Microstructure and texture of high manganese steel subjected to dynamic impact loading

ABSTRACT

Dynamic impact response of high Mn-steel at a strain rate of 3000?s^?1 was investigated using the Split Hopkinson Pressure bar. The investigated steel depicted continuous yielding at high strain rates. Additionally, the yield stress displayed a positive strain-rate sensitivity with an increasing strain rate. Microstructural evaluations displayed that strain-induced martensitic transformation and dislocation multiplication during slip were dominant plastic deformation mechanisms in the absence of deformation twinning which contributes to the strain hardening. Adiabatic shear band and martensite to austenite reversion or dynamic recrystallisation were also attributed to strain softening during impact deformation. The {001}<110> R-cube, {011}<110> R-Goss, and ({111}<110>) E texture components were strengthened after impact loading compared with as-received condition, while the intensities of Cube, Cupper, Brass, and S texture components were decreased.     

Morphology of adiabatic shear bands in cylindrical specimens of AISI 4340 steel impacted by Hopkinson pressure bar

Abstract

Cylindrical specimens of AISI 4340 steel, which were heat treated by quenching in oil followed by tempering at either 315 or 425°C, were impacted in a Hopkinson pressure bar at different impacting speeds. It was found that when strain and strain rate reached certain values, adiabatic shear bands (or plastic deformation zones) were formed in the specimens. The adiabatic shear bands appeared either in a circle on the transverse section, a hyperbola on different longitudinal sections without the central axis of the cylinder, and a triangle on the longitudinal section through the central axis of the cylinder. From these observations, it can be concluded that the plastic deformation localisation zone is limited in a thin conical shell in three dimensions. It was further confirmed that the adiabatic shear bands initiated along the maximum shear stress directions. In addition, the adiabatic shear bands in the specimens tempered at 315°C appeared white, while those in specimens tempered at 425°C had deformation characteristics. This indicates that the appearance of adiabatic shear bands is related to the hardness and microstructure of the tested steel.     

Investigation on hot deformation of 20Cr–25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

Abstract

The deformation behaviour of a 20Cr–25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000–1200°C and strain rates of 0·01–10 s^?1. It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516·7 kJ mol^?1. Based on the dynamic materials model and the Murty’s instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025–1120°C and 0·01–0·03 s^?1 or 1140–1200°C and 0·08–1 s^?1, where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

High strain rate superplastic flow stress and post-deformation mechanical properties of mechanically alloyed 2024 aluminium alloy reinforced with SiC particles

Abstract

Composites consisting of 2024 aluminium alloys reinforced with volume fractions of 0, 5, 10, and 15 vol.-% of SiC particles were fabricatedfrom the mechanically alloyed powders by an optimised hot compaction and prestraining process. Fine and equiaxed grain structures with grain sizes of <1 μm were observed within the matrix of each alloy. The composite specimens were compressed at temperatures between 733 and 813 K with a wide strain rate range from 10^?3 to 10 s^?1. Two strain rate regions with different slopes from ～ 5 × 10^?1 s^?1 were found in log (true stress–log (strain rate) curves. In the lower strain rate region of each alloy, the strain rate sensitivity values m were 0.03–0.16. The threshold stress σ_th for each alloy was estimated using an extrapolation procedure. A linear relationship was found between <disp-formula><graphic href="splitsection10-m1.tif"/></disp-formula> and σ_th where V_f is the volume fraction of SiC particles. In the higher strain rate region of each alloy, m values greater than 0.3 were obtained at 773 K, which is very close to the solidus temperature of 775 K for 2024 aluminium alloy. Moreover, the maximum yield strength and elongation for each alloy at room temperature were also obtained in the specimens compressed at 773 K. Thus, it was found that the optimum temperature for the high strain rate superplastic processing of the composites was just below the solidus temperature of the 2024 aluminium alloy. The grain coarsening resulted in the decrease of post-deformation strength and ductility as well as the m value in hot compression above the solidus temperature.     

Thermal Effects During Tensile Deformation of Ti‐6Al‐4V at Different Strain Rates

An in‐depth analysis of the effect of heat generated by plastic work on the observed tensile behaviour of Ti­6Al­4V at different strain rates is presented. Special emphasis is put on the transition from isothermal to adiabatic conditions and how this transition is affected by several process parameters such as material properties, environmental conditions and sample geometry. Experiments are performed in isothermal conditions at moderate temperatures, from ?10 to 70 °C, as well as at strain rates from quasi‐static speeds to more than 1000 s^?1 using a split Hopkinson tensile bar setup. This experimental data is used in conjunction with numerical simulations to determine the evolution of temperature during the experiments and the temperature and strain rate sensitivity of the material, as well as the Taylor–Quinney coefficient. Finally, a full model of the material behaviour is presented and used to define clear limits for adiabatic and isothermal conditions.     

Response of titanium alloys to high strain rate deformation

Abstract

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10^?1 s^?1 to 10³ s^?1 and samples of Ti811 and Ti153 at a strain rate of 10³ s^?1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.     

On the evaluation of parameters of the constitutive equation for 7475 Al alloy

From the mechanical data on 7475 Al alloy, it is evident that flow stress is significantly dependent on the strain during superplastic flow. This is due to its ability to strain-harden during superplasticity. The rate of increase in the flow stress is much higher at 457° C than at 517° C. This gives rise to non-unique values for the parameters of the constitutive equation. At 457° C, whereas the stress exponent (n) and activation energy for superplastic flow at 1 × 10^–4 sec^–1 increase only slightly with strain, the grain size sensitivity parameter (p) and structure parameter (A) decrease significantly with strain. These changes in the constitutive parameters are associated with dislocation activity occurring within the grain interior, leading to grain elongation without significant changes in the grain size, through the parameter, (b/d) ^p , of the constitutive equation.     

数字图像相关方法辅助的IM7/8552碳纤维/环氧树脂复合材料单向带层合板沿厚度方向非线性本构参数识别

提出了采用数字图像相关(DIC)方法和有限元模型修正(FEMU)技术相结合,通过短梁剪切(SBS)试验获得碳纤维增强环氧树脂(IM7/8552)正交各向异性复合材料单向带层合板沿厚度方向压缩本构关系参数的试验方法.该方法根据假设材料初始本构,采用3D有限元模型(FEM)计算获得主平面压头下方沿厚度方向的应力和应变分布,...     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Deformation of sintered copper and 50Cu–50Fe mixture to large strains by cyclic extrusion and compression

Abstract

Sintered compacts of copper and a 50Cu–50Fe mixture have been plastically deformed to large strains (total strain ?_t=13·8) by cyclic extrusion and compression. The hardness changes after deformation indicate that no further work hardening occurs with either material when ?_t>4·6. With copper, strain accommodation at large strains would appear to occur solely by dynamic recovery and recrystallisation. With the Cu–Fe mixture, shear banding is still found at the highest strains used.

MST/1609     

The uniaxial stress versus strain response of pig skin and silicone rubber at low and high strain rates

The uniaxial compressive responses of silicone rubber (B452 and Sil8800) and pig skin have been measured over a wide range of strain rates (0.004–4000 s⁻¹). The uniaxial tensile response of the silicone rubbers was also measured at low strain rates. The high strain rate compression tests were performed using a split-Hopkinson pressure bar made from AZM magnesium alloy. High gain semi-conductor strain gauges were used to detect the low levels of stress (1–10 MPa), and a pulse shaper increased the rise time of dynamic loading on the specimen. The experiments reveal that pig skin strain hardens more rapidly than silicone rubbers and has a greater strain rate sensitivity: pig skin stiffens and strengthens with increasing strain rate over the full range explored, whereas silicone rubber stiffens and strengthens at strain rates in excess of 40 s⁻¹. A one term Ogden strain energy density function adequately describes the measured constitutive response of each solid, and a strategy is outlined for determining the associated material constants (strain hardening exponent and a shear modulus). The strain rate sensitivities of the pig skin and two silicone rubbers are each quantified by an increase in the shear modulus with increasing strain rate, with no attendant change in the strain hardening exponent. It is shown that the Mooney-Rivlin model is unable to describe the strong strain hardening capacity of these rubber-like solids.     

Bending vector sensor based on Mach–Zehnder interferometer using S type fibre taper and lateral-offset

Abstract

A novel, simple and highly sensitive bend vector sensor is proposed and experimentally demonstrated for directional bending measurement. This sensor consists of a lateral-offset and an S fibre taper processed through special fusion splicing method. The asymmetrical fibre structure of S fibre taper and lateral-offset determined a pair of directions along which the bending response to the transmission spectrum is different. Thus, it can be used for bending vector measurement. For a curvature range from ?4 to +4 m^?1, the bending sensitivities near 1550 nm reach 0.70807 and 0.99695 nm/m^?1, respectively.     

Residual stress distributions in laser and gas-metal-arc welded high-strength steel plates

S700 is a high-strength steel recently developed by Tata Steel. This paper describes the residual stress distributions in a multi-pass gas-metal-arc weld (GMAW), a single pass autogenous laser weld (ALW) and a multi-pass ultra-narrow gap laser weld (NGLW) in 13?mm thick high-strength (S700) steel plates, as measured by X-ray diffraction and the contour method. The relationships between microstructural variation and residual stress distributions were investigated. It was found that solid-sate phase transformations from austenite to ferrite, bainite and martensite changed both the magnitude and distribution of residual stresses in the three different welded specimens. The width of the regions sustaining tensile stress in each specimen decreased in the following order: GMAW?>?ALW?>?ultra-NGLW.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

Mechanical properties of 304L stainless steel SMAW joints under dynamic impact loading

The impact properties of 304L Stainless Steel Shielded Metal Arc Welded (SMAW) joints are studied at strain rates between 10^{− 3} and 7.5× 10³ s^{− 1} using a compressive split-Hopkinson bar. The effects of strain rate on the flow response and fracture characteristics are fully evaluated. The results show that the tested weldments exhibit a pronounced strain rate sensitivity, and that changes in the strain rate result in a difference in the flow stress, fracture strain, and work hardening rate. Furthermore, it is noted that the strain rate sensitivity and activation volume vary with the magnitude of the strain rate, and are related to different work hardening stress levels. At all values of strain rate, the tested weldments fail as a result of adiabatic shearing, in which cracks initiate within the shear band and then propagate along this shear band until failure occurs. Observation of the fractured specimens reveals that the fracture surfaces of the fusion zone and base metal regions are characterized by the presence of elongated dimples. The variation in the observed dimple features with strain rate is consistent with the results of the impact stress-strain curves.     

A computational investigation of adiabatic shear plugging based on thermo-viscoplastic instability

Summary The adiabatic shear plugging of a titanium alloy target impacted by a steel projectile is numerically studied by a recently developed dynamic Lagrangian finite element code ETVP. The elastic-plastic wave propagation effect in the long projectile is taken into account. The rate-dependent behaviour of the titanium alloy target is also taken into consideration by using a thermo-viscoplastic constitutive equation. Moreover, the adiabatic shear failure criterion with two control variables (both strain and strain rate) based on thermo-viscoplastic constitutive instability of materials is generalized to three-dimensional stress states and incorporated into the code. The results clearly show the developing process of adiabatic shear plugging of the target including the initiation of the localization, the widening and deepening of adiabatic shearing zone, and finally the forming of a plug. The numerical results for the residual velocities are found to be in good agreement with those measured experimentally. The corresponding energy relation in the plugging process is then analyzed from this new plugging model.     

Study based on the analysis of valence electron structure for adiabatic shear sensitivity of 30CrMnMo and C45E4 steel in process of high‐speed penetration

In this paper, the influence of valence electron structure parameters on the adiabatic shearing sensitivity has been studied for two grade steels (30CrMnMo and C45E4) as a function of covalent pair number and lattice electron number by empirical electron theory of solids and molecules in high‐speed penetration process. The research shows that the adiabatic shearing sensitivity increases with the increase of the covalent electron pair number n_A, decreases with the increase of the lattice electron number n_l. The n_A of each structure unit in 30CrMnMo steel is larger than that in C45E4steel, the n_l of each structure unit in 30CrMnMo steel is smaller than that in C45E4steel. Therefore, the adiabatic shearing sensitivity is higher for 30CrMnMo targets damaged by adiabatic shear failure, and yet C45E4 targets were damaged by ductile fracture without any adiabatic shear band. The basis can be provided for appropriately selecting and designing materials with different adiabatic shearing sensitivity by studying the specific alloy elements on the influence of adiabatic shearing sensitivity in valence electron structure level.     

Adiabatic Shear in annealed and shock-hardened iron and in quenched and tempered 4340 steel

Adiabatic shear localization is a catastrophic failure mechanism which can occur in ductile metals under high strain rate loading. This mechanism is driven by a thermal instability process in which rapid temperature rise due to plastic work couples with thermal softening to cause uniform deformation to collapse into narrow bands of intense shear within which material ductility is exhausted. Adiabatic shear localization is studied in three ferrous metals: annealed Armco and as-received Remco iron, both of which are high purity alpha iron; shock-hardened Remco iron; and 4340 steel quenched and tempered to a range of hardness levels. Using a compressive split-Hopkinson bar, punching-shear experiments were performed at room and elevated initial temperatures at shear strain rates of up to 45000 s^–1. Optical and scanning electron microscopy was performed on the deformed shear specimens to determine the extent of shear localization and mode of failure. Experimental evidence showed that the tempered 4340 steels were susceptible to localization through adiabatic shear banding; however, as-received and shock-hardened Remco iron and annealed Armco iron were not. Finite element simulations of the experiments were performed utilizing a user material subroutine developed as part of this research. This constitutive routine incorporates two adiabatic shear failure criteria, namely (i) maximum shear stress with a minimum critical shear strain rate and (ii) flow localization. These criteria proved to be capable of predicting the onset of an instability; however, the deformation which follows the instability was not predicted well.     

Initiation and growth of microfractures along adiabatic shear bands in Ti–6AI–4V

Abstract

An experimental study has been made of the manner in which microfractures initiate and grow along adiabatic shear bands formed in the titanium alloy Ti–6AI–4V by the normal impact of hard steel spheres at velocities up to 340 m s^?1. It is suggested that a critical shear strain must be exceeded along the shear bands for microvoids to nucleate, or to cause significant local thermal softening in the bands, leading to the formation of single voids or arrays of voids and smooth-sided cracks when the stress state became predominantly tensile. The final shape of the micro fractures within the shear bands and the morphology of the resultant fracture surfaces are explained in terms of the density of void nucleation sites and the tensile-stress state across the shear bands.

MST/179     

AISI D2钢力学性能尺寸效应实验研究

为研究AISI D2钢力学性能尺寸效应现象,在常温下采用电子万能试验机和分离式霍普金森压杆(SHPB)实验装置对3种不同剪切带宽度(分别为800,400,50μm)的帽形试样进行了准静态和动态加载实验.实验结果表明,流动应力和失效应变随着剪切带宽度的减小而增大,但产生流动应力和失效应变尺寸效应现象的剪切带宽度不同.基于应变率强化项修正的Johnson-Cook本构模型,通过实验数据拟合得到材料的本构关系.研究表明,修正的Johnson-Cook本构模型与实验结果吻合较好.