Al-5.4Zn-2.6Mg-1.4Cu合金板材的低周疲劳行为

进行Al-5.4Zn-2.6Mg-1.4Cu合金板材的室温低周疲劳实验,对比研究了轴向平行于轧制方向(RD方向)和垂直于轧制方向(TD方向)试样的低周疲劳行为。结果表明：对于0.4%~0.8%的外加总应变幅,RD和TD方向合金试样的循环应力响应行为均呈现出循环稳定;对于相同的外加总应变幅,TD方向合金的循环应力幅值高于RD方向,而RD方向合金的疲劳寿命高于TD方向。对于RD和TD方向,Al-5.4Zn-2.6Mg-1.4Cu合金的塑性应变幅、弹性应变幅与载荷反向周次均呈线性关系。在低周疲劳加载条件下,裂纹在疲劳试样的自由表面以穿晶方式萌生和扩展。     

热处理对Ti65钛合金板材的显微组织、织构及拉伸性能的影响

研究了不同热处理条件下Ti65钛合金板材的显微组织和织构的变化规律,分析了板材织构的类型和热处理影响拉伸强度的机制。结果表明,热处理对板材的显微组织和织构类型具有显著的影响。通过热处理可分别得到具有等轴组织、双态组织或片层组织的板材。等轴组织板材的织构为晶体c轴与板材RD方向呈现70°~90°夹角的B/T型织构,双态组织和片层组织板材的主要织构类型与等轴组织类似,且出现晶体学c轴与RD方向平行的织构。双态组织板材内的位错和亚结构使板材的室温拉伸强度提高,但是对高温拉伸变形的阻碍能力有限。板材中的织构是影响合金力学性能各向异性的主要因素。经980℃/1 h/AC+700℃/4 h/AC热处理后的板材横、纵向拉伸强度的差异最小,且都具有较高的室温拉伸性能和最佳的650℃拉伸性能。     

Ti65合金的初级蠕变和稳态蠕变

使用透射电镜(TEM)研究了Ti65合金在600~650℃、120~160 MPa条件下的蠕变变形行为及其微观变形机制。结果表明：初级蠕变变形机制主要由受攀移控制的位错越过α₂相的过程主导;稳态蠕变阶段蠕变机制主要由受界面处扩散控制的位错攀移的过程主导,且应力指数为5~7。在初级蠕变阶段α₂相与位错的相互作用是α₂相对合金高温强化的主要方式,在稳态蠕变阶段沿α/β相界分布的硅化物阻碍位错运动与限制晶界滑移是硅化物对合金强化的主要方式。     

Effect of anisotropy in commercial purity titanium on deep drawability at elevated temperatures

Abstract

Commercial purity Ti sheets (0.88 mm thick) were deep drawn by a complex working process at two temperatures using a hot die and a cool punch. The working temperature ranged from 25 to 400°C. Generally, the limiting drawing ratios (LDRs) along both the rolling direction (RD) and transverse direction (TD) increased with increasing operating temperature, while the optimal blank holding force decreased with temperature. The LDR(RD) is larger than the LDR(TD) at all temperatures in the range tested and the difference (ΔLDR) is largest at 200°C or higher. The deep drawn cups failed at the cup wall near the die throat along the TD. The thickness strain along the TD was consistently smaller than that along the RD. The anisotropy index δ _011¯1 increased significantly at 100°C and the largest δ _hkil values were found in δ ₀₀₀₂ along the RD and δ _112¯0 along the TD. The reorientation bands that appeared when drawn at 25°C are a result of work hardening, but the microshear bands that occurred in the 400°C specimen are a result of large deformation. The difference in microstructures along the TD and RD may explain the anisotropy in deep drawing properties.     

The effect of rolling direction on the creep process of Al–Cu bimetallic sheet

The study of the mechanical properties of aluminium–copper (Al–Cu) metal layered composite, formed by joining aluminium and copper sheets in the process of rolling have been presented in this paper. The influence of the rolling direction on the basic strength parameters and rheological properties of the composite was analysed. All tests were carried out on flat specimens cut from a sheet in the direction compatible with the rolling direction (RD) and transverse direction (TD). Preliminary tests of monotonic uniaxial tension at a temperature of 293 K were carried out and the basic mechanical properties of Al–Cu bimetal were determined. The hardening process of the material was described by the three-parameter Swift’s equation. The essential creep tests were carried out at a temperature of 523 K in the range of stress 88.5–137.9 MPa. The relation between minimum creep rate and applied stress for the specimens cut from the RD and TD directions were determined. The relationships between the time to fracture, stress, and rupture elongation, obtained from the creep tests, were determined as well. Variations of the steady creep rate with time to fracture by using the Monkman–Grant’s model and its modifications were analysed. It was found that the rolling process strongly affected the short-time monotonic deformation at 293 K and the creep process at 523 K temperature.     

Role of initial texture on the plastic anisotropy of Mg-3Al-1Zn alloy at various temperatures

The effect of initial texture on the anisotropic properties of AZ31 Mg alloys was investigated on the basis of microstructure evolution after compression tests and Lankford parameter (r-value) experiments at various temperatures. Two kinds of sheets were used: one is the cast-and-rolled sheet, and the other is the strip-cast sheet. Compression tests were conducted up to a strain of 0.3 at temperatures of 200-400 °C, and tensile tests were performed to obtain the r-value at various temperatures (25-450 °C). The results showed that, at all test temperatures, the average r-value of the RD plane were greater than those of the ND and TD planes in the cast-and-rolled material. When comparing the average r-value of the RD plane, the cast-and-rolled material revealed much higher values than those of strip-cast material. It was observed that the cross-sectional shape of RD compressive specimens (the compression axis was parallel to the rolling direction) of cast-and-rolled materials changed from an initial circular shape to an ellipsoidal shape due to the plastic anisotropy. Compression processes of specimens were simulated using a finite element method where the Hill's anisotropic yield criterion was adopted. The simulated results were in a good agreement with experimental data.     

A different approach for considering the effect of non-proportional loading path on the forming limit diagram of AA5083

In this paper, the effects of strain path change on the forming limit diagram (FLD) of AA5083 sheet were investigated. The aim is to predict the forming limit curve (FLC) with non-proportional loading path by ductile fracture criteria. For this purpose, some square blanks were pre-strained by uniaxial tension in rolling direction (RD) and transverse direction (TD), and some others were pre-strained by biaxial stretching over a hemispherical punch. Then, the FLD test specimens were prepared by trimming the pre-strained blanks with the longitudinal axis in the RD and TD directions. The out-of-plane formability test was used for obtaining the FLD. The commercial finite element software ABAQUSE 6.9 was used for simulation in accordance with the experimental procedure. For trimming in the simulation environment, a program was written in MATLAB 7.6 that could determine the elements and introduce their properties to the new simulation model. Ductile fracture criteria were used for predicting the failure, and the Hill’79 criterion was used for applying the anisotropic coefficients. The results show that pre-straining in biaxial tension generally reduces the FLC and shifts it to the right-hand side of the FLD, whereas pre-straining in uniaxial tension raises the FLC and shifts it to the left-hand side. The numerical results were compared with the experimental findings, and relatively good agreement was achieved.     

Warm forming limits of rare earth-magnesium alloy ZEK100 sheet

Forming limit curves were developed for a rare earth-magnesium alloy, ZEK100-F, at temperatures between 25 and 350 °C in both the rolling (RD) and transverse directions (TD) of the sheet. ZEK100-F contains additions of zirconium (Zr) as a grain refining alloying element and a rare earth addition, neodymium (Nd), that promotes a weakened basal texture allowing enhanced slip activity at lower temperatures. Warm formability measurements were also performed on non-rare earth containing AZ31B-O to examine the relative performance of these two alloys. The ZEK100 material exhibited significantly better room temperature formability over AZ31B-O with a limiting dome height of 29 mm for ZEK100 compared to only 12 mm for the AZ31B-O. At elevated temperatures (250 °C) the difference in formability between the two alloys becomes less pronounced with a LDH of 40 and 36 mm for ZEK100 and AZ31B-O, respectively. What is particularly striking is the pronounced benefit of the rare earth alloyed material at intermediate temperatures, with an LDH of 37 mm at 150 °C which equals the LDH of AZ31B at 250 °C. Similar trends were determined in the measured forming limit curves reported here for the two alloys. The relative performance of the two alloys is largely attributed their initial textures. ZEK100-F also exhibits strongly anisotropic formability (RD versus TD) which can again be attributed to its’ initial texture.     

高能球磨时间和退火温度对制备TiNi合金粉的影响

为获得高能球磨时间和退火温度对TiNi机械合金粉特性的影响机制,采用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱仪(EDS)、差示扫描量热法(DSC)等分析方法对TiNi合金粉进行了研究。结果表明,机械合金的相成分随着在氩气保护气氛中的球磨时间和退火温度的不同而发生变化。球磨22h的产物是非晶态TiNi合金、Ti的固溶体、Ni的固溶体,球磨27h的产物是非晶态TiNi合金粉和Ni固溶体相,球磨30h发生了明显的固相反应,生成了TiNi、Ni3Ti、Ti3Ni4等物相;在650℃/5h和1000℃/5h下的退火产物都是Ni3Ti、Ti2Ni、TiNi2、TiNi和TiC,但在上述2个退火温度下TiNi并不是主要物相,其中在650℃退火时TiNi的含量明显更低。     

Reversible motion of twin boundaries in AZ31 alloy and new design of magnesium alloys as smart materials

Attractiveness of magnesium alloys for structural applications is caused by their intrinsic properties i.e. low density and high specific strength. The main challenge in development of magnesium alloys is connected with requirement to fulfill the main function of structural materials i.e. to bear load. Wrought magnesium alloys possess strong basal texture which causes anisotropy of mechanical properties. It would be interesting to find out the way how to benefit from this anisotropic behavior of magnesium alloys. One way is to take into account strong basal texture and {1 0 –1 2} twinning in magnesium alloys under compressive deformation. Parallelepiped samples of AZ31 magnesium alloy were successively deformed in compression with 3.5% strain along two perpendicular directions. During first compression the sample contracts along the RD direction parallel to compression axis, elongates only in one perpendicular ND direction and no deformation is observed in third perpendicular TD direction. Subsequent compression along the ND direction recovers the initial shape of the sample. Microstructure analyses shows that the {1 0 –1 2} twinning is the main deformation mode during compression along the RD direction and twin variants which gives 0% strain to TD direction are predominant in microstructure. Twin-free microstructure is observed after subsequent compression along the ND direction. Crystallographic analyzes and calculations explain why reversible motion of twin boundaries is more favorable than nucleation of other twin variants in matrix grains during compression along the ND direction. The experiment presented in this article profile wrought magnesium alloy as smart material and emphasize the importance of strong {0 0 0 1} <1 0 –1 0> texture and {1 0 –1 2} twinning in obtaining the properties characteristic for smart materials. In the presented case, it is the ability to produce and recover significant strains in a controlled manner under compressive stress.     

Modified twinning behaviour induced by texture evolution in hot rolled Mg–3Al–1Zn alloy

Abstract

Compressive deformation along the rolling direction (RD) of a hot rolled Mg–3Al–1Zn alloy is applied to investigate the texture evolution and the recompressive yield strength (RYST) along the transverse direction (TD). Preferential orientation of the basal and prismatic planes is generated by the plastic deformation. Precompression along RD results in one plane of {10–10} aligned nearly perpendicular to the normal direction to the rolling plane. As the compressive strain along RD increases, the RYST shows an earlier raised and later decreased trend. The modified twinning mechanism is investigated using X-ray diffraction and electron backscattered diffraction observations. The results reveal that {10–12} twinning in the matrix dominates the recompression along TD, while the formation of {10–12}–{10–12} twins becomes comparatively easier to occur in the previous {10–12} twins for large precompressed samples.     

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al₄Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ_{(0002)[-1100]}, which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.     

Weakened anisotropy of mechanical properties in rolled ZK60 magnesium alloy sheets with elevated deformation temperature

The rolling direction (RD) and the transverse direction (TD) samples were obtained from an as-rolled ZK60 magnesium alloy sheet with strong anisotropy of initial texture and their mechanical properties were tested at various deformation temperatures. Meanwhile, the microstructure and texture of these samples after fracture were investigated. Results revealed that a higher flow stress along the RD than that along the TD at room temperature were ascribed to the strong anisotropy of transitional texture, and this texture effect was remarkably weakened with the increase of deformation temperature. Deformation structure was dominant at 100 °C, and was replaced by dynamic recrystallization structure when the deformation temperature increased to 200 °C and 300 °C. The texture presented a strong texture (transitional texture in the RD sample and basal texture in the TD sample) at 100 °C, but its intensity visibly decreased and texture components became more disperse at 200 °C and 300 °C. These microstructure and texture results were employed in conjunction with calculated results to argue that raising deformation temperature could increase the activity of non-basal slip by tailoring the relative critical resolved shear stress of each deformation mode and finally result in low texture effect on mechanical anisotropy.     

Superplasticity of Ti2448 Alloy with Nanostructured Grains

Ti-24Nb-4Zr-8Sn, abbreviated as Ti2448 from its chemical composition in weight percent, is a multifunctional β type titanium alloy with body centered cubic (bcc) crystal structure, and its highly localized plastic deformation behavior contributes significantly to grain refinement during conventional cold processing. In the paper, the nanostructured (NS) alloy with grain size less than 50 nm produced by cold rolling has been used to investigate its superplastic deformation behavior by uniaxial tensile tests at initial strain rates of 1.5×10-2, 1.5×10-3 and 1.6×10-4 s-1 and temperatures of 600, 650 and 700℃. The results show that, in comparison with the coarse-grained alloy with size of 50 μm, the NS alloy has better superplasticity with elongation up to ～275% and ultimate strength of 50–100 MPa. Strain rate sensitivity (m) of the NS alloy is 0.21, 0.30 and 0.29 for 600, 650 and 700℃, respectively. These results demonstrate that grain refinement is a valid way to enhance the superplasticity of Ti2448 alloy.     

Anisotropic Creep Rupture Properties of a Nickel-base Single Crystal Superalloy at High Temperature

The creep rupture properties of a single crystal superalloy were tested at 975℃/255 MPa as a function of the deviation degrees from [001].The misorientation of the specimens away from [001] distributed approximately along a line between [001]-[011] and [001]-[111] boundaries in the triangle of the stereographic projection.Creep rupture lifetimes of the specimens were not sensitive to the misorientation until the deviation degree exceeded ～30 deg.Two steps of lattice rotation were found in all specimens during creep,first towards the [001]-[111] boundary,and then to [001] or [111] along the boundary.Single slip and strong asymmetric deformation were observed during the first stage of lattice rotation in specimens with large misorientation.The rotation mechanism was associated with the activated slip systems according to the calculated Schmid factors.The impact of lattice rotation on the rupture properties was also discussed.     

Investigations of ferritic/martensitic super heat resistant 11‐12 %Cr steels for 650 °C power plants

The investigations of advanced ferritic/martensitic 11–12 %Cr steels for 650 °C power plant components focus on the improvement of high‐temperature creep properties with respect to chemical composition. The claim of the DFG research work was the development of new heat‐resistant 12 %Cr ferritic‐martensitic steels with sufficient creep and oxidation resistance for a 650 °C application by using basic principles and concepts of physical metallurgy on the basis of the state of art and to overcome the usual trial and error industrial alloy development. Efforts are focussed on a 100,000h creep strength of 100MPa at 650 °C in combination with a sufficient corrosion resistance by a Cr content of 12 % with contents 4‐5 %W, 3.4‐5,5 %Co, V, B and 1 %Cu as well as the choice of Ta or Ti instead of Nb. The results demonstrate that the aim is not to realize with the used alloying concept. In the long term range all 12 %Cr melts have a lower creep rupture strength than the advanced 9 %Cr piping steel P92. A high creep strength could be reached with a 0.06 % Ta alloyed 11 %Cr melt, which is in addition alloyed with a higher C and B content and as well as with lower W and Co portions. The results indicate in accordance with the finding of other steel researcher that a lower Cr content allows more effectiveness for the alloying partners respectively for the generation of more stable precipitates.     

Effect of temperature on the creep behavior of a Ni–Cr–Fe–Al alloy: a comparison of the experimental data and a model

In order to characterise the mechanical behaviour of sandwich structures, which combine an interlayer of a woven wire mesh between two thin walled sheet metals, creep tests at 650, 680 and 750 °C were carried out on sheet metals made of the nickel based alloy Nicrofer 6025 HT (2.4633). In addition to the tests the creep behaviour was simulated by a model, which considers the creep rate as a function of the applied stress σ and the internal deformation resistance including an internal back stress σ_i and a particle resistance σ_P. The damage is included by a damage parameter D, which converges to “one” with increasing damage. A concluding comparison with the creep test results shows that the model is able to describe the creep behaviour of the investigated sheet metals.     

Partially alloyed filler sheet for brazing of Ti and its alloys fabricated by spark plasma sintering method

Partially alloyed filler metals in the form of powders and laminated foils were used for the brazing of Ti and Ti alloys to lower the manufacturing cost. In this study, by using a raw elemental powder mixture, a multi-component filler sheet with a nominal composition of 37.5Ti–37.5Zr–15Cu–10Ni was fabricated using a Spark Plasma Sintering (SPS) machine in the temperature range from 650 °C to 785 °C for 1 min. As the sintering temperature was increased from 650 °C to 750 °C, the bending strength of the sheets tended to rise, but the bending strength at 785 °C was drastically reduced. The melting range of the sheets became similar to that of the as-cast alloy. The sheets sintered at 750 °C showed the highest bending strength of 259 MPa, which was much higher than that of the as-cast material, and the melting range of this sheet was from 800 °C to 852 °C. The relatively high strength of the sheet was due to the remaining elemental powders such as Ti or Zr, but the brittle intermetallics, such as Ti₂Cu and (Ti,Zr)₂Ni Laves phases, formed in the sheet during the sintering process deteriorated its mechanical strength. The partially developed eutectic phase between the remaining Ti or Zr powder caused the sheet to exhibit melting behavior similar to that of the as-cast alloy. The brazability of the sheet sintered at 750 °C was examined with commercially pure Ti at 870 °C for 5–60 min. The tensile strength of the Ti joint brazed for 30 min was 431 MPa, which was close to that of the base metal.     

Thin film interaction between low-k dielectric hydrogen silsesquioxane (HSQ) and Ti barrier layer

The interaction between low-k dielectric hydrogen silsesquioxane (HSQ) and Ti barrier layer has been studied using four-point-probe sheet resistance measurement, X-ray diffraction, conventional Rutherford backscattering spectrometry (RBS), nuclear resonance analysis (NRA), elastic recoil detection (ERD), secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS). The conventional intermetal dielectrics SiO₂ and plasma-enhanced tetraethylorthosilicate (PETEOS) have been studied also for the purpose of comparison with HSQ. In the low temperature regime (300–550°C), a considerable amount of oxygen atoms, from various sources, diffuses into Ti film to form a Ti(O) solid solution, raising the resistivity of Ti significantly and causing the expansion of the Ti lattice. A good correlation between the oxygen composition in the Ti film, the sheet resistance variation of Ti and the change of Ti lattice parameter C₀ have been observed. At the same temperature, there are more oxygen atoms incorporated into the Ti film in Ti/HSQ than those for Ti/PETEOS, suggesting that additional HSQ-related oxygen sources, such as the moisture uptake and the conversion reaction of HSQ, may be attributed to this. In the high temperature regime (550–700°C), HSQ reacts with Ti to form a final TiO/Ti₅Si₃/HSQ stack structure. It is assumed that a few competing reactions occur in this regime. At 550–650°C, HSQ reacts directly with Ti; in the meantime, part of HSQ undergoes conversion reactions, with the reaction products SiO₂ and SiH₄ reacting with Ti to form Ti silicide. At 650–700°C, HSQ is almost completely converted into SiO₂, so the dominant mechanism is Ti reaction with SiO₂. Before HSQ is completely turned into SiO₂, the Ti/HSQ system is more reactive than both Ti/PETEOS and Ti/SiO₂. The initiating temperature for the Ti/HSQ reaction exhibits no obvious Ti thickness dependence.