Effect of local strain distribution on concurrent microstructural evolution during superplastic deformation of Al–Li 8090 alloy
25 Jun 2003-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing (Elsevier)-Vol. 351, Iss: 1, pp 174-182
TL;DR: In this article, the effect of local strain developed on the microstructural variation along the gauge length of tensile specimens was studied and a model was proposed for cavity nucleation on the basis of inhomogeneity in microstructure and its implication in deformation mechanism.
Abstract: Superplastic deformation of Al–Li 8090 alloy was carried out to study the effect of local strain developed on the microstructural variation along the gauge length of tensile specimens. For this, separate specimens were deformed to failure at a constant temperature of 530 °C and at the strain rates within the superplastic regime. The strain distribution was found to be non-uniform with more deformation towards fracture tip and less towards the shoulder section of specimens. The grain size was found to decrease with increase in local strain whereas cavity size and cavity volume fraction were found to increase. The cavity growth in longitudinal direction is suggested to be controlled by power law but the same in transverse direction is controlled by diffusional process. A model is proposed for cavity nucleation on the basis of inhomogeneity in microstructure and its implication in deformation mechanism.
Citations
More filters
TL;DR: In this article, the effect of tensile deformation at 530°C at a constant strain rate of 5 × 10−4 −1 on the microstructure, texture and mechanical characteristics of the central layer of commercially processed superplastic Al-Li alloy (AA8090) sheet has been investigated.
43 citations
20 Jan 2013-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, microstructures required for superplasticity were fabricated by intermittent multipass friction stir processing (FSP) in a 6mm 5086 aluminium alloy plate.
Abstract: Microstructures required for superplasticity were fabricated by intermittent multipass friction stir processing (FSP) in a 6 mm 5086 aluminium alloy plate. Two processing parameters corresponding to two different heat inputs were used. Multipass FSP created a gradient microstructure with fine and coarse grain-depth features on the processed plates. Three sheets of 1.5 mm thickness with different microstructural features were extracted for deformation testing under superplastic conditions: a layer with only fine grains from the nugget layer (NL), a layer with a thermomechanical-/heat-affected layer containing coarse grains (TL), and a composite layer (CL) having both fine and coarse grains in equal proportions. High temperature tensile testing was conducted for different layers between 450–550 °C with strain rates ranging from 5×10−4 s−1 to 1×10−2 s−1 to determine the optimum superplastic conditions. The NL and CL were comparable in terms of ductility with a high m value of 0.44. The maximum ductility values were 325% for NL, 355% for CL and 230% for TL. The high ductility of the composite layers, despite their microstructural inhomogeneity, establishes multipass FSP as an effective bulk processing technique.
38 citations
TL;DR: In this paper, the superplastic properties of two high strength aluminium alloys are investigated and the effective activation energy of super-plastic deformation is calculated, and the alloy containing both fine and coarse particles exhibit 1100%-1200% elongation at strain rates of 1.
38 citations
16 May 2018-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity as discussed by the authors.
Abstract: Multipass friction stir processing (FSP) was performed in as-cast AZ91 Mg alloy (AC) to generate layered microstructure through the thickness and study its effect on superplasticity. The FSP tools with different tool pin length were used to develop three kinds of layered microstructured materials. A full thickness fine grained microstructure (FFG), a half thickness fine grained with remaining half in as-cast condition (HFG). The last variation was one third thickness modified into fine grain from both the surfaces and the middle section having as-cast microstructure (SFG). FSP was performed at a tool rotational speed of 720 rpm and at a transverse speed of 150 mm/min. The coarse α-Mg dendrites with large plate like interconnected β-Mg17Al12 interdendrites were the characteristics microstructure of as-cast AZ91 alloy. Fine grains and uniformly distributed precipitates were the characteristics of FSPed microstructure. High temperature tensile tests were carried out at 350 °C using three different initial strain rates i.e. 5 × 10−3 s−1, 1 × 10−3 s−1 and 5 × 10−4 s−1. The FFG material showed superplasticity at all strain rates and highest ductility of 680% was achieved at the strain rate of 5 × 10−4 s−1. The AC and HFG material displayed very low elongation while SFG material exhibited superplasticity of 388%. The superplastic behaviour in SFG was due to increase in the fraction of fine grained microstructure and modification of as-cast microstructure on both the surfaces. Microstructure and texture studies revealed that grain boundary sliding accommodated by grain boundary migration and grain rotation was responsible for superplasticity in FSPed region.
32 citations
TL;DR: In this paper, different Al-based alloys were evaluated as materials for cladding on a high-strength Al-Zn-Mg-Cu-Ni-Zr alloy with high-strain rate superplasticity.
12 citations
References
More filters
971 citations
TL;DR: In this paper, the authors compare the conditions under which hole growth without vacancy condensation is faster than hole growth by diffusion and show that low values of the ratio σ/e, where σ is stress and e is the strain rate, as well as large voids favour the strain process.
Abstract: In a creeping solid holes may grow by vacancy condensation or by the action of the applied stress producing strains at the surface of the hole which cause it to grow. The latter mechanism does not involve a vacancy flux to the hole. A comparison of the two processes indicates the conditions under which hole growth without vacancy condensation is faster than hole growth by diffusion. Low values of the ratio σ/e, where σ is stress and e is the strain rate, as well as large voids favour the strain process. Such conditions usually arise in tertiary creep but may also occur earlier in the creep life. Experimental examples of cavitation in which vacancy condensation is shown to be the minor process are given, and the relevance of such a mechanism to hole growth in grain-boundary sliding and regions of localized flow is indicated.
221 citations
31 Jan 2000-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, a new thermomechanical process was developed which produced a fine grain structure in an Al-Mg-Si-Cu alloy, with an average grain diameter of approximately 10 μm and an average aspect ratio near 1.6.
Abstract: A new thermomechanical process has been developed which produced a fine grain structure in an Al–Mg–Si–Cu alloy. The alloy under investigation falls within the composition limits of both 6013 and 6111. The refined microstructure has an average grain diameter of approximately 10 μm and an average aspect ratio near 1.6. Superplasticity was investigated using ambient pressure, uniaxial tensile tests and cone tests with backpressure. The refined material exhibits superplasticity above 500°C. Uniaxial tests indicated a strain rate sensitivity of 0.5 at 540°C, where the elongation reached 375% for a flow stress of 680 psi (4.7 MPa). Cone tests revealed excellent overall formability, and the suppression of cavitation with backpressure. The effects of strain on grain size and porosity were determined.
183 citations
151 citations
15 Apr 2001-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, low temperature superplastic behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized.
Abstract: Low temperature superplastic (SP) behavior (mechanical and deformation mechanisms) of two commercial Mg-based alloys (AZ31 and ZK60) was characterized. The two alloys were tested in the as extruded condition with initial grain size of 15 μm (AZ31) and fine (2 μm) and coarse (25 μm) grains mixed randomly for the ZK60. Strain rate was activated in the range 10−5–1 s−1 at 450 K (0.49Tm) in order to determine the deformation capacity curves (elongation to failure vs. strain rate), and to evaluate the strain rate sensitivity coefficient, m, from the stress versus strain rate curves. Optical, scanning and transmission electron microscopy observations (SEM and TEM) were performed to elaborate on the dynamic recrystallization (DRX) grain growth, fracture modes and deformation mechanisms at the SP mode. In addition, X-ray diffraction was utilized to track for microstructural classification. Although low temperature was applied, the ZK60 exhibited superplastic-like behavior and the maximum peak of elongation (220%) was detected at 1×10−5 s−1 with m equal to 0.2. In AZ31 SP behavior was suppressed due to grain growth, while for ZK60, DRX was detected. However, for the latter alloy, it was observed that the coarse/fine grain interface was the trigger for microcracking initiation. Actually, this phenomenon reduces the SP capacity of the ZK60 alloy. Surface observations and TEM findings indicate that grain boundary sliding with homogeneous character is the controlling SP deformation mode. Typical dislocation features have supported this deformation mode, mainly by grain boundary dislocation pile-ups. More sophisticated extrusion processes as equal angular channel extrusion (EACE) is likely to be considered in the future as a mean to improve grain homogeneity and produce ultra fine grain microstructure.
144 citations