Fraction Of Ultrafine Grains Research Articles

Extending the mechanical property regime of laser powder bed fusion Sc- and Zr-modified Al6061 alloy by manipulating process parameters and heat treatment

Highly dense compositionally modified Al6061 alloys with Sc and Zr (Al6061MOD) were fabricated using the laser powder bed fusion. The as-built AL6061MOD alloy exhibits a bimodal grain size distribution consisting of coarse columnar grains and ultrafine grains, which is tunable by either varying the printing parameters or post-heat treatment. A high laser energy density along with a lower cooling rate, leaving more time for nucleating primary Al3Sc particles, triggers a higher area fraction of ultrafine grains. The heat treatment-induced evolution of microstructure and mechanical properties is jointly influenced by the size of grains and (Al,Si)3(Sc,Zr) particles. The conventional T6 heat treatment triggers coarsening of grains (∼5.3 μm) and (Al,Si)3(Sc,Zr) particles (∼42 nm), lowering the yield strength (∼280 MPa) compared with that of as-built counterparts. On the contrary, direct aging enhances the yield strength (∼383 MPa) due to the precipitation of nano-sized secondary Al3(Sc,Zr) particles (∼5 nm) as well as retaining the grain size (∼690 nm) of ultrafine grains. Concurrent with the tunable strength, the plasticity behaviors (Lüders band and Portevin-Le Chatelier effect) are manipulated correspondingly with varying microstructure. The broad tailoring of microstructure and mechanical property regime can shed light on designing Al alloys targeting the desired applications by manipulating the printing parameters and heat treatment.

Effects of annealing and extrusion on the microstructure and tensile properties of ultrafine-grained Al fabricated by spark plasma sintering

ABSTRACT Ultrafine-grained Al samples were fabricated by spark plasma sintering (SPS) of a mechanically milled nanocrystalline powder. The effects of annealing and extrusion on the microstructure and tensile properties of the as-SPSed samples were investigated. Annealing removed the nanopores and caused significant coarsening of Al grains in the interparticle boundary (IPB) regions, leading to the formation of a heterogeneous structure consisting of coarse grains, ultrafine grains and Al4C3/Al2O3 nanoparticles. Hot extrusion also eliminated the nanopores and resulted in the formation of a heterogeneous structure containing enhanced volume fraction of ultrafine grains and finer nanoparticles. These microstructural changes rendered a much improved YS and UTS of 251 and 288 MPa, respectively, and excellent tensile ductility (elongation to fracture: 12.7%). The improvement in mechanical properties can be attributed to grain boundary and nanoparticle strengthening, strain delocalisation of the heterogeneous structure and improvement of the bonding strength of IPBs.

Optimizing the mechanical properties of a Mg–Y–Nd–Zn–Zr alloy via interrupted compression with an intermediate annealing

This work proposed a practical method, interrupted compression with an intermediate annealing, for optimizing the mechanical properties of Mg–4Y-2Nd-0.2Zn-0.5Zr alloy. This method contained the first compressive step for an interrupted strain of 0.25, the intermediate annealing for 5 min, and the second compressive step for a total true strain of 1.6. The temperatures for the two steps of hot compression and intermediate annealing were the same. The obtained sample was named IC sample. As a comparison, the continuous compression was also performed (CC sample). During hot compression, the work softening of the CC sample was dominated by the rotational dynamic recrystallization (rDRX) and the twin-assisted DRX (tDRX). With the strain increasing, the rDRX acted a more dominant role compared to the tDRX. In the IC sample, the intermediate annealing triggered the formation of deformation bands. During the second compressive step of the IC sample, the work softening was dominated by the DRX assisted by deformation bands besides the rDRX. As a result, the volume fraction of ultra-fine grains (V UFG ) was much higher in comparison to the CC sample. Compared to the grain size of UFGs (D UFG ) and the basal pole distribution, the V UFG had a more important role in the resultant mechanical properties. The higher V UFG of the IC sample led to the better combination of strength and ductility at each compressive temperature. Comparing the IC sample to the CC sample after compressing at temperatures from 420 °C to 510 °C, the increments of the UTS were 7.4%, 2.4%, 1.2% and 0.4%, and the increments of the FE were 4.0%, 13.9%, 22.8% and 21.3%. The interrupted compression with an intermediate annealing had a deep potential in applying to other Mg alloys despite the initial texture. • Both continuous compression (CC) and interrupted compression (IC) are performed. • In CC sample, work softening is controlled by both rDRX and tDRX. • Intermediate annealing triggers the formation of deformation bands in IC sample. • In IC sample, work softening is dominated by the DRX assisted by deformation bands. • Interrupted compression successfully improves DRX degree and mechanical properties.

EFFECT OF THE BIMODAL STRUCTURE PROCESSED BY ECAP AND SUBSEQUENT ROLLING ON STATIC STRENGTH AND SUPERPLASTICITY OF Al-Mg-Sc-Zr ALLOY

Microstructure and mechanical properties of the 1570C aluminum alloy were studied after equal channel angular pressing (ECAP) to the strain of 3 at 325°C and subsequent warm and cold rolling with near 80% reductions at 325°C and 20°C, respectively. Even containing a partially recrystallized bimodal structure with a volume fraction of ultrafine grains of 0.3 and their size not exceeding 2 mm, the alloy after ECAP demonstrated an excellent balance of room temperature static strength parameters (yield strength (YS) » 300 MPa, tensile strength (UTS) » 400 MPa and elongation (El) » 26%), and high strain rate superplasticity (with maximum elongation exceeding 2500% at 520°C and a strain rate of 1.4 × 10-2 s- 1). Subsequent warm and cold rolling resulted in an increase in YS to 340 and 430 MPa and UTS to 415 and 485 MPa amid El decreased to 24 and 11%, respectively. Despite the difference in the deformation structures formed in both rolling states, similar superplastic behavior was observed with maximum elongations of up to 3000% at temperatures of 500-520°C and strain rates of about 10-2 s-1. It was concluded that the initial processing of the alloy to relatively low ECAP strains before warm/cold rolling, leading to bimodal structure with a low fraction of ultrafine grains, is sufficient to ensure a favorable combination of both service and technological properties of the sheets obtained.

Microstructure evolution of the TiB2/Al composites fabricated by powder metallurgy during hot extrusion

The microstructure evolution near the central axis during hot extrusion for the fine-grained TiB2/Al-Zn-Mg-Cu composites fabricated by powder metallurgy is studied by experimental observation at different locations in the remaining die. The significant increase of the fraction of ultrafine grains and texture evolution indicates the extensive dynamic recrystallization at the narrowest position of the die. The deformed grains are distinguished from the undeformed grains with the parameter of grain orientation spread (GOS). And the deformed grains are generally rotated around the extrusion direction. Additionally, the introduced submicron TiB2 particles have the potential to stimulate the dynamic recrystallization associated with progressively increased misorientation of subgrain boundaries, and even change the spatial distribution of the low angle grain boundaries. The preferential activation of {111} 〈1−10〉 system and non-octahedral (NOC) slip system {001} 〈110〉 contributes to the formation of strong deformation fiber texture according to the analysis of the Schmid factor. It is supposed that grain boundary sliding accommodated dislocation slip mechanism controls the microstructure evolution.

Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

ABSTRACTThe microstructure/texture evolution and strengthening of 316 L-type and 304 L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.

On Kinetics of Grain Refinement and Strengthening by Dynamic Recrystallization

The grain refinement and strengthening of an austenitic stainless steel are studied under multiple multidirectional forging at 1073 K up to strains of 4. The structural changes are characterized by the development of discontinuous and continuous dynamic recrystallization (DRX) leading to the grain refinement down to submicrometer level. The new ultrafine grains develop primarily along original grain boundaries and deformation microbands, leading to heterogeneous necklace‐like microstructures at intermediate strains followed by rapid expansion of the DRX grains occupying the whole worked sample at large strains. The change in the fraction of ultrafine grains is commonly characterized by a sigmoid‐type dependence on strain and can be expressed by a modified Jonson–Mehl–Avrami–Kolmogorov equation. The DRX development is accompanied by a stepwise decrease in the flow stress at reloading in each subsequent forging pass especially in intermediate strains. This stepwise softening results from rapid growth of freshly nucleated grains. In contrast, after forging, both the yield strength and ultimate tensile strength at room temperature increase progressively with an increase in the number of forging passes. The strengthening during DRX development can be attributed to concurrent contribution of work‐hardening and grain refinement and can be expressed by a summation of recovered and recrystallized fractions.

Microstructure of titanium on complex deformation paths: Comparison of ECAP, KOBO and HE techniques

In this work samples of commercial purity grade 2 titanium were severely deformed by three different extrusion techniques: equal-channel angular pressing (ECAP), extrusion through oscillating die (KOBO), and hydrostatic extrusion (HE). Resulting microstructures and mechanical properties of processed titanium samples are studied and compared with regard to potential application as a material for dental implants. The obtained results demonstrate the effect of different strain paths, involved by three applied deformation methods, on microstructure refinement, mechanical properties and textures of the studied material.Heterogeneous microstructures with bimodal grain size distributions are observed for all deformation techniques in the examined material, however differences in their internal structures and grain boundary distributions point to different grain refinement mechanisms. The fraction of ultra-fine grains is the highest in titanium deformed by HE. Also the internal grain fragmentation is the most important after HE deformation. This leads to the best mechanical and microstructural properties obtained in this type of deformation. Consequently, the HE process seems to be very interesting for production of material for medical implants.

Formability and fracture behaviour of cryorolled Al-3 Mg-0.25 Sc alloy

The rolling of an Al-3Mg-0.25Sc alloy at room and cryogenic temperature to 50% and 75% reduction in thickness resulted in the formation of a bimodal microstructure. The grain size distribution based on electron backscattered diffraction (EBSD) analyses showed an enhanced fraction of ultra-fine grains with nearly 100 nm in cryorolled samples whereas; room temperature rolled samples exhibited sub-micron grains with 300 nm. The transmission electron microscopy (TEM) studies revealed the dense dislocation cell structures for cryorolled samples due to the restriction of dynamic recovery. The better forming and fracture limit strains were noted for cryorolled samples compared to room temperature rolled ones in the forming and fracture limit diagrams. A good correlation of formability with void coalescence parameters based on larger void size, lower ligament thickness and lower d-factor were obtained. This indicated better accumulation of plastic deformation and improved formability of cryorolled samples. Further, the reduced aspect ratio (L/W) of void signified delayed fracture behaviour of cryorolled samples compared to room temperature rolled samples and showed better fracture resistance. The improved fracture limit strain in combined forming and fracture limit diagram exhibited by cryorolled samples was consistent with the void coalescence parameters.

Forming Mechanism of High‐Speed Cold Roll Beating of Spline Tooth

The spline tooth of ASTM 1045 was fabricated by high‐speed cold roll‐beating (HSCRB) process at room temperature. Microhardness of the spline tooth was examined by a nanoindenter. The grains and misorientation angle distributions were measured by electron backscatter diffraction (EBSD). The results showed that the microhardness was improved up to 1280 μm deep from the surface of the spine tooth. The microhardness and the grain sizes gradually decreased in the direction away from the surface. On the surface, the fraction of ultrafine grains increased up to about 90%, and the average grain diameter (which was ∼0.56 µm) decreased by 71.4%. The model of grain evolution during HSCRB process is proposed in this work. New grains appear first on the boundaries of the elongated grains within numerous subgrains. The elongated grains are refined as a result of subgrain rotation. By analyzing the HSCRB technical principle, we concluded that the process parameters affect the refinement degree of studied steel by determining beating pass, beating pass interval time, and strain rate.

Kinetics of Submicrocrystalline Structure Formation in a Cu-Cr-Zr Alloy during Large Plastic Deformation

The grain refinement and kinetics of submicrocrystalline structure formation in a Cu-0.3%Cr - 0.5%Zr alloy during large plastic deformation were investigated. The fraction of high-angle boundaries and the fraction of ultrafine grains were used to estimate the kinetics of grain refinement and submicrocrystalline structure evolution during large plastic deformation. The multidirectional forging (MDF), equal channel angular pressing (ECAP), and high pressure torsion (HPT) were used as methods of large plastic deformation. Comparative analysis showed that the grain refinement process occurred faster during HPT and MDF in comparison with ECAP. The fraction of ultrafine grains achieved almost 1 after 3 HPT turns and after MDF to the total strain of 4; while the one reached only 0.29 after 4 ECAP passes. The modified Johnson-Mehl-Avrami-Kolmogorov equation could be applied to determine the kinetics of grain refinement in copper alloy during large plastic deformation as a function of true strain.

Effects of Initial Microstructure and Deformation Method on Grain Refinement in a Cu-Cr-Zr Alloy

The development of submicrocrystalline structure in a Cu-0.3wt.%Cr-0.5wt.%Zr during multidirectional forging (MDF) and equal channel angular pressing (ECAP) was investigated in comparison. A large number of strain-induced subboundaries with low-angle misorientations appeared at early deformation. The subsequent straining led to an increase in the misorientations of these subboundaries, resulting in the formation of submicrocrystalline structure at sufficiently large strains. The process of microstructural evolution can be considered as continuous dynamic recrystallization. MDF provided faster kinetics of new ultrafine grain formation as compared to ECAP. The fraction of ultrafine grains with a size below 2 μm comprised 0.59 or 0.23 after MDF or ECAP to a total strain of 4, respectively. The grain refinement kinetics could be accelerated by the presence of second phase precipitates. The fraction of ultrafine grains after MDF to a strain of 4 achieved 0.36 or 0.59 in the solution treated or aged samples, respectively.

Influence of warm deformation on the formation of a fragmented structure in low-carbon martensitic steels

Methods of optical metallography and scanning and transmission electron microscopy were used to investigate the structure of low-carbon steels of martensitic classes VKS-7 and VKS-10 subjected to warm rolling or upsetting at temperatures of 600 and 700°C (in the α state) and 800°C (in the γ state). It has been shown that the deformation by rolling at 600°C to degrees of 40 and 60% does not lead to the destruction of the lath structure of the initial martensite; an increase in the rolling temperature to 700°C and of the degree of deformation to 80% favors the development of recrystallization in situ. It has been found that, upon warm deformation by upsetting, recrystallization occurs at lower temperatures than in the case of the warm rolling. It has been shown that warm deformation by upsetting at a temperature of 700°C leads to the formation of a fragmented structure with a high fraction of ultrafine grains with a size less than 2 μm.

Microstructure evolution in a Cu-Cr-Zr alloy during warm intense plastic straining

The effect of equal channel angular pressing at a temperature of 200 °C to a total strain of 12 on microstructure evolution and mechanical properties of a Cu-0.87wt.%Cr- 0.06wt.%Zr was investigated. New ultrafine grains resulted from gradual increase in the misorientations of strain-induced low-angle boundaries with increasing number of passes. Therefore, the development of ultrafine grains is considered as a kind of dynamic recrystallization. The equal channel angular pressing to a total strain of 12 resulted in the formation of almost equiaxed ultrafine grained structure with an average grain size of 0.5 dm and 0.7 dm in the solution treated and aged samples, respectively. At the same time, the fraction of ultrafine grains comprises 0.77 in the solution treated samples and 0.72 in the aged samples. Significant grain refinement led to the remarkable increase of the ultimate tensile strength up to 550 MPa.

Effect of Hot Torsion Parameters on Development of Ultrafine Ferrite Grains in Microalloyed Steel

Hot torsion testing was performed on a low carbon Nb-Ti microalloyed steel to study the effects of hot torsion parameters, strain and strain rate, on ultrafine ferrite grains production through dynamic strain-induced transformation, at a deformation temperature just above Ar3. The initiation and evolution of ultrafine ferrite grains were studied. The results show that the amount of strain and strain rate has conversely effect on the volume fraction and grain size of ultrafine ferrite grains. With increasing strain, the interior of austenite grains become activated as nucleation sites for fine ferrite grains. As a result, ferrite grains continuously nucleate not only at the former austenite grain boundaries but also inside the austenite grains which leads to a rapid increase in volume fraction of ultrafine grains. Increasing of strain rate reduces the tendency of ferrite grains coarsening so that ultrafine ferrite grains are achieved, while the volume fraction of ultrafine grains decreases at the same strain level.

High Strain Rate Superplasticity in an Al-Mg-Sc-Zr Alloy Produced by Equal Channel Angular Pressing and Subsequent Cold and Warm Rolling

Superplastic (SP) properties of an Al−5%Mg−0.2%Sc−0.08%Zr alloy subjected to equal channel angular pressing (ECAP) at T=325°C with an effective stain of ~ 8 and subsequent rolling at the same and ambient temperatures were studied. It has been shown that the formation of ultrafine grained (UFG) structure with the grain size of about 1 μm and the volume fraction 0.6-0.7 under ECAP resulted in exceptionally high SP ductilities in a wide temperature - strain rate range. Maximum elongations ~3300% appeared at 475°C and the strain rate () of 5.6×10-2s-1. Subsequent warm rolling with a total reduction of 86% led to increased to 0.8-0.85 volume fraction of ultrafine grains with no changes in grain size. Cold rolling with reduction of 80%, in contrast, provided a heavily deformed structure with high dislocation density. In spite of the difference in the alloy microstructures, the SP properties in both rolled conditions were close to similar. The both states exhibited SP behavior with maximum elongation of ~ 2800% at 520°C and = 1.4 ×10-2s-1.

On the processing of hetero-nanostructured metals for improved strength/ductility balance by ECAE and SPS techniques

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by equal channel angular extrusion) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) techniques for producing multi-scale microstructures when a conducting milled powder is used is demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of spark plasma sintering), altering the nature of the milled powder – by Y 2O 3 addition during the milling stage – is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

Processing Dense Hetero-Nanostructured Metallic Materials for Improved Strength/Ductility Balance through High Strain Deformation and Electrical Current Assisted Sintering (ECAS)

This paper has examined some recent findings concerning the processing of fully dense hetero-nanostructured materials (i.e. consisting of nano, ultrafine and micrometric grains) which can be produced by using the interplay between heavy deformation and recrystallization. By plastic deformation of bulk materials, an improved strength/ductility balance can be obtained directly by imparting high strain deformation (by ECAE) until the occurrence of recrystallization. Using a powder metallurgy route, the strong potential of electric field assisted sintering (ECAS) for producing multi-scale microstructures when a milled powder is used is also demonstrated. In this case, in addition to modify the classic processing parameters (time/temperature of ECAS), altering the nature of the milled powder - by Y2O3 addition during the milling stage - is also a good way to delay the onset of recrystallization and, thereby, increase the fraction of ultrafine grains.

Ultrafine Grain Formation in Ferritic Stainless Steel during Severe Plastic Deformation

The development of submicrocrystalline structures in Fe-20 pct Cr ferritic stainless steel was studied in multidirectional forging to large total strains. The structural changes are characterized by the development of microshear bands in high density dislocation substructures. The multidirectional deformation promotes the multiple shearing, which results in the formation of a spatial net of mutually crossed microshear bands subdividing the original grains. The new grains with high-angle boundaries appear primarily at the microshear band intersections and subsequently along the bands. The fraction of ultrafine grains gradually increases with increasing the density of microshear bands as a result of continuous increase in misorientations among deformation subgrains during processing. An increase in the processing temperature can accelerate remarkably the kinetics of ultrafine grain evolution at large strains. The mechanism of strain-induced grain formation is discussed in detail.

The Effect of Chemistry on the Formation of Ultrafine Ferrite in Steel.

A thermomechanical process has been developed to produce ultrafine (1-2 μm) ferrite grains in hot rolled steel strip. This process involves rolling the steel at or just above the austenite to ferrite transformation temperature, and was applied to a variety of steel chemistries, including plain low and high carbon grades and steels containing microalloying elements such as Nb, Ti, B and Mo. Significant volume fractions of ultrafine grains were observed in all samples. However, it was found that the chemical composition of the steel slightly influenced the morphology and volume fraction of ultrafine grains formed in the surface layers of the strip, and significantly altered the microstructure formed in the core of the strip. In plain carbon grades, the level of ferrite refinement increased slightly as carbon content increased. In the case of the microalloyed steels, those containing Nb and Ti additions produced the greatest level of refinement. The volume fraction and level of refinement of the ultrafine ferrite grains also influenced the strengths of the steels.