Reverse-transformation austenite structure control with micro/nanometer size

To control the reverse-transformation austenite structure through manipulation of the micro/nanometer grain structure, the influences of cold deformation and annealing parameters on the microstructure evolution and mechanical properties of 316L austenitic stainless steel were investigated. The samples were first cold-rolled, and then samples deformed to different extents were annealed at different temperatures. The microstructure evolutions were analyzed by optical microscopy, scanning electron microscopy (SEM), magnetic measurements, and X-ray diffraction (XRD); the mechanical properties are also determined by tensile tests. The results showed that the fraction of stain-induced martensite was approximately 72% in the 90% cold-rolled steel. The micro/nanometric microstructure was obtained after reversion annealing at 820-870℃ for 60 s. Nearly 100% reversed austenite was obtained in samples annealed at 850℃, where grains with a diameter ≤ 500 nm accounted for 30% and those with a diameter >0.5 μm accounted for 70%. The micro/nanometer-grain steel exhibited not only a high strength level (approximately 959 MPa) but also a desirable elongation of approximately 45%.     

Improved cold rolling workability of warm rolled Fe?6.5wt%Si electrical steel with columnar grains by annealing

The effects of annealing temperature(with the annealing time being constant at 1 h) on the microstructure, ordering, residual stress, mechanical properties, and subsequent cold rolling workability of Fe?6.5wt%Si electrical steel with columnar grains were investigated, where the steel was warm rolled at 500°C with a reduction of 95%. The results show that recrystallization began to occur in the sample annealed at 575°C and that full recrystallization occurred in the sample annealed at 625°C. When the annealing temperature was 500°C or greater, the extent of reordering in the sample was high, which reduced the room-temperature plasticity. However, annealing at temperatures below 300°C did not significantly reduce the residual tensile stress on the edge of the warm rolled samples. Considering the comprehensive effects of annealing temperature on the recrystallization, reordering, residual stress, and mechanical properties of the warm rolled Fe?6.5wt%Si electrical steel with columnar grains, the appropriate annealing temperature range is 300°C?400°C. Unlike the serious edge cracks that appeared in the sample after direct cold rolling, the annealed samples could be cold rolled to a total reduction of more than 71.4% without the formation of obvious edge cracks, and bright-surface Fe?6.5wt%Si electrical steel strips with a thickness less than 0.1 mm could be fabricated by cold rolling.     

Grain boundary characteristics and tensile properties of Ti14 alloy after semi-solid deformation

The microstructure and room-temperature tensile properties of Ti14, a new α+Ti₂Cu alloy, were investigated after conventional forging at 950℃ and semi-solid forging at 1000 and 1050℃, respectively. Results show that coarse grains and grain boundaries are obtained in the semi-solid alloys. The coarse grain boundaries are attributed to Ti₂Cu phase precipitations occurred on the grain boundaries during the solidification. It is found that more Ti₂Cu phase precipitates on the grain boundaries at a higher semi-solid forging temperature, which forms precipitated zones and coarsens the grain boundaries. Tensile tests exhibit high strength and low ductility for the semi-solid forged alloys, especially after forging at 1000℃. Fracture analysis reveals the evidence of ductile failure mechanisms for the conventional forged alloy and cleavage fracture mechanisms for the alloy after semi-solid forging at 1050℃.     

Monte Carlo study on abnormal growth of Goss grains in Fe-3%Si steel induced by second-phase particles

The selective abnormal growth of Goss grains in magnetic sheets of Fe-3%Si (grade Hi-B) induced by second-phase particles (AlN and MnS) was studied using a modified Monte Carlo Potts model. The starting microstructures for the simulations were generated from electron backscatter diffraction (EBSD) orientation imaging maps of recrystallized samples. In the simulation, second-phase particles were assumed to be randomly distributed in the initial microstructures and the Zener drag effect of particles on Goss grain boundaries was assumed to be selectively invalid because of the unique properties of Goss grain boundaries. The simulation results suggest that normal growth of the matrix grains stagnates because of the pinning effect of particles on their boundaries. During the onset of abnormal grain growth, some Goss grains with concave boundaries in the initial microstructure grow fast abnormally and other Goss grains with convex boundaries shrink and eventually disappear.     

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al-Mg2Si in situ composites

The effect of high-speed direct-chill (DC) casting on the microstructure and mechanical properties of Al-Mg₂Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al-Mg₂Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg₂Si particles (from 28 to 12 μm), the spacing of eutectic Mg₂Si (from 3 to 0.5 μm), and the grains of AA6061 alloy (from 102 to 22 μm). The morphology of the eutectic Mg₂Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg₂Si phase and the grains in the Al-Mg₂Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al-Mg₂Si composites and AA6061 alloy billets.     

Recrystallization and mechanical properties of WE43 magnesium alloy processed via cyclic expansion extrusion

In this study, cyclic expansion extrusion (CEE), as a relatively new severe plastic deformation (SPD) process, is applied to a rare earth (RE) containing Mg alloy WE43. The effects of the processing temperature and the number of passes are also investigated. The results showed that dynamic recrystallization (DRX) occurred after CEE processing at 400℃, and a bimodal structure with ultrafine DRXed grains surrounded the unrecrystallized grains. However, the DRX at 330℃ was retarded because of the existence of RE elements. The tensile tests showed that a simultaneous increase in the strength and the ductility of WE43 is obtained after CEE processing at 400℃ via two passes. Furthermore, the highest ultimate tensile strength of 440 MPa was achieved after the second pass of CEE at 330℃, and the highest ductility of 21% was attained after the second pass of CEE at 400℃. The microhardness measurements showed that the hardness increased from HV 80 to HV 114 and HV 98 after two passes of CEE processing at 330 and 400℃, respectively. In conclusion, increasing the processing passes could increase the mechanical properties and the volume fraction of the recrystallized grains. Moreover, increasing the temperature reduced the strength and the microhardness even if the elongation increased.     

Comparison of Ferrite Refinement Mechanisms by Different Processing Schedules in 08 and 20Mn Steels

The influence of deforming temperature on ferrite refinement was analyzed by comparing the microstructures obtained by deformation at above A_r3, in two-phase region of (α + γ) and at below A₁ in clean 08 and 20Mn steels. The results indicate that ferrite refinement through strain induced transformation by deformation at above A_r3 is more effective than that by deformation simply through ferrite dynamic recrystallization. The main problem of ferrite refinement by deformation at below A_r3 is the inhomogeneity of microstructure which is controlled by the orientations and sizes of ferrite grains and the distribution of second phases. Ferrite dynamic recrystallization after strain induced transformation can further effectively refine ferrite.     

Dynamic recrystallization during hot torsion of Al-4Mg alloy

Binary Al-4Mg alloy have been deformed by hot torsion at 300-500℃ and strain rates of 0,006-1.587 s-1 to a true strain of 5.5. The specimens were annealed in vacuum for 1.5 h at 500℃ and then water quenched. The study indicates that the dynamic recrystallization occurs during hot torsion of Al-4Mg alloy in a certain range of Z parameter (Zener-Hollmon Parameter), i.e. 19.3 ≤ lnZ ≤ 24.8. Increasing the strain rate at higher deformation temperature or reducing the strain rate at lower deformation temperature accelerates the occurrence of dynamic recrystallization in the alloy.     

Influence of microstructure on the corrosion resistance of Fe–44Ni thin films

An Fe–44Ni nanocrystalline (NC) alloy thin film was prepared through electrodeposition. The relation between the microstructure and corrosion behavior of the NC film was investigated using electrochemical methods and chemical analysis approaches. The results show that the NC film is composed of a face-centered cubic phase (γ-(Fe,Ni)) and a body-centered cubic phase (α-(Fe,Ni)) when it is annealed at temperatures less than 400℃. The corrosion resistance increases with the increase in grain size, and the corresponding corrosion process is controlled by oxygen reduction. The NC films annealed at 500℃ and 600℃ do not exhibit the same pattern, although their grain sizes are considerably large. This result is attributed to the existence of an anodic phase, Fe_0.947Ni_0.054, in these films. Under this condition, the related corrosion process is synthetically controlled by anodic dissolution and depolarization.     

Electrical annealing of severely deformed copper: microstructure and hardness

Commercial pure copper sheets were severely deformed after primary annealing to a strain magnitude of 2.32 through constrained groove pressing. After induction of an electrical current, the sheets were heated for 0.5, 1, 2, or 3 s up to maximum temperatures of 150, 200, 250, or 300℃. To compare the annealing process in the current-carrying system with that in the current-free system, four other samples were heated to 300℃ at holding times of 60, 90, 120, or 150 s in a salt bath. The microstructural evolution and hardness values of the samples were then investigated. The results generally indicated that induction of an electrical current could accelerate the recrystallization process by decreasing the thermodynamic barriers for nucleation. In other words, the current effect, in addition to the thermal effect, enhanced the diffusion rate and dislocation climb velocity. During the primary stages of recrystallization, the grown nuclei of electrically annealed samples showed greater numbers and a more homogeneous distribution than those of the samples annealed in the salt bath. In the fully recrystallized condition, the grain size of electrically annealed samples was smaller than that of conventionally annealed samples. The hardness values and metallographic images obtained indicate that, unlike the conventional annealing process, which promotes restoration phenomena with increasing heating time, the electrical annealing process does not necessarily promote these phenomena. This difference is hypothesized to stem from conflicts between thermal and athermal effects during recrystallization.     

取向硅钢薄带形变再结晶组织及织构演变

利用背散射电子衍射微织构分析技术及X射线衍射织构分析技术,结合对取向硅钢薄带再结晶各阶段退火板磁性能的分析,系统研究了其形变再结晶过程中的组织及织构演变。结果表明,薄带内原始高斯晶粒取向发生绕TD轴向{111}<112>的转变,同时晶粒取向还表现出绕RD轴的附加转动,这种附加转动及其导致的表层微弱立方形变组织可为再结晶立方织构的形成提供核心。退火各阶段样品磁性能的变化对应了{110}-{100}<001>有益织构及其他织构的强弱转变以及再结晶晶粒不均匀程度的变化,综合织构类型及晶粒尺寸的变化推断发生了二次及三次再结晶过程。升温过程再结晶织构演变主要体现了织构诱发机制,也即与基体存在绕<001>轴取向关系的晶粒长大优势结合高斯织构的抑制效应发挥作用;而在高温长时间保温后三次再结晶过程,{110}低表面能诱发异常长大发挥主要作用使得最终得到锋锐的高斯织构。     

退火温度对单向纤维晶纯铜线材组织性能影响

对退火前后单向纤维晶纯铜线材组织进行了观察，测试了其力学与导电性能，结果表明：在300℃以下退火时，线材的金相显微组织与退火前相比没有明显变化，仍然保持为连续纤维晶，其抗拉强度有所下降，延伸率有所增加；在400℃退火时，线材发生完全再结晶，且有孪晶生成；500％退火时，再结晶晶粒长大；在400℃以上退火时，抗拉强度大幅度下降，延伸率显著增加；在200～400℃的范围退火时，电导率增加，但增幅较小。     

挤压温度对Mg-Zn-Mn变形镁合金组织和性能的影响

对新型变形镁合金Mg-6%Zn-1%Mn铸锭在320、360、420℃等不同温度下进行挤压实验,成型后实施不同热处理,并分析不同状态下合金的微观组织和力学性能.结果表明:在320~420℃条件下,该合金能实现平稳地挤压成型并完成动态再结晶.挤压温度越低,再结晶晶粒越细小,挤压棒材性能越好.高温(420℃)挤压成型,动态再结晶越易进行,且再结晶晶粒越均匀,更有利于后期通过热处理改善合金性能.     

取向硅钢中高斯晶粒异常长大选择性行为

对CGO取向硅钢二次再结晶中断实验进行了研究,发现二次再结晶升温过程中,仅在异常长大开始前,高斯晶粒尺寸明显大于其他晶粒,且不同取向晶粒的数量与脱碳退火时的特征一致.高斯晶粒晶界上MnS等抑制剂的优先粗化使高斯晶粒能够率先发生异常长大,且只有晶界弯曲严重或经过很小的生长几个晶粒就能合并的高斯晶粒才能成为二次晶核.在高斯晶粒异常长大过程中,晶界形貌参差不齐,呈岛屿状.研究表明:高斯晶粒独特的生长方式,可能是使二次再结晶能很快完成的原因.     

Mn对Al-Mg-Si-Cu铝合金车身板组织和性能的影响

通过拉伸和埃里克森实验以及扫描电镜/能谱、透射电镜和金相分析,研究Mn的质量分数对Al Mg Si Cu铝合金汽车板显微组织、力学性能和成形性的影响·研究表明,随Mn质量分数增加,Al Mg Si Cu汽车板铝合金不可溶结晶相及弥散相粒子数量均增加,不可溶结晶相使合金组织纤维化对板材冲压成形性不利,弥散相粒子阻碍再结晶晶粒长大;提高Mn的质量分数,Al Mg Si Cu汽车板铝合金的强度增加,但延伸率和冲压成形性降低·     

变形量对Cu-Ni-Si合金再结晶行为的影响

对经过二级变形时效处理后的Cu-Ni-Si合金进行不同变形量的冷变形后,再进行退火处理,研究了变形量对合金抗软化性、再结晶组织和再结晶动力学行为的影响。研究结果表明:再结晶晶粒在晶界处形成,大的变形量能够提高再结晶的形核速率。经40%变形的合金的软化温度为470℃,再结晶温度在550℃左右;在400℃退火时,合金发生再结晶的激活能为162 kJ/mol。再结晶的激活能随变形量的增加而降低,当变形量由40%增至80%时,再结晶激活能由162 kJ/mol降至140 kJ/mol。     

薄板坯连铸连轧工艺下Hi-B钢的组织及织构

采用金相显微镜和扫描电镜研究实验室模拟薄板坯连铸连轧( TSCR)工艺试制的高磁感取向硅钢( Hi- B钢)组织、织构的演变特征. 研究发现实验室模拟薄板坯连铸连轧工艺试制的Hi-B钢热轧板显微组织及织构在厚度方向上存在不均匀性. 常化板表面脱碳层铁素体晶粒明显粗化,常化板织构基本继承了热轧板相应的织构类型,仅织构强度不同. 一次大压下率冷轧后,晶粒及其晶界沿轧向被拉长形成鲜明的纤维组织,织构主要为α纤维织构和γ纤维织构,脱碳退火后试样发生回复和再结晶现象并形成初次晶粒组织,脱碳退火后织构分布较为集中. 温度升高至1000℃时二次再结晶开始,1010℃时钢中晶粒发生异常长大,高斯织构强度达到61. 779. 成品磁感为1. 915 T,铁损为1. 067 W·kg-1 .     

3004铝合金再结晶织构及其显微组织

应用取向分布函数(ODF)和透射电镜(TEM)研究了3004铝合金再结晶织构及其显微组织·结果表明:在退火样品中存在较强的再结晶U({001}〈100〉)立方织构,且该织构随退火温度的升高而增强,在350℃和450℃时强度级别分别达8级和12级,再结晶R/S({124}〈211〉)织构、S({123}〈634〉)和C({112}〈111〉)织构组分强度较低·TEM形貌像表明:经250℃,120min退火后,样品中回复过程已基本完成;在300℃以上退火时,随着退火温度的升高,再结晶晶粒不断长大,第二相粒子继续析出,弥散地分布于晶粒内的亚晶界上,并被位错所包围,在再结晶过程中起到了促进形核的作用·     

Al-Mg系铝合金超塑性薄壁管材

采用熔铸—均匀化退火—挤压工艺研制Al-5.60Mg-0.30Zr-0.07Cr-0.16Mn（质量分数,%）合金管材。对该合金管材进行析出退火处理后,采用热旋—退火—冷旋工艺制备薄壁旋压管。采用金相显微镜、扫描电镜、拉伸性能测试等手段研究该铝合金旋压管冷变形态与完全再结晶退火态的组织与性能,测试其超塑性能,讨论其超塑性变形与断裂行为。研究结果表明,在Al-Mg铝合金中加入微量锆、铬、锰,可以促使试验合金中第二相颗粒弥散分布,减小后续加工的变形不均匀性;Al-5.6Mg-0.30Zr合金经析出退火—旋压变形后,于500℃退火1 h的再结晶退火组织晶粒平均粒径小于10μm。