Processing to ultrafine grain structures by conventional routes

Abstract

The aim of this project was to achieve grain sizes of the order of 1 µm in Al–Mg–Cr alloys by deformation at elevated temperatures to large strains using conventional rolling or plane strain compression. The ‘processing window’ for generating such ultrafine grain sizes by continuous recrystallisation has also been investigated. The necessary processing conditions for achieving a 1–3 µm grain size in alloys containing 2–3 wt-%Mg, deformed to strains of 3, were found to be strain rates of less than 10 s^-1 and temperatures between 300 and 350°C. The restricted range of conditions under which such fine grain microstructures can be achieved in these alloys by plane strain deformation is seen to be a limiting factor in commercial exploitation of such processing methods.     

Microstructure and microtexture of ultrafine ferrite formed in 0.1%C steel using new strip rolling process

Abstract

A new hot strip rolling process is discussed which is capable of producing ultrafine, equiaxed ferrite grains (i.e. less than 2 µm)in the surface region of steel strip. Both microstructural and texture analysis of low carbon steel strip that has been rolled using this method are used to show that the ferrite forms by strain induced transformation. Analysis by electron backscatter diffraction (EBSD) indicates that a strong ferrite microtexture exists within the individual austenite grains in which the ferrite nucleates. The results from bulk X-ray texture analysis confirm that the ferrite forms as a result of transformation from austenite that has undergone heavy shearing during rolling, with nucleation occurring on the austenite substructure. In the centre region of the strip, a bainitic microstructure forms after rolling during air cooling. In the transition region between the surface and the centre of the strip, ferrite is shown to nucleate to form closely spaced parallel ‘rafts’ of ferrite grains traversing individual austenite grains. Again, EBSD is used to show that the ferrite located within these rafts is strongly textured, which, in combination with microstructural evidence, suggests that this ferrite nucleates along intragranular shear bands that form in the austenite in this region of the strip during rolling.     

Effect of strain on formation of ultrafine ferrite in surface of hot rolled microalloyed steel

Abstract

The phenomenon of ultra grain refinement of ferrite in surface layers of hot rolled strip has been studied in a low carbon, niobium microalloyed steel. Wedge specimens were used, to vary the nominal equivalent strain applied during rolling from zero to approximately unity, and the cooling rate after rolling was varied from ~ 20 to 1 K s ^-1. In contrast with previous work, which contended that a very coarse austenite grain size and a low rolling temperature near the Ar ₃ were essential to obtain ultrafine ferrite in surface layers, such ultrafine layers were observed after rolling coarse austenite at up to 150 K above the Ar ₃ and after rolling fine grained austenite near the Ar ₃. In the case of coarse grained austenite, a critical nominal rolling strain needed to be exceeded to trigger the surface layer phenomenon, upon which cooling rate had little effect on the surface layer's grain size. Refining the prior austenite grain size had the further beneficial effect of refining the grain size at the centre of the rolled product, for example to 2·6 μm, while the surface layer was refined to 0·7 μm.     

Microstructural control by secondary processing of strip cast low carbon steel

Abstract

The secondary processing of low carbon steel strip produced by twin roll casting was investigated to examine its effect on microstructural development and mechanical properties. The as cast microstructure is predominantly acicular ferrite with regions of bainitepearlite and polygonal ferrite. Deformation at temperatures below Ar1 produces a heterogeneous microstructure with regions of moderately deformed acicular ferrite adjacent to highly deformed regions containing shear bands. Cold rolled and warm rolled steels show similar behaviour to conventional hot band in that dynamic recovery during warm rolling results in sluggish recrystallisation and produces a coarse final grain size. However, the initial as cast microstructure recrystallises at a slower rate than conventional hot band and produces a weaker recrystallisation texture. This can be attributed to the heterogeneous microstructure of the as cast strip such that, after rolling, nucleation occurs within shear bands and more ill defined sites, which results in nucleation of randomly oriented grains thereby producing a weak final texture. It was found that austenitising the as cast strip followed by rolling in the vicinity of Ar3 produces a uniform distribution of equiaxed, ultrafine ferrite UFF grains throughout the thickness of the strip. The production of UFF by twin roll casting and subsequent rolling represents a simple processing route for the production of fine grained low carbon sheet steel products.     

Effect of Nb on dynamic strain induced austenite to ferrite transformation

Abstract

Dynamic strain induced transformation (DSIT) is an interesting processing route to obtain ultrafine ferrite grains. In the present work, the effect of Nb on DSIT was investigated. Samples of low C–Mn steels, with and without Nb, were intensively deformed in hot torsion, aiming at the production of ultrafine ferrite grains. After soaking at 1200°C, the samples were cooled to 1100°C, submitted to hot torsion deformation to decrease the grain size and then cooled to 900, 850 or 800°C for further hot torsion deformation. In the steel without Nb, recrystallisation took place before enough deformation could be accumulated to induce ferrite formation, so DSIT would only take place at the lowest temperature investigated, 800°C. In the Nb steel, Nb addition delayed austenite recrystallisation, allowing DSIT ferrite to form at higher temperature than in the steel without Nb, 850°C.     

Microstructure and texture evolution in low carbon steel deformed by differential speed rolling (DSR) method

The study examined the microstructural and textural evolution of low carbon steel samples fabricated using a differential speed rolling (DSR) process with respect to the number of operations. For this purpose, the samples were deformed by up to 4-pass of DSR at room temperature with a roll speed ratio of 1:4 for the lower and upper rolls, respectively. The DSR technique applied to low carbon steel samples resulted in a microstructure composed of ultrafine ferrite grains, approximately 0.4 µm in size, after 4-pass with a high-angle grain boundary fraction of ~65 %. The microstructural features of the ferrite phase indicated the occurrence of continuous dynamic recrystallization, beginning with the formation of a necklace-like structure of ultrafine equiaxed grains around the elongated grains, which were formed in the early stages of deformation, and ending with ultrafine recrystallized grains surrounded by boundaries with high angles of misorientations. In the pearlite phase, the microstructural changes associated with DSR deformation were presented by the occurrence of bending, kinking, and breaking of the cementite lamellar plates. In addition, the evolution of texture after DSR processing was affected by shear deformation and rolling deformation, leading to the formation of a texture composed of fractions of components with shear texture orientations such as {110} 〈001〉 (Goss) and orientations close to {112} 〈111〉, in addition to rolling texture components consisting mainly of α-fiber and γ-fiber.     

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

Abstract

In the present study, wedge-shape sa mples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.     

Surface integrity and removal mechanism in grinding sapphire wafers with novel vitrified bond diamond plates

In order to improve machining efficiency of sapphire wafer machining using the conventional loose abrasive process, fixed-abrasive diamond plates are investigated in this study for sapphire wafer grinding. Four vitrified bond diamond plates of different grain sizes (40?µm, 20?µm, 7?µm, and 2.5?µm) are developed and evaluated for grinding performance including surface roughness, surface topography, surface and subsurface damage, and material removal rate (MRR) of sapphire wafers. The material removal mechanisms, wafer surface finish, and quality of the diamond plates are also compared and discussed. The experiment results demonstrate that the surface material is removed in brittle mode when sapphire wafers are ground by the diamond plates with a grain size of 40?µm and 20?µm, and in ductile mode when that are ground by the diamond plates of grain sizes of 7?µm and 2.5?µm. The highest MRR value of 145.7?µm/min is acquired with the diamond plate with an abrasive size of 40?µm and the lowest surface roughness values of 3.5?nm in Ra is achieved with the 2.5?µm size.     

Recrystallisation textures in cold rolled low carbon steel containing ferritic and bainitic microstructures

Abstract

Low carbon steel strip was heat treated to generate four different starting microstructures (fine and coarse polygonal ferrite, acicular ferrite and bainite) for investigating their influence on texture development during cold rolling and annealing. The starting materials were cold rolled to 50–90% reduction and annealed for various times in the temperature range 853–953 K. The resultant microstructures and textures were examined mainly by electron backscatter diffraction and X-ray diffraction. The initial microstructure strongly influenced the crystallographic rotation paths during cold rolling, whereby high strain deformation generated strong {223}〈110〉 texture components in the polygonal ferritic microstructures, whereas a strong {001}〈110〉 texture was produced in the acicular/bainitic microstructures. Subsequent annealing generated, to varying degrees, the classic {111}〈uvw〉 (γ-fibre) recrystallisation texture in all materials. Unexpectedly, coarse polygonal ferrite produced the strongest γ-fibre recrystallisation texture after 70–90% cold rolling reduction. Based on arguments involving the effect of carbon in solution, initial grain size and deformation textures on recrystallisation texture development, it was shown that a strong γ-fibre texture can indeed be generated in coarse polygonal ferrite.     

Extrusion upsetting multiple processing in sandglass die

Abstract

A new method of producing ultrafine grain sizes, known as extrusion upsetting multiple processing in sandglass die or sandglass extrusion, has been investigated using a Zn–5Al (wt-%) alloy. Since the shape of the test billet can remain unchanged after sandglass extrusion, the billet can be extruded repeatedly in order to obtain a large plastic strain. Ultrafine grain size can be achieved in the billet material due to the large plastic strain and dynamic recrystallisation during sandglass extrusion. The process technology, and the microstructures, superplasticity, and microhardness of the test material after sandglass extrusion have been studied. The experimental results show that equiaxial ultrafine microstructures can be introduced into the bulk test material during sandglass extrusion and high strain rate superplasticity can be realised.     

Effect of process variables on formation of dynamic strain induced ultrafine ferrite during hot torsion testing

Abstract

Ultrafine grain sizes were produced using hot torsion testing of a 0.11C-1.68Mn-0.20Si wt- steel, with ultrafine ferrite <1 m nucleating intragranularly during testing by dynamic strain induced transformation. A systematic study was made of the effect of isothermal deformation temperature, strain level, strain rate, and accelerated cooling during deformation on the formation of ultrafine ferrite by this process. Decreasing the isothermal testing temperature below the Ae3 temperature led to a greater driving force for ferrite nucleation and thus more extensive nucleation during testing; the formation of Widmanstatten ferrite prior to, or early during, deformation imposed a lower temperature limit. Increasing the strain above that where ferrite first began 0.8 at 675C and a strain rate of 3 s1 increased the intragranular nucleation of ferrite. Strain rate appeared to have little effect on the amount of ferrite formed. However, slower strain rates led to extensive polygonisation of the ferrite formed because more time was available for ferrite recovery. Accelerated cooling during deformation followed by air cooling to room temperature led to a uniform microstructure consisting of very fine ferrite grains and fine spherical carbides located in the grain boundaries regions. Air cooling after isothermal testing led to carbide bands and a larger ferrite grain size.     

Effects of Sn on microstructure and mechanical properties of as-rolled Mg–5Zn–1Mn alloy

Effects of Sn on microstructure and mechanical properties of Mg–5Zn–1Mn alloy subjected to high strain rate rolling (9.1?s^-1), 300°C and 80% pass reduction are investigated. With higher Sn content, the dynamic recrystallisation (DRX) grain size gradually decreases due to the stronger pinning of nano-scale precipitates at grain boundaries and the DRX fraction first increases due to the enhanced effect on DRX by decreasing stacking fault energy and then decreases due to more precipitates at grain boundaries. Ultimate tensile strength (UTS) and elongation to rupture (Er) of as-rolled alloys increase and then decrease. Alloy with 0.9 mass% Sn exhibits the highest DRX fraction (95?vol.-%), the finer DRX grain size (1.22?µm), UTS of 358?MPa and Er of 20.4%.     

Differential scanning calorimetry study of constrained groove pressed low carbon steel: recovery,recrystallisation and ferrite to austenite phase transformation

Abstract

Low carbon steel sheets are subjected to severe plastic deformation (SPD) via constrained groove pressing (CGP) up to five passes. As a result of this process, strain magnitude up to 5·8 is imposed to the sheets, which leads to grain size of 225 nm. These nanostructured steel sheets, due to their high dislocation density and ultrafine microstructure, are very sensitive to heating. In the present study, recovery, recrystallisation and ferrite to austenite phase transformation phenomena for the SPD steel are investigated using differential scanning calorimetry method. The results show that with increasing the strain in steel sheets, the deformed stored energy (released through recovery and recrystallisation) and enthalpy of ferrite to austenite phase transformation are significantly increased and varied in 38·5–85·8 and 109–156·1 MJ m^?3 ranges respectively. In addition, transformation temperature is decreased from 761 to 750°C after five CGP passes. However, recovery stored energy, recovery and recrystallisation peak temperatures are not changed, considerably. Experimental data show that with increasing the hardness, the stored energy is increased. One empirical equation is developed for relationship between hardness and stored energy of severely deformed low carbon steel. In addition, using the dislocation model, this mentioned relationship is justified.     

Optimizing strength and ductility in Cu–Al alloy with recrystallized nanostructures formed by simple cold rolling and annealing

A single-phase Cu–Al alloy with a low stacking fault energy was processed by cold rolling and subsequent annealing. Fully recrystallized microstructures composed of ultrafine grains were obtained after isothermal annealing at different temperatures. The minimum mean grain sizes achieved were below 1 μm. It was found that the microstructures were homogeneous after annealing at 400 °C, but somehow inhomogeneous after annealing at lower temperature of 300 °C and higher temperature of 550 °C. Superior strength and ductility were obtained by controlling the grain size of the microstructures. After annealing at 400 °C for 10 s, a fully and homogeneously recrystallized material with mean grain size of 770 nm was produced, which had a high yield strength of 524.2 MPa and a remarkable uniform elongation of 15.7 %.     

Role of silicon in influencing strength and impact behaviour of ferrite and its likely influence at ultrafine grain size

Abstract

Plates of fine grained Nb containing steels having Si contents of 0.03, 0.1, and 0.51% were austenitised at 920°C and cooled at 40 and 7.5 K min^-1 through the γ to αtransformation temperature range. Increasing the Si level from 0.03 to 0.1%resulted in a decrease in the yield strength and an improvement in impact performance for both cooling rates. Raising the Si level further to 0.51%Si caused the strength to increase and the impact behaviour to deteriorate. The results are explained in terms of Si segregating to the ferrite boundaries, increasing the activity of C and N, and thereby displacing these atoms from the boundary regions. In this way, although Si increases σ₀in the Hall–Petch relationship the K _y value is reduced. Consequently, at fine grain size, Si additions can actually reduce the strength of steels and, at ultrafine grain sizes (1 µm), this would be expected to result in very substantial softening.     

Reactive friction-stir processing of nanocomposites: effects of thermal history on microstructure–mechanical property relationships

The effects of thermal history on the microstructure and mechanical properties of a friction-stir-processed Al–Mg–TiO₂ (3?vol.-%, 20?nm) nanocomposite were studied. It is shown that, with increases in peak temperature, a more uniform distribution of nanoparticles in the metal matrix, and a refined grain structure, are attained. Transmission electron microscopy indicated that the mechanism of grain refinement is influenced by the hard inclusions, changing from discontinuous to continuous dynamic recrystallisation. A fine-grained nanocomposite (average grain size of 3?µm) with a uniform distribution of nanoparticles is obtained after four fully-overlapping passes at 1200?rev?min^?1 and 100?mm?min^?1. Under these circumstances the mechanical properties, including yield stress (95%), tensile strength (36%) and hardness (72%) are significantly enhanced relative to the untreated alloy.     

Achieving superplasticity at high strain rates using equal channel angular pressing

Abstract

Equal channel angular pressing (ECAP) is a processing procedure in which a sample is pressed through a die containing a channel bent into an L shaped configuration. This procedure introduces a high strain into the sample without any change in the cross-sectional area and it may be used to attain an ultrafine grain size with dimensions lying typically within the submicrometer range. This paper describes a series of experiments where ECAP was applied to a commercial Al–Mg–Li–Zr alloy having an initial grain size of ～400 µm. The results demonstrate a refinement in the grain size of this alloy to a size of ～1 µm and it is shown that these small grains are stable up to temperatures >600 K because of the presence of β′-Al₃Zr particles. The stability of these ultrafine grains at elevated temperatures provides an opportunity to achieve superplastic ductilities in this alloy at very high strain rates: for example, the measured elongations to failure under optimum pressing conditions exceed 1000% at a strain rate of 10^-1 s^-1 when testing at temperatures above 600 K.     

Tensile and compressive test in nanocrystalline and ultrafine carbon steel

Plastic deformation behavior was investigated in near fully dense nanostructured and ultrafine-grained bulk samples of carbon steel (0.55 wt% C) under compression and tension tests. The specimens were obtained by hot pressure from mechanically milled powder at 400 and 500 °C. Subsequent heat treatments at temperatures going from 600 to 900 °C produced samples with ferrite grain sizes from 30 nm to 17 μm. Nanocrystalline grained steel samples presented very high strength with low ductility. Once, in the ultrafine range, as the ferritic grain size was increased, the strength was decreased and the ductility was improved. The porosity and carbon atoms within the structure were analyzed in order to explain the results of strength and strain obtained.     

Influence of warm working and tempering on fissure formation

Abstract

The influence of warm working and tempering on the formation of fissures on the fractured faces of Charpy V-notch samples has been examined for a variety of ferrite–pearlite steels and iron alloys which had been rolled in the temperature range 600–400°C and tempered in the range 600–725°C. In accordance with fissures being initiated by the ease of intergranular failure along the ferrite grain boundaries, the number of deep fissures produced on warm working increased with the degree of grain boundary alignment in the rolling direction and the grain aspect ratio (maximum grain diameter/minimum grain diameter). Pearlite banding and the presence of grain boundary carbides were found not to influence the number of fissures formed, fissuring behaviour being the same for the Fe–Mn alloys and plain C–Mn steels. The presence of low levels of S and P also did not influence fissure formation. At a given average grain aspect ratio it was found that the introduction of a two phase rolling sequence (760–720°C) into the rolling schedule encouraged fissure formation. This is probably due to a small number of elongated grains not recrystallising during the two phase rolling sequence and being further elongated at the lower rolling temperatures, combined with a greater alignment of the ferrite boundaries in the rolling direction. By rolling the steels and Fe alloys with the same reduction at temperatures insufficient to allow recrystallisation (600–400°C), it has been possible to keep the aspect ratio constant and vary the dislocation density. At constant aspect ratio, increasing the dislocation density by warm working increased the number of deep fissures formed. On the basis of these results, it is suggested that the weakness at the grain boundaries which gives rise to these fissures may be caused by dislocation interaction with the boundary together with boundary alignment giving a well defined crack path. Subsequent tempering at 600°C which allowed some recovery to take place without grain boundary movement did not reduce the number of fissures. Fissuring was only removed when the tempering temperature was high enough to allow grain boundary movement.

MST/769     

Thermal stability of ultrafine grained aluminium alloy prepared by large strain extrusion machining

Abstract

In the present study, ultrafine grained (UFG) Al alloy chips with average grain sizes of ～200 nm were successfully prepared by large strain extrusion machining (LSEM) process using a combined cutting tool with rake angle of 10° and chip compression ratio of 1.0. The tests showed that the Vickers hardness of the UFG Al alloy is significantly improved due to grain size reduction. To understand effect of heat on the microstructure and mechanical properties, the UFG chips were subjected to heat treatment at different temperatures and different annealing time durations. When annealed <100°C, most of fine grains within the UFG chips were found to be replaced by elongated grains whose grain sizes increased with a significant increase in the aspect ratio as the annealing time increased. Despite such increase in grain size, the Vickers hardness was not reduced as expected because of the precipitation of secondary phases. When annealed at temperatures up to 200°C, recrystallisation occurred, along with grain growth, but the Vickers hardness did not deteriorate because of precipitation of secondary phases, as before. However, annealing at temperatures of 300°C and above resulted in significant reduction in hardness of the chips due to dominance of grain growth over secondary precipitation. These results indicated that UFG Al alloy chips have a good thermal stability at temperatures <200°C.