Static and dynamic compression strength of hot-pressed boron carbide using a dumbbell-shaped specimen

Dumbbell-shaped specimens were machined from a tile of a commercially available boron carbide and subjected to static and dynamic compressive loads. Static experiments were performed using a screw-driven load frame, and the fracture process was recorded with a high-speed camera. Dynamic experiments were performed using a split-Hopkinson pressure bar and an ultra-high-speed camera to record the fracture process. The average static (~10^?1 s^?1) compressive strength of this boron carbide was determined to be 6.1 ± 0.3 GPa, and the dynamic compression strength, up to a strain rate of 300 s^?1, was 6.2 ± 0.3 GPa. These strength values are twice the value reported by the manufacturer and approximately 30% higher than the values reported by others for the same boron carbide but when using a cuboidal specimen geometry.     

Fatigue life estimation of notched specimens of modified 9Cr-1Mo steel under uniaxial cyclic loading

Accuracy in the estimation of low cycle fatigue life of modified 9Cr-1Mo steel notched specimen by different analytical methods such as linear rule, Neuber’s rule, strain energy density method and numerical method such as finite element analysis have been studied in this investigation. The fatigue tests on notched specimens having notch radius of 1.25 mm, 2.5 mm and 5.0 mm were carried out at 823 K with net stress amplitudes of 250 MPa, 300 MPa and 350 MPa. The fatigue tests on smooth specimens were carried out with strain amplitudes ranging from ±0.3% to ±0.8% with a strain rate of 3 × 10^?3 s^?1 at 823 K to evaluate the fatigue life of notched specimen through strain-life approach. In order to predict the cyclic stress response of the material, Chaboche non-linear hardening model was employed considering two back stress components. Predicted hysteresis loops for smooth specimen were well in agreement with experimental results. Estimated fatigue lives of notched specimens by analytical methods and finite element analysis were within a factor ±16 and ±2.5 of the experimental lives respectively.     

Effects of directional grain structure on impact properties and dislocation substructure of 6061-T6 aluminium alloy

Abstract

The effects of the grain structure direction on the impact properties and dislocation substructure of 6061-T6 aluminium alloy are investigated under room temperature conditions and strain rates of 1×10³, 3×10³ and 5×10³ s^?1 using a split-Hopkinson pressure bar system. The impact tests are performed using specimens machined from rolled 6061-T6 plates in the longitudinal, transverse and through thickness directions respectively. The results show that for all specimens, the flow stress increases with increasing strain rate. Furthermore, for all strain rates, the highest flow stress occurs in the transverse specimen. For strain rates of 1×10³ and 3×10³ s^?1, the flow stress in the through thickness specimen is greater than that in the longitudinal specimen. However, at a strain rate of 5×10³ s^?1, the flow stress in the longitudinal specimen is higher than that in the through thickness specimen due to a greater dislocation multiplication rate. For all three grain structure directions, the strain rate sensitivity increases with increasing strain rate, but decreases with increasing true strain. The highest strain rate sensitivity is observed in the longitudinal specimen at strain rates of 3×10³ to 5×10³ s^?1. The dislocation density increases markedly with increasing strain rate. Moreover, the square root of the dislocation density varies as a linear function of the flow stress in accordance with the Bailey–Hirsch relationship. The strengthening effect produced by the increased dislocation density is particularly evident in the transverse specimen, followed by the longitudinal specimen and the through thickness specimen.     

Effect of aluminum on microstructural evolution during hot deformation of TX32 magnesium alloy

The effect of Al (0.4 and 1 wt%) addition on the hot deformation behavior of the Mg–3Sn–2Ca (TX32) alloy has been studied with the help of processing maps generated in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1. The deformed specimens have been examined as regards changes in texture and microstructure using electron back scatter diffraction and transmission electron microscopy, respectively. The map for the TX32 base alloy exhibited two dynamic recrystallization (DRX) domains in the temperature and strain rate ranges: (1) 300–350 °C and 0.0003–0.001 s^?1, and (2) 390–500 °C and 0.005–0.6 s^?1. While 0.4 wt% Al addition to TX32 did not result in any significant change in the processing map, the map for the alloy with 1 wt% Al (TX32-1Al) exhibited four domains in the ranges: (1) 300–325 °C and 0.0003–0.001 s^?1, (2) 325–430 °C and 0.001–0.04 s^?1, (3) 430–500 °C and 0.01–0.5 s^?1, and (4) 430–500 °C and 0.0003–0.002 s^?1. In the first three domains, DRX has occurred, whereas in the fourth domain, grain boundary sliding takes place causing intercrystalline cracking in tension. In Domain 1 for all the alloys, DRX has occurred predominantly by basal slip and recovery by climb as confirmed by the resulting basal texture and tilt type sub-boundary structure. In Domain 2 of the base alloy and Domain 3 of the alloy with 1 wt% Al, second-order pyramidal slip dominates associated with cross-slip which randomizes the texture, and forms tangled dislocations and twist type sub-boundaries in the microstructure. The addition of 1 wt% Al causes solid solution strengthening and results in Domain 2 of the map of TX32-1Al alloy and in this domain basal+prismatic slip dominate.     

Hot workability analysis with processing map and texture characteristics of as-cast TX32 magnesium alloy

Hot deformation behavior of as-cast TX32 (Mg–3Sn–2Ca) alloy has been studied in uniaxial compression in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1 with a view to characterize the evolution of microstructure and texture. On the basis of the temperature and strain rate dependence of flow stress, a processing map has been developed and the crystallographic orientation information on the deformed specimens has been obtained from electron back scatter diffraction micro-texture analysis. The processing map revealed two domains of dynamic recrystallization in the temperature and strain rate ranges of (1) 300–350 °C and 0.0003–0.001 s^?1 and (2) 390–500 °C and 0.005–0.6 s^?1. Specimens deformed at peak in Domain 1 exhibited maximum intensity of basal poles located at about 35–45° to the compression axis while those deformed at peak in Domain 2 showed near-random texture. Schmid factor analysis of different slip systems operating in the two domains suggests that basal + prismatic slip causes the basal texture in Domain 1 while second-order pyramidal slip randomizes the texture in Domain 2.     

Deformation behavior and constitutive model for high temperature compression of a newly type of Ni3Al‐based superalloy

In order to investigate the hot deformation mechanism of a newly development Ni₃Al‐based superalloy, hot compression tests at temperatures between 1100 °C–1250 °C and the strain rates of 0.001 s^?1–1.0 s^?1 were conducted. The results show that the curves of true stress‐strain indicate the thermal deformation is a typical dynamic recrystallization process, which the peak stresses and steady‐state stresses increase with decreasing temperatures and increasing strain rates. The softening mechanism is mainly dynamic recrystallization. The experimental data of peak stresses and steady‐state stresses is employed to calculate the constants in the Arrhenius equation. The steady‐state stresses are considered more reasonable for solving the parameters in the Arrhenius equation. Based on the constitutive equation obtained, the calculated values of steady‐state stresses match well with the experimental values at the strain rates of 0.001 s^?1, 0.01 s^?1 and 0.1 s^?1, whereas there exists much deviation at 1.0 s^?1. For the sake of accuracy of predicted results at 1.0 s^?1 strain rate, a modified Zener‐Hollomon parameter Z’ is introduced. The results show that the modified constitutive equations established in this study could well predict the value of steady‐state stress in hot deformation of the newly development Ni₃Al‐based alloy.     

Photothermal Beam Deflection Spectroscopy for the Determination of Thermal Diffusivity of Soils and Soil Aggregates

Photothermal beam deflection spectroscopy (BDS) with a red He–Ne laser (632.8 nm, 35 mW) as an excitation beam source and a green He–Ne laser (543.1 nm, 2 mW) as a probe was used for estimating thermal diffusivity of several types of soil samples and individual soil aggregates with small surfaces (2?×?2 mm). It is shown that BDS can be used on demand for studies of changes in properties of soil entities of different hierarchical levels under the action of agrogenesis. It is presented that BDS clearly distinguishes between thermal diffusivities of different soil types: Sod-podzolic [Umbric Albeluvisols, Abruptic], 29?±?3; Chernozem typical [Voronic Chernozems, Pachic], 9.9?±?0.9; and Light Chestnut [Haplic Kastanozems, Chromic], 9.7?±?0.9 cm²·h^?1. Aggregates of chernozem soil show a significantly higher thermal diffusivity compared to the bulk soil. Thermal diffusivities of aggregates of Chernozem for virgin and bare fallow samples differ, 53?±?4 cm²·h^?1 and 45?±?4 cm²·h^?1, respectively. Micromonoliths of different Sod-podzolic soil horizons within the same profile (topsoil, depth 10–14 cm, and a parent rock with Fe illuviation, depth 180–185 cm) also show a significant difference, thermal diffusivities are 9.5?±?0.8 cm²·h^?1 and 27?±?2 cm²·h^?1, respectively. For soil micromonoliths, BDS is capable to distinguish the difference in thermal diffusivity resulting from the changes in the structure of aggregates.     

High Strain Rate Compressive Tests on Wood

Abstract: The penetrating split Hopkinson pressure bar was used to study the response of dry maple wood under high strain rate impact load. Using longer bar and shorter specimens utilised the assumption of one‐dimensional stress waves travelling along the bars and specimen because the experiment fulfilled the ratio of diameter to length of bars condition in Kolsky bar experiments. The stress–strain relationships and behaviour of the fibre structure materials’ failure were investigated during the compressive dynamic tests at strain rates between 950¹ and 2000 s^?1. The mechanics of dynamic failure was studied and it was confirmed that deformation of specimen is a linear function of energy absorption by specimens.     

Rate-dependent low cycle fatigue and ratcheting of 25Cr2MoVA steel under cyclic pulsating tension

Low cycle fatigue and ratcheting deformation of 25Cr2MoVA steel under cyclic tension were tested at 550°C. The effects of stress rates for 0·1, 0·5, 2·5, 5, 10, and 40 MPa s^?1 on ratcheting effect and fracture appearance were discussed systematically. Results indicate that the steady static creep rate very approaches to the ratcheting strain rates for 0·1 MPa s^?1, but it is obviously greater than those of greater stress rates tested. Moreover, the steady ratcheting strain rate reduces rapidly with increasing the frequency when the frequency is less than 36·8 h^?1, and then changes slightly for the greater frequency. This implies the steady ratcheting rate of 25Cr2MoVA is approximately rate-independent when the stress rate is greater than 10 MPa s^?1. Additionally, there are some microvoids in the central fibrous regions of specimens under static creep and cyclic tension with 0·1, 0·5, and 5 MPa s^?1, but very few microvoids for the specimen under cyclic tension with 10 MPa s^?1 can be observed. Furthermore, the low cycle fatigue life is relatively short when the stress rate is less than 10 MPa s^?1, but it enhances exponentially subsequently.     

The Hot Workability of SiCp/2024 Al Composite by Stir Casting

The hot workability of SiC_p/2024 Al composite was explored by conducting hot compression simulation experiments on Gleeble-3500 under temperatures of 300–500 °C and strain rates of 10^?3–1 s^?1. Constitutive equation was developed through hyperbolic sine function, and the activation energy was calculated to be 151 kJ mol^?1. The hot processing maps referring to dynamic material model were drawn in a true strain range from ?0.2 to ?0.8. At the strain of ?0.8, the recommended regions in processing map contained two domains: superplastic domain (500 °C, 10^?3 s^?1) with an efficiency of about 0.72 and DRX domain (500°C, 1 s^?1) with an efficiency of about 0.45. Together with macrostructure and microstructure observations, it was suggested to remove the DRX region.     

Possible ways of enhancing high temperature ductility in CuO doped cubic zirconia (8Y–CSZ)

Abstract

The effect of initial density and rapid prestraining on superplastic ductility of 1 wt-%CuO doped cubic zirconia (8Y–CSZ) was investigated. To obtain a range of initial densities, the tensile test specimens were slip cast to net shape and pressureless sintered over a range of temperatures in air. The specimens were then superplastically tested at a temperature of 1500 K and at a constant strain rate of 1×10^-4 s^-1. The results showed that specimens with low initial densities had lower flow stresses and higher superplastic elongations to failure than higher density specimens. The reasons for the ductility change were discussed with reference to the presence of porosity and grain growth. For the prestraining test, a specimen with an initial density of 95% was prestrained to 30% at a temperature of 1550 K and at a prestrain rate ? · ₁ of 1×10^-3 s^-1, followed by elongation to failure at a slower test strain rate ? · ₂ of 1×10^-4 s^-1. It was seen that prestraining at the above test conditions considerably improved superplastic ductility. The reasons for this ductility enhancement were explained in terms of suppression of grain growth.     

Effect of notch root radius on tensile behaviour of 316L(N) stainless steel

Abstract

Type 316L(N) stainless steel (SS) is used as the major structural material for high temperature components of sodium cooled fast reactors. The influence of notch root radius on the tensile behaviour of 316L(N) SS under multi-axial stress state was investigated. Double U-notches with five different kinds of notch geometry were incorporated symmetrically into the tensile testing specimens by changing the notch root radius while keeping the gross diameter, net diameter and notch depth as the same for all the notches. The notch root radius was varied as 0·25, 0·5, 1·25, 2·5 and 5 mm. Tensile tests were carried out on the notched specimens at room temperature (298 K) and at 923 K at a constant strain rate of 3×10^?3 s^?1. The tensile strength and yield strength of notched specimen of 316L(N) SS increased with decrease in notch radius at both the temperatures and the notch severity was less pronounced at high temperature. The fractured notch surface was analysed using scanning electron microscope and unfractured notch was sliced along the axis and observed under optical microscope. Finite element analysis was performed on the models of notched specimens with various notch root radii. These results showed that Von Mises equivalent stress which was derived from triaxial stresses decreased with decrease in notch radius. The shift of location of peak values of maximum principal stress and hydrostatic stress towards the axis of the specimen, leading to formation of cracks, occurred at a lower nominal stress when the notch radius was increased.     

Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

Characterization of High‐Strain Rate Mechanical Behavior of AZ31 Magnesium Alloy Using 3D Digital Image Correlation

Characterization of the material mechanical behavior at sub‐Hopkinson regime (0.1 to 1 000 s^?1) is very challenging due to instrumentation limitations and the complexity of data analysis involved in dynamic loading. In this study, AZ31 magnesium alloy sheet specimens are tested using a custom designed servo‐hydraulic machine in tension at nominal strain rates up to 1 000 s^?1. In order to resolve strain measurement artifacts, the specimen displacement is measured using 3D Digital Image correlation instead from actuator motion. The total strain is measured up to ≈ 30%, which is far beyond the measurable range of electric resistance strain gages. Stresses are calculated based on the elastic strains in the tab of a standard dog‐bone shaped specimen. Using this technique, the stresses measured for strain rates of 100 s^?1 and lower show little or no noise comparing to load cell signals. When the strain rates are higher than 250 s^?1, the noises and oscillations in the stress measurements are significantly decreased from ≈ 250 to 50 MPa. Overall, it is found that there are no significant differences in the elongation, although the material exhibits slight work hardening when the strain rate is increased from 1 to 100 s^?1.     

The thermal expansion coefficient as a key design parameter for thermoelectric materials and its relationship to processing-dependent bloating

The coefficient of thermal expansion (CTE) is a key design parameter for thermoelectric (TE) materials, especially in energy harvesting applications since stresses generated by CTE mismatch, thermal gradients, and thermal transients scale with the CTE of the TE material. For the PbTe–PbS-based TE material (Pb_0.95Sn_0.05Te)_0.92(PbS)_0.08—0.055 % PbI₂ over the temperature ranges of 293–543 and 293–773 K, a CTE, α_avg, of 21.4 ± 0.3 × 10^?6 K^?1 was measured using (1) dilatometry and (2) high-temperature X-ray diffraction (HT-XRD) for powder and bulk specimens. The CTE values measured via dilatometry and HT-XRD are similar to the literature values for other Pb-based chalcogenides. However, the processing technique was found to impact the thermal expansion such that bloating (which leads to a hysteresis in thermal expansion) occurred for hot pressed billets heated to temperatures >603 K while specimens fabricated by pulsed electric current sintering and as-cast specimens did not show a bloating-modified thermal expansion even for temperatures up to 663 K. The relationship of bloating to the processing techniques is discussed, along with a possible mechanism for inhibiting bloating in powder processed specimens.     

Study on mechanical properties and constitutive model of 5052 aluminium alloy

The stress–strain relationship of 5052 aluminium alloy was investigated via quasi-static tensile tests and split Hopkinson pressure bar tests. The specimens were exposed to various temperatures (25–500°C) and strain rates (10^?4–0.7?×?10^4?s^?1). At strain rates ranging from 0.001 to 3000?s^?1, the material underwent significant work hardening. When the strain rate exceeded 5000?s^?1, the work hardening effect decreased and the flow stress was relatively constant. The Johnson–Cook constitutive model was modified to describe the deformation behaviour of the material subjected to high temperatures and strain rates. The accuracy of the modified model was verified through ballistic impact testing.     

Compression stress–strain and creep properties of the 52In–48Sn and 97In–3Ag low-temperature Pb-free solders

Lead (Pb)-free, low melting temperature solders are required for step-soldering processes used to assemble micro-electrical mechanical system (MEMS) and optoelectronic (OE) devices. Stress–strain and creep studies, which provide solder mechanical properties for unified creep-plasticity (UCP) predictive models, were performed on the Pb-free 97In–3Ag (wt.%) and 58In–42Sn solders and counterpart Pb-bearing 80In–15Pb–5Ag and 70In–15Sn–9.6Pb–5.4Cd alloys. Stress–strain tests were performed at 4.4 × 10^?5 s^?1 and 8.8 × 10^?4 s^?1. Stress–strain and creep tests were performed at ?25, 25, 75, and 100°C or 125°C. The samples were evaluated in the as-fabricated and post-annealed conditions. The In–Ag solder had yield stress values of 0.5–8.5 MPa. The values of ΔH for steady-state creep were 99 ± 14 kJ/mol and 46 ± 11 kJ/mol, indicating that bulk diffusion controlled creep in the as-fabricated samples (former) and fast-diffusion controlled creep in the annealed samples (latter). The In–Sn yield stresses were 1.0–22 MPa and were not dependent on an annealed condition. The steady-state creep ΔH values were 55 ± 11 kJ/mol and 48 ± 13 kJ/mol for the as-fabricated and annealed samples, respectively, indicating the fast-diffusion controlled creep for the two conditions. The UCP constitutive models were derived for the In–Ag solder in the as-fabricated and annealed conditions.     

Effect of strain rate and saturation on uniaxial dynamic compressive behaviours of mortar

The effect of moisture content on the compressive mechanical behaviours of cement mortar under different high strain rates is studied in this paper. The rapid impact testing, i.e. the strain rates of 80, 100, 150, 200 and 250 s^? 1 by Split Hopkinson pressure bar, on number of specimens with special water/cement ratio of 0.50 and saturations as 0%, 25%, 50%, 75% and 100%, respectively, was executed. The dynamic compressive behaviours were analysed in terms of the maximum stresses, elastic modulus, critical strain at maximum stresses and ultimate strains at failure. Results indicated that similarity existed in the shape of strain–stress curves of mortars with different moisture subjected to different strain rates of impact loading, i.e. the upward section presented bilinear characteristics, while the descending stage was almost linear. As strain rate increases, the dynamic compressive strength, elastic modulus and critical strain at maximum stress increase which can be ascribed to the dynamic fracture effect and the microscope inertia effect. Besides, it was shown that desiccation provokes an increase in mortar strength and deformation behaviour of the studied mortar with different saturation caused by capillary depression and microcracking. Drying effect has to be considered in modelling of the coupling between desiccation and mechanical behaviour of the mortar. Finally, the multi-parametric statistical analysis of water content and strain rate on the mechanical behaviours of cement mortar subjected to dynamic loading is detailed.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.