tensile tests were conducted to evaluate the influence of equal-channel angular pressing (ECAP) on the mechanical properties at room temperature of overaged Al 7075-O alloy. ECAP processing was performed using route BC at different temperatures and number of passes, i.e. different processing severity conditions. The maximum load (Fmax) recorded during the last pass of each ECAP path considered in this study is a very good estimation of the processing severity. The mechanical properties were studied in terms of the balance between tensile strength and ductility. In the processed Al 7075-O alloy, the grain size was reduced down to ∼150 nm. Consequently, tensile testing at room temperature revealed a significant increase in the maximum tensile strength after ECAP with respect to the as start material. In the present study, as the processing severity increases with the number of ECAP passes or with the decrease in processing temperature, there is a consistent trend of increment in ultimate tensile strength with minor decrease in uniform plastic elongation respect to the first ECAP pass at room temperature. This is in contrast to the behaviour after more severe plastic deformation conditions, where an increase in strength together with a strong decrease in elongation would be expected.Financial support from MICINN (Project MAT2009-14452) is gratefully acknowledged. Authors thank Professor Edgar Rauch from the Materials and Processes Science and Engineering Laboratory (SIMaP) in Grenoble, France, for the assistance with the ACOM-TEM technique.Peer reviewe

Grain shape and microstructural evolution during equal channel angular pressing

The congruence between theoretical and experimental shape changes in flow plane microstructures after repetitive equal channel angular pressing (ECAP) by route A is demonstrated for initially spheroidal grains. The model has been extended to predict shape changes of initially ellipsoidal grains of various aspect ratios during repetitive ECAP by route A. The potential role of initial grain morphology on the grain refinement is suggested

High strain rate superplasticity at intermediate temperatures of the Al 7075 alloy severely processed by equal channel angular pressing

The mechanical properties of an overaged Al 7075-O alloy processed by ECAP were examined by tensile tests at intermediate-high temperatures ranging from 250 to 400ºC and strain rates from 10−5 to 10−1 s−1. The influence of the number of ECAP passes on the ductility enhancement was evaluated. Elongation to failure, eF, significantly increased with increasing the number of ECAP passes up to 8 at 130ºC. The alloy processed under these conditions exhibited a maximum value of 322% at 300ºC and an initial strain rate of 10−3 s−1. High strain rate superplasticity was obtained at this temperature, eF=210%, at strain rate as high as 10−2 s−1. The large elongations together with lower stresses and lower stress exponents than those for the start material confirm that grain boundary sliding (GBS) is the operative deformation mechanism. A loss of superplastic behaviour at above temperatures of 350ºC is related to abnormal grain growth and a change of deformation mechanism.Financial support from MICINN (Project MAT2009-14452) is gratefully acknowledged.Peer reviewe

Achieving microstructures prone to superplastic deformation in an Al-Zn-Mg-Cu alloy by equal channel angular pressing

An Al-Zn-Mg-Cu alloy without Sc or Zr additions was subjected to overaging and processing by equal channel angular pressing, obtaining a grain size of ∿160 nm and a fraction of high-angle boundaries of 56%. The high elongations attained at high strain rate (10-2-10-3 s-1) and low test temperature (250-300 °C) are attributed to the formation of new recrystallized grains, ∿500 nm in size, during heating at test temperature. The presence of equiaxed grains and texture randomization in the gauge length confirms that grain boundary sliding is the operative deformation mechanism in the mentioned range of testing conditions. © 2012 Elsevier B.V. All rights reserved.Peer Reviewe

Influence of the thermal treatment on the deformation-induced precipitation of a hypoeutectic Al-7wt% Si casting alloy deformed by high-pressure torsion

A hypoeutectic Al-7wt%Si alloy was subjected to high-pressure torsion (HPT) at room temperature using an imposed pressure of 6 GPa and torsional straining through five revolutions. Different thermal treatments, prior to the HPT processing, resulted in reducing supersaturated Si concentration in comparison to the as-cast materials. Microstructural parameters and microhardness were evaluated in the present work. Processing by HPT produced significant grain refinement with grain sizes of about 200- 400 nm. Furthermore, fine deformation-induced Si precipitates from the supersaturated solid solution were observed, which are very useful in retaining the fine microstructure during HPT processing. The microhardness increase was outstanding, with values for processed samples twice higher (84HV) than that for the as-cast Al-7wt%Si alloy (44HV). The results demonstrate that the refining and strengthening of the Al matrix by HPT processing depend on the available supersaturated solid solution during the processing.Financial support from MICINN (Project MAT2009-14452) is gratefully acknowledged. C.M.Cepeda-Jiménez. thanks the Spanish National Research Council (CSIC) for a I3P contract, and A.P. Zhilyaev to the Spanish Ministry of Education and Science for a Ramón y Cajal contract.Peer reviewe

Impact toughness improvement of high strength aluminium alloy by intrinsic and extrinsic fracture mechanisms via hot roll bonding

A multilayer aluminium laminate comprising ten layers of Al-Zn-Mg-Cu alloy (82 vol.%) and nine layers of pure aluminium (18 vol.%) has been processed by hot rolling. The rolled laminate was characterized by electron backscattering diffraction, Charpy impact and shear tests. The multilayer laminate showed an outstanding Charpy impact toughness, being eighteen times higher than that for the as-received Al-Zn-Mg-Cu alloy. Damage tolerance improvement was due to the high volume fraction of the high strength aluminium and extrinsic fracture mechanisms.Financial support from CICYT (Project MAT2003-01172 and MAT2006-11202) is gratefully acknowledged. C.M. Cepeda-Jiménez thanks the CSIC for a I3P contract. We also thank F.F. González-Rodríguez for assistance during hot rolling and J. Chao- Hermida for assistance with the Charpy impact test.Peer reviewe

Influence of the supersaturated silicon solid solution concentration on the effectiveness of severe plastic deformation processing in Al–7 wt.% Si casting alloy

A comparative study of room temperature severe plastic deformation (SPD) of a hypoeutectic Al–7wt% Si casting alloy by high pressure torsion (HPT) and equal channel angular pressing (ECAP) has been performed. Microstructural parameters and microhardness were evaluated in the present work. Three different initial Si solid solution contents have been considered: as cast (C sample, 1.6 wt% Si), annealed and quenched (Q sample, 1.2 wt% Si) and annealed and furnace cooled (S sample, 0.7 wt% Si). The samples processed by ECAP have smaller average Si particle sizes (0.9-1.7 μm), than those for samples processed by HPT (2.4-4.4 μm). The initial supersaturated Si solid solution is the major factor affecting the microstructure and the mechanical properties of the material. Fine deformation-induced Si precipitates from the supersaturated solid solution were responsible of the large grain refinement obtained by both SPD processing methods, which was considerably higher than that reported for pure aluminium. Q samples, processed by both SPD methods, containing an intermediate concentration of Si in solid solution, show the highest hardness due to the finest and most homogeneous microstructure. The finest and homogeneous grain size was ∼0.2 μm for the HPTed and ∼0.4 μm for the ECAPed, Q samples.Financial support from MICINN (Project MAT2009-14452) is gratefully acknowledged. A.P. Zhilyaev thanks the Spanish Ministry of Science and Innovation for a Ramón y Cajal contract.Peer reviewe

Assessment of homogeneity of the shear-strain pattern in Al–7 wt%Si casting alloy processed by high-pressure torsion

An as-cast Al–7 wt%Si alloy was subjected to processing by high-pressure torsion (HPT) at room temperature, through 1/4, 1/2, 1 and 5 turns at a pressure of 6 GPa and two rotation speeds, 0.1 and 1 rpm. Vickers microhardness was measured along diameters of HPT disk surfaces. The final hardness values were higher than in the initial as-cast condition and, unexpectedly, nearly constant under all different processing conditions, and along the disk diameter. The microstructure was characterised by optical and scanning electron microscopy. The as-cast microstructure comprises equiaxed primary α dendrite cells embedded in the Al–Si eutectic constituent. The morphology and distribution of the eutectic constituent in the HPT processed materials is used to delineate the shear strain, which was analysed in the cross-section planes of the disks. A high degree of homogeneity in the imposed shear strain throughout the samples was observed, being congruent with the ideal rigid-body torsion. In addition, the high compressive pressure applied, causing compressive strain prior to the torsional strain, is responsible for the deformation-induced precipitation of small Si particles and for the (sub)grain refinement in the primary Al constituent. The role of torsional strain is that of increasing monothonically the redistribution of the eutectic silicon and the misorientation of the (sub)grainsFinancial support from MICINN (Projects MAT2009-14452 and MAT2012-38962)Peer reviewe

Influence of processing severity during equal-channel angular pressing on the microstructure of an Al-Zn-Mg-Cu alloy

A commercial Al-Zn-Mg-Cu alloy, Al 7075, was overaged at 553 K (280 °C) for 5 hours and processed by equal-channel angular pressing (ECAP) using route BC. Different temperatures and number of passes, which determine the processing severity, were considered. The processing severity has been estimated by the maximum stress (σ Proc) recorded during each ECAP pass. The higher the number of passes or the lower the processing temperature, i.e., the higher is the processing severity, the finer the (sub)grain size is obtained. A minimum ultrafine (sub)grain size of approximately 150 nm after three passes at 353 K (80 °C) or eight passes at 403 K (130 °C) was obtained. The microhardness exhibited an instant increase from 76 HV for the overaged initial state to 115 HV after only the first pass. The coarsened precipitates in the overaged alloy lead to larger structural refinement than in pure aluminum.Financial support from MICINN (Project MAT2009-14452)Peer reviewe