Mechanical behavior of Gamma-Met PX under uniaxial loading at elevated temperatures and high strain rates

Gamma titanium aluminides have received considerable attention over the last decade. These alloys are known to have low density, good high temperature strength retention and good oxidation and corrosion resistance. However, poor ductility and low fracture toughness have been the key limiting factors in the full utilization of these alloys. More recently, a new generation of gamma titanium aluminide alloys, commonly referred to as Gamma-Met PX, has been developed by GKSS, Germany. These alloys have been observed to have superior strength and better oxidation resistance at elevated temperatures when compared with conventional gamma titanium aluminides.The present paper discusses results of a study to understand the uniaxial mechanical behavior in both compression and tension of Gamma-Met PX at elevated temperatures and high strain rates. The compression and tensile tests are conducted using a modified Split-Hopkinson Bar apparatus at test temperatures ranging from room temperature to 900 °C and strain rates of up to 3500 s⁻¹. Under uniaxial compression, in the temperature range from room to 600 °C, the flow stress is observed to be nearly independent of test temperature. However, at temperatures higher than 600 °C thermal softening is observed at all strain rates with the rate of thermal softening increasing dramatically between 800 and 900 °C. The room temperature tensile tests show negligible strain-rate dependence on both yield stress and flow stress. With an increase in test temperature from room to 900 °C, the material shows a drop in both yield and flow stress at all levels of plastic strain. However, the measured flow stress is still higher when compared to nickel based super-alloys and other gamma titanium aluminides under similar test conditions. Also, no anomaly in yield stress is observed up to 900 °C.     

确定材料在高温高应变率下动态性能的Hopkinson杆系统

描述了一种利用Hopkinson杆装置确定在高温（温度可高达1 173 K）、高应变率下材料动态性能的试验方法。在试样加温过程中,试样不与入射杆及透射杆接触。当试样加热到预定温度时,气压驱动同步组装系统,推动透射杆及试样,使得应力波到达入射杆与试样接触面时,入射杆、试样及透射杆紧密接触。利用以上系统,完成了连铸单晶铜及上引法连铸多晶铜从室温到1 085 K范围内的应力应变曲线。测试结果表明,不论是上引法连铸多晶铜还是连铸单晶铜,流动应力随温度的升高而下降,在温度低于585 K时,材料的应变硬化率明显大于在温度高于585 K时的应变硬化率。     

Mechanical characterisation of aluminium alloy 7449-T7651 at high strain rates and elevated temperatures using split hopkinson bar testing

Experimental Techniques - The mechanical behaviour of a 7 series high-strength aluminium alloy that is mainly used by the aerospace industry is under investigation. Aluminium alloy AA7449-T7651 is...     

Failure analysis of stainless steel at elevated temperatures

In this paper, particular emphasis has been put on gathering information on the phenomena that take place at the crack tip of a crack propagating at 1100°F. Since the experimental program was directed toward studying crack propagation in tubing, the tests were conducted on rings.From the experimentally obtained data and from the correlation with the theoretically predicted values, the following picture emerges for the fracture behavior with full plasticity present. There is a region surrounding the crack tip where very large plastic deformations take place. This region is surrounded by a much larger region where the loading is nearly proportional and the behavior can be predicted well by the results of the deformation theory of plasticity and the theory of singularity fields. As the crack propagation initiates, there is a drastic change in the crack-tip configuration. The crack tip does not blunt and a fairly sharp crack-tip region is observed. The crack tip carries a large deformation field of a far more localized nature than that observed at the initiation of the crack growth.Paper was presented at 1978 SESA Spring Meeting held in Wichita, KS on May 14–19.     

Apparatus for studying cyclic stress-strain and fatigue at elevated temperatures

An apparatus is described which permits tubular specimens to be heated to a uniform temperature, while being cyclically strained with a constant amplitude of alternating strain about a fixed mean strain. Under these conditions, tests were performed in which the mean stress was measured as a function of the number of cycles of repeated strain for the alloys Udimet 700 and Rene 41 at 1300°F. These data along with fatigue fracture data are reported.     

Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel (AISI 316L) and one nickel-base alloy (Alloy 617) have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromechanisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650°C.     

A multi-scale algorithm for dislocation creep at elevated temperatures

Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals. When using traditional discrete dislocation dynamics(DDD) to capture this process, we often need to update the forces on N dislocations involving ～N~2 interactions. In this letter, we introduce a multi-scale algorithm to speed up the calculations by dividing a sample of interest into sub-domain grids:dislocations within a characteristic area interact following the conventional way, but their interaction with dislocations in other grids are simplified by lumping all dislocations in another grid as a super one. Such a multi-scale algorithm lowers the computational load to ～N 1.5. We employed this algorithm to model dislocation creep in Al-Mg alloy. The simulation leads to a power-law creep rate in consistent with experimental observations. The stress exponent of the power-law creep is a resultant of dislocations climb for ～5 and viscous dislocations glide for ～3.     

碳纤维增韧的陶瓷基复合材料在高温高应变率下的压缩力学行为

利用高温电子万能试验机和具有高温同步自组装功能的Hopkinson压杆对二维C/SiC复合材料进行了应变率为10-4~103s-1,温度为293~1273K下的单轴压缩力学性能测试。实验结果表明:二维C/SiC复合材料破坏时并未表现出典型的脆性破坏,而是在应力达到压缩强度时出现了显著的应变软化,在经历了较大的变形后才最终破坏,同时材料还表现出良好的高温承载能力及一定的温度和应变率依赖性。随着温度的升高,复合材料的压缩强度呈降低的趋势。与准静态下室温压缩时相比,材料在1273K 时的压缩强度的降低程度不超过30%,但压缩强度对应变率的敏感性随着温度的升高而增大。由于高温下试样氧化,C/SiC复合材料压缩强度对应变率的敏感性在温度为1073K时显著增大。     

高温高应变率下异种不锈钢激光焊接件的力学性能

利用改进的SHTB实验设备,对316L和304不锈钢焊接结构的动态力学性能进行了实验.提出了一种适用于焊接结构件的新型的SHTB夹持装置,并在应变率约103 s-1、温度为25～500℃的环境下获得了焊接结构件动态应力-应变曲线.研究表明:随应变率的升高,结构件的屈服强度和抗拉强度随应变率的增大而增大,随温度的升高而降...     

高流态地质聚合物混凝土的高应变率动态压缩变形特性

以矿渣、粉煤灰为原材料,以NaOH、Na2CO3为碱激发剂,制备了强度等级为C30的高流态地质聚合物混凝土（highly-fluidized geopolymer concrete,HFGC）,运用波形整形技术改进了Ø100mm SHPB实验装置,通过参数控制保证应力均匀和恒应变率加载,对HFGC开展了动态压缩实验,分析了HFGC在冲击压缩荷载下的变形特性。HFGC属于应变率敏感材料和脆性材料,高应变率作用下,HFGC的典型应力应变曲线包括压实挤密阶段、弹性阶段和软化、屈服阶段。在10~100s-1的应变率范围内,HFGC的峰值应变εc随应变率的变化表现出明显的冲击韧化效应,εc随应变率的升高先增大后减小,满足二次函数关系εc＝-1.2×10-6+1.6×10-4+0.001 7,变形特性变化的临界应变率为66.7s-1。HFGC的动态弹性模量均低于其在准静态下的弹性模量。     

Investigation of strain gages for use at cryogenic temperatures

Strain sensitivity and resistance changes in Advance, Karma, Budd Alloy, Nichrome V, and stabilized Armour D foil gages, and Nichrome V-platinum temperature-compensated gages were evaluated at cryogenic temperatures. The more promising gage types, as determined from these studies, were tested to strain levels up to 11,000 μin./in. with tensile specimens. The other performance characteristics that were investigated included zero drift, creep, hysteresis, linearity and gage element size effects. A mounting method developed for foil gages to be used at cryogenic temperatures is described.     

A laser extensometer for measuring strain at incandescent temperatures

A laser-extensometer system has been developed and installed in an electron-beam tensile vacuum furnace to measure the strain of tensile-test specimens at incandescent temperatures. The laser extensometer operates by measuring the length of time during which the laser light can be “seen” by a photosensitive device while the light beam is rotated at a constant angular velocity. The light can be “seen” by the photosensitive device only when the light passes through a gap between two opaque flags which are affixed to the specimen so that the change in the gap width is proportional to the strain in the specimen. Tests indicate that the laser extensometer is capable of measuring strain with a maximum error of less than 100 μin./in. based on a 1-in. specimen gage length. The sensitivity and accuracy of the first model of the laser extensometer increase with a decreasing strain rate and are well within the range of requirements for metallurgical evaluation of modern metals and alloys for use at very high temperatures. Operation of the extensometer has indicated that greatly improved sensitivity and reduction of electronic noise and drift can be achieved by simple modifications yet to be made.     

Strain analysis for extrusion of powder metals

A method is described for determining the strain distribution during axisymmetric extrusion of powder metals. Density variations in these materials are included in the analysis. The method examines these changes together with the laminar flow pattern to determine strain rates and, ultimately, strains.     

Grain boundary oxidation and fatigue crack growth at elevated temperatures

Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and statistical distribution of grain boundary oxide penetration depth were studied.At a constant ΔK-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, ν. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consustent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to ν, and fatigue crack growth is intergranular.     

A work-hardening rule for finite elastic-plastic deformation of metals at elevated temperatures

An extension of Prager-Ziegler's work-hardening rule for infinitesimal elastic-plastic deformation to a work-hardening rule for finite elastic-plastic deformation of polycrystalline metals at elevated temperatures is developed. Attention is given to the development to satisfy certain invariant, continuity and thermodynamic requirements.