微观组织状态对Ti-6Al-4V合金动态力学性能和绝热剪切敏感性的影响

采用分离式霍普金森压杆装置(SHPB)技术,对Ti-6Al-4V合金的等轴组织、双态组织、片层组织进行高应变速率下的动态压缩实验,研究了不同组织状态的动态力学性能及其绝热剪切敏感性,并进行了金相观察及分析。结果表明:在动态压缩载荷条件下,Ti-6Al-4V合金3种组织的真应力-应变曲线大致分为弹性阶段和塑性阶段,没有出现明显的屈服阶段,3种组织试样在高应变率条件下,表现出一定的应变率强化效应,但是应变强化效应不明显;在较大的应变率下,不同组织状态下的试样表现出一个共同的特征,即平均应变都较小,但其应力值却较大;等轴组织的动态压缩力学性能优于其他两种组织;从试样的横剖面和轴剖面的分析来看,不同组织对Ti-6Al-4V合金的绝热剪切敏感性有较大的影响,层片组织具有最大的绝热剪切敏感性,而等轴组织具有最小的绝热剪切敏感性,双态组织介于两者之间。     

不同组织Ti6321合金的绝热剪切行为研究

通过热处理获得等轴组织、双态组织和魏氏组织的Ti6321合金,研究不同组织的Ti6321合金在动态压缩下的绝热剪切行为.利用分离式霍普金森压杆(SHPB)试验装置对帽形试样进行强迫剪切加载,结合扫描电子显微镜和金相显微镜,对其绝热剪切带和微观组织演化进行观察和分析.结果表明:Ti6321合金的绝热剪切敏感性与其组织密切...     

Mn-Si-Cr系Q&P钢的准静态、动态力学行为对比

利用万能试验机和分离式霍普金森压杆装置(SHPB)对Mn-Si-Cr系Q&P钢分别进行了准静态和动态压缩试验。在应变速率为0.001、0.01、0.1 s-1和900、1 500、2 200、3 000 s-1情况下分别得到了准静态和动态压缩真应力-真应变曲线,并利用扫描电子显微镜进行压缩后的显微组织和断口分析,利用X射线衍射仪(XRD)对压缩变形试样进行物相分析。结果表明,准静态和动态压缩变形条件下,试验钢的真应力-真应变曲线均可大致分为弹性变形和塑性变形2个阶段,且没有明显的屈服平台。准静态压缩条件下应变速率强化效果不明显但应变强化效应较显著。动态压缩条件下应变强化效应不明显,但展现出一定的应变速率强化效应。准静态变形后,试样中心区域板条组织倾向沿近水平方向(垂直于压缩方向)定向排布。动态变形后,约有1/3试样发生了断裂,未发生断裂的试样中心出现45°方向剪切带,其附近板条组织发生了“屈曲”。准静态变形后残余奥氏体含量下降明显,而动态压缩试样中,残余奥氏体含量只有略微下降,且块状M/A岛内部出现扭曲变形与开裂,这可能是导致部分试样断裂的诱因。动态压缩破坏试样断口整体呈现45°剪切断裂,一端发生微孔聚集性断裂,另外一端发生剪切断裂。     

Mn-Si-Cr系Q&P钢的准静态、动态力学行为对比

利用万能试验机和分离式霍普金森压杆装置(SHPB)对Mn-Si-Cr系Q&P钢分别进行了准静态和动态压缩试验。在应变速率为0.001、0.01、0.1 s-1和900、1 500、2 200、3 000 s-1情况下分别得到了准静态和动态压缩真应力-真应变曲线,并利用扫描电子显微镜进行压缩后的显微组织和断口分析,利用X射线衍射仪(XRD)对压缩变形试样进行物相分析。结果表明,准静态和动态压缩变形条件下,试验钢的真应力-真应变曲线均可大致分为弹性变形和塑性变形2个阶段,且没有明显的屈服平台。准静态压缩条件下应变速率强化效果不明显但应变强化效应较显著。动态压缩条件下应变强化效应不明显,但展现出一定的应变速率强化效应。准静态变形后,试样中心区域板条组织倾向沿近水平方向(垂直于压缩方向)定向排布。动态变形后,约有1/3试样发生了断裂,未发生断裂的试样中心出现45°方向剪切带,其附近板条组织发生了“屈曲”。准静态变形后残余奥氏体含量下降明显,而动态压缩试样中,残余奥氏体含量只有略微下降,且块状M/A岛内部出现扭曲变形与开裂,这可能是导致部分试样断裂的诱因。动态压缩破坏试样断口整体呈现45°剪切断裂,一端发生微孔聚集性断裂,另外一端发生剪切断裂。     

热处理制度对7039铝合金冲击性能及组织的影响

将7039铝合金热轧板材在470℃/2 h条件下固溶后,分别进行T6处理、T73处理以及RRA处理.利用Hopkinson压杆技术对3种热处理态的7039铝合金进行冲击压缩实验,用光学显微镜和透射电镜对冲击后的试样进行组织观察,分析热处理制度对合金动态应力-应变行为和微观组织的影响.结果表明:经T6处理的7039铝合金在高速冲击加载时的绝热剪切敏感性明显低于T73和RRA处理的合金,经RRA处理的合金绝热剪切敏感性最大;不同热处珲状态的合金在应变率为3 000 s-1时,组织中均产生绝热剪切带以及变形带;合金在高速冲击加载过程中产生的绝热剪切带内部组织主要是一种强同复组织.     

TB10钛合金的动态力学性能及绝热剪切分析

利用分离式的Hopkinson压杆,得到了不同组织状态的近β型TB10钛合金在高应变率下的动态压缩应力-应变曲线,结果表明:TB10钛合金的三种组织状态都表现出应变率敏感性。通过光学显微镜分析其显微组织的变化规律,结果表明:两相区固溶 时效、两相区固溶 双重时效的试样中均可观察到明显的绝热剪切带,且沿剪切带出现裂纹;单一固溶态试样金相观察表明晶粒的变形是均匀的,没有观察到绝热剪切带。在单一固溶态试样中出现了应力诱发马氏体相。     

真空热压烧结制备SiCp/2024Al复合材料动态力学性能北大核心

通过真空热压烧结制备30%(体积分数)SiCp/2024Al复合材料,采用分离式霍普金森压杆(SHPB)对其进行动态压缩实验,得到应变速率为1600~3800 s-1的动态应力-应变曲线。结果表明:在一定的高应变率范围内承受动态载荷时,30%SiCp/2024Al复合材料在不同应变率下,应力-应变曲线趋势变化不大,基本表现为应变率不敏感材料。试样在动态冲击下均未出现宏观剪切破坏,采用扫描电镜(SEM)对压缩试样微观组织进行表征,高应变率试样内部出现孔洞和微裂纹等损伤,试样边界出现增强体颗粒脱粘现象,材料表现出良好的塑韧性。还讨论了动态载荷压缩变形机制。     

2195铝锂合金绝热剪切带内纳米结构的热稳定性

利用分离式Hopkinson压杆(split hopkinson pressure bar,简称SHPB)技术对T6时效态2195铝锂合金帽型试样进行动态加载获得绝热剪切带(adiabatic shear band,ASB),利用透射电镜(TEM)和光学显微镜(OM)观察动态加载前后剪切带的微观结构特征,利用电子背散射衍射(EBSD)分析合金在100~400℃温度下退火后绝热剪切带微观结构的变化,研究剪切带内纳米结构的热稳定性。结果表明:在动态加载过程中,帽型试样的剪切区域形成绝热剪切带,剪切带内的晶粒为50~100 nm左右的纳米等轴晶,在绝热剪切形变过程中析出相已完全溶解于基体中,纳米晶内部和晶界不存在析出相。在不同温度下退火时,剪切带内的晶粒随温度升高而长大,100~200℃温度下退火后晶粒未发生显著长大,在300℃退火后晶粒急剧长大到0.22μm,400℃退火后晶粒尺寸为1.77μm;在300℃左右温度下剪切带的硬度显著下降,此温度正是剪切带内纳米晶粒急剧长大的临界温度。     

SCM435钢热变形动态再结晶动力学模型参数的确定

通过分析冷镦钢SCM435在温度为950～1150℃、应变速率为0.1～1s-1范围内发生动态再结晶的热/力模拟试验数据,利用其应变硬化速率θ与流变应力σ的θ-σ曲线,准确确定了其发生动态再结晶的临界应变εc、峰值应变εp、临界应力σc和峰值应力σp,用应力-应变(σ-ε)曲线方法计算SCM435钢的动态再结晶Avrami动力学曲线和时间指数n.结果表明:SCM435钢发生动态再结晶的临界应变与峰值应变的平均比值εc/εp=0.73,动态再结晶Avrami时间指数平均值n=1.91;在温度950～1150℃,应变速率0.1～1s-1范围内,应变速率是SCM435钢的动态再结晶动力学敏感因素,温度对其影响不大;动态再结晶率50%的时间t50与应变速率成反比.     

AZ91合金的压缩行为

Z91合金的压缩行为研究表明 ,在室温和 350℃下 ,选取应变速率在 5 .6× 1 0 - 3～ 1 .8× 1 0 - 2 s- 1 之间变化时 ,真应力 -真应变曲线变化规律呈现一致性 ,真应力值差距不明显 ,压缩屈服强度σ0 .2 比较接近 ;在 2 50℃下 ,真应力值因应变速率的不同差距比较明显 ,随着应变的增加 ,真应力趋向稳定值。压缩后的试样形貌从室温到 350℃之间的变化规律为 :剪切破碎 -部分饼状 -完全饼状。观察变形组织发现 ,变形后的AZ91合金中出现了大量的动态再结晶晶粒。     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy having equiaxed and bimodal microstructures

Effects of microstructural morphology on dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy plates were investigated in this study. Dynamic torsional and ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures and tensile properties. According to the dynamic torsional test data, maximum shear stress and fracture shear strain of the bimodal microstructure were higher than those of the equiaxed microstructure, and the possibility of the adiabatic shear band formation was more likely in the equiaxed microstructure than in the bimodal microstructure. In the ballistically impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited areas near the penetrated surface without occurring delamination, and their number was smaller than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure, which was consistent with the dynamic torsional test results.     

Microstructural study of adiabatic shear band

This article presents a study of the microstructural development of the adiabatic shear band in an HY-100 steel. The steel was deformed at a high strain rate by ballistic impact, and subsequent metallographic observations along with electron microscopy were performed. A number of white- etched shear bands were found near the perforated region, and three typical microstructural features of the adiabatic shear band were observed: elongated grain structure at the boundary between the shear band and matrix, fine equiaxed grain structure with high dislocation densities in the middle of the shear band, and relatively coarse-grained structure located between the above two structures. These microstructures might be formed in an extremely short time by the combined effects of the large temperature rise and the highly localized deformation. Since very complex phenomena might occur within the shear band, possible mechanisms, such as dynamic recovery and strain-induced dynamic phase transformation, are suggested to explain the micro- structural development of the adiabatic shear band.     

Dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing

The dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests, using a torsional Kolsky bar, were conducted on four steel specimens, two of which were annealed at 480 °C after ECAP, and then the test data were compared in terms of microstructures, tensile properties, and adiabatic shear-band formation. The equal-channel angular pressed specimen consisted of very fine, equiaxed grains of 0.2 to 0.3 μm in size, which were slightly coarsened after annealing. The dynamic torsional test results indicated that maximum shear stress decreased with increasing annealing time, whereas fracture shear strain increased. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen. Their width was smaller in the equal-channel angular pressed specimen than in the 1-hour-annealed specimen, but they were not found in the 24-hour-annealed specimen. Ultrafine, equiaxed grains of 0.05 to 0.2 μm in size were formed inside the adiabatic shear band, and their boundaries had characteristics of high-angle grain boundaries. These phenomena were explained by dynamic recrystallization due to a highly localized plastic strain and temperature rise during dynamic deformation.     

Analysis and prevention of cracking phenomenon occurring during cold forging of two AISI 1010 steel pulleys

This study is concerned with the effects of microstructural parameters on the cracking phenomenon occurring during cold forging of two AISI 1010 steels that were fabricated by converter steel making and electric furnace steel making, respectively. This allowed a comparison between microstructures that contained a small or large amount of nitrogen. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks initiated and propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using a torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. From the dynamic shear stress-strain curves, the steel containing a considerable amount of nitrogen showed a smaller shear strain of 0.2 at the maximum shear stress point, after which the shear stress decreased rapidly prior to fracture, whereas the other steel containing a smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with the yield-point phenomenon occurring in the steel containing more nitrogen. Because the cracking occurring during cold forging was associated with the adiabatic shear banding and the yield-point phenomenon, the minimization of nitrogen and the fast cooling rate after hot rolling were suggested to prevent the cracking.     

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 μm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.     

Effects of Strain Rate and Test Temperature on Torsional Deformation Behavior of API X70 and X80 Linepipe Steels

This study aimed at investigating effects of strain rate and test temperature on deformation and fracture behavior of three API X70 and X80 linepipe steels fabricated by varying alloying elements and hot-rolling conditions. Quasi-static and dynamic torsional tests were conducted on these steels having different grain sizes and volume fractions of acicular ferrite and polygonal ferrite, using a torsional Kolsky bar, and then the test data were compared via microstructures, tensile properties, and adiabatic shear band formation. The dynamic torsional test results indicated that the steels rolled in the single-phase region had the higher maximum shear stress than the steel rolled in the two-phase region, because their microstructures were composed mainly of acicular ferrites. Particularly in the API X80 steel rolled in the single-phase region, increased dynamic torsional properties could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. The possibility of the adiabatic shear band formation at low temperatures was also analyzed by the energy required for void initiation and difference in effective grain size.     

Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si

The stable and unstable plastic flow of Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) has been investigated at temperatures from 816 to 1010 °C (1500 to 1850 °F) and at strain rates from 0.001 to 10 s^-1 in order to establish its hot forging characteristics. In hot, isothermal compression, Ti-6242 with an equiaxed a structure deforms stably and has a flow stress which decreases with straining due to adiabatic heating. With a transformed-β microstructure, unstable flow in hot compression is observed and concluded to arise from large degrees of flow softening caused by microstructural modification during deformation and, to a small extent, by adiabatic heating. Both microstructures have a sharp dependence of flow stress on temperature. Using the concepts of thermally-activated processes, it was shown analytically that this dependence is related to the large strain-rate sensitivity of the flow stress exhibited by the alloy. From lateral sidepressing results, the large dependence of flow stress on temperature was surmised to be a major factor leading to the shear bands occurring in nonisothermal forging of the alloy. Shear bands were also observed in isothermal forging. A model was developed to define the effect of material properties such as flow softening rate and strain-rate sensitivity on shear band development and was applied successfully to predict the occurrence of shear bands in isothermal forging.     

Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-martensite volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing tempered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at α-phase/tempered-martensite interfaces, and that the number of voids increased with increasing martensite volume fraction. Adiabatic shear bands of 6 to 10 μm in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation. jointly appointed with the Materials Science and Engineering Department, Pohang University of Science and Technology     

Numerical modeling of the propagation of an adiabatic shear band

The critical phenomena determining the propagation of an adiabatic shear band occur at its extremity. The stress and strain distributions at the tip of a shear band are calculated as a function of applied shear strain using the finite element method for an elasto-plastic material. Three assumptions simplify the calculations considerably: (a) the mechanical response of the material follows an adiabatic stress-strain curve; (b) the material within the shear band has zero shear strength; (c) the body is taken to be in equilibrium. The distribution of stresses and strains in the adiabatically-deformed material is compared to that of a quasi-statically deformed material. While the stress-strain curve for an isothermally deformed material is monotonic with continuous work-hardening, the adiabatic work-hardening curve reaches a plateau followed by work-softening (due to thermal softening). The stress and strain fields for both cases are nearly identical, except in the region directly in front of the shear band. In the adiabatically-deformed material a thin region (~5 μm) with large strains and lowered stresses is produced. This region, in which accelerated deformation takes place as the applied shear deformation increases, is absent in the isothermally-deformed material. The formation of this instability region, ahead of the shear band, is considered to be the mechanism for the propagation of an adiabatic shear band.