Deformation Structures of Pure Titanium during Shear Deformation

The deformation microstructure of commercial pure (CP) titanium formed in the theoretical shear zone of an equal channel angular pressing (ECAP) die during 3 or 4 passes is investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The typical feature of the microstructure is that ultrafine grains coexist with coarse elongated grains with a high density of low-angle grain boundaries (LAGBs). Dislocation tangle zones (DTZs), dislocation cells (DCs), and subgrains are generated during shear deformation. The primary twin type has been found to be { 10[`1]2 }. \left\{ {10\bar{1}2} \right\}. Grain refinement appears to progress by continuous dynamic recrystallization (CDRX), in which dislocation movement to LAGBs leads to their evolution into high-angle grain boundaries (HAGBs).     

Dynamic restoration mechanisms in Al-5.8 At. Pct Mg deformed to large strains in the solute drag regime

An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains, nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains as they spiral around the torsion axis. No evidence was found fordiscontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei. It is concluded that dynamic recovery in the solute drag regime gives rise togeometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Aspects of orientation-dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an important selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grains is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy.     

Aspects of Orientation-Dependent grain growth in extra-low carbon and interstitial-free steels during continuous annealing

The present work concentrates on the application of orientation imaging microscopy (OIM) based on the electron backscattered diffraction (EBSD) technique to the investigation of the microstructural evolution of an extra-low carbon (ELC) steel and a Ti-Nb-bearing interstitial-free (IF) steel, during continuous annealing. Aspects like the nucleation, the evolution of the recrystallized volume fraction and grain size of grains with different orientations, the interface area limiting recrystallized {111} regions, and the apparent growth rates have been considered. Different criteria have been applied in order to identify crystallites produced during annealing. During the first stages of annealing, a network of grain boundaries with misorientations higher than 10 deg is produced, mainly inside the deformed γ-fiber grains. The crystallites formed within this network, free from cells or subgrains at their interiors, can be considered as potential nuclei. However, among all, only some of them become effective due to an improtant selection. The {111} recrystallized grains have a significant size and number advantage as compared with other texture components, and a hard impingement between clusters of {111} grians is produced during grain growth. The effect of grain growth behind the recrystallization front seems to be negligible as compared with the grain coarsening produced by the migration of this front, driven by the cold-work stored energy. J.L. Bocos, formerly Researcher with CEIT     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced ?110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the ?110? drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced «110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the «110» drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

Study of grain boundary fracture surfaces in doped tungsten-rhenium alloys

The combination of Auger electron spectroscopy and scanning electron microscopy has identified the source of the unique interlocked elongated grains responsible for the high temperature sag resistance in doped tungsten and tungsten-rhenium alloys as due to bubbles which are formed by the volatilization of potassium during sintering. By pinning grain boundaries these bubbles raise the recrystallization temperature (from 1300†C to 2100†C) and their distribution within the material controls the recrystallized grain morphology. There is a thin layer of potassium remaining on the bubble surfaces. The size and distribution of the bubbles is related to the amount of material deformation during processing. Increasing rhenium content does not affect the concentration or distribution of residual potassium. It has no noticeable effect on bubble size, distribution or density. The presence of a thermal gradient during annealing does affect bubble density and recrystallization temperature. Formerly with the Air Force Materials Laboratory, Wright-Patterson AFB, Ohio Formerly with the Aerospace Research Laboratory, Wright-Patterson AFB, Ohio     

Initial stages of recrystallization in aluminum of commercial purity

In commercial aluminum with a purity of 99.4 pct, the formation and growth of recrystallization nuclei were studied by techniques such asin-situ annealing in a high voltage electron microscope, transmission electron microscopy and light microscopy. Sample parameters were the initial grain size (370 and 19 microns) and the degree of deformation (50 and 90 pct reduction in thickness by cold-rolling). It was found that the initial grain boundaries and high angle boundaries within the original grains are preferential sites for recrystallization nuclei, and that the effect of such sites is enhanced by the FeAl₃ particles present in the commercial aluminum as impurities. The nucleation temperatures determined by high voltage electron microscopy and transmission electron microscopy decrease markedly when the initial grain size is decreased both after 50 and 90 pct cold rolling; a less pronounced temperature decrease is obtained by increasing the degree of deformation. The size of the recrystallization nuclei, the recrystallization temperature and the recrystallized grain size are reported for the four sample states, and finally the structural and kinetic observations are discussed.     

优质GH738合金热变形过程中的再结晶机制

摘要：结合MTS压缩实验,分析了不同变形温度、应变速率、变形量及变形后保温时间对优质GH738合金再结晶的影响规律;进而,利用光学金相显微镜（OM）、透射电子显微镜（TEM）和电子背散射衍射（EBSD）分析,系统研究了该合金热变形前后的合金的晶粒组织、晶内亚结构、晶粒取向差异和弯曲晶界。结果表明：在实验参数范围内,优质GH738合金在热压缩过程中的再结晶以晶界弓弯方式形核的非连续动态再结晶机制为主,以亚晶旋转方式形核的连续动态再结晶机制为辅;热变形及之后的保温过程中形成的弯曲晶界大部分为普通晶界,少部分为孪晶界;弯曲晶界是取向微偏离CSL点阵的临位晶界发生小面化的结果,其本质是借助晶界位错运动形成的。     

Recrystallization and grain growth in titanium: I. characterization of the structure

Transmission electron microscopy, quantitative optical microscopy, and texture studies were made on swaged and recrystallized titanium wire of three impurity contents: zone refined, a special lot of intermediate purity, and commercial A-70. The electron microscopy studies revealed that a) during recrystallization a number of processes overlap, and b) during grain growth there occurs a decrease in the dislocation density within the grains along with the increase in the average grain size. The quantitative microscopy studies indicated that the linear intercept grain size distribution is approximately log normal and that for a given mean grain size the distribution is relatively independent of the combination of annealing time and temperature used to obtain it. Moreover, there exists a range of grain sizes in space, the numbers of grains in each class interval changing with increase in grain size. The so-called grain shape factor decreases with increase in mean grain size (annealing time) at a constant temperature and with decrease in temperature for a constant grain size. The texture of the as-swaged wire and the changes in the texture during grain growth are in qualitative accord with those previously reported for deformed and recrystallized titanium. Impurity content influences the degree of these various structural characteristics but not their substance. K. Okazaki, Formerly Visiting Research Associate, Metallurgical Engineering and Materials Science Department, University of Kentucky, Lexington, Ky.     

On deformation-induced continuous recrystallization in a superplastic AlLiCuMgZr alloy

The microstructural changes of a warm rolled AlLi alloy occurring during static annealing and superplastic deformation at 515°C were studied by means of transmission electron microscopy. Deformation induces a continuous recrystallization with a rapid subgrain growth and a rapid increase in boundary misorientations. The higher strain rate results in a faster subgrain growth and a finer recrystallized grain size. The increasing rate of boundary misorientations and the strain at which the average misorientation reaches about 20° increase with increasing strain rate. The increase in boundary misorientations is proportional to the subgrain growth during the whole static annealing process. Deformation results in a more rapid increase in boundary misorientation with subgrain size than static annealing. Dislocation gliding plays an important role before the formation of high angle grain boundaries during superplastic deformation. The absorption of dislocations into subgrain boundaries results in a more rapid increase in boundary misorientation during deformation. Thus, the mechanism of the deformation-induced continuous recrystallization is suggested to be the generation of dislocations in grains and the absorption of gliding dislocations into subgrain boundaries.     

FGH98合金的再结晶行为

利用扫描电镜和透射电镜对热挤压态和退火态FGH98合金的相析出规律及组织特征进行了分析,并对其再结晶机制进行了深入研究.结果表明:热挤压态FGH98合金在空冷过程中已经发生了少量的再结晶现象,随着挤压温度的提高,体系内位错密度下降.退火态FGH98合金中再结晶机制主要与体系内未回溶的一次γ'相有关,在一次γ'相聚集区再结晶主要以依靠亚晶的聚合和亚晶的长大,或两者的混合机制进行形核;随着一次γ'相的减少,合金还可以通过应变诱发晶界迁移、多晶粒交汇区形核、孪晶叠加等多种方式形核.需要指出的是,FGH98合金中未回溶的一次γ'相在退火处理过程中也会通过部分的回复和再结晶发生软化效应.      

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.     

Fe-6.5%Si合金温轧后退火过程中再结晶行为

用电子背散射技术观察了700℃温轧板在退火过程中的组织及织构演变以了解其再结晶行为.结果表明,温轧织构由强的（111）〈112〉、较弱的〈110〉∥RD及Goss组成,再结晶织构与之相似.〈110〉∥RD及（111）〈112〉新晶粒首先形成于与之构成小角度晶界的形变晶粒的晶界附近,而在角隅及组织不均匀区等位置孕育出与周围晶粒构成大角度晶界的晶核,择优取向不明显.退火过程中（111）〈112〉在形变组织中累积,最终转化为（111）〈112〉再结晶晶粒.分析认为,温轧后退火是不均匀组织在低储存能驱动下的再结晶过程.（112）〈110〉及（111）〈112〉形变拉长晶粒多发生连续再结晶从而退火织构与形变态相似.在角隅区形成核心进而发生不连续再结晶,核心取向的统计性及不连续晶核的长大弱化再结晶织构,其中Goss晶粒多以此方式形成于（111）〈112〉晶粒内部.      

Microstructure and Microtexture Evolution of Shear Localization in Dynamic Deformation with Different Strains in Annealed Copper

Shear localizations with different strains in annealed copper were obtained by a modified split Hopkinson pressure bar. Microstructure and microtexture evolution of the shear localization regions were examined using optical microscopy, electron backscatter diffraction technique, and transmission electron microscopy. The results show that both the mechanical response and deformation behavior are correlated closely to the shear strains. The elongated dislocation cells, stretched subgrains, and the refinement of subgrains are observed within shear localizations during dynamic deformation. Ultrafine grains of 100 to 300 nm with high-angle-boundaries are produced within the shear band with the shear strain of 5.8. Microtexture characterization reveals that a stable orientation, in which 〈110〉 directions of the crystals tend to align with the shear direction, develops both in the deformation and recrystallization areas. The {111} planes of the crystals tend to parallel to the shear plane in the deformation area, whereas the aggregated extent of this orientation becomes weak in the recrystallization area. In addition, some grains exist with the {100} planes parallel to the shear plane in the deformation and recrystallization areas. The rotational dynamic recrystallization is a reasonable mechanism for the microstructure evolution. The effects of cooling stage on the growth of grains and the change of dislocation density are estimated as a complementarity to this mechanism.     

Electron backscatter diffraction analysis of dynamically recrystallized grain structures in a Ni-Cr-Fe base alloy

Dynamic variations in grain structures and grain boundary characteristics of NiCrFe-based alloy 718 during hot uniaxial compression as well as stress relaxation after the compression were investigated in this article. An electron backscatter diffraction (EBSD) technique was used for the specimens that were compressed at temperatures of 1010 °C and 1066 °C and strain rates of 0.5 and 0.005 s⁻¹, up to a strain of 0.7. Stress relaxation was observed by keeping the upper die in position at the test temperatures as soon as the compression was completed. The variations in the CSL boundary distribution and in the misorientation angle distribution during compression and stress relaxation were thoroughly analyzed to characterize the dynamically recrystallized grain (DRX) boundaries. During deformation at a high strain rate of 0.5 s⁻¹, dynamically recrystallized grains were formed by progressive subgrain rotation. Active dynamic recovery (DRV) at 1066 °C was inferred from the similar degree of strain softening in spite of the different fraction of dynamic recrystallization, which is supported by the high frequency of low misorientation angle boundaries. Stress relaxation was caused by a coalescence of subgrains having very small misorientation angles. Directional grain growth and a redistribution of the grain boundary character caused by the grain rotation occur during the stress relaxation, resulting in reduced total boundary energy. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee.     

Quantitative Evaluation of Bulk and Interface Microstructures in Al-3003 Alloy Builds Made by Very High Power Ultrasonic Additive Manufacturing

Ultrasonically consolidated 3003 aluminum alloy builds were prepared with constituent tapes by using a very high power ultrasonic additive manufacturing (UAM) process. Microstructures of interface and bulk regions were quantitatively characterized using the electron backscattered diffraction technique. The interface microstructure consists of equiaxed grains. The 〈111〉 crystallographic directions of these grains were aligned with the normal direction of the specimen, confirming a shear deformation mode at these regions. In addition, due to recrystallization, the density of low-angle grain boundaries also significantly decreased. In contrast, original elongated grains and partially recrystallized grains were observed in the bulk region of the tape. These elongated grains correspond to rolling texture components of face-centered-cubic materials. The preceding microstructure gradients are rationalized based on the accumulated thermomechanical cycles during processing.     

Evolution of Microstructure and Texture during Annealing of Aluminum AA1050 Cold Rolled to High and Ultrahigh Strains

The microstructure and texture of commercial purity aluminum (AA1050) have been investigated after cold rolling to von Mises strains of 3.6 to 6.4 followed by recovery and recrystallization during annealing. The evolution of structural parameters of the deformed microstructure, such as boundary spacing and fraction of high-angle boundaries (HABs), did not reach saturation in the given strain range. Recovery was accompanied by structural coarsening and by a decrease in the fraction of HABs. The coarsening rate increased with increasing strain prior to annealing. Recrystallization nuclei were found to form both in deformation zones around coarse particles and in recovered lamellar structures. The process of recrystallization in the present material can thus be characterized as discontinuous recrystallization. In recrystallized conditions, the average grain size was related to the grain orientation: the mean size of grains having orientations of the rolling texture was smaller than the size of grains with other orientations. The orientation dependence of the recrystallized grain size was more pronounced in the samples rolled to ultrahigh strains.     

Recrystallization Behavior of a Heavily Deformed Austenitic Stainless Steel During Iterative Type Annealing

The study describes evolution of the recrystallization microstructure in an austenitic stainless steel during iterative or repetitive type annealing process. The starting heavily cold deformed microstructure consisted of a dual phase structure i.e., strain-induced martensite (SIM) (43 pct in volume) and heavily deformed large grained retained austenite. Recrystallization behavior was compared with Johnson Mehl Avrami and Kolmogorov model. Early annealing iterations led to reversion of SIM to reversed austenite. The microstructure changes observed in the retained austenite and in the reverted austenite were mapped by electron backscatter diffraction technique and transmission electron microscope. The reversed austenite was characterized by a fine polygonal substructure consisting of low-angle grain boundaries. With an increasing number of annealing repetitions, these boundaries were gradually replaced by high-angle grain boundaries and recrystallized into ultrafine-grained microstructure. On the other hand, recrystallization of retained austenite grains was sluggish in nature. Progress of recrystallization in these grains was found to take place by a gradual evolution of subgrains and their subsequent transformation into fine grains. The observed recrystallization characteristics suggest continuous recrystallization type process. The analysis provided basic insight into the recrystallization mechanisms that enable the processing of ultrafine-grained fcc steels by iterative type annealing. Tensile properties of the processed material showed a good combination of strength and ductility.