Alloy Grain Research Articles

Characterization of Anomalous Grains in FeCo Magnetic Alloys.

Heat-treated FeCo-based magnetic alloys were characterized using a suite of electron microscopy techniques to gain insight into their structural properties. Electron channeling contrast imaging (ECCI) in the scanning electron microscope (SEM) found unique grains towards the outer edge of a FeCo sample with nonuniform background contrast. High-magnification ECCI imaging of these nonuniform grains revealed a weblike network of defects that were not observed in standard uniform background contrast grains. High-resolution electron backscattered diffraction (HR-EBSD) confirmed these defect structures to be dislocation networks and additionally found subgrain boundaries within the nonuniform contrast grains. The defect content within these grains suggests that they are unrecrystallized grains, and ECCI can be used as a rapid method to quantify unrecrystallized grains. To demonstrate the insight that can be garnered via ECCI on these unique grains, the sample was imaged before and after micro indentation. This experiment showed that slip bands propagate throughout the material until interacting with the dislocation networks, suggesting that these specific defects provide a barrier to plastic deformation. Taken together, these results show how ECCI can be used to better understand failure mechanisms in alloys and provides further evidence that dislocation networks play a critical role in the brittle failure of FeCo alloys.

Die-cast multicomponent near-eutectic and hypoeutectic Al–Si–Ni–Fe–Mn alloys: Microstructures and mechanical properties

Microstructures and mechanical properties of near-eutectic and hypoeutectic Al–Si–Ni–Fe–Mn alloys were studied under as-cast condition. Near-eutectic 78Al15Si4Ni2.5Fe0.5Mn (wt.%) and hypoeutectic 83.6Al11Si3Ni1.9Fe0.5Mn (wt.%) alloys were processed by high pressure die casting. These two alloys have the same ternary eutectic mixture of α-Al, Si and Al6(FeNiMn) with a melting point of 567 °C. Primary intermetallic phases of block-like Al17(FeNiMn)4Si and plate-like Al4(FeNiMn)Si2 in these two alloys were observed. Precipitates of Si and Ni-contained phase coexist in primary α-Al grains of hypoeutectic alloy. The lattice parameters of Al6(FeNiMn), Al17(FeNiMn)4Si, and Al4(FeNiMn)Si2 phases are slightly changed compared with orthorhombic Al3Ni, orthorhombic Al5.6Fe2 and tetragonal FeAl3Si2 phases from the database, respectively. These two alloys have excellent mechanical properties at high temperatures. The ultimate tensile strength (UTS) at 300 °C ranges from 135 ± 6 MPa (hypoeutectic alloy) to 157 ± 6 MPa (near-eutectic alloy), which is superior to commercial A383 alloy and some other heat-resistant alloys. Therefore, these near-eutectic and hypoeutectic Al–Si–Ni–Fe–Mn alloys have huge potential for industrial application at elevated temperatures.

Effect of Solution Treatment Time on Microstructure Evolution and Properties of Mg-3Y-4Nd-2Al Alloy

In order to explore the microstructure evolution of an Mg-RE alloy refined by Al during solution treatment, an Mg-3Y-4Nd-2Al alloy was treated at 545 °C for different time periods. Phase evolution of the alloy was investigated. After solution treatment, the Mg-RE eutectic phase in the Mg-3Y-4Nd-2Al alloy dissolves, the granular Al2RE phase does not change, the acicular Al11RE3 phase breaks into the short rod-like Al2RE phase, and the lamellar Al2RE phase precipitates in the grains. With the extension of solution time, the precipitated phase of the lamellar Al2RE increased at first and then decreased, and its orientation relationship with the matrix is <112>Al2RE//<21¯1¯0>Mg and {111}Al2RE//{0002}Mg. The undissolved granular Al2RE phase can improve the thermal stability of the alloy grain by pinning the grain boundary, and the grain size did not change after solution treatment. Solution treatment significantly improved the plasticity of the alloy. After 48 h of solution treatment, the elongation increased to 17.5% from 8.5% in the as-cast state.

Mechanical properties and strengthening mechanism of the nano-sized m-ZrO2 ceramic particle reinforced NbMoTaW refractory high-entropy alloy

Body-centered cubic (BCC) refractory high-entropy alloys (RHEAs) possess the typical advantage of high strength. However, RHEAs often exhibit apparent brittleness at room temperature, which remains great challenges for their engineering applications. In this study, NbMoTaW with 0 and 1 wt% m-ZrO2 nano-sized ceramic particles RHEAs have been successfully fabricated by ball milling and spark plasma sintering (SPS). The effect of m-ZrO2 ceramic particles on the microstructures and mechanical properties of NbMoTaW RHEAs was investigated. Results show that the matrix of alloys was characterized by single-phase BCC. Microstructural analysis shows that the grains in the single-phase BCC alloy have been refined by 39.0% through introduction of oxide nano-ceramic particles. The m-ZrO2 ceramic addition could effectively enhance both the hardness and strength of the NbMoTaW RHEA, accompanied by a slight decrease in plasticity. Compared with the vacuum arc-melting (VAM) method, the sintered NbMoTaW-1ZrO2 RHEA shows a significant increase in comprehensive compressive properties, especially the ductility. The yield strength, ultimate compressive strength and fracture strain of sintered NbMoTaW-1ZrO2 RHEA are 1775 MPa, 2118 MPa and 12.4%, respectively, surprisingly increased by 67.8%, 74.9% and 376.9%. A quantitative yield strength model correlating m-ZrO2 ceramic particle size, volume fraction and NbMoTaW-1ZrO2 grain size was built. The addition of fine m-ZrO2 ceramic resulted in Orowan strengthening and fine grains, thereby increasing yield strength of alloy. The superior combination of mechanical properties of the NbMoTaW-1ZrO2 RHEA could promote it to be a promising alloy in engineering applications.

W–Ta–Nb solid solutions prepared by vacuum arc melting: Microstructure evolution and mechanical properties

In order to further improve the strength and toughness of W–Ta solid solutions and ensure the high specific gravity characteristics of the material, the effect of low content of Nb on the microstructure evolution and performance of W–30Ta solid solutions was performed. The results showed that the phase composition of W–Ta–Nb alloys was single phase W–Ta–Nb solid solutions. The compressive strength of W–Ta–Nb solid solutions increased first and then decreased upon increasing the Nb content. The amount of solid solution of Nb in W–Ta–Nb solid solutions increased and the interplanar spacing increased upon increasing the Nb content. Nb has the effect of refining the grain of tungsten alloy. The grain size gradually decreased upon increasing the Nb content. The addition of a small amount of Nb can effectively improve the strength and toughness of W–Ta–Nb solid solutions. The addition of a small amount of Nb reduced the energy required for the initiation of ductile dislocations in W–Ta–Nb solid solutions.

Improved microstructure, mechanical properties and electrical conductivity of the Cu–Ni–Sn–Ti–Cr alloy due to Ce micro-addition

In this paper, new Cu–Ni–Sn–Ti–Cr and Cu–Ni–Sn–Ti–Cr–Ce alloys were prepared and studied. After solution treatment and 60% cold rolling, the Cu–Ni–Sn–Ti–Cr alloy with micro-hardness of 221 HV, a tensile strength of 598.1 MPa, elongation of 9.9% and electrical conductivity of 45.2% IACS was obtained after aging at 500 °C for 60 min. Furthermore, the new Cu–Ni–Sn–Ti–Cr–Ce alloy was also obtained using aging at 500 °C for 60 min, having 245 HV micro-hardness, 645.2 MPa tensile strength, elongation 9.4% and 44.6% IACS conductivity. The comprehensive properties of high strength, hardness, and electrical conductivity of the alloy are attributed to the composite effects of solution, deformation, grain boundary, and precipitation strengthening. Among them, precipitation strengthening plays a major role, and the precipitates are mainly nanometer CuNi2Ti, Cr, and NiTi phases. The interface between the nanometer CuNi2Ti phase and Cu matrix is a coherent, which can improve the strength of the alloy without sacrificing plasticity. In addition, texture type transformation and texture strength change of the alloy are closely related to the increase of micro-hardness. The micro-addition of trace rare earth Ce can refine the alloy grains, and promote dynamic recrystallization to produce finer grains, significantly improving the alloy’s micro-hardness.

Crack-free and high-strength AA2024 alloy obtained by additive manufacturing with controlled columnar-equiaxed-transition

Laser powder bed fusion (LPBF) of high-strength Al alloys is challenging due to the formation of both hot and cold cracks. In the present work, highly dense and crack-free AA2024 samples could be additively manufactured via inoculation treatment of Zr-based metallic glass (MG) powders. The columnar grains in the LPBF-fabricated AA2024 alloy were significantly refined and almost completely transformed to the equiaxed grains with a bimodal grain size distribution consisting of ultrafine grains with a size smaller than 1 μm and relatively coarser grains. The grain refinement can be associated with the formation of Al3Zr particles, serving as the heterogeneous nucleation sites for the α-Al matrix. Complete routes for columnar-equiaxed-transition (CET) have been revealed by tailoring the concentration of nucleation particles and solidification conditions. CET occurs both at the melt pool boundary due to the sufficiently high concentration of Al3Zr particles and at the topmost of the melt pool due to the heterogeneous nucleation driven by constitutional undercooling. Between these two regions, columnar grains epitaxially grow with orientations determined by the thermal gradient. The as-built Zr-based MG inoculated AA2024 specimens are robust in healing hot cracks due to a more tortuous propagation path of cracks for equiaxed grains. The as-fabricated AA2024/5% MG specimens exhibit a high ultimate tensile strength of 531 MPa due to crack elimination and grain refinement, surpassing most of the reported values for the LPBF-fabricated AA2024 alloy inoculated with other inoculated powders. The present work could provide a novel inoculation agent to fabricate high-strength Al alloys and the CET can be used to precisely control the grain morphology.

Effect of Isothermal Treatment on Microstructure and Performance of Micro/Nanostructure 2205 Duplex Stainless Steel Prepared by Aluminothermic Reaction

Using an appropriate heat treatment process to control the precipitation of the σ phase, while regulating the alloy structure and grain size, is one of the effective ways to obtain good comprehensive properties of duplex stainless steel (DSS). Herein, the feasibility and preparation parameters of micro/nanostructure 2205 DSS prepared by the aluminothermic reaction are explored. The microstructure and properties are further controlled by isothermal treatment at 1000 °C for a different time. The results show that the volume percentages of α and γ phases in the 1.0 h isothermal treated steel obtain a maximum value and a minimum value, corresponding to 60% and 36%, respectively. The evolution of microstructure with isothermal treatment also affects the content and morphology of the precipitated σ phase, which changes from continuous precipitation to granular precipitation at the α/γ phase boundaries, and its volume percentage obtains a minimum value when the isothermal treatment is performed for 1.0 h. With the increase of heat treatment time, the average grain size and volume fraction of nanocrystals increase gradually. Corresponding to the changes in microstructure and grain size, the maximum values of Vickers hardness and tensile properties of the alloy are obtained after 1.0 h isothermal treatment.

Grain and twin boundaries dependent mechanical behavior of FeCoCrNiCu high-entropy alloy

The nanoindentation response of FeCoCrNiCu high-entropy alloy is revealed through molecular dynamics simulation. The influences of various grain sizes and twin lamellae thicknesses on the mechanical characteristics and plastic deformation are carefully studied. The result indicates that the indentation force and substrate hardness increase as the grain size increases from 55.16 Å to 114.46 Å and the twin layer thickness rises from 6.74 Å to 30.04 Å, which reveals the inverse Hall-Petch effect in the relationship between the material strength versus grain size and the twin lamellae thickness. The shear strain and residual stress are converged around the indentation regions and then enhanced deep into the substrate along the grain boundary and twin boundary. The presence of grain boundaries significantly affects the movement of the atoms, resulting in an asymmetric dispersion of the atomic displacement vector during the deformation. Moreover, the surface morphology reveals that the pile-up height generally increases with increasing the grain size and decreasing the twin lamellae spacing. The microstructural evolution suggests that the rotation of the grain related to grain boundary sliding and the migration of the grain boundary are the dominant mechanism in the deformation with grain size reduction. Moreover, the dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic, which significantly affects the plastic deformation of the material. The result also shows that the dislocation density tends to increase with increasing grain size and twin lamellae thickness. The figure shows the simulation model of FeCoCrNiCu high-entropy alloy (a), atomic displacement vectors of samples with different grain sizes (b), atomic configuration of the specimens with various twin lamellar thicknesses (c). ● The inverse Hall-Petch effect is recognized with the variation of grain size and twin lamellae thickness. ● The deformation mechanism of FeCoCrNiCu HEA is significantly dependent on the grain/twin boundaries. ● The rotation of grain related to grain boundary sliding and the grain boundary migration are dominant mechanism. ● The dislocation nucleation at the intersections of the twin and grain boundaries is also a remarkable characteristic.

Growth kinetics of the sputtered Ni-30Cr-5Al alloy nanograins: Effects of Y addition and oxidation

The grain growth of the sputtered Ni-30Cr-5Al and Ni-30Cr-5Al-1Y (wt%) nanocrystalline alloys during temperature rising to and isothermally holding at 1000 °C in Ar has been investigated by transmission electron microscope (TEM). The thermal-induced grain growth of the two alloys at 1000 °C follows a law of dn − d0n = kt but the Y presence results in an increase of n from 3.46 to 3.95. This is attributed to Y segregation at the alloy grain boundaries (GBs) and Y-rich phase precipitation, which retards the GB curvature-driven grain growth thermodynamically by a solute drag effect and kinetically by second-phase pinning effect, respectively. Moreover, the Y-free alloy grains grow much faster in air than in Ar, because the surface oxidation in air generates additional tensile stresses in the alloy which facilitate the GB mobility. The Y-containing alloy nanograins remain similar in size during heating in air and in Ar, plausibly due to the formation of more Y-rich oxide particles which hinder the GB mobility through the Zener pinning effect.

Effect of Sr microalloying on microstructure and corrosion resistance of Al–4.6Mg alloy

AbstractThe effect of Sr on the microstructure of Al–4.6Mg alloy is studied via optical microscopy, scanning electron microscopy‐energy‐dispersive analysis, electron backscattered diffraction, transmission electron microscopy, and the intergranular corrosion susceptibility is evaluated according to ASTM G67. The main precipitation phases of Al–4.6Mg alloy are β‐Al3Mg2 and Al6Mn, with a new square phase of Al3Mn also being formed. After the addition of Sr, the β‐Al3Mg2 phase of the alloy decreases, the morphology changes, and Al4Sr and τ‐Al38Mg58Sr4 phases are formed. Accordingly, the nanoscale precipitation phase τ‐Al38Mg58Sr4 is uniformly distributed inside the grains. Microstructure analysis shows that Sr promotes the dynamic recovery and dynamic recrystallization of the alloy, inhibits the growth of recrystallized grains, and reduces the alloy grain size from 14.294 to 10.273 μm. Additionally, the low‐angle grain boundaries increase, the high‐angle grain boundaries decrease, the texture of the rotated cube r‐{001}&lt;110&gt; improves, and the deformation texture decreases. The IGC susceptibility of the alloy is reduced with the addition of Sr, and the corrosion resistance in an electrolyte containing NaCl is enhanced.

The autocatalytic evolution of martensitic microstructure

This work considers the autocatalytic spread of the martensitic transformation throughout the austenite grains in FeNiC and FeNiMn alloys. Formal descriptions were reviewed. The microstructural path concept was re-introduced. The model’s analysis of scale factors reiterated the importance of the martensite–austenite interaction. We compared the initial heterogeneous martensite nucleation in a polycrystalline FeNiC with the nucleation in a Fe30Ni particulate alloy. A cluster of tetrakaidecahedral austenite grains might be a reasonable choice to describe the martensite spread in the FeNiC.

Microstructure evolution, tensile properties and internal structure transformation study on Ti6Al4V–5Cu anti-bacterial alloy during uniaxial tensile straining at low forming temperature

The tensile behavior of Ti6Al4V–5Cu alloy studied in the temperature range of 450–550 °C at the strain rate interval of 10−3-10−1 s−1. Alloy deformation behavior, microstructure evolution, and its relative internal structure changes due to deformation were investigated. The tensile behavior of the alloy had elucidated in terms of change in true stress-strain relationship. At 450 °C, the alloy showed the decrease of peak stress at the strain rate interval of 10−3-10−2 s−1, which suggests Ti6Al4V–5Cu plastic flow behavior, was quite different. The alloy exhibited an elongation (El) of 28% at 450 °C in a strain rate of 10−3 s−1. At 10−3 s−1, the El of the alloy decreased with the increase of forming temperature. However, at 10−2 s−1 and 10−1 s−1, the El increased with the rising of temperature. Scanning electron microscope (SEM) was used to investigate the microstructure of Ti6Al4V–5Cu alloy. The alloy grain boundaries (GBs) were found as continuous and discontinuous structural features, especially the high angle grain boundaries (HAGBs). The GB feature has contributed to the initiation of micro-cracks because of the creation of weakest regions in the material during deformation. The nature of GB geometry has also a tremendous contribution to the workability of Ti6Al4V–5Cu alloy at low forming temperatures. The presence of a high amount (%) LAGBs and the serrated nature of HAGBs structures determined the pull to grain pieces. The deformation mechanism of the Ti6Al4V–5Cu alloy at low forming temperatures was accomplished initially by boundary serration phenomenon due to strain-induced GB migration related to heat treatment at low strain condition, then by initiation of recrystallized grains-triggered by the plasticity behavior of strain induced sub-boundaries, and finally by the development of deformed grains generated by the reduction in corrugation propensity of GBs due to high strain condition.

Effect of Cu Content on the Alloy Tensile Properties of Al-Cu Based Alloys Tested at 25 °C and 250 °C: Application of the Concept of Quality Index

The present work was performed on three versions of a newly developed alloy coded T200 containing 6.5% Cu, 0.1% Fe, 0.45% Mg, and 0.18% Zr in addition to A319 and A356 alloys (grain refined and Sr-modified). Tensile bars were subjected to 13 different heat treatments prior to testing at either 25 °C or 250 °C. The tensile data were analyzed using the quality index method. The results obtained showed that, due to the high copper content in the T200 alloy coupled with proper grain refining, the alloy possesses the highest quality as well as improved resistance to softening when tested at 250 °C among the five alloys. The results also demonstrate the best heat treatment condition to maximize the use of the T200 alloy for automotive applications. Grain-refined alloy B, treated in the T6 temper and tested at 250 °C, exhibited the best combination of the four tensile parameters, i.e., UTS, YS, %El, and Q-values: 308 MPa, 304 MPa, 2.3%, and 352 MPa, respectively, which are comparable with those obtained from the 356 alloy: 309 MPa, 305 MPa, 2.8%, and 375 MPa in the same order.

Enhancing mechanical properties and degradation performance of Mg−0.8wt.%Ca alloy by directional solidification

The mechanical properties and degradation performance in Hank's solution of the directionally solidified (DSed) Mg−0.8Ca (wt.%) alloys were investigated and compared with those of the as-cast alloy. The microstructure was studied by OM, SEM, TEM, and EBSD. It is found that the columnar grains in DSed alloys have a consistent growth orientation of <1120>. The mechanical strength and ductility of Mg−0.8Ca alloy are significantly improved after directional solidification. Besides, the corrosion resistance of Mg−0.8Ca alloy in Hank's solution is dramatically enhanced after directional solidification. The Mg−0.8Ca alloys in Hank's solution mainly undergo microgalvanic corrosion, and the corrosion products consist of Mg(OH)2, (Ca, Mg)3(PO4)2, and hydroxyapatite (HA). The superior properties of the DSed Mg−Ca alloy are beneficial from the columnar microstructure and the redistribution of the eutectics. The DSed Mg−0.8Ca alloy with columnar dendritic structure is expected to be used as biodegradable materials.

Investigation of Microstructures and Mechanical Properties of Ultra-High Strength Al-Zn-Mg-Cu Alloy Prepared by Rapid Solidification and Hot Extrusion

Al-Zn-Mg-Cu aluminum alloys have the advantages of high specific strength, easy processing, and high toughness, showing great potential application in the aerospace field. However, ultra-high strength aluminum alloys usually contain coarse microstructures, micro-segregation, and casting defects that seriously deteriorate mechanical properties. Here, we report a high-strength aluminum alloy (Al-10.5Zn-2.0Mg-1.2Cu-0.12Zr-0.1Er) prepared by rapid solidification and hot extrusion to explore the microstructure modification of the alloy based on this strategy. The results show that: rapid-solidification technology can significantly refine alloy grains, alloy ribbons were composed of α (Al) equiaxed fine grains, and the average grain size was less than 6 μm. After extrusion, the alloy had partially recrystallized, existing coarse second-phase (T-phase) and needle-shaped precipitates were MgZn2 (η-phase), and the tensile strength and elongation of the extruded bar were 466.4 MPa and 12.9%, respectively. After T6 heat treatment, the tensile strength of the alloy reached 635.8 MPa, while elongation decreased to 10.5%. According to microstructure analysis and considering the contributions of grain boundary, dislocation, and precipitation-strengthening to the improvement of the mechanical properties, it was found that precipitation-strengthening is the main strengthening mechanism. Our research shows that rapid-solidification and hot-extrusion technology have great potential for improving the microstructures and mechanical properties of aluminum alloys.

The Effect of Rhenium Content on Microstructural Changes and Irradiated Hardening in W-Re Alloy under High-Dose Ion Irradiation.

An amount of 100 dpa Si2+ irradiation was used to study the effect of transmutation rhenium content on irradiated microscopic defects and hardening in W-xRe (x = 0, 1, 3, 5 and 10 wt.%) alloys at 550 °C. The increase in Re content could significantly refine the grain in the W-xRe alloys, and no obvious surface topography change could be found after high-dose irradiation via the scanning electron microscope (SEM). The micro defects induced by high-dose irradiation in W and W-3Re alloys were observed using a transmission electron microscope (TEM). Dislocation loops with a size larger than 10 nm could be found in both W and W-3Re alloy, but the distribution of them was different. The distribution of the dislocation loops was more uniform in pure W, while they seemed to be clustered around some locations in W-3Re alloy. Voids (~2.4 nm) were observed in W-3Re alloy, while no void was investigated in W. High-dose irradiation induced obvious hardening with the hardening rate between 75% and 155% in all W-xRe alloys, but W-3Re alloy had the lowest hardening rate (75%). The main reasons might be related to the smallest grain size in W-3Re alloy, which suppressed the formation of defect clusters and induced smaller hardening than that in other samples.

Effect of rare earth Ce addition on microstructure and mechanical properties of titanium alloy Ti-6Al-4V

• Effect of Ce on microstructure and mechanical properties of Ti6Al4V are studied. • Ti6Al4V alloy grains are significantly refined after adding Ce. • Ce combines with O in matrix to form CeO 2 , which is coherent with the alloy matrix. • The strength and ductility of Ti6Al4V containing 0.5wt.% Ce are obviously improved. • The frictional performance of Ti6Al4V containing 0.3wt.% Ce is obviously optimized. These Ti6Al4V-xCe (x=0, 0.1, 0.3, 0.5, 0.7 wt.%) were prepared by vacuum non consumable arc furnace. And the microstructure and comprehensive mechanical properties of these Ti6Al4V were systematically explored. The results show that the tensile strength and ductility of Ti6Al4V containing 0.5 wt.% Ce is significantly improved (Ultimate tensile strength: 978.1MPa; Elongation: 7.1%). And the wear resistance of Ti6Al4V containing 0.3 wt.% Ce is significantly optimized (Wear quality: 0.55mg). The excellent balance between strength and plasticity of Ti6Al4V is due to the significant grain refinement and the complete coherent relationship between ceria and Ti6Al4V matrix.

Observation of grain motion during semisolid deformation by using 4D-CT and 3DXRD

A technique combining time-resolved computed tomography (4D-CT) and three-dimensional X-ray diffraction (3DXRD) was employed to investigate semisolid deformation in an Al-10mass%Cu alloy with the equiaxed grain structure. The diffraction patterns and reconstructed 3D images were used to quantitatively analyse the translational and rotational motions of solid grains in the semisolid alloy. Finally, the motion of solid grains and the expansion of porosity during the compression tests are discussed on the basis of in situ observations.

Role of the alloy grain boundaries in the high-temperature oxidation and Cr volatilization of 22 wt% Cr ferritic stainless steel for SOFC applications

The effects of alloy grain size of high-Cr ferritic stainless steel were investigated using a sheet (t = 2 mm) and plate (t = 22 mm) of Cr ferritic stainless steel. The average grain size of each sample was measured as ∼ 25 µm and ∼ 500 µm, respectively. The large-grained plate sample exhibited a 41% oxidation rate reduction, and a slight decrease in Cr volatilization was observed in the small-grained sheet sample. A numerical examination revealed that inhomogeneous GB-related oxidation is a major cause of the different oxidation kinetics affected by metal grain size.