Mechanical properties and microstructural evolution of Al 6061 alloy processed by multidirectional forging at liquid nitrogen temperature

Al–Mg–Si alloy was subjected to multidirectional forging (MDF) at liquid nitrogen temperature (LNT), to cumulative strains of 1.8, 3.6 and 5.4. The deformed microstructures were examined by optical microscopy under polarized light and transmission electron microscopy (TEM). The deformed samples showed the formation of dislocation cells structure with high dislocation density at lower strains. Composite structure consisting of lamellar morphology at deformation bands and equiaxed grain morphology was observed. Significant differences in microstructure of the deformed samples were observed with increasing strain at LNT. At cumulative strain of 5.4, the microstructure shows nearly equiaxed subgrain structure with an average size of 250 nm with high angle grain boundaries. The mechanical properties were studied through Vickers hardness testing machine and tensile tester. The hardness value of MDFed alloy at LNT has increased from 50 Hv to 115 Hv for cumulative strain of 5.4. Tensile strength has increased from 180 MPa to 388 MPa with 4.5% percentage of elongation to failure. The improvement in hardness and tensile strength of forged alloy is attributed to the formation of equiaxed sub-grain structures and the presence of high dislocation density.     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

Dynamic recrystallization behavior of GH4169G alloy during hot compressive deformation

The microstructure evolutions and nucleation mechanisms of GH4169 G alloy were studied by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The hot compression tests were performed different imposed reductions in the range of true strain from 0.12 to 1.2 at the temperatures of 930 ℃-1050 ℃ with strain rates of 0.01 s⁻¹-1 s⁻¹. It is found that cumulative and local misorientation increase firstly and then decrease when the strain is increased due to the progress of dynamic recrystallization (DRX). The low angle boundaries (LAGBs) rapidly develop to high angle boundaries (HAGBs) at relatively high deformation temperature or the low strain rate. There are three DRX mechanisms observed for GH4169 G alloy during hot deformation. Discontinuous dynamic recrystallization (DDRX) as the dominant mechanism for GH4169 G alloy is characterized by typical necklace structures and bulged-original boundaries. Besides, different deformation bands with dislocation cells formed in deformed matrix at low temperature and large strain, which indicates that continuous dynamic recrystallization (CDRX) contributed to the DRX process. The twin boundaries lost their coherent characteristics and provide sites for nucleation, which also accelerates the nucleation of DRX.     

Structural and mechanical properties in AA 5083 processed by ECAE

Abstract

Equal channel angular extrusion has been used to analyse refining of grains in an industrial 5083 aluminium alloy during severe plastic deformation. The influence of the total strain as well as of the processing route were studied by tensile tests and TEM. The room temperature behaviour and the high temperature properties suggest that large strains increase the density of high angle boundaries in the material. The optimal processing route to achieve grain refinement appears to be route B for the present investigation.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.     

Heterogeneous deformation of a two phase nickel – tungsten alloy

Abstract

The pattern of co-deformation of micrometre sized W particles in a spherodised Ni - W alloy has been studied in a series of plane strain deformed specimens. The microstructural changes were characterised by optical microscopy, SEM and TEM. A pronounced heterogeneity was found in the strains sustained by the particles, with some particles remaining equiaxed at high imposed strains while others sustained strains equal to the imposed strain. Two factors were shown to be important in this behaviour: the spacing of the slip bands in the matrix relative to the spacing and the size of the particles, and the heterogeneous distribution of W particles in the starting microstructure. The results of plane strain deformation have been compared with those reported for wire drawing of two phase structures.     

Enhanced work hardening in Hadfield steel during explosive treatment

This paper presents results concerning Hadfield steel subjected to explosive treatment and compression, respectively, with the purpose of clarifying the difference between dynamic and static deformation behaviors. A Hadfield steel sample that was deformed to a lower strain at an exceptionally high strain rate exhibited the same hardness as a sample that was deformed to a higher strain at a low strain rate. A deformation model based on in situ deformation has been developed, whereby the enhanced work hardening during explosive treatment is attributed to the deformation of the grains being mainly accommodated by the curvature of the grain boundaries and shape change of the surrounding grains in their original positions, without obvious macroscopic deformation.     

Deformation twinning in AISI 316L austenitic stainless steel: role of strain and strain path

Abstract

AISI 316L austenitic stainless steel was deformed at different strain and strain paths. The twin boundaries in the deformed microstructure had two possible origins: decay of original annealing twins and generation of deformation twins. Assuming that rotations of grains, specifically grains on both sides of a twin boundary, are responsible for the twin decay, a simple model was proposed to bring out the domain of relative twin generation. A biaxial strain path, in general, was associated with strong twin generation – an association or dependency linked to the texture estimated values of Taylor factor. Formation of strain induced martensite was also observed to be strain and strain path dependent and was more in biaxial strain path.     

Ultrafine-grained steel fabricated using warm multiaxial forging: Microstructure and mechanical properties

Grain refinement of bulk metals using severe plastic deformation (SPD) is a popular approach to improve both strength and toughness. In this paper, grain refinement of steel processed by warm multiaxial forging (MAF) and its mechanical behavior has been investigated. Coarse-grained, plain low carbon steel was deformed using MAF at 500 °C. Microstructural evolution is characterized using electron backscattered diffraction and mechanical behavior has been studied. Fraction of low angle grain boundaries (LAB) is observed to increase with strain up to total engineering strain of 1.3 thereafter it starts decreasing whereas, high angle grain boundaries showed just the opposite trend. It appears that initially grain subdivision takes place with imposition of strain thereby increasing the fraction of LAB. After a critical strain these LAB transforms into the high angle boundaries (HAB). The initial coarser grains of average 30 μm size subdivided into grains of the size finer than 0.5 μm. This has been confirmed by TEM micrographs. Improved tensile strengths and hardness values are obtained after warm MAF.     

Recrystallisation behaviour of supersaturated Al–Mn alloys Part 1 – Al–1.3 wt-%Mn

Abstract

The effects of concurrent precipitation on recrystallisation during the isothermal annealing of a supersaturated and deformed Al – Mn alloy have been investigated. At low annealing temperatures precipitation on the prior boundaries prevents recrystallisation whereas at high temperatures recrystallisation is complete before precipitation occurs. In the temperature range 375 – 500°C, recrystallisation is affected by precipitation and complex microstructures containing high fractions of low angle boundaries are formed. It is shown that strain induced boundary migration of very large boundary areas is an important recrystallisation mechanism in this temperature range. The interactions between precipitation and recrystallisation are analysed in terms of a simple model. The heating rate is shown to substantially affect the recrystallisation behaviour.     

Microstructure and texture evolution during incremental sheet forming of AA1050 alloy

In incremental sheet forming higher limiting strain can be achieved compared to the conventional sheet metal forming process, which results in increased formability. The higher level of strain may be accompanied by non-uniform thinning. Thus, the different sections in a component may undergo different levels of deformation. In the present work a truncated cone of AA1050 H14 alloy was formed using the incremental sheetmetal forming (ISF) technique. The deformation mechanism during ISF was studied by investigating the microstructural and texture evolution in the truncated cone along the thickness of the cone wall. High resolution electron backscatter diffraction was performed at different sections of the formed truncated cone. The results show the formation of subgrains in different sections of the cone. At higher strains, grains become thin and elongated which results in grain fragmentation and formation of small grains. These small grains undergo complete recovery process and new grain boundaries (low and high angle) are formed within the thin elongated grains. Further, the evolution of shear texture shows the evidence of shear mode of deformation during incremental sheet forming. Thus, the presence of through thickness shear could be used for understanding the higher forming limit in the ISF process.

     

Recrystallization and Grain Growth in Ultrafine‐Grained Materials Produced by High Pressure Torsion

Ultrafine‐grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu–0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8 GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries.     

High temperature deformation of aluminium bronze

Abstract

Hot compression tests were carried out on commercial Cu–8 wt-%Al alloy to test the effect of the deformation conditions on high temperature deformation characteristics and the final structure of the hot deformed material. Dynamic recrystallisation of the material was found to operate at deformation temperatures above ～900 K. Nucleation and growth of recrystallised grains were observed for specimens deformed at temperatures below ～1000 K. However, the flow stress peaks that usually mark the onset of dynamic recrystallisation were hardly seen on the stress–strain curves. During hot deformation of Cu–8 wt-%Al alloy above ～1000 K the interaction of →β phase transformation and deformation processes affected both the flow stress value and the structure of the material. In particular, post-deformation water quenching of the specimens resulted in martensitic transformation within pre-existing β grains. Moreover, local coherent iron particles were detected within β and neighbouring grains.     

The effect of SiC nanoparticles on deformation texture of ARB-processed steel-based nanocomposite

In this study, the influence of SiC nanoparticles on deformation texture of steel-based nanocomposite fabricated by accumulative roll bonding process was investigated. It was found that there was a texture transition from the rolling texture to the shear texture for both pure interstitial free steel and steel-based nanocomposite. However, the texture transition occurred in different cycles for the pure steel (the third cycle) and steel-based nanocomposite (the first cycle). It was realized that the fraction of low misorientation angle grain boundaries was decreased and the fraction of high misorientation angle grain boundaries was increased by the number of cycles. Also, recrystallization occurred in the pure steel and steel-based nanocomposite samples after the third and first cycles, respectively. In addition, the occurrence of recrystallization in steel-based nanocomposite was sooner than that of pure steel. At the early stage of dynamic recrystallization in processed steels, the {011}< 100 >-oriented grains were evolved and the fraction of grains with α-fiber and γ-fiber orientations was slightly decreased. The formation of the rolling texture in the steel-based nanocomposite samples was different from the typical rolling texture for the pure steel samples, due to the presence of the SiC nanoparticles in the nanocomposite. The weak rolling texture was attributed to the high stored energy of deformation, which was, in turn, due to low deformation temperature.     

EBSD investigation of the microstructure and microtexture evolution of 1050 aluminum cross deformed from ECAP to plane strain compression

Electron backscattered diffraction (EBSD) was used to document the microstructure and texture developed due to cross deformation of commercial purity 1050 aluminum alloy. The materials are first deformed in equal channel angular pressing die (ECAP) to different number of passes; 1,4, 8, 12, and 16 passes, via route B_C and then deformed in plane strain compression (PSC) to two axial true plastic strain values of 0.5 and 1.0. Deformation path change was proven to be a very effective tool for manipulating the evolution of microstructure and microtexture. The study provides a documentation of the evolution of microstructure parameters namely cell size, misorientation angle, fraction of submicron grain size, and fraction of high angle grain boundaries. These microstructure parameters were investigated on two planes; the plane normal to the loading direction in PSC (RD–TD) and that plane normal to the transverse direction (RD–ND). These microstructure parameters are compared to those achieved due to the ECAP process only. The ideal rolling texture orientations are depicted and crystal orientation maps were generated. The spatial distribution of grains having these orientations is revealed through these maps. The fraction of the main texture components for a 10° spread around the specified orientations is experimentally calculated and a quantitative idea on the evolution of microtexture is also presented.     

Fatigue Damage Evolution in Ultrafine‐Grained Interstitial‐Free Steel

The current work presents the crack propagation behavior in ultrafine‐grained (UFG) interstitial‐free (IF) steel, and in particular, focuses on the damage evolution in UFG IF steel under cyclic loading. The current results indicate that equal‐channel angular pressing (ECAP) has a major influence on the cyclic deformation response of the UFG IF steel, such that the failure and the crack path depend on the inclination plane during ECAP. Furthermore, the UFG IF steel demonstrates significant notch sensitivity in comparison to its coarse‐grained counterpart. This is attributed to the ultrafine grains with a large volume fraction of high‐angle grain boundaries, where glide of dislocations is hindered and the resulting internal stresses increase the stress concentration further in the presence of a pre‐existing notch.     

Grain refinement in as cast Al–Mg–Mn alloy during high temperature equal channel angular pressing

Abstract

An as cast Al–Mg–Mn alloy with coarse equiaxed grain structure was processed by equal channel angular pressing (ECAP) at 350°C up to eight passes. Systematic studies were made on the microstructural evolution during ECAP by optical microscopy, electron backscattered diffraction and TEM. Equal channel angular pressing led to a considerable grain refinement, resulting in an average cell size of about 1 μm and a fraction of high angle boundaries of 75% after eight pressing passes. Deformation bands were not developed during the ECAP process, and a reasonably equiaxed substructure was obtained even after one pass. The main mechanism of grain refinement was attributed to the continuous dynamic recrystallisation based on the motion of deformation induced dislocations. Discontinuous recrystallisation at grain boundaries and triple junctions also contributed to the refinement, which played an important role especially at high strain of eight passes.     

Microstructure in adiabatic shear bands in a pearlitic ultrahigh carbon steel

Abstract

Adiabatic shear bands, obtained in compression deformation at a strain rate of 4000 s^?1, in a pearlitic 1·3%C steel, were investigated. Shear bands initiated at 55% compression deformation with the width of the band equal to 14 μm. Nano-indentor hardness of the shear band was 11·5 GPa in contrast to the initial matrix hardness of 3·5 GPa. The high strength of the shear band is attributed to its creation from two sequential events. First, large strain deformation, at a high strain rate, accompanied by adiabatic heating, led to phase transformation to austenite. Second, retransformation upon rapid cooling occurred by a divorced eutectoid transformation (DET). The result is a predicted microstructure consisting of nano size carbide particles within a matrix of fine ferrite grains. It is proposed that the DET occurs in iron–carbon steels during high rate deformation in ball milling, ball drop tests and in commercial wire drawing.     

Ultrafine grain formation in face centered cubic metals during severe plastic deformation

The evolution mechanisms of new high-angle boundaries as well as ultrafine grains at large strains were studied by means of multidirectional forging (MDF) of pure copper at low temperature and aluminum alloy at high temperature, where dynamic recovery operates as a main restoration process. The structural changes can be characterized by the evolution of deformation bands such as microshear or kink bands at moderate strains. Multidirectional forging accelerates the evolution of many mutually crossing microshear or kink bands developed in various directions. The misorientations between (sub)grains increased gradually with increasing cumulative strain, finally leading to the development of a new fine-grained structure. The dynamic grain formation can be resulted from in situ or continuous dynamic recrystallization which is discussed in detail.     

High-strain deformation of dual-phase steel

Abstract

A commercial type dual-phase steel has been heat treated to develop a conventional dual-phase structure and, by a double-quench heat treatment, a dual-phase structure with a small martensite island size. These specially heat treated materials together with the normalized material have been plastically deformed by rolling to a reduction of 98% (ε_t = 4·0). The tensile properties have been determined after deformation and correlated with the microstructure. It has been found that within the strain range ε_t = 0·5–1·5 the work hardening modulus is similar to that of pure iron. Over a narrow strain range little work hardening occurs but within the range ε_t = 2·5–4·0 the work hardening modulus is greater than that of ferrite. The increase in modulus seems to be associated with the plastic deformation of the martensite islands which, at the highest strains, give a fibre reinforcing effect. The results are discussed in relation to the work hardening mechanisms involved. It is concluded that changes in the ferrite grain size, established during the development of deformation bands at lower strains and subsequently deformed at higher strains, greatly influence the flow stress through a Hall-Petch relationship.

MST/241