Fraction Of High-angle Boundaries Research Articles

Characterization of the hot deformation microstructure of AISI 321 austenitic stainless steel

In the present research, the hot deformation microstructure of AISI 321 austenitic stainless steel was studied by performing hot compression tests in the temperature range of 800–1200 °C and strain rates of 0.001–1 s−1. Microstructural evolutions were studied using transmission and scanning electron microscope equipped with EBSD camera. Results show that the main restoration process in the temperature range of 800–950 °C is dynamic recovery and dynamic recrystallization is the prevailing softening mechanism in the temperature range of 1000–1200 °C. TEM observations confirm the occurrence of dynamic recovery and recrystallization at the specified temperatures. Increasing deformation temperature leads to the increase in the fraction of high angle boundaries and simultaneous decrease in the fraction of low angle boundaries. Also, deformation at 800 and 850 °C is accompanied by the formation of M23C6 precipitates at austenite grain boundaries that increases the deformation activation energy and results in grain boundary sensitization detected using scanning electron microscopy.

Compression with oscillatory torsion applied after solution treatment and aging treatment of CuCr0.6 alloy for grain refinement: Microstructure, mechanical and electrical properties

Microstructure, mechanical properties and electrical conductivity of the CuCr0.6 alloy processed by compression with oscillatory torsion (COT) at room temperature were investigated. The COT processing was applied for the samples processed with the solution treatment followed by quenching into iced water at 1000 °C for 3 h and for the samples processed using the solution treatment followed by aging treatment at 500 °C for 2 h. Application of the solution and aging processes prior to the COT deformation results in the partial dissolution of Cr precipitates into the Cu matrix and precipitation of the coherent second phase precipitates, respectively. COT processing with different values of the total effective strain (εft) of 10, 20, 40 was introduced to the material. It was found that the presence of the second phase precipitates has a significant effect on the formation of the ultrafine grain (UFG) structure during the COT deformation. After the COT deformation with maximal effective strain value of εft = 40 of the samples processed by aging, the obtained grain size was about 300 nm and the high angle boundary fraction of about 30%. Application of the COT deformation at the effective strain value of εft = 40 for the samples after solid-solution treatment leads to the formation of a UFG microstructure with the average grain size of about 450 nm and the high angle boundary fraction of 13%. Formation of the more refined UFG structure and the presence the dispersed chromium precipitates in the samples after aging contributes to the enhanced ultimate tensile strength (UTS) of 521 MPa, with the electrical conductivity of 85% IACS, which are much higher than the corresponding values of solid-solution-treated samples prior to COT of 353 MPa and 40% IACS.

Sulfide Stress Cracking Behavior of a Martensitic Steel Controlled by Tempering Temperature.

A medium-carbon Cr–Mo–V martensitic steel was thermally processed by quenching (Q) at 890 °C and tempering (T) at increasing temperatures from 650 °C to 720 °C and the effect of tempering temperature, Tt, on sulfide stress cracking (SSC) behaviors was estimated mainly via double cantilever beam (DCB) and electrochemical hydrogen permeation (EHP) tests and microstructure characterization. The results indicate that the threshold stress intensity factor for SSC, KISSC, increased with increasing Tt. The overall and local H concentration around the inclusions decreased with increasing Tt, due to reductions in the amounts of solute atoms, grain boundaries and dislocations, which effectively prevented SSC initiation. Also, increasing Tt caused an increased fraction of high-angle boundaries, which evidently lowered the SSC propagation rate by more frequently diverting the propagating direction and accordingly restricted SSC propagation. The overall SSC resistance of this Q&T–treated steel was therefore significantly enhanced.

Effect of temperature of isothermal multidirectional forging on microstructure development in the Al-Mg alloy with nano-size aluminides of Sc and Zr

The effect of deformation temperature on structural changes in the homogenized ingot of a complex-alloyed aluminum alloy 1570C (Al-5% Mg-0.2% Sc-0.1% Zr) during hot isothermal multidirectional forging (IMF) was investigated. The alloy with equiaxial grains of 25 μm and coherent precipitates of Al3(Sc, Zr) 5–10 nm in diameter was subjected to IMF at the temperatures of 325° and 450 °C (∼0.6 and 0.8 Tm, respectively) at a strain rate of 10−2 s−1 to a total strain of ε = 8.4 with a pass strain of 0.7. It has been shown that grain refinement takes place during IMF at both temperatures. The formation of new fine (sub)grains surrounded by low- and high-angle boundaries started in the vicinity of the original grain boundaries in the earlier stages of deformation. With increasing strain, the volume fraction and boundary misorientations of such crystallites increased; that results in almost complete replacement of the original structure with a new fine-grained one via continuous dynamic recrystallization. The alloy grain refinement was controlled by the interaction of lattice dislocations and/or grain boundaries with precipitates, effectively inhibiting migration of the boundaries as well as dynamic recovery, leading to rearrangement of lattice dislocations, and their annihilation. Herewith, the kinetics of the fine-grained structure formation was practically independent on the processing temperature, resulting in roughly similar microstructures after the same strains, which characterized by the closed mean misorientation angles and fractions of high-angle boundaries. However, the size of (sub)grains processed at 325 °C was significantly lower than at 450 °C, i.e., 1.2 and 2.5 μm, respectively. This indicated that parameters of the developed structure were thermally activated and controlled by precipitates. The features of microstructure formation and the nature of grain refinement of a complex-alloyed aluminum alloy under high-temperature severe plastic deformation conditions are discussed.

A comprehensive investigation of the strengthening effects of dislocations, texture and low and high angle grain boundaries in ultrafine grained AA6063 aluminum alloy

The effect of equal channel angular pressing (ECAP) on the microstructure and mechanical properties of AA6063 aluminum alloy was investigated. For this purpose, samples of AA6063 aluminum alloy were deformed up to 10 passes using ECAP and the evolution of microstructure, texture and dislocation density was investigated. It was found that the dislocation density increased and cells mostly surrounded by low angle boundaries (LABs) formed after 2 passes ECAP. Increasing the dislocation density continued with further processing and reached a maximum at the 4th ECAP pass. With further deformation to 6 passes, the dislocation density reduced, the fraction of high angle boundaries (HABs) increased and the averages of cell size and grain size reduced, significantly, which indicated the occurrence of grain refinement. However, a slight increase in dislocation density, average cell size (ACS) and average grain size (AGS), indicating coarsening, occurred when 10 passes of ECAP are imposed. Hardness, yield (YS) and ultimate tensile strength (UTS) increased continuously although the most significant enhancement occurred after 2 passes of ECAP. Investigating the correlation between the microstructure and mechanical properties indicated that Hall-Petch relationship was valid for ultrafine grained AA6063 before the coarsening started provided that the property of interest was correlated to average cell size but not the average grain size.

Effect of the Size of Al3(Sc,Zr) Precipitates on the Structure of Multi-Directionally Isothermally Forged Al-Mg-Sc-Zr Alloy

The effect of Al3(Sc,Zr) dispersoids on the evolution of the cast Al-Mg-Sc-Zr alloy structure under multi-directional isothermal forging (MIF) has been investigated. The alloy, which has an equiaxed grain structure with a grain size of ~25 μm and contains dispersoids 5–10 and 20–50 nm in size after onestage (at 360°C for 6 h) and two-stage (360°C for 6 h + 520°C for 1 h) annealing, respectively, was deformed at 325°C (~0.65 Tm) up to cumulative strains of e = 8.4. In the initial stages of MIF, new fine (sub)grains surrounded by low-angle and high-angle boundaries (HABs) were formed near the initial grain boundaries. With increasing strain, the volume fraction and misorientation of these crystallites increased, which led to the replacement of a coarse-grained structure with a fine-grained one with a grain size of ~1.5-2.0 μm. Dynamic recrystallization occurred in accordance to a continuous mechanism and was controlled by the interaction of lattice dislocations and/or (sub)grain boundaries with dispersoids that effectively inhibited the migration of boundaries, as well as the rearrangement of lattice dislocations and their annihilation. The particle size and the density of their distribution significantly affected the parameters of the evolved structure; in an alloy with smaller particles, a structure with a smaller grain size and a larger HAB fraction developed.

Continuous dynamic recrystallization (CDRX) model for aluminum alloys

The control of a microstructure, during and after hot forming, is crucial to tailor optimum mechanical properties for specific applications. Recrystallization is a key process which may contribute to a great extent to microstructure development. Dynamic recrystallization is becoming an attracting research area to investigate novel hot forming routes in order to maximize the performance of aluminum products while shortening the time required for manufacturing. A continuous dynamic recrystallization (CDRX) mathematical model was developed by Gourdet–Montheillet (GM) to predict the inherent phenomena of an AA1200 alloy. In the present work, the original GM model has been extended and applied to study CDRX in a 5052 aluminum alloy. The proposed model embodies a solid solution and second phase strengthening, through newly estimated kinetic factors and a kinetic constant, respectively, to discern the CDRX behavior of 5052 aluminum alloy compared to AA1200. The latter kinetic constant relies on the Kocks–Mecking–Estrin (KME) theory. The input law of the fraction of high angle boundaries (f HAB), as a function of strain (e) (but independent of temperature and strain rate), is defined as the best fitting function of the experimental data. The results are presented in terms of stress–strain curves, dislocation density, and (sub) grain size, as these are important design parameters from an industrial and engineering viewpoint. The model has been validated successfully, from both a qualitative and quantitative point of view, against various literature data sources and tests (e.g., hot compression, hot plane strain compression, and equal channel angular pressing) pertaining to the 5052 alloy and other similar Al–Mg alloys.

Through-thickness microstructural evolution during grain boundary engineering type thermomechanical processing and its implication on sensitization behavior in austenitic stainless steel

Through-thickness microstructural variations and their synergistic influence on degree of sensitization in grain boundary engineering (GBE) treated 304L stainless steel was compared with solution-annealed and marginally cold-worked (~3% thickness reduction) specimens. The solution-annealed specimen has shown higher degree of sensitization due to the presence of lower fraction of Σ3 (as well as other low Σ CSL) boundaries and dominant random high-angle boundaries (RHAGBs) connectivity. However, it has shown thickness-independent sensitization behavior as grain boundary character distribution, residual strain and grain boundary network topology towards thickness direction are similar. Owing to higher residual strain, as revealed by local misorientation analysis, the cold-worked specimen showed maximum degree of sensitization (higher than solution-annealed condition) at surface. The GBE specimen has shown much lower degree of sensitization at surface due to higher-fraction of Σ3 boundaries and discontinuous RHAGBs connectivity. However, increasing fraction of RHAGBs and their connectivity, as confirmed by twinning-related domain and fractal analysis, cause more sensitization at interior of this specimen. In spite of this, the degree of sensitization at the interior of GBE specimen is lower than the solution-annealed or cold-worked specimen. Even though the residual strain in GBE specimen is higher than the solution-annealed condition, the lower degree of sensitization indicates that the grain boundary character distribution and grain boundary network topology have higher implication than residual strain in controlling sensitization.

Microstructure changes in ultrafine-grained nickel processed by high pressure torsion under ultrasonic treatment

Commercially pure nickel was processed by high pressure torsion (HPT) and subjected to ultrasonic treatment (UST) with different amplitudes of compression-tension stresses in the zone of stress antinode of a standing wave. It was found that microstructure parameters such as the dislocation density, low- and high-angle grain boundary fractions, microhardness, and the stored excess energy as well, non-monotonically depend on the ultrasound amplitude. A structure relaxation leading to a reduction of internal stresses and stored energy and increase of the fraction of high-angle boundaries was observed at some intermediate amplitudes of the oscillating stress. The maximum relaxation effect was observed in the samples after UST with the amplitude of 60MPa. Possible mechanisms of the influence of ultrasound on the microstructure of deformed materials are discussed.

Dynamic reverse phase transformation induced high-strain-rate superplasticity in low carbon low alloy steels with commercial potential

Superplastic materials are capable of exhibiting large tensile elongation at elevated temperature, which is of great industrial significance because it forms the basis of a fabrication method to produce complex shapes. Superplasticity with elongation larger than 500% has been widely realized in many metals and alloys, but seldomly been succeeded in low carbon low alloy steel, even though it is commercially applied in the largest quantity. Here we report ultrahigh superplastic elongation of 900–1200% in the FeMnAl low carbon steels at high strain rate of 10−2–10−3 s−1. Such high-strain-rate superplasticity was attributed to dynamic austenite reverse phase transformation from a heavily cold rolled ferrite to fine-grained ferrite/austenite duplex microstructure and subsequent limited dynamic grain coarsening, under which a large fraction of high angle boundaries can be resulted for superplastic deformation. It is believed that this finding of the low carbon low alloy steel with ultrahigh superplasticity and relative low cost would remarkably promote the application of superplastic forming technique in automobile, aeronautical, astronautical and other fields.

Local microstructure and flow stress in deformed metals

The microstructure and flow stress of metals are related through many well-known strength-structure relationships based on structural parameters, where grain size and dislocation density are examples. In heterogeneous structures, the local stress and strain are important as they will affect the bulk properties. A microstructural method is presented which allows the local stress in a deformed metal to be estimated based on microstructural parameters determined by an EBSD analysis. These parameters are the average spacing of deformation introduced boundaries and the fraction of high angle boundaries. The method is demonstrated for two heterogeneous structures: (i) a gradient (sub)surface structure in steel deformed by shot peening; (ii) a heterogeneous structure introduced by friction between a tool and a workpiece of aluminum. Flow stress data are calculated based on the microstructural analysis, and validated by hardness measurement and 2D numerical simulations. A good agreement is found over a plastic strain range from ∼1 to 5.

Microstructure evolution and strengthening mechanisms in commercial-purity titanium subjected to equal-channel angular pressing

High-resolution electron backscatter diffraction (EBSD) was applied to examine grain refinement in commercial-purity titanium Grade 4 subjected to equal-channel angular pressing (ECAP) via the Conform technique. This approach enables the production of long-length billets and thus has the potential for commercial application. Microstructure evolution was found to be a relatively-complex process which included several stages. At relatively-low accumulated strains, microstructure changes were markedly influenced by mechanical twinning. However, the concomitant grain refinement suppressed this mechanism, and subsequent microstructure development was dictated by the evolution of deformation-induced boundaries which developed preferentially near the original grain boundaries. The final material produced after an effective strain of ~ 8.4 was characterized by a mean grain size of 0.3µm, high-angle boundary fraction of 55 pct., a texture of moderate strength, and a yield strength of ~ 1050MPa. Based on the detailed microstructural analysis, the contributions of various strengthening mechanisms were quantified. The rapid material strengthening during the early stages of ECAP was explained in the terms of a major increase in dislocation density and the extensive formation of the deformation-induced boundaries. With further increments in accumulated strain, however, the dislocation as well as grain-boundary density reached a saturation, thus reducing the hardening efficiency of ECAP at high strains.

Defect structure and thermomechanical stability of nano- and microcrystalline titanium obtained by different methods of intense plastic deformation

Mechanical stability under prolonged loading and thermostability under annealing have been studied for nano- and microcrystalline titanium obtained by different methods of intense plastic deformation. The effect of nanoporosity and the fraction of high angle boundaries formed due to intense plastic deformation has been revealed and analyzed. It has been established that, depending on the loading or the annealing temperature, thermomechanical stability of titanium can be affected, apart from the above structural characteristics, by either twin grain boundaries or titanium-carbide disperse particles.

Structure and microstructure evolution of Al–Mg–Si alloy processed by equal-channel angular pressing

An ultrafine grained Al–Mg–Si alloy was prepared by severe plastic deformation using the equal-channel angular pressing (ECAP) method. Samples were ECAPed through a die with an inner angle of Φ = 90° and outer arc of curvature of ψ = 37° from 1 to 12 ECAP passes at room temperature following route Bc. To analyze the evolution of the microstructure at increasing ECAP passes, X-ray diffraction and electron backscatter diffraction analyses were carried out. The results revealed two distinct processing regimes, namely (i) from 1 to 5 passes, the microstructure evolved from elongated grains and sub-grains to a rather equiaxed array of ultrafine grains and (ii) from 5 to 12 passes where no change in the morphology and average grain size was noticed. In the overall behavior, the boundary misorientation angle and the fraction of high-angle boundaries increase rapidly up to 5 passes and at a lower rate from 5 to 12 passes. The crystallite size decreased down to about 45 nm with the increase in deformation. The influence of deformation on precipitate evolution in the Al–Mg–Si alloy was also studied by differential scanning calorimetry. A significant decrease in the peak temperature associated to the 50% of recrystallization was observed at increasing ECAP passes.

Friction stir modification of GTA 7075-T6 Al alloy weld joints: EBSD study and microstructural evolutions

As known, mechanical properties of gas tungsten arc welded 7075 Al alloys are not desirable and some techniques should be utilized in order to refine the microstructure and hence to improve the mechanical properties of weld joints. In this research work, the microstructure of gas tungsten arc welded 7075 Al alloy was modified by friction stir processing. Evaluation of the tensile strength of the welded joints showed that the tensile strength of the welded joint (228MPa) increases up to 320MPa after friction stir processing. In addition, electron backscattered diffractometry (EBSD) was used in order to study the microstructure and grain boundary character evolutions during arc welding and friction stir processing. It was revealed that as-cast dendritic microstructure of gas tungsten arc welded joint completely disappears during friction stir processing and very fine equiaxed grains are formed in welded joints. Analysis of EBSD data showed that friction stir processing of gas tungsten arc welded joints leads to increase of specific boundaries from 0.7% up to 7.8%. In addition, fraction of high angle boundaries increases after friction stir processing which is resulted from dynamic recrystallization occurring during friction stir processing.

Achieving good strength-ductility synergy of friction stir welded Cu joint by using large load with extremely low welding speed and rotation rate

Cu plates with 2mm in thickness were joined by large load friction stir welding with extremely low welding speed and rotation rate. Heat-affect zone was successfully eliminated for the improved thermal cycle. The grain refinement in the stir zone is attributed to the combination of continuous dynamic recrystallization, discontinuous dynamic recrystallization, dynamic recovery as well as twinning-induce dynamic recrystallization. The stir zone exhibited ultra-refined grains with low dislocation density and a high fraction of high angle boundaries. Abundant twin boundaries were introduced into the stir zone as well. As a result, the stir zone showed a superior strength-ductility synergy with a strength of 548MPa and a uniform elongation of 36%. This work also provides an effective strategy to enhance the strength of friction stir welded Cu joint without ductility loss.

The Strength-Ductility Balance of an Ultrafine-Grained Medium-Carbon Steel Produced by Severe Plastic Deformation

By processing with different high pressure torsion procedures, we obtained ultrafine-grained structures in medium-carbon steel and pure iron with different microstructural features. The UFG carbon steel exhibited a better balance of tensile strength and ductility in comparison with the UFG pure iron. The higher strength of the carbon steel is ascribed to the finer ferrite grain size, the dispersed carbide particles and the presence of alloying elements. The reasonable ductility of carbon steel is attributed to the maintaining of positive work hardening rate at high strain stage of tension, owing to the presence of intragranular particles and high fraction of high-angle boundaries.

Microstructure-property relation and evolution in friction stir welding of naturally aged 6063 aluminium alloy

The development of the microstructure and mechanical properties during friction stir welding of 6063-T4 aluminium alloy was investigated through electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques and mechanical testing. Grain boundary and dislocation strengthening were observed to be the dominant strengthening mechanisms occurring in the stir zone (SZ). While the precipitates morphologies were sensitive to the weld zone temperatures, the grain size and grain boundary misorientations were related to strain (𝜖), strain rates (\(\dot {\epsilon }\)), and peak temperatures (Tp). Severe gradients in the crystallographic texture were observed at the top and bottom and on the retreating and advancing sides of the weld obtained at 48 mm/min welding speed. The optimal strength and ductility of a transverse tensile specimen were achieved when the cold defects, overaging in heat affected zones (HAZs), and grain size in stir zone (SZ) were minimized. While the yield strength of the longitudinal tensile specimens was controlled by the grain size, dislocation density and fraction of low angle boundaries (LABs), the uniform elongation was controlled by the fraction of high angle boundaries (HABs). In addition, a successful implementation of more simple geometry weld zone finite element model than earlier reported work for use in forming simulations was also proposed.

Microstructure and mechanical strength of near- and sub-micrometre grain size copper prepared by spark plasma sintering

Spark plasma sintering (SPS) has been used to prepare fully dense samples of copper in a fully recrystallized condition with grain sizes in the near- and sub-micrometre regime. Two synthesis routes have been investigated to achieve grain size control: (i) SPS at different temperatures from 800 to 1000°C, and (ii) SPS at 800°C followed by annealing at temperatures from 950 to 1050°C. By use of an initial spherical powder with an average particle diameter of ≈0.5μm, samples with average grain sizes in the range from 0.5 to 3μm have been prepared. Microstructural examination based on both transmission electron microscope, and on electron back-scatter diffraction studies, confirms the samples are in a nearly fully recrystallized condition, with grains that are dislocation-free, and have a random texture, with a high fraction of high angle boundaries. The mechanical strength of the samples has been probed using hardness measurements and tensile testing, revealing an enhanced strength for samples with grain sizes less than ≈1μm.

The microstructure-strength relationship in severely-deformed commercial-purity titanium

Electron backscatter diffraction was applied to examine the microstructure developed in commercial-purity titanium subjected to equal channel angular pressing at 200°C to a true strain of 8.4. The microstructure was characterized by a mean grain size of 0.4μm and fraction of high-angle boundaries of 55%. However, it also contained a significant fraction of relatively coarse (>1μm) remnants of original grains. A microstructure-properties analysis indicated that strengthening effect of ECAP was essentially contributed by substructure.