Woei-Shyan Lee

Plastic deformation and fracture behaviour of Ti–6Al–4V alloy loaded with high strain rate under various temperatures

Abstract This study investigates the plastic deformation and fracture behaviour of titanium alloy (Ti–6Al–4V) under high strain rates and various temperature conditions. Mechanical tests are performed at constant strain rates ranging from 5×102 to 3×103 s−1 at temperatures ranging from room temperature to 1100°C by means of the compressive split-Hopkinson bar technique. The materials dynamic stress–strain response, strain rate, temperature effects and possible deformation mechanisms are discussed. Furthermore, the plastic flow response of this material is described by a deformation constitutive equation incorporating the effects of temperature, strain rate, strain and work hardening rate. The simulated results based on this constitutive equation are verified. The fracture behaviour and variations of adiabatic shear band produced by deformation at each test condition are investigated with optical microscopy and scanning electron microscopy. The results show that the flow stress of Ti–6Al–4V alloy is sensitive to both temperature and strain rate. Nevertheless, the effect on flow stress of temperature is greater than that of strain rate. Fracture observations reveal that adiabatic shear banding turns out to be the major fracture mode when the material is deformed to large plastic strain at high temperature and high strain rate.

High-temperature deformation behaviour of Ti6Al4V alloy evaluated by high strain-rate compression tests

Abstract The high-temperature deformation behaviour of Ti6Al4V alloy has been investigated using the split Hopkinson bar. The specimens were deformed under a constant strain-rate of 2×10 3 s −1 at an initial temperature varying from 700–1100°C at intervals of 100°C. In order to compare the effect of temperature on the flow characteristics, room-temperature tests were also performed to determine the basic mechanical properties. Parallel with this work, the fracture features and microstructures of the deformed specimens were studied by means of different microscopy techniques to understand the formation of adiabatic shear bands and the variations of dislocation features. In addition, for the purpose of describing the flow behaviour of this material in terms of strain, strain rate and temperature, a deformation constitutive equation is proposed. The present results demonstrate that temperature has a significant effect on the flow behaviour of the material. The strength of the material and the work-hardening coefficient decrease rapidly with an increase in temperature. Furthermore, the proposed constitutive equation describes and predicts excellently the observed flow response. From the fracture analysis localized adiabatic shearing forming due to impact is found to be favoured with Ti6Al4V alloy and increases with increasing temperature of deformation. Microstructural observations show that dislocation cells form under all conditions, and that the dislocation density and cell size vary with the temperature. A quantitative relationship between the flow stress and observed dislocation structure is characterized.

Mechanical properties and microstructural features of AISI 4340 high-strength alloy steel under quenched and tempered conditions

Abstract In this work, the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different tempering conditions are investigated. The specimens are quenched and tempered to a martensite structure and loaded to fracture at a constant strain-rate of 3.3×10−4 s−1 by means of a dynamic material testing machine (MTS 810). The mechanical properties and strain-hardening exponent are considered as function of the tempering conditions. The morphological features of the as-quenched martensite and their evolution during tempering are described. Fractographs of the specimens are also made in order to analyse their fracture and embrittlement mechanisms. The results indicate that the mechanical properties and microstructural features are affected significantly by tempering temperature and holding time. The strength and hardness of tempered martensite drop as the tempering temperature and holding time are increased. However, the ductility increases with increasing tempering temperature and holding time, except when tempered martensite embrittlement occurs. Microstructural observations reveal that the carbide precipitates have a plate-like structure at low temperatures, but are spheroid-like at high temperatures. Under the tested conditions, the fracture appearances show that the material failed in a ductile manner except for the case of tempering at 300°C, where tempering martensite embrittlement occurs due to the existence of retained interlath austenite.

Impact properties and microstructure evolution of 304L stainless steel

Abstract The impact properties and microstructure evolution of 304L stainless steel have been studied systematically by means of a split Hopkinson bar and TEM metallographic techniques. Cylindrical specimens were deformed at room temperature under strain rates ranging from 102 to 5×103 s−1, with the true strains varying from 0.05 to 0.3. Results indicate that the mechanical properties and the microstructure largely depend on impact loading. Increasing the strain rate of impact loading increases both flow stress and strain rate sensitivity. However, the inverse tendency is observed for the activation volume. The effect of loading rate on mechanical response and impacted substructure of 304L stainless steel is found to be directly related to the amount of dislocation and the amount of transformed α′ martensite. By using the proposed constitutive equation with the experimentally determined specific material parameters, the flow behaviour of tested material can be described successfully for the range of test conditions. Microstructural observations reveal that the morphologies and characteristics of both dislocation and α′ martensite are sensitive to changes of loading conditions. At high strain rates and large deformation, greater dislocation density, more shear bands and more α′ martensite transformation are observed. Significant strengthening was found to result from dislocation multiplication and α′ martensite transformation, but the latters effect is more evident as high rate loading is imposed. Correlation between dislocation density, fraction of α′ martensite, flow stress and strain rate is confirmed and is discussed in terms of the observed microstructure.

The effects of strain rate and temperature on the compressive deformation behaviour of Ti6Al4V alloy

Abstract The plastic deformation behaviour and compressive characteristics of Ti6Al4V alloy are investigated by means of compression tests under constant strain rates of 0.02, 0.1, and 1.0 s −1 and at temperatures ranging from 25 to 500°C. The height reductions, varying from 10 to 70% with intervals of 20%, are set to investigate the workability of this material. The flow stress data are analyzed in terms of strain rate and temperature sensitivities. A constitutive equation studying the dependence of the flow stress on strain, strain rate and temperature is presented and evaluated. For microstructural analysis, the deformed specimens are polished and etched in order to analyze the characteristics of the deformation patterns. A scanning electron microscope is also used to observe the fracture appearance of failed specimens. The experimental results show that the flow stress decreases directly with the increasing of temperature and that the strain rate affects the flow stress only slighty. Flow softening phenomena are observed at room temperature under a strain rate of 1.0 s −1 . When the reduction in height exceeds 50% at room temperature, inner cracks will initiate inside the specimen. Oblique outer cracks appear on the equatorial surface of the cylinder until the reduction in height increases to 70%. It is noticeable that the characteristics of the flow lines observed in the deformation pattern under isothermal conditions are similar to those of the Dead Metal Zone of non-isothermal deformation. Further, the fracture appearance tends to show that the formation of shear bands induces fragmentary failure. Finally, comparing the calculated flow stress and the measured flow stress, the results show that constitutive equation predicts an acceptable flow stress within a reasonable extent of error.

The strain rate and temperature dependence of the dynamic impact response of tungsten composite

Abstract Liquid sintered tungsten (W) heavy alloy with a 92.5W–5.25Ni–2.25Fe composition was examined using a compression split-Hopkinson bar to realize the effects of strain rate and temperature on dynamic impact deformation behaviour. Both stress and strain were measured for specimens tested at temperatures ranging from 25 to 1100°C and strain rates ranging from 8×10 2 to 4×10 3 s −1 . The relationship between flow stress, strain rate and temperature was determined and the results have been successfully modeled by a proposed constitutive equation incorporating the effect of strain, strain rate and temperature. Our results show that flow stress increases with increasing strain rate. Alternatively, high temperature reduces flow stress significantly and improves the degree of thermal softening. During impact, initial cracking occurs preferentially either at tungsten–tungsten grain boundaries or at the tungsten–matrix interface, and failure is dominated principally by a mixture fracture model. Metallographic examinations show a dramatic increase in microcrack density and deformation of tungsten grains as strain rate and temperature are increased. Additionally, changes in microhardness are also found to correlate with changes in strain rate and temperature.

The deformation behaviour and microstructure evolution of high-strength alloy steel at high rate of strain

Abstract The deformation behaviour and dislocation sub-structure of AISI 4340 alloy steel under high-rate loading conditions are studied. The mechanical characterization of the specimens is performed over a wide range of strain-rate (10 −3 −5 × 10 3 s −1 ) at different strain levels by means of the compression split-Hopkinson bar technique. The dynamic stress-strain response, strain-rate sensitivities, activation volume and deformation mechanisms are determined. The results of these experiments and how they correlate with the effects of strain rate on mechanical behaviour are discussed also. Based on the stress-strain results, a constitutive equation which accounts for the effect of work hardening and strain-rate sensitivity on the stress is suggested. The variations of the dislocation configurations produced by deformation for each test condition are investigated using the transmission electron microscopy technique (TEM). Experimental observations reveal structural differences between specimens deformed at very high strain-rate, the dislocation density, cell structure and cell size being found to be related to the strain, strain-rate and flow-stress levels.

Effects of strain rate and temperature on mechanical behaviour of Ti–15Mo–5Zr–3Al alloy

This study uses the compressive split-Hopkinson pressure bar to investigate the mechanical behaviour of Ti-15 Mo-5 Zr-3 Al alloy deformed at strain rates ranging from 8 x 10(2) to 8 x 10(3) s(-1) and temperatures between 298 and 1173 K. The results indicate that the mechanical behaviour of the alloy is highly sensitive to both the strain rate and the temperature. The flow stress curves are found to include a work hardening region and a work softening region. The strain rate sensitivity parameter, m, increases with increasing strain and strain rate, but decreases with increasing temperature. The activation energy varies inversely with the flow stress, and has a low value at high deformation strain rates or low temperatures. Correlating the mechanical properties of the Ti alloy with the transmission electron microscope (TEM) observations, it is concluded that the precipitation of alpha phase dominates the fracture strain. TEM observations reveal that the amount of alpha phase increases with increasing temperature below the beta transus temperature. The maximum amount of alpha phase is formed at a temperature of 973 K and results in the minimum fracture strain observed under the current loading conditions.

The effects of temperature and strain rate on the properties of carbon-fiber-reinforced 7075 aluminum alloy metal-matrix composite

Abstract The mechanical properties and fracture behavior of 7075-T6 aluminum alloy reinforced with 0.15 V f of laminated carbon fiber were studied for the effects of strain rate between 10 −1 s −1 and 3.3×10 3 s −1 and temperature between 25 and 300 by using a Saginomiya 100 metal-forming machine and a compressive split-Hopkinson bar. The effects of strain rate and temperature on flow behavior of the composites tested have been evaluated and fractographic observations made of the fracture surfaces in order to understand the fracture mechanisms. Results show that the composite flow stress is sensitive to both strain rate and temperature, and that temperature increase noticeably reduces the strain-rate dependence of the flow stress. Fracture strain is found to be greater at low strain rates, increasing, however, with strain rate and temperature in the dynamic range. Changes of fracture features and damage process appear to relate directly to both strain rate and temperature. Damage initiation and propagation processes are identified.

The plastic deformation behaviour of AISI 4340 alloy steel subjected to high temperature and high strain rate loading conditions

Abstract The purpose of this study is to investigate the deformation behaviour of AISI 4340 alloy steel under high strain rates ranging from 500 to 3300 s−1 and at constant temperatures in the range of 25–1100°C by means of a split Hopkinson bar. The tested temperatures are obtained by enclosing the specimen in a clam-shell radiant-furnace. The S.E.M. and T.E.M. techniques are also used to analyze the fracture and microstructure characteristics of the deformed specimens. With the macroscopic and microscopic results, a constitutive equation incorporating the effects of temperature, strain rate and work-hardening rate is proposed to describe the plastic deformation behaviour of the material. The experimental results indicate that the flow stress of AISI 4340 alloy steel increases with strain rate, but decreases with the augmentation of temperature. Also, the work-hardening coefficient, strain rate and temperature sensitivities change with the variation of strain rate, strain and temperature levels. Using the parameter values of the materials obtained from mechanical testing in the Johnson-Cook constitutive equation, the simulated results show that the proposed constitutive equation predicts an acceptable flow stress value with a reasonable extent of error. The adiabatic shear failure mode predominates in the fracture behaviour of the material over the whole range of strain rates and temperatures studied. Microstructural observations reveal that the features of dislocations and the precipitation of particles change in accordance with the variation of the strain rates and the loading temperatures.