Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

Abstract

The effects of reheating temperature and of the strain and temperature of roughing deformation passes on the kinetics of strain induced precipitation after a finishing deformation pass have been investigated for several niobium high strength low alloy (HSLA) steels. Strain induced precipitation was detected via its strengthening effect on a second finishing deformation carried out either by experimental rolling or by plane strain compression tests. Precipitates were also observed using thin foil electron microscopy. Decreasing reheating/roughing temperature and increasing roughing strain were found to significantly accelerate precipitation – this acceleration is attributed to clustering of niobium in solution.

MST/1409     

Electron microscopy of binder phase deformation in WC–Co alloys

Abstract

The deformation behaviour of binder phase regions in cemented carbides is studied using transmission electron microscopy. Stacking fault densities vary strongly up to the limit of complete transformation of the cubic into the hexagonal modification. Undissociated dislocations are rarely observed. Stacking fault intersections are limited to regions of low stacking fault density. All types of deformation defect are observed in undeformed specimens. These defects are caused by deformation resulting from the relaxation of residual hydrostatic stresses during foil preparation.

MST/808     

Nucleation and growth during recrystallisation of aluminium alloys investigated by local texture analysis

Abstract

This paper presents a systematic survey on the formation of recrystallisation textures in cold rolled Al alloys. In particular, the nucleation of new grains in cube bands, grain boundaries, shear bands, and the deformation zones around large pProceedings of Conference 'Texture on a Mircoscale' as well as their subsequent growth into the deformed matrix are examined and discussed in the context of their contribution to the final recrystallisation textures. In addition to X-ray macrotexture analysis, particular attention is focused on the determination of local orientations by electron diffraction either in the transmission electron microscope (microbeam electron diffraction) or, at later stages of recrystallisation, in the scanning electron microscope (electron backscattered diffraction). A combination of these complementary techniques for orientation determination with increasing spatial resolution yields much more valuable information on the underlying mechanisms of recrystallisation than is accessible by the sole use of the individual techniques.

MST/3676     

Deformation characteristics of Ti–6Al–4V

Abstract

The deformation characteristics of Ti–6Al–4V have been established by torsion testing in the temperature range 800–1150°C. Constitutive equations are proposed for both the β-region and the α+β-region which, it is suggested, may have some practical applications. Extensive optical and electron microscopy have established that dynamic recovery is the operative deformation mode in the β-region, while dynamic recrystallisation predominates in the α+β-region.

MST/806     

Strain driven grain boundary sliding and/or rotation during tensile deformation of ultrafine grained C–Mn steel

Abstract

The microstructure evolution of ultrafine grained C–Mn steel during tensile deformation was investigated using scanning electron microscopy. The surface morphologies and orientation imaging micrographs at different locations near the fractures were discussed. No obvious evident work hardening was identified and partially attributed to the strain driven grain boundary motion of grain rotation and/or grain boundary sliding, especially at the initial stage, while the dislocation activities gradually participate in as deformation proceeds.     

SEM/EBSD based in situ studies of deformation induced phase transformations in supermartensitic stainless steels

Abstract

Recent year's equipment design has enabled the combination of in situ deformation tests with near real time electron backscatter diffraction (EBSD) mapping of the microstructure evolution in the scanning electron microscope (SEM). The present work involves studies of deformation induced phase transformations in supermartensitic steel containing ～40 vol.-% retained austenite at room temperature. The martensite formation was initiated already at low strains, and increased gradually with increasing plastic strains up to ～10%. It was observed that the martensite formed homogeneously within the microstructure, independent of the crystallographic orientations of the retained austenite. But no new martensite variants, besides those already present in the as received condition, did form during deformation. At the same time, the mutual distribution of these variants remained approximately constant throughout the deformation process.     

Deformation bands in Inconel 718

Abstract

The deformation bands formed under monotonic loading in the homogenised and the aged Inconel 718 have been investigated using transmission electron microscopy. The crystallographic behaviour of the deformation bands is discussed. The orientations of the deformation bands were observed in tranmission electron micrographs as follows: two orientations of <112> bands and one orientation of <110> bands on the {O 11} plane; two orientations of <110> bands on the {DOl} plane, and three orientations of <110> bands on the {111} plane. Deformation bands were classified into <112> and <110> bands, theformer corresponding to the projections of edge traces of {111} planes, and the latter corresponding to the projections of inclined deformation {111} planes. Both twins and dislocations were observed in the deformation bands.

MST/2002     

Deformation microstructure and texture in hot worked aluminium alloys

Abstract

Three non-heat-treatable aluminium based materials (AA 1050, AA 1050+1%Mn, and AA 1050+1%Mg) were deformed by plane strain compression (strains of 0·5 to 2, strain rates of 0·25 to 25 s^?1) at elevated temperature (300 to 500°C). The resulting microstructures and textures were studied using optical and back scattered electron microscopy and neutron diffraction. Trends in the development of the deformation microstructure and texture with deformation parameters were noted. It was found that the amount of cube texture in the deformed material decreases as the strain increases. The Zener–Hollomon parameter is not suitable for describing the evolution of cube texture during hot deformation in AA 1050. The addition of 1%Mn or 1%Mg to AA 1050 has little effect on the trends of texture development during hot working. The subgrain size in these alloys decreases with increasing Zener–Hollomon parameter, but the strain has little effect. The misorientation between neighbouring subgrains appears to be approximately independent of deformation parameters in the range of deformation conditions studied.

MST/3472     

Microstructural changes in austenitic stainless steels resulting from cathodic charging of hydrogen

Abstract

The microstructures of two austenitic stainless steels after cathodic charging of hydrogen and room temperature out gassing have been examined in the transmission electron microscope. The charging and aging cycle results in a considerable degree of faulting and transformation to α′- and ε-martensites. Although the extent of hydrogen induced transformation is atypical of the material behaviour after deformation at the same temperature, the morphology of the transformation products is similar. Observations have indicated that the surface cracking commonly encountered in austenitic stainless steels after cathodic hydrogen charging is associated primarily with the hydrogen induced ε-martensite. In an alloy which did not transform as a result of deformation, the defect structures resulting from prior cold work did not appreciably reduce the extent of faulting, twinning, and ε-martensite formation after hydrogen charging and aging.

MST/430     

Cyclic deformation behaviour of ferritic Fe-19Cr-4Ni-2Al alloy at elevated temperatures

Abstract

Cyclic stress response of ferritic Fe-19Cr-4Ni-2Al alloy was investigated under low cycle fatigue at elevated temperatures. The deformation microstructure developed was examined by transmission electron microscopy. The results showed that the alloy exhibited marked cyclic hardening at 773 K but continuous cyclic softening at 923 K. The cyclic hardening was considered to be associated with dynamic strain aging while the cyclic softening was interpreted in terms of the precipitate-dislocation interaction.     

High temperature deformation of nickel base superalloy Udimet 520

Abstract

The high temperature deformation behaviour of nickel base superalloy Udimet 520 was characterised using hot compression isothermal tests. Hot compression tests were conducted between 900 and 1150°C with strain rates of 0.001, 0.01, 0.1 and 1 s^-1. Testing at ≤ 950°C led to sample fracture for all the applied strain rates. The flow behaviour at 1000, 1050 and 1075°C indicated the occurrence of dynamic recovery. For specimens tested at 1100, 1125 and 1150°C, recrystallisation is the softening mechanism. The strain rate sensitivity factor m was estimated for various thermomechanical histories. The activation energy for the hot deformation was determined to be 780 kJ mol¹. The Zener–Hollomon parameter was also determined and its variation with grain size was studied with deformation conditions. The microstructures of all samples were examined by both optical and scanning electron microscopy. The presence and variations in the morphology and size distribution of deformed and recrystallised grains were determined and related to the deformation conditions.     

Micro- and nano-scale deformation processes during scratch damage in high density polyethylene

Abstract

Atomic force microscopy and scanning electron microscopy techniques have been used to examine the surface deformation experienced by high density polyethylene during the scratch test. The scratch deformation process involves stretching of fibrils and microfibrils resulting in the formation of surface openings. At the molecular level the chains of molecules unfold and align in the direction of the moving indenter. In the scratch test, the scratch velocity may suggest that low strain rates are valid, but the local strain rates can be many orders of magnitiude higher as exemplified by atomic force microscopy. A number of modes of deformation are encountered during scratching. They include deformation bands, crazing, tearing, microcracking, regular cracking, and grooving. Crazing-tearing is the predominant mode of scratch deformation. It is envisaged that the sequence of tearing along the craze involves formation of deformation bands, development of craze, followed by tearing. Atomic force and scanning electron microscopy of scratch surface damage indicated that the nature and modes of scratch deformation are qualitatively similar to the case of uniaxial tensile deformation, implying similarities in the deformation behaviour between scratch and tensile deformation.     

Characteristics of shear banding in dual phase steel

Abstract

The evolution process of shear banding in a ferrite–martensite dual phase steel has been investigated via in situ tensile testing in a scanning electron microscope. Shear band type deformation localisation occurs at the maximum loading point of uniaxial tension. Necking occurs simultaneously and locally. Voids nucleate in ferrite domains and on the interfaces between the two component phases or grain boundaries. The void volume fraction is greater within the shear band than away from the band and is greater in the interior than at the specimen surface. The results also show that void damage promotes the initiation of shear bands and the development of shear banding stimulates further void damage.

MST/1819     

Grain refinement of high purity aluminium by asymmetric rolling

Abstract

For the purpose of grain refinement, development of the microstructure of coarse grain, high purity aluminium during cold asymmetric rolling has been studied by electron backscatter pattern (EBSP) analysis, as well as optical and transmission electron microscopy, and compared with that developed during conventional rolling. In 91.3% asymmetrically rolled sheet, new fine equiaxed grains with an average size of ～2 µm are evolved almost uniformly throughout the thickness. On the other hand, in conventionally rolled sheet, the coarse fibrous structure is predominant. A change of grain boundary misorientation distribution with an increase in reduction shows that the fraction of sub-boundaries below 10° decreases linearly, and that of the high angle boundaries above 15° increases linearly. The fine grain evolution during asymmetric rolling seems to result from the development of sub-boundaries into high angle boundaries promoted by a simultaneous action of two deformation modes, namely compression and additional shear deformation. Fine grains evolved during asymmetric rolling are stable at temperatures below 423 K. Annealing at temperatures above 473 K results in remarkable grain growth.     

Effect of isothermal deformation on microstructure and properties of electron beam welded joint of Ti2AlNb/TC11

Abstract

The present paper reports the influence of hot work (isothermal deformation accompanied with heat treatment) on microstructure and properties of electron beam welded dissimilar joint. Ti₂AlNb alloy and TC11 alloy were used to fabricate the joints. Isothermal deformation and heat treatment were performed under certain conditions. The structures were analysed using optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the as welded metal of Ti₂AlNb/TC11 joint is mainly composed of α₂ and β phases. The metastable β phase transformed into α+β phases during deformation and heat treatment processes. There are no big differences in tensile strength of joints under different conditions. However, the impact toughness of the weld has improved 72% after hot work.     

Deformation and recrystallisation studied by EBSP: breakthroughs and limitations

Abstract

The use of microtexture investigations by the electron backscattering pattern (EBSP) technique in the SEM for investigations of plastic deformation and recrystallisation is described. The focus is on grain subdivision during deformation and on nucleation and growth during recrystallisation. Specific investigations in which the EBSP technique has been essential to achieve a scientific breakthrough are reviewed. Also the limitations of the EBSP technique are discussed and illustrated by examples of incorrect results which might have been accepted if the EBSP measurements had not been complemented by other methods.     

Surface modification of D36 steel by ultrasonic impact with electropulsing

ABSTRACT

The present work investigated the effect of ultrasonic impact treatment (UIT) assisted with electropulsing (EP) on the microstructure of D36 low-carbon steel. Optical microscope and scanning electron microscope were employed to reveal the microstructural evolution of the specimen subjected to the treatment. In comparison with UIT solely, a higher hardness on the superficial layer was obtained by EP-UIT and the fracture on treated surface was avoided effectively which meant a better plasticity. The electron back scatter diffraction analysis indicated a high density of low-angle boundary network owing to the large strain introduced. The distinguishing features of EP-UIT were ascribed to the athermal effect of drifting electrons which facilitate the mobilising of dislocations in the deformation.

This paper is part of a themed issue on Materials in External Fields.     

Experimental investigations into Kinetics of recrystallisation of cold rolled nickel

Abstract

The kinetics of recrystallisation of cold rolled nickel (99.98% purity) have been investigated by means of in situ heating in a scanning electron microscope and quantitative post-mortem metallographic examinations as well as microhardness measurements. The specimens were deformed over a wide range of deformation ratio (ε = 0.25–1.12) and isothermally annealed at 455°C. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory has been tested, and it has beenfound that measured JMAK exponents (n = 1.3–2.1) are lower than those predicted theoretically and are strongly dependent on deformation ratio. Therefore, it is confirmed that the JMAK theory fails to apply to the recrystallisation of plastically deformed nickel. Two hypotheses, competition between recovery and recrystallisation and non-random distribution of nucleation sites, are discussed to explain the low values of the JMAK exponent.     

Initiation of voiding at second-phase particles in a quaternary Al–Li alloy

Abstract

The intermetallic phases within an as-cast and rolled Al–Li–Cu–Mg alloy have been characterized using quantitative X-ray microanalysis and back-scattered electron imaging. An Al–Cu–Mg eutectic and an Al/Cu-rich intermetallic resembling A1₆( Fe, Cu) are the principal constituents of the as-cast alloy. During homogenization and fabrication, the eutectic dissolves but the iron-rich phase is broken up into stringers which lie along the high-angle grain boundaries. Void initiation at the particles during deformation of the wrought material has been observed and quantified. Voids nucleate following either particle cracking or particle/matrix decohesion. Evidence for particle cracking caused by coplanar slip band impingement is presented based on electron microscopical observations of specimens deformed conventionally and in situ.

MST/569