The effect of deformation texture on the thermal stability of UFG HSLA steel

The microstructures of ultrafine grained (UFG) metals processed by severe plastic deformation are far from the thermodynamic equilibrium thus being prone to undergo coarsening processes. Theoretical and experimental investigations revealed that the stability against discontinuous grain growth in UFG metals with high stacking fault energy strongly depends on the fraction of high angle grain boundaries (HAGBs). This means that discontinuous grain growth does not occur if the fraction of HAGBs exceeds a certain level. The present work focuses on the impact of strong deformation textures on the thermal stability of UFG microstructures in a ferritic steel processed by linear flow splitting. It shows that the expected correlation between thermal stability and fraction of HAGBs is valid up to moderate texture intensities, whereas a strong deformation texture promotes discontinuous grain growth in spite of a high fraction of HAGBs. EBSD measurements reveal that this behavior is attributed to a strain-induced grain boundary migration causing a progressive orientation pinning effect with ongoing grain growth. Thereby, a large fraction of HAGBs is transformed into low angle grain boundaries (LAGBs) with low mobility. Consequently, a microstructure with a majority of LAGBs evolves being unstable against discontinuous grain growth.     

Modelling of continuous recrystallization in aluminium alloys

Microstructure and microtexture analyses have been made of three aluminium alloys after annealing alone and after concurrent straining and annealing, and simulative models of microstructure/microtexture evolution processes have been formulated. Both experimental and modelling results are presented as boundary misorientation distributions. For each alloy, the results show that annealing alone does not significantly alter the boundary misorientation distribution, while concurrent straining and annealing (up to a strain of 0.5) decreases the fraction of low-angle boundaries. To understand the mechanisms by which concurrent straining and annealing alter the boundary misorientation distribution, three simulative models of microstructure/microtexture evolution during concurrent straining and annealing have been formulated. Application of the models to experimentally determined initial microstructure/microtexture states shows that the boundary sliding (sub)grain rotation model decreases the fraction of low-angle boundaries, the dislocation glide (sub)grain rotation model increases the fraction of low-angle boundaries, and the (sub)grain neighbour switching model has a modest effect on the boundary misorientation distribution. A combination of the boundary sliding (sub)grain rotation model and the (sub)grain neighbour switching model most closely reproduces the boundary misorientation distributions found experimentally.     

Recrystallization and Grain Growth in Ultrafine‐Grained Materials Produced by High Pressure Torsion

Ultrafine‐grained (UFG) materials processed by severe plastic deformation are known to exhibit good mechanical properties. Much about the annealing behavior of such materials is still unknown, and this work aims to provide a better understanding of the thermal properties of UFG materials. For this purpose a Cu–0.17 wt%Zr alloy was subjected to high pressure torsion (HPT) with a maximal pressure of 4.8 GPa at room temperature. The microstructures of the specimens were characterized using electron back scatter (EBSD) measurements, transmission electron microscopy (TEM), and hardness measurements. During annealing of the samples, dispersoids were formed which improved the thermal stability of the alloy. At higher strain levels the fraction of high angle grain boundaries (HAGBs) increased above 70% of the total grain boundaries.     

Quantitative study of grain refinement in Al–Mg alloy processed by equal channel angular pressing at cryogenic temperature

Strain induced grain refinement of an Al–1 wt.% Mg alloy processed by equal channel angular pressing (ECAP) at cryogenic temperature is investigated quantitatively. The results show that both mean grain and subgrain sizes are reduced gradually with increasing ECAP pass. ECAP at cryogenic temperature increases the rate of grain refinement by promoting the fraction of high angle grain boundaries (HAGBs) and misorientation at each pass. The fraction of HAGBs and the misorientation of Al–1 wt.% Mg alloy during ECAP at cryogenic temperature increase continuously as a function of equivalent strain. Both {110} and {111} twins at ultrafine-grained size are observed firstly in Al–Mg alloy during ECAP. The analysis of grain boundaries and misorientation gradients demonstrates the grain refinement mechanism of continuous dynamic recrystallization.     

Microstructure evolution of CuZr polycrystals processed by high-pressure torsion

The microstructure evolution of extruded Cu–0.18 wt% Zr polycrystals processed by high-pressure torsion (HPT) at room temperature at the pressure of 4 GPa and the different number of the HPT revolutions (i.e. different strain) was investigated using the combination of the electron back-scatter diffraction, microhardness measurements and the X-ray diffraction. A significant transition from the inhomogeneous microstructure after few HPT revolutions into the homogeneous equiaxed microstructure with increasing number of HPT rotations was observed. HPT straining leads to the grain size refinement by a factor more than 100 after the 25 HPT revolutions. Moreover, the EBSD revealed an increase in the fraction of high-angle grain boundaries (HAGBs) with increasing HPT straining reaching the value of 70% after 25 revolutions. Additionally, a slight increase of the twin-related CSL Σ3 grain boundaries occurred during the microstructure refinement. The microhardness measurements confirmed the billet radial inhomogeneity at early stages of the HPT straining, whereas with increasing number of the HPT rotations, causing the specimen fragmentation and homogenization, the microhardness values increased. The average crystallite size and the average dislocation density in individual specimens determined by the XRD diffraction were in the range of approximately 100–200 nm and 2 × 10¹⁵ m⁻², respectively. Moreover, XRD measurements confirmed the absence of residual stresses in all specimens.     

Structure and properties of AA3003 alloy produced by accumulative roll bonding process

The investigation of the microstructure and mechanical properties has been conducted on AA3003 alloy produced by a novel intense plastic straining process named accumulativev roll-bonding (ARB). The results show that ultra-fine grained 3003 alloy having mean grain size of 700–800 nm was successfully produced by the 250°C-ARB. The average grain sizes of 250°C-ARB samples were reduced greatly from about 10.2 m initially to 700–800 nm. After 6 cycles of ARB, the whole volume of the material was filled with ultra-fine grains with high angle boundaries. The tensile strength of the ARB processed 3003 alloy (after 6 cycles) is considerably higher than that of the initial material, and about 1.5 times higher than that of commercially available fully-hardened (H18) 3003 alloy. Strengthening in ARB processed 3003 alloy may be attributed to strain hardening and grain refinement hardening.     

Using Post‐Deformation Annealing to Optimize the Properties of a ZK60 Magnesium Alloy Processed by High‐Pressure Torsion

A ZK60 magnesium alloy with an initial grain size of ≈10 µm is processed by high‐pressure torsion (HPT) through 5 revolutions under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. An average grain size of ≈700 nm is achieved after HPT with a high fraction of high‐angle grain boundaries. Tensile experiments at room temperature show poor ductility. However, a combination of reasonable ductility and good strength is achieved with post‐HPT annealing by subjecting samples to high temperatures in the range of 473–548 K for 10 or 20 min. The grain size and texture changes are also examined by electron back scattered diffraction (EBSD) and the results compared to long‐term annealing for 2500 min at 450 K. The results of this study suggest that a post‐HPT annealing for a short period of time may be effective in achieving a reasonable combination of strength and ductility.

     

Influence of minor Ce additions on the microstructure and mechanical properties of Mg-1.0Sn-0.6Ca alloy

The microstructure and mechanical properties of Mg-Sn-Ca-Ce alloys with different Ce contents(0.0,0.2, 0.5, 1.0 wt%) were studied at room temperature. Ce additions to ternary Mg-Sn-Ca alloy resulted in grain refinement as well as a change in the category of second phase from CaMgSn to(Ca, Ce)Mg Sn and Mg_(12)Ce. The volume fraction of second phase increased with rising Ce content, which aggravated the restriction of DRXed grain growth during the extrusion process and eventually led to texture weakening of as-extruded Mg-Sn-Ca based alloys. In terms of plasticity, owing to vigorously activated basal slip and homogeneous distributed tensile strain in tension, the tensile ductility of as-extruded alloys reached the maximum value of 27.6% after adding 0.2 wt% Ce, which enhanced by about 26% than that of ternary MgSn-Ca alloy. However, further Ce additions(0.5 and 1.0 wt%) would coarsen the second phase particles and then impair ductility. The tension-compression yield asymmetry of as-extruded Mg-Sn-Ca ternary alloy was alleviated greatly via Ce additions, due to the joint effects of grain refinement, increased amount of strip distributed second phase particles and texture weakening.     

Annealing behaviour of a nanostructured Cu–45 at.%Ni alloy

The microstructure and crystallographic texture have been investigated in a Cu–45 at.%Ni alloy after heavy rolling and subsequent annealing at different temperatures. Cold-rolling to a von Mises strain of 5.7 produced a sample with an average boundary spacing along the normal direction of ~70 nm and a large fraction of high-angle boundaries (HABs), ~70 %. Annealing of this sample for 1 h at temperatures ≤450 °C causes structural coarsening, during which the fraction of HABs decreases. Annealing at higher temperatures results in pronounced discontinuous recrystallization accompanied by twinning. Large frequencies of twin boundaries contribute to high HAB fractions measured in the as-recrystallized condition. Cube-oriented grains demonstrate a size advantage compared to grains of other orientations, thus creating a strong cube texture in the recrystallized material. Further annealing of the recrystallized microstructure promotes grain growth, which leads to a significant strengthening of the cube texture and to a dramatic loss of HABs. After 1 h of annealing at 1000 °C the fraction of the cube texture reaches 99 % and the HAB fraction is 12 %.     

Microstructure tailoring of Al0.5CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation (SPD) have high strength but often poor uniform ductility.SPD via high-ratio differential speed rolling (HRDSR) followed by an annealing treatment was applied to Al0.5CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved.The optimized microstructure was composed of an ultrafine-grained FCC matrix (1.7-2 μm) with a high fraction of high-angle grain boundaries (61 ％-66 ％) and ultrafine BCC particles (with a size of 0.6-1 μm and a volume fraction of 8 ％-9.3 ％) distributed uniformly at the grain boundaries of the FCC matrix.In the severely plastically deformed microstructure,the nucleation kinetics of the BCC phase was accelerated.Continuous static recrystallization (CSRX) took place during the annealing process at 1273 K.Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains.The precipitation of the hard and brittle σ phase was,however,suppressed.The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14 ％-20 ％.These tensile properties are comparable to those of transformation-induced plasticity steel.Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified,and each strengthening mechanism contribution was estimated.The calculated results matched well with the experimental results.     

Microstructure and mechanical behavior of friction stir processed ultrafine grained Al-Mg-Sc alloy

Twin-roll cast (TRC) Al-Mg-Sc alloy was friction stir processed (FSP) to obtain ultrafine grained (UFG) microstructure. Average grain size of TRC alloy in as-received (AR) condition was 19.0 ± 27.2 μm. The grain size reduced to 0.73 ± 0.44 μm after FSP. About 80% of the grains were smaller than 1 μm in FSP condition. FSP resulted into 80% of the grain boundaries to have high angle grain boundary (HAGBs) character. Uniaxial tensile testing of UFG alloy showed an increase in yield strength (YS) and ultimate tensile strength (UTS) (by ∼100 MPa each) of the alloy with a very marginal decrease in total and uniform elongation (total - 27% in AR and 24% in UFG and uniform - 19% in AR and 14% in UFG). A theoretical model predicted that the grain refinement cannot take place via discontinuous dynamic recrystallization. Zener pinning model correctly predicted the grain size distribution for UFG alloy. From work hardening behaviors in both the conditions, it was concluded that grain boundary spacing is more important than the character of grain boundaries for influencing extent of uniform deformation of an alloy.     

挤压比对Al-0.68Mg-0.60Si合金组织和性能的影响

采用光学显微镜(OM)、扫描电子显微镜(SEM)、能谱仪(EDS)、电子背散射衍射系统(EBSD)、透射电子显微镜(TEM)、硬度测试、室温拉伸测试等研究挤压比对Al-0.68Mg-0.60Si合金组织与性能的影响。结果表明:随着挤压比的增大,T6态Al-0.68Mg-0.60Si合金型材基体内的强化相弥散质点的尺寸逐渐减小,弥散程度增加,小角度晶界占比呈下降趋势,但再结晶分数有所提高,当挤压比达到39.6以上,合金内部基本为立方织构。此外,在挤压变形过程中,随着挤压比(λ=26.8~55.7)的增大,合金型材的硬度、抗拉强度先上升再下降;当λ=39.6时,合金的抗拉强度达到最大值284.00MPa。     

Microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity

The microstructure and mechanical properties of novel Al-Y-Sc alloys with high thermal stability and electrical conductivity were investigated.Eutectic Al3 Y-phase particles of size 100-200 nm were detected in the as-cast microstructure of the alloys.Al3 Y-phase particles provided a higher hardness to as cast alloys than homogenized alloys in the temperature range of 370-440℃.L12 precipitates of the Al3(ScxYy) phase were nucleated homogenously within the aluminium matrix and heterogeneously on the dislocations during annealing at 400℃.The average size of the L12 precipitates was 11±2 nm after annealing for 1 h,and 25-30 nm after annealing for 5 h,which led to a decrease in the hardness of the Al-0.2 Y-0.2 Sc alloy to15 HV.The recrystallization temperature exceeded 350℃and 450℃for the Al-0.2 Y-0.05 Sc and Al-0.2 Y-0.2 Sc alloys,respectively.The investigated alloys demonstrated good thermal stability of the hardness and tensile properties after annealing the rolled alloys at 200 and 300℃,due to fixing of the dislocations and grain boundaries by L12 precipitates and eutectic Al3 Y-phase particles.The good combination of strength,plasticity,and electrical conductivity of the investigated Al-0.2 Y-0.2 Sc alloys make it a promising candidate for electrical conductors.The alloys exhibited a yield stress of 177-183 MPa,ultimate tensile stress of 199-202 MPa,elongation of 15.2-15.8%,and electrical conductivity of 60.8%-61.5% IACS.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

135°ECAP+旋锻变形工业纯钛的热稳定性研究

对经过135°ECAP+旋锻变形后的工业纯钛100,150,200,250,300,350,400,450℃和500℃下保温1h退火。采用透射电子显微镜(TEM)、扫描电子显微镜(SEM)、拉伸试验机及显微硬度仪等技术研究ECAP+旋锻变形工业纯钛退火后的组织与性能变化。结果表明:在400℃以下退火时,显微组织中位错密度降低,晶界逐渐清晰,变形组织未发生明显变化,材料的抗拉强度和显微硬度略有降低,伸长率增加不明显;在400℃以上退火时,随着退火温度的升高,发生再结晶,晶粒尺寸明显增大,平均晶粒尺寸约为5μm,材料的抗拉强度和显微硬度均出现明显降低,伸长率增加。拉伸断口表明,ECAP+旋锻变形退火后工业纯钛的拉伸断裂均为韧性断裂。随着退火温度的升高,韧窝尺寸变大,韧窝深度变深。     

Annealing behaviour of dilute aluminium alloys following hot deformation

Abstract

The microstructure and texture of three dilute aluminium alloys after hot deformation and annealing was assessed; In particular, the influence of deformation temperature, strain rate, and strain on the annealed texture was examined, as well as the effect of alloy composition. The microstructures of the commercially pure materials studied (Al, Al+1%Mn and Al+1%Mg) varied in the volume fraction of coarse intermetallic particles, the type of dispersoid present, and the level and type of solute in solid solution. Furthermore, the initial stages of recovery and recrystallisation were studied in detail for one of the alloys (commercially pure Al). It was found that the main recrystallisation texture component was the cube and its strength, as well as the recrystallised grain size, depended strongly on the deformation strain. The deformation strain rate and temperature, and the alloy composition also strongly influenced the grain size and cube texture strength. These results are discussed in the context of current theories for cube nucleation within cube bands in the hot deformed microstructure. The present work was carried out as part of a wider research programme, partially supported by the European Union (Brite/Euram funded), to develop micromechanical models to describe the evolution of microstructure and texture during hot deformation and annealing of aluminium alloys.

MST/3376     

High strain rate superplasticity in a nano-structured Al–Mg/SiCP composite severely deformed by equal channel angular extrusion

High strain rate superplastic deformation potential of an Al–4.5%Mg matrix composite reinforced with 10% SiC particles of 3 μm nominal size was investigated. The material was manufactured using powder metallurgical route and mechanical alloying which was then processed by equal channel angular extrusion (ECAE). The composite showed a high resistance to static recrystallization. The manufacturing operations atomized SiC particles to nanoscale particles and the severe plastic deformation process resulted in a dynamically recrystallized microstructure with oxide dispersoids distributed homogeneously throughout the matrix. These particles stabilized the ultra-fine grained microstructure during superplastic (SP) deformation. Testing under optimum conditions at constant strain rates led to tensile elongations >360%, but it could be further increased by control of the strain rate path. Transmission electron microscope (TEM) studies showed that the low angle boundary sub-grain structure obtained on heating to the SP deformation temperature developed on straining into a microstructure containing high angle boundaries capable of sustaining grain boundary sliding.     

Stability against coarsening in ultra-fine grained aluminum alloy AA 3103 sheet fabricated by continuous confined strip shearing

The stability of ultra-fine grained Al–Mn alloy AA 3103 against coarsening at elevated temperature is analyzed. AA 3103 sheets were produced by means of continuous confined strip shearing (CCSS), which represents an adaptation of equal channel angular pressing to impose severe plastic deformation on sheet samples. With increasing number of CCSS passages, finer and more uniform grains with an increasing fraction of high-angle grain boundaries (HAGBs) were observed. In particular, the sample subjected to 12 passages of CCSS displayed fairly uniform equiaxed grains with an average size of 1.6 μm. During subsequent annealing at 300 and 350 °C, these grains were quite stable, exhibiting a rather low growth rate. This stability is explained in terms of Humphreys’ unified theory of the stability of cellular structures, which has shown that under conditions of very large strains when HAGBs prevail intrinsically stable microstructure will be formed.     

冷轧前热处理对6016-T4P铝合金组织和力学性能的影响

通过拉伸试验、电子背散射衍射(EBSD)和透射电子显微镜(TEM)等检测手段,研究了在冷轧前分别进行固溶处理和过时效处理工艺对6016-T4P板材组织和力学性能的影响.结果表明:冷轧前进行固溶处理的6016-T4P板材比冷轧前进行过时效处理的6016-T4P板材塑性、强度和平面各向异性指数更高,晶粒度更小,晶界分布更均匀;而冷轧前进行过时效处理会弱化6016-T4P板材的再结晶织构强度并改变再结晶织构类型,进而改变板材平面内不同方向上的屈服强度并减小平面各向异性指数.     

Effect of high-pressure torsion processing on the microstructure evolution and mechanical properties of consolidated micro size Cu and Cu-SiC powders

In this paper, micro size Cu and Cu-SiC composites powders were consolidated by powder metallurgy (PM) followed by sintering or high-pressure torsion (HPT) to study the effect of the different processing methods on microstructure evolution and mechanical properties. HPT contributes in producing fully dense samples with a relative density higher than those processed by PM followed by sintering. Bimodal and trimodal microstructures with a mixture of ultrafine grain (UFG) and micro or nano grain sizes were noted in the case of Cu and Cu-SiC HPTed samples, respectively. The increase of the SiC volume fraction (SiC%) produces smaller grain size with higher fractions of high angle grain boundaries (HAGBs) in the HPTed Cu-SiC samples than that in the case of HPTed Cu sample. The HPT under a pressure of 10 GPa and 15 revolutions was effective to achieve a complete fragmentation of SiC particles down to ultrafine particle size. HPT processing of Cu and Cu-SiC composites enhanced the mechanical properties (hardness and tensile strength) with conserving a reasonable degree of ductility (elongation%). The yield strength of the samples was estimated based on the microstructure observations and processing parameters by different models correctly with an error range of 5.1–1% from the experiential results.