Straight Grain Research Articles

Stress-induced phase transformation characteristics and its effect on the enhanced ductility in continuous columnar-grained polycrystalline Cu–12 wt % Al alloy

The microstructure evolution and stress-induced phase transformation (SIPT) characteristics of continuous columnar-grained (CCG) polycrystalline Cu–12wt%Al alloy during the tensile deformation process were investigated to understand the ductility enhancement mechanism, which shows a tensile elongation of 28%, nine times as high as that of the conventional as-cast polycrystalline counterpart. The results show that the CCG alloy has highly-oriented 〈001〉β texture and straight low-energy grain boundaries (GBs) along solidification direction, which significantly promote grains compatibility during plastic deformation. Additionally, besides β1'→α1' transformation, β1'→γ1'→α1' SIPT was also observed simultaneously in the CCG polycrystalline Cu–12wt%Al alloy, which is different from single crystalline and conventional polycrystalline counterparts. As a consequence of the special phase transformations, high phase transformation plasticity (15–20%) and prominent subsequent plastic deformation (8–13%) of the phase transformation product α1' martensite can be obtained.

Highly dense and fine-grained SiC with a Sc-nitrate sintering additive

Abstract To explain the sintering behavior of 5 wt.% Sc-nitrate-added SiC, which showed a 99.3% density with the fine 156 nm-sized grains, a microstructural study using high resolution electron microscopy (HREM) was performed and the results were compared with those of Tm-added SiC, which had a mean grain size of 753 nm. Contact flattening was proposed as a governing sintering mechanism for Sc-added SiC based on the experimental observations of an inter-granular phase thinner than 1 nm along with the straight grain boundaries. On the other hand, the Tm-added SiC showed a several nm-thick inter-granular phase with a curved grain boundary morphology, which is the typical microstructure that can be obtained by a solution–reprecipitation mechanism combined with Ostwald ripening.

Twinning in multicrystalline silicon for solar cells

Twinning in multicrystalline silicon was studied by observing the surface of as-cut wafers and by EBSD. By tracing twin structures downwards to their first point of origin, the conditions at the point of generation were identified. Twins covering the whole width of a grain predominantly originates at junctions between three grain boundaries. Twins also originate at straight grain boundary segments of alternating direction, indicative of faceted grain boundaries. The phenomena are proposed to occur because they reduce grain boundary energy, which is substantiated by pole figure analysis. In addition to the CSL relationship, the orientation of the grain boundary in low energy configurations turned out to be essential.

Deformation fabrics of natural blueschists and implications for seismic anisotropy in subducting oceanic crust

Investigations of microstructures are crucial if we are to understand the seismic anisotropy of subducting oceanic crust, and here we report on our systematic fabric analyses of glaucophane, lawsonite, and epidote in naturally deformed blueschists from the Diablo Range and Franciscan Complex in California, and the Hida Mountains in Japan. Glaucophanes in the analyzed samples consist of very fine grains that are well aligned along the foliation and have high aspect ratios and strong crystal preferred orientations (CPOs) characterized by a (100)[001] pattern. These characteristics, together with a bimodal distribution of grain sizes from some samples, possibly indicate the occurrence of dynamic recrystallization for glaucophane. Although lawsonite and epidote display high aspect ratios and a strong CPO of (001)[010], the occurrence of straight grain boundaries and euhedral crystals indicates that rigid body rotation was the dominant deformation mechanism. The P-wave (AVP) and S-wave (AVS) seismic anisotropies of glaucophane (AVP=20.4%, AVS=11.5%) and epidote (AVP=9.0%, AVS=8.0%) are typical of the crust; consequently, the fastest propagation of P-waves is parallel to the [001] maxima, and the polarization of S-waves parallel to the foliation can form a trench-parallel seismic anisotropy owing to the slowest VS polarization being normal to the subducting slab. The seismic anisotropy of lawsonite (AVP=9.6%, AVS=19.9%) is characterized by the fast propagation of P-waves subnormal to the lawsonite [001] maxima and polarization of S-waves perpendicular to the foliation and lineation, which can generate a trench-normal anisotropy. The AVS of lawsonite blueschist (5.6–9.2%) is weak compared with that of epidote blueschist (8.4–11.1%). Calculations of the thickness of the anisotropic layer indicate that glaucophane and lawsonite contribute to the trench-parallel and trench-normal seismic anisotropy beneath NE Japan, but not to that beneath the Ryukyu arc. Our results demonstrate, therefore, that lawsonite has a strong influence on seismic velocities in the oceanic crust, and that lawsonite might be the cause of complex anisotropic patterns in subduction zones.

Retardation of (Cu,Ni)6Sn5 spalling in Sn–Ag–Cu/Ni solder joints via controlling the grain structure of Ni metallization layer

In the Sn–3.0Ag–0.5 Cu/Ni (wt%; SAC305/Ni) solder joint, the (Cu,Ni)6Sn5 spalling was suppressed by altering the microstructure of Ni films. Ni films with intertwined and straight grain boundaries (GBs) were fabricated by the diverse target powers of the direct current magnetron sputter. The intertwined GB and the straight GB of Ni films determined the degree of the (Cu,Ni)6Sn5 spalling. Ni films with straight GB accelerated the (Cu,Ni)6Sn5 spalling due to the fast diffusion path of Ni and Cu in straight GB of Ni films. In contrast, the winding grain structure of Ni film, called intertwined GB-Ni film, can suppress the interdiffusion of Cu and Ni. Thus, the (Cu,Ni)6Sn5 spalling was successfully retarded by the Ni structure with intertwined GB.

Marble decay induced by thermal strains: simulations and experiments

Thermoelastic behavior of different marble types was analyzed using computational modeling and experimental measurements. Eight marble samples with different composition, grain size, grain boundary geometry, and texture were investigated. Calcitic and dolomitic marbles were considered. The average grain size varies from 75 μm to 1.75 mm; grain boundary geometry differs from nearly equigranular straight grain boundaries to inequigranular-interlobate grain boundaries. Four typical marble texture types were observed by EBSD measurements: weak texture; strong texture; girdle texture and high-temperature texture. These crystallographic orientations were used in conjunction with microstructure-based finite element analysis to compute the thermoelastic responses of marble upon heating. Microstructural response maps highlight regions and conditions in the marble fabric that are susceptible to degradation phenomena. This behavior was compared to the measured thermal expansion behavior, which shows increasing residual strains upon repetitive heating–cooling cycles. The thermal expansion behavior as a function of temperature changes can be classified into four categories: (a) isotropic thermal expansion with small or no residual strain; (b) anisotropic thermal expansion with small or no residual strain; (c) isotropic thermal expansion with a residual strain; and (d) anisotropic thermal expansion with residual strain. Thermal expansion coefficients were calculated for both simulated and experimental data and also modeled from the texture using the MTEX software. Fabric parameters control the amount and directional dependence of the thermal expansion. Marbles with strong texture show higher directional dependence of the thermal expansion coefficients and have smaller microstructural values of the maximum principal stress and strain energy density, the main precursors of microcracking throughout the marble fabric. In contrast, marbles with weak texture show isotropic thermal expansion behavior, have a higher propensity to microcracking, and exhibit higher values of maximum principal stress and strain energy density. Good agreement between the experimental and computational results is observed, demonstrating that microstructure-based finite-element simulations are an excellent tool for elucidating influences of rock fabric on thermoelastic behavior.

Figured grain in aspen is heritable and not affected by graft-transmissible signals

Figure can add value to wood products, but its occurrence is unpredictable. A first step in reliably producing figured wood is determining whether it is faithfully transmitted to progeny via sexual and asexual reproduction. We describe a 26-year-old male aspen genotype, designated ‘Curly Poplar’, which was shown to be a Populus × canescens hybrid using microsatellite markers. All rooted cuttings of this genotype exhibited an undulating pattern on the radial surface that was not seen in the control trees, all of which showed a smooth radial surface and straight grain. We observed spiral grain with a magnitude of 2.77 ± 0.12°/cm from vertical in 11-month-old, field-grown rooted Curly Poplar cuttings, but spiral grain was not apparent in wood from the 26-year-old mature ortet that supplied these cuttings. Veneer cut from the mature tree exhibited a novel type of figure that we called ‘Scattered Moiré’. Reciprocal grafts between Curly Poplar and various non-figured aspens showed that a graft-transmissible signals did not appear to be involved in figure formation in Curly Poplar or the induction of figure in straight-grained trees. Curly Poplar was crossed to a straight-grained clone to test the inheritance of the gene(s) responsible for figure. Samples from the resulting population revealed that 79 out of 377 seedlings exhibited figure. A Chi-square test led to the rejection of a 1:1 segregation ratio between figured and non-figured phenotypes (p < 0.01), but not of a 1:3 segregation ratio (p 0.0793). Overall, these analyses showed that figure in Curly Poplar is under genetic control, but its inheritance may not be simple.

Rheological contrast between glaucophane and lawsonite in naturally deformed blueschist from Diablo Range, California

AbstractDeformation microstructures for a lawsonite blueschist from the New Idria serpentinite body, Diablo Range, are investigated to clarify rheological behaviors of glaucophane and lawsonite, which are main mineral assemblages of subducting oceanic crust at relatively cold geotherm. Developments of crystal‐preferred orientations (CPOs) with small grain size, irregular grain boundary and high aspect ratio of glaucophane indicate deformation mechanism as recovery and dynamic recrystallization possibly accommodated by dislocation creep, while lawsonite deforms by rigid body rotation based on euhedral grains with angular or straight grain boundaries. Higher aspect ratios, lower angle to foliation, and stronger CPOs of both minerals in the glaucophane‐rich layer rather than those in the lawsonite‐rich layer suggest the strain localization into the glaucophane‐rich layer. Additionally fabric strength (the degree of crystal alignment) and seismic anisotropy are higher in the glaucophane‐rich layer than that of the lawsonite‐rich layer, which is consistent with the microstructural analyses. All our results imply, therefore, the dominant role of glaucophane rather than lawsonite for rheological behavior and seismic anisotropy of blueschist.

Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks, southwest Japan

In the Sambagawa schist, southwest Japan, while ductile deformation pervasively occurred at D1 phase during exhumation, low-angle normal faulting was locally intensive at D2 phase under the conditions of frictional–viscous transition of quartz (c. 300 °C) during further exhumation into the upper crustal level. Accordingly, the formation of D2 shear bands was overprinted on type I crossed girdle quartz c-axis fabrics and microstructures formed by intracrystalline plasticity at D1 phase in some quartz schists. The quartz c-axis fabrics became weak and finally random with increasing shear, accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries, which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica. We interpreted these facts as caused by increasing activity of pressure solution: the quartz grains were dissolved mostly at platy quartz–mica interface, and precipitated with random orientation and pinned by mica, thus having led to the obliteration of existing quartz c-axis fabrics. In the sheared quartz schist, the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands, but also to enhanced dissolution at the interphase boundaries.

Role of grain boundaries in the high-temperature performance of a highly stabilized beta titanium alloy II: Creep behavior

Two different microstructures, with straight and bending grain boundaries, were obtained after two typical heat treatments: (1) annealing treatment (AnT) at 600°C for 4h followed by air cooling and (2) solution treatment (S-AT) at 850°C for 1h, followed by water quenching, an aging treatment at 550°C for 6h and air cooling. Creep performance was tested for samples of the alloys with different grain boundary morphologies, and the results show that the grain boundary morphology plays an important role in creep performance. The alloy with a bending grain boundary microstructure after AnT showed better creep resistance at lower temperature levels, while the alloy with a straight grain boundary microstructure after S-AT showed better creep resistance at higher temperature levels. These observations mainly result from the changes in creep cavity behavior, internal stresses and precipitation strength for curved and straight grain boundary microstructures with increasing creep temperature.

The process and mechanisms of metamorphism and deformation of hornblende—An example from the Hengshan Mountains, Shanxi Province

Hornblende’s plastic and superplastic deformation mechanisms were studied on the basis of some new information obtained from amphibolite samples in the Hengshan Mountains (Hengshan, Shanxi, China). For this purpose, the samples were measured and analyzed by optical microscope (OM), electron probe microanalysis (EPMA), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) respectively. Because localized strong strain is more than 1000%, it is presumed that structural superplastic deformation was developed during the ductile shearing process. It is calculated that deformation occurred at 650–679°C and 0.770–0.914 GPa, based on the study of plagioclase-hornblende geothermometry and geobarometry. TEM images show that new hornblende grains developed few dislocations and hornblende crystals exhibit straight grain boundaries. EBSD of strongly deformed hornblendes reveals a fabric dominated by a {100} 〈001〉 lattice preferred orientation (LPO), indicative of new grain recrystallization along the lineation direction (X axis). Thus superplastic deformation of hornblendes from the Hengshan Mountains was developed by mainly solid-state diffusive mass transfer and grain boundary sliding.

Role of grain boundaries in the high temperature performance of a highly stabilized beta titanium alloy I: Thermal stability

Two different microstructures with straight and bending grain boundaries were obtained after two typical heat treatments (1) annealing treatment (AnT) at 600°C for 4h followed by air cooling and (2) solution treatment at 850°C for 1h, water quenching followed by aging treatment at 550°C for 6h, air cooling (S-AT). The mechanical properties under room-temperature and thermal exposure conditions were tested for the alloy with different grain boundary morphologies. The results show that the grain boundaries morphology plays important role in thermal stability. The alloy with bending grain boundaries microstructure after AnT shows higher thermal plasticity in comparison to the ones with straight grain boundaries microstructure after S-AT, which is mainly resulting from the relatively small quantity and intermittent precipitations along bending grain boundaries in comparison to straight grain boundaries. The sequent microstructure evolution experiment for solution treatment at different temperatures shows the critical transformation temperature from bending grain boundaries to straight boundaries is in temperature range from 800°C to 820°C.

Multiple growth mechanisms of jadeite in Cuban metabasite

Samples of rocks reported in the literature to be jadeite jade from the subduction-zone complex of the Escambray Massif in central Cuba have been studied by optical and transmission electron microscopy, electron microprobe and hot-cathode cathodoluminescence (CL) microscopy. Although these rocks are indeed rich in jadeite, the bulk rock composition generally conforms to MORB, with Na2O enriched by . 3 wt% and CaO depleted by .2 wt%. Al2O3 contents are unchanged. These changes are attributed to early pre-subduction spilitization of the ocean-floor protolith. Relics of magmatic augite preserving an ophitic texture are common. Disequilibrium textures are the rule. Extensively recrystallized rocks show fine, felty intergrowths of predominantly Al-rich glaucophane and jadeite, the latter with rims and patches of omphacite. TEM observations indicate extensive replacement of pyroxene by amphibole. Glaucophane developed rims of magnesiokatophorite and edenite. Chlorite and epidote are also present. Late development of actinolite, chlorite, epidote and albite is observed. Quartz is present. Less recrystallized samples with numerous large (.1.5 mm) grains of augite show several types of sodic and sodic-calcic clinopyroxene development: (1) Topotactic replacement of magmatic pyroxene by jadeite and omphacite along a broad front encroaching upon the augite grain from the rock matrix. Jadeite dominates where presumably plagioclase was formerly present. Omphacite dominates where augite is internally replaced along cleavage and fractures. Late chlorite, taramite and ferropargasite replace these pseudomorphs. (2) Former plagioclase laths of the ophitic fabric are replaced by jadeite together with lesser omphacite in epitactic relationship with the enclosing augite. Former plagioclase-augite grain boundaries remain preserved. Late pumpellyite is associated with the omphacite. (3) Jadeite þ omphacite þ pumpellyite þ chlorite with irregular grain boundaries dominate in the rock matrix between the augite relics, with idiomorphic crystals of epidote scattered throughout and in chlorite–epidote clusters. Pumpellyite is interpreted to be a late retrograde product. Quartz is present. (4) Jadeite þ omphacite þ chlorite assemblages, in which monomineralic sheaf-like jadeite aggregates are common, fill very thin (500–1500 mm) fractures criss-crossing the sample, including ophitic augite remnants. Cathodoluminescence microscopy shows that jadeite in the veins is distinctly different from CL in the other types of jadeite, showing features like oscillatory growth zoning indicative of crystallization from a fluid. Generally omphacite develops irregularly along jadeite rims, but recrystallization may lead to pairs with straight grain boundaries suggestive of phase equilibration. Comparison with published solvus relationships suggests temperatures of 425–500 C. This unusual occurrence of different types of jadeite in a metabasic rock suggests two contrasting sources. The first – in the rock matrix, as topotactic alteration of igneous pyroxene and as plagioclase replacement epitactically growing on augite – can be explained as due to local domain equilibration in a rapidly subducted ‘‘spilitized’’ gabbroic rock. The second, in very thin fracture fillings, conforms to an origin as a crystallization product from a pervasive fluid. Conceivably, ‘‘pooling’’ of the fluids flowing through the fractures in larger cavities could lead to larger masses of jadeitite. These have not yet been conclusively documented in the Escambray Massif.

Indented rings (hazel growth) of Norway spruce reduce anisotropy of mechanical properties

This study presents three-point bending test results of Norway spruce clear wood specimens loaded on the radial-longitudinal plane in two different load cases. The tested samples were graded as resonance wood for instrument making and were characterised by narrow annual rings and relatively low density. The modulus of elasticity (MOE) and the corresponding modulus of rupture (MOR) are illustrated separately for the samples with straight grain and the group showing the specific growth pattern of indented rings (‘hazel growth’). With the longitudinal wood anatomical direction parallel to span width, the fibre deviation caused by the indents reduces MOE and MOR values, whereas a ‘reinforcing’ effect of the indents could be observed for the load case with span width parallel to the radial direction. Both aspects lead to a reduction in anisotropy for hazel-growth Norway spruce (anisotropy MOE: indented rings 11.6, straight grain 14.7, anisotropy MOR: indented rings 6.9, straight grain 8.9), which partly explains the exceptional position of this growth pattern for the construction of high-class musical instruments with outstanding mechanical and acoustical performance.

Investigation of grain boundaries in BaSi2 epitaxial films on Si(1 1 1) substrates using transmission electron microscopy and electron-beam-induced current technique

a-Axis-oriented undoped n-BaSi2 epitaxial films were grown on Si(111) substrates by molecular beam epitaxy, and the crystalline quality and grain boundaries were investigated by means of reflection high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy (TEM). The grain size of the BaSi2 films was estimated to be approximately 0.1–0.3μm, and straight grain boundaries (GBs) were observed in the plan-view TEM images. Dark-field TEM images under a two-beam diffraction condition showed that these GBs consist mostly of BaSi2 {011} planes. The diffusion length of minority carriers in n-BaSi2 was found to be approximately 10μm by an electron-beam-induced current technique.

ANATOMIA DA MADEIRA DE Toona ciliata CARACTERÍSTICAS DAS FIBRAS PARA PRODUÇÃO DE PAPEL

A madeira de cedro-australiano, plantado no Brasil visando à produção de serrados, foi caracterizada anatomicamente, avaliando-se os elementos constituintes no sentido medula–casca, além das relações entre as dimensões das fibras, índices indicadores da qualidade da polpa celulósica a ser produzida. Foram utilizados discos retirados do diâmetro à altura do peito (DAP) de cinco árvores com 18 anos de idade. A madeira apresenta cerne e alburno distintos, anéis de crescimento distintos a olho nu, textura fina a média, com brilho e grã regular. O diâmetro tangencial dos poros e a largura dos raios aumentaram no sentido medula casca, ao contrário do número de poros por milímetro quadrado e a quantidade de raios por milímetro, que diminuíram. Os valores médios do comprimento dos elementos de vaso, das fibras, a largura da fibra e o diâmetro do lume aumentaram no sentido medula–casca, sendo que a espessura da parede teve uma pequena variação, mas não significativa estatisticamente. Os valores obtidos para os índices indicativos da qualidade da polpa celulósica não sugerem a madeira de Toona ciliata M. Roem para a produção de papel do tipo embalagem.Palavras chave: Cedro-australiano; características anatômicas; polpa celulósica. AbstractToona ciliate wood anatomy and fiber characteristics for paper production. The Australian cedar wood, planted in Brazil to aim at saw wood production, was anatomically characterized, evaluating the constituent elements variation from pith to bark, as well the relationship between fiber dimensions, index indicating the quality of cellulose produced. Were obtained discs at breast high (DBH) of five trees with 18 years old. The wood has heartwood and sapwood distinct, growth rings distinct without lens, fine to medium texture, with brightness and straight grain. Tangential diameter of vessel lumina and ray width increased from pith to bark; unlike vessels per square millimeter and rays per millimeter that decreased. The mean values of vessel element length, fiber length, fiber width, lumen diameter increased from pith to bark, and fiber wall thickness had a little variation, but not statistically significant. The obtained values for quality pulp indicative index don’t suggest Toona ciliata M. Roem wood for package paper production. Keywords: Australian cedar; anatomic characteristic; cellulose pulp.

Anatomical Properties of Shorea mujongensis P.S. Ashton, a Critically Endangered Species of Dipterocarps from Kalimantan

Wood anatomy of Shorea mujongensis P.S. Ashton was investigated in order to ensure this species belongs to yellow meranti group. Such study is very important since this species is already listed in the red list of IUCN and classified as critically endangered species. The microscopic slides were prepared according to the Johansen's method, while the anatomical features observed according to the IAWA List. The results show that S. mujongensis wood exhibit brown heartwood, light brown sapwood, rough texture, straight grain sometimes interlocked and somewhat rough. The main microscopic characters are growth rings indistinct; vessel diffuse, mostly solitary, rounded to oval; simple perforation plate and alternate intervessel pits; parenchyma scanty paratracheal to thin vasicentric; axial intercellular canals in long tangential line, radial intercellular canal and vasicentric tracheids present; rays uniseriate and multiseriate, prismatic crystal in procumbent cells; fiber length 1,294 µm, diameter 26 µm and wall thickness 4µm. Macroscopic and microscopic observation of S. mujongensis wood confirms the species belongs to yellow meranti group. The assesment on fiber dimensions and derived values of the wood fibers classified the wood into class quality II. It indicates that this species is moderately favorable as raw material for pulp and paper manufacture.

Complex fringes around magnetite porphyroclasts: Growth and deformation history

Deformed strain fringes in iron formation rocks show complex quartz fiber patterns that grew alongside magnetite porphyroclasts embedded in a matrix of quartz and iron oxides during coaxial to non-coaxial deformation. These rocks have been deformed by a combination of processes involving microfracturing, pressure solution and dislocation glide at temperatures of approximately 300 °C. Detailed microstructural observation and crystallographic analysis show that quartz fiber growth is not controlled by the crystal faces. Quartz c-axis orientations of fibrous quartz indicate that these grains initially grew with their c-axes parallel to the fiber length. Late solid-state deformation of crystallized fibers comprised dislocation glide along basal plane and subsequent recrystallization. This produces aggregates of recrystallized grains consisting of equant to elongated quartz grains with straight and orthogonal grain boundaries.

Electron microscopy characterization of the weld line zones of an inertia friction welded superalloy

Microstructure in the weld line (WL) zone produced by inertial friction welding has been studied for a series of RR1000–RR1000 welds using transmission electron and scanning electron microscopy. A fully recrystallised fine grain structure was found to form throughout the WL zone, characterized by straight and smooth grain boundaries, high energy status and modified grain boundary chemistry due to very fast cooling after welding. Very fine γ′ particles with unimodal size distribution were reprecipitated in the WL zone. The fine γ′ particles are spherical in shape, high in number density and characterized by an imbalanced chemistry, containing less Al, Ti, Ni, Ta γ′-forming elements but more Cr, Co, Mo γ-forming elements than all types of parent γ′ particles. As a result, a hard and strong WL zone was produced by the inertial friction welding.

The Microstructure Evolution during Annealing in Large Strain Deformed Fe-32%Ni Alloy

The microstructure evolution during annealing in large strain deformed Fe-32%Ni alloy was investigated by transmission electron microscope (TEM). Firstly, the Fe-32%Ni alloy was subjected to multi-axial forging at temperature of 773K and a strain rate of 10-3 s-1 to cumulative strain of 9.0, and then the large strain deformed specimens were annealed at temperature of 973K with different time. The results show that the grains of Fe-32%Ni alloy were obviously refined by severe plastic deformation, and the ultra-fine grains with mean size of about 300nm were obtained when the cumulative strain amounted to 9.0. The large strain deformed microstructure evolves homogeneously and gradually to equiaxed structures with straight and smooth grain boundaries when annealed at temperature of 973K, and there is no observable nucleation stage found during annealing. The annealing process involves two sequential processes i.e. recovery followed by normal grain growth, and the microstructure evolution mechanism is considered as continuous recrystallization.