The Microstructure Evolution of Dual-Phase Pipeline Steel with Plastic Deformation at Different Strain Rates

Tensile properties of the high-deformability dual-phase ferrite-bainite X70 pipeline steel have been investigated at room temperature under the strain rates of 2.5 × 10^?5, 1.25 × 10^?4, 2.5 × 10^?3, and 1.25 × 10^?2 s^?1. The microstructures at different amount of plastic deformation were examined by using scanning and transmission electron microscopy. Generally, the ductility of typical body-centered cubic steels is reduced when its stain rate increases. However, we observed a different ductility dependence on strain rates in the dual-phase X70 pipeline steel. The uniform elongation (UEL%) and elongation to fracture (EL%) at the strain rate of 2.5 × 10^?3 s^?1 increase about 54 and 74%, respectively, compared to those at 2.5 × 10^?5 s^?1. The UEL% and EL% reach to their maximum at the strain rate of 2.5 × 10^?3 s^?1. This phenomenon was explained by the observed grain structures and dislocation configurations. Whether or not the ductility can be enhanced with increasing strain rates depends on the competition between the homogenization of plastic deformation among the microconstituents (ultra-fine ferrite grains, relatively coarse ferrite grains as well as bainite) and the progress of cracks formed as a consequence of localized inconsistent plastic deformation.     

Concentration dependence of Cr for alleviating environmental embrittlement in Fe30Ni20Mn35Al15

Previously, it has been shown that Fe₃₀Ni₂₀Mn₃₅Al_15, which consists of alternating submicron B2 and f.c.c. lamellae, exhibits a room temperature yield strength of 770 MPa and an elongation of ∼10% at a strain rate of 3 × 10⁻³ s⁻¹ in air, but at the slower strain rate of 3 × 10⁻⁶ s⁻¹ the alloy exhibits an elongation <1% [1]. An addition of 6 at% Cr has been proven to not only solve this environmental problem, but also to increase the elongation to 15–18% irrespective of strain rate [2]. Since we do not know whether Cr additions less than 6 at% can suppress this environmental embrittlement, in this paper we examined the room temperature mechanical properties of several alloys based on Fe₃₀Ni₂₀Mn₃₅Al₁₅ with Cr additions ≤6 at%. We show that additions as low as 0.5 at% Cr alleviate the environmental embrittlement and that additions of ≥2 at% completely suppress the embrittlement with little change in microstructure. X-ray photoelectron spectroscopy examination suggested that the suppression is mostly due to the formation of protective oxide scales on the surface that provide rapid passivation. The lower yield strength when Cr is present may also contribute to the improved ductility, possibly by easing dislocation cross-slip in the deforming f.c.c. phase where most of the Cr resides.     

On the Change in Work Hardening Characteristics of Molybdenum Polycrystals Due to Natural Aging

Strips of 99.95 at.% Mo polycrystals annealed at 700 °C as well as the ones annealed and then aged for 6 months at room temperature were deformed in tension at various strain-rates in the range 2.1 × 10⁻⁴ to 4.2 × 10⁻³ s⁻¹ till fracture. It is found that natural aging of the annealed specimens for 6 months leads to 20-30% reduction in the yield stress (YS), 18-22% reduction in the ultimate tensile strength (UTS), and 72-76% reduction in the ductility, i.e. the tensile strain ε_max corresponding to UTS, depending on the value of [\upvarepsilon \dot] \dot{\upvarepsilon } in the tensile strain-rate range referred to. Data analysis in terms of the kink-pair nucleation model of flow stress shows that the reduction in YS of the aged Mo specimens is a consequence of lowering of the Peierls energy per interatomic spacing along the length of screw-dislocation segments trapped in the Peierls valleys on the migration of point defects to the dislocation cores during the course of natural aging. The reduction in UTS and ε_max is attributed to the variation in the relative contribution of the processes of dislocation multiplication and annihilation together with the reduction in the Peierls stress of the metal.     

Superplasticity in cast A356 induced via friction stir processing

Superplasticity was investigated in friction stir processed A356 alloy at temperatures of 470–570 °C and initial strain rates of 3 × 10⁻⁴–1 × 10⁻¹ s⁻¹. Maximum superplastic elongation of 650% was obtained at 530 °C and an initial strain rate of 1 × 10⁻³ s⁻¹ where a maximum strain rate sensitivity of 0.45 was observed.     

High temperature deformation behavior of Mg-Sn(-Zn) alloy

In the present study, we have investigated the high temperature deformation behavior of Mg-Sn(-Zn) based alloy systems in comparison with that of Mg-Al alloy. Compared with Mg-Al alloy, Mg-Sn alloy exhibits significantly refined grain structure and high ductility due to the presence of fine Mg₂Sn particles in the α-Mg matrix, for example, 184 % at 350 °C under a strain rate of 1 × 10⁻³ s⁻¹. When Zn is added to the Mg-Sn alloy, the elongation to failure remarkably increases from 184 % (Mg-Sn alloy) to 310 % under a strain rate of 1 × 10⁻³ s⁻¹. Such an improvement in ductility is due to the significantly refined grain structure that results from the addition of Zn.     

Hot deformation behavior of TiAl alloys prepared by blended elemental powders

A nearly full dense Ti-45Al-7Nb-0.4W (at.%) alloy billet with dimension of 120 mm in diameter and 50 mm in height was fabricated by reactive sintering of blended elemental powders. The high temperature deformation behavior was investigated by isothermal compressive tests, performed at temperature in 1000–1200 °C with strain rates from 1 × 10^?3 s^?1 to 1 × 10^?1 s^?1. Results indicate that the dependence of flow stress on temperature and strain rate is well fit for a hyperbolic-sine relationship using the Zener–Hollomon parameter. The measured apparent activation energy Q and stress exponent are determined as 420 kJ mol^?1 and 3.7, respectively. High oxygen content, high Nb content and fine grain size are main reasons for the high activation energy and high strength of PM TiAl alloy. An appropriate set of deformation processing parameters of 1200 °C and 1 × 10^?3 s^?1 are recommended for the present TiAl alloy.     

AZ41镁合金超塑性成形特性

研究AZ41镁合金在热轧(无后续热变形)条件下的显微组织变化,以确定其在超塑性成形工艺中的适用性,并确定最佳成形参数.采用高温拉伸试验和热气体胀形试验对材料在不同应变速率(1×10?1～1×10?3 s?1)和温度(350～450℃)下的成形性进行评估.利用GOM Aramis相机进行圆形网格分析,了解峰值应变和材料减...     

Strain rate sensitivity of a high strain rate superplastic TiNp/2014 Al composite

The effects of deformation temperature and strain in hot rolling deformation on strain rate sensitivity of the TiN_p/2014 Al composite were studied by tensile tests conducted out at 773, 798, 818 and 838 K with the strain rates from 1.7 ×10^?3 to 1.7 × 10⁰ s^?1. It is shown that the curves of m value of the TiN_p/2014Al composite deformed at different temperatures can be divided into two stages with the variation of strain rate, and the critical strain rates are 10^?1 s^?1. The optimum deformation temperature of the TiN_p/2014 Al composite is near incipient melting temperature of 816 K and the optimum strain rate is a little higher than the critical strain rate. The effect of deformation temperature on strain rate sensitivity is relative to liquid phase helper accommodation. The effect of strain in hot rolling deformation on strain rate sensitivity attributes to change of microstructure and deformation mechanism.     

Superplastic Behavior of Al-4.5Mg-0.46Mn-0.44Sc Alloy Sheet Produced by a Conventional Rolling Process

This article describes the superplastic behavior of the Al-4.5Mg-0.46Mn-0.44Sc alloy. The investigated alloy was produced by casting and was conventionally processed to form a sheet with a thickness of 1.9 mm and an average grain size of 11 μm. The superplastic properties of the alloy were investigated using a uniaxial tensile testing with a constant cross-head speed and with a constant strain rate in the range 1 × 10⁻⁴ to 5 × 10⁻² s⁻¹ at temperatures from 390 to 550 °C. The investigations included determinations of the true-stress, true-strain characteristics, the maximum elongations to failure, the strain-rate sensitivity index m, and the microstructure of the alloy. The m-values determined with the strain-rate jump test varied from 0.35 to 0.70 in the temperature interval from 390 to 550°C and strain rates up to 2 × 10⁻² s⁻¹. The m-values decreased with increased strain during pulling. The elongations to failure were in accordance with the m-values. They increased with the temperature and were over 1000%, up to 1 × 10⁻³ s⁻¹ at 480 °C and up to 1 × 10⁻² s⁻¹ at 550 °C. A maximum elongation of 1969% was achieved at an initial strain rate of 5 × 10⁻³ s⁻¹ and 550 °C. The results show that the addition of about 0.4 wt.% of Sc to the standard Al-Mg-Mn alloy, fabricated by a conventional manufacturing route, including hot and cold rolling with subsequent recrystallization annealing, results in good superplastic ductility.     

High temperature deformation and processing map of a NiTi intermetallic alloy

The deformation behavior of a 49.8 Ni-50.2 Ti (at pct) alloy was investigated using the hot compression test in the temperature range of 700 °C–1100 °C, and strain rate of 0.001 s^?1 to 1 s^?1. The hot tensile test of the alloy was also considered to assist explaining the related deformation mechanism within the same temperature range and the strain rate of 0.1 s^?1. The processing map of the alloy was developed to evaluate the efficiency of hot deformation and to identify the instability regions of the flow. The peak efficiency of 24–28% was achieved at temperature range of 900 °C–1000 °C, and strain rates higher than 0.01 s^?1 in the processing map. The hot ductility and the deformation efficiency of the alloy exhibit almost similar variation with temperature, showing maximum at temperature range of 900 °C–1000 °C and minimum at 700 °C and 1100 °C. Besides, the minimum hot ductility lies in the instability regions of the processing map. The peak efficiency of 28% and microstructural analysis suggests that dynamic recovery (DRV) can occur during hot working of the alloy. At strain rates higher than 0.1 s^?1, the peak efficiency domain shifts from the temperature range of 850 °C–1000 °C to lower temperature range of 800 °C–950 °C which is desirable for hot working of the NiTi alloy. The regions of flow instability have been observed at high Z values and at low temperature of 700 °C and low strain rate of 0.001 s^?1. Further instability region has been found at temperature of 1000 °C and strain rates higher than 1 s^?1 and at temperature of 1100 °C and all range of strain rates.     

Tensile ductility of an Fe–40Al–0.6C alloy

Binary Fe–40Al and ternary Fe–40Al–0.6C alloys were cast, hot-extruded into rods, annealed at low temperatures to reduce the non-equilibrium vacancy concentration and tested in uniaxial tension at room temperature in air, over a range of strain rates from 4.2×10⁻¹ s⁻¹ to 4.2×10⁻⁸ s⁻¹. Yield strength, fracture strength, tensile ductility and the work-hardening behavior in the 0.2–1.0% plastic deformation range were monitored. Resulting fracture surfaces were examined at low and high magnifications, and the change in the fraction transgranular cleavage as a function of test strain rate was correlated with the observed mechanical properties. Prior to testing, both alloys exhibited fairly coarse grain size (∼80–100 μm); whereas the binary alloy was single phase, the ternary alloy contained a dispersion of lath-shaped perovskite carbides (Fe₃AlC_0.5) in the grain interior and at grain boundaries. In the binary alloy, ductility decreases continuously with decreasing strain rate and this behavior has been previously attributed to an environmental effect. For a given strain rate, over the range of strain rates examined, the ternary alloy demonstrates improved ductility over the binary alloy; furthermore, at the extremely slow strain rates (<4×10⁻⁷ s⁻¹), the ductility of the ternary alloy increases with decreasing strain rate after reaching a minimum. Whereas in the binary alloy, fracture mode remains completely intergranular over the entire strain rate regime, in the ternary alloy, fracture mode is completely intergranular at the fastest strain rate but gradually transitions to a predominantly transgranular cleavage mode with decreasing strain rate. A maximum in the fraction transgranular cleavage is reached coincident with the ductility minimum, beyond which (i.e. lower strain rates) the fraction transgranular cleavage decreases sharply. These observations are discussed in terms of the possible role of these carbides as hydrogen traps and their consequential effects on mechanical properties.     

Strain rate sensitivity of nanocrystalline Au films at room temperature

The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 μm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 × 10^?6 and 20 s^?1. The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575–895 MPa and 675–940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 10^?1 s^?1. The activation volumes for the two film thicknesses were 4.5 and 8.1 b³, at strain rates smaller than 10^?4 s^?1 and 12.5 and 14.6 b³ at strain rates higher than 10^?4 s^?1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 10^?4 s^?1. The latter trends indicated that the strain rate regime 10^?5–10^?4 s^?1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5–6 h) and significant primary creep with initial strain rate of the order of 10^?7 s^?1.     

The high-speed deformation behavior of TRIP steels

The high-speed deformation behavior of TRIP steel was investigated at strain rates ranging from 10⁻² s⁻¹ to 10³ s⁻¹. The effects of metallurgical factors, such as the rolling direction, thickness, and gage length, on the tensile properties at various strain rates were evaluated. The ultimate tensile strength, uniform elongation, strain rate sensitivity, absorbed energy, and strain-hardening exponent are reported. In general, the strength increases and the ductility decreases as the strain rate increases. The samples with a high amount of retained austenite had two distinct regions of strain rate sensitivity, showing high strain rate sensitivity over a strain rate of 10² s⁻¹. The tensile properties were not affected by the gage length and thickness of the tensile samples; however, the rolling direction of the tensile samples affected the UTS values slightly. The absorbed energy of the TRIP steel greatly exceeded that of HSLA steel.     

Strain rate-dependent compressive deformation behavior of Nd-based bulk metallic glass

Compressive deformation behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass was characterized over a wide strain rate range (6.0×10⁻⁴ to 1.0×10³ s⁻¹) at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate. Serrated flow and a large number of shear bands were observed at the quasi-static strain rate (6.0×10⁻⁴ s⁻¹). The results suggest that the appearance of a large number of shear bands is probably associated with flow serration observed during compression; and both shear banding and flow serration are a strain accommodation and stress relaxation process. At dynamic strain rates (1.0×10³ s⁻¹), the rate of shear band nucleation is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample. The fracture behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass is sensitive to strain rate.     

Superplastic deformation of a heat-resistant Al–Cu–Mg–Ag–Mn alloy

The superplastic behavior and deformation mechanism of a heat-resistant Al–Cu–Mg–Ag–Mn alloy prepared by ingot metallurgy was investigated by using optical microscopy, scanning electron microscopy and transmission electron microscopy. It is shown that the Al–Cu–Mg–Ag–Mn alloy shows good superplastic properties at temperatures higher than 450 °C and strain rates lower than 10^?2 s^?1. A maximum elongation-to-failure of 320% was observed at 500 °C and 5 × 10^?4 s^?1, where the corresponding strain rate sensitivity index m is 0.58 and the flow stress σ is 5.7 MPa. Microstructure studies revealed that the observed superplastic behavior resulted from severe grain elongation rather than grain boundary sliding. It is suggested that this phenomenon may provide a new concept for developing superplastic materials.     

Tensile behavior of a free-standing Pt-aluminide (PtAl) bond coat

The tensile behavior of a high activity stand-alone Pt-aluminide (PtAl) bond coat was evaluated by the micro-tensile test method at various temperatures (room temperature to 1100 °C) and strain rates (10^?5 s^?1–10^?1 s^?1). At all strain rates, the stress–strain behavior of the stand-alone coating was significantly affected by the variation in temperature. The stress–strain response was linear, indicating brittle behavior, at temperatures below the brittle–ductile transition temperature (BDTT). The coating exhibited appreciable ductility (up to 2%) above the BDTT. The strength (both yield stress and ultimate tensile strength) of the coating decreased and its ductility increased with increasing temperature above the BDTT. The tensile behavior of the coating was sensitive to strain rate in the ductile regime, with its strength increasing with increasing strain rate at any given temperature. The BDTT of the coating was found to increase with increasing with increasing strain rate. The coating exhibited two distinct mechanisms of deformation above the BDTT. The transition temperature for the change of deformation mechanism also increased with increasing strain rate.     

Microstructural evolution and grain boundary sliding in a superplastic magnesium AZ31 alloy

Tensile experiments on a fine-grained single-phase Mg–Zn–Al alloy (AZ31) at 673 K revealed superplastic behavior with an elongation to failure of 475% at 1 × 10^?4 s^?1 and non-superplastic behavior with an elongation to failure of 160% at 1 × 10^?2 s^?1; the corresponding strain rate sensitivities under these conditions were ～0.5 and ～0.2, respectively. Measurements indicated that the grain boundary sliding (GBS) contribution to strain ξ was ～30% under non-superplastic conditions; there was also a significant sharpening in texture during such deformation. Under superplastic conditions, ξ was ～50% at both low and high elongations of ～20% and 120%; the initial texture became more random under such conditions. In non-superplastic conditions, deformation occurred under steady-state conditions without grain growth before significant flow localization whereas, under superplastic conditions, there was grain growth during the early stages of deformation, leading to strain hardening. The grains retained equiaxed shapes under all experimental conditions. Superplastic deformation is attributed to GBS, while non-superplastic deformation is attributed to intragranular dislocation creep with some contribution from GBS. The retention of equiaxed grain shapes during dislocation creep is consistent with a model based on local recovery related to the disturbance of triple junctions.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

High-rate tensile properties of Si-reduced TRIP sheet steels

There have been efforts to develop Si-reduced TRIP steels to improve the wettability of Zn coatings, since the conventional CMnSi-TRIP steels suffer from poor galvanizability. In addition, for the development of potential applications of Si-reduced TRIP steels in vehicle crash management, a better understanding of high strain rate properties is required. In the present study, the effects of alloying elements, such as Cu, Al, Si, and P, on the high-rate tensile properties of Si-reduced TRIP sheet steels were investigated. Tensile tests were performed with a servo-hydraulic tensile testing machine at strain rates ranging from 10⁻² to 6 × 10² s⁻¹, and the ultimate tensile strength, elongation, strain rate sensitivity, and absorbed energy were evaluated. The retained austenite volume fractions and carbon content of the specimens were measured using neutron diffraction. The UTS was increased with Cu, Al, Si, and P alloying throughout the strain rate range, and the alloying effect on UTS was considerable with Cu and P. The effects of alloying on the microstructure were not significant. All the steels tested in this study exhibited positive strain rate sensitivity, and the m value at strain rates higher than 10 s⁻¹ was at least two times higher than that at lower strain rates.     

On the Stress corrosion cracking behaviour of aluminium alloy sheet in an aqueous solution of 3% NaCl + 0.3% H2O2

The stress corrosion cracking (SCC) behaviour of aluminium alloy sheet was investigated in the long transverse direction using the slow strain rate testing technique. The synthetic environment used was an aqueous solution of 3% NaCl + 0.3% H₂O₂. No indications of SCC sensitivity are observed for the alloys 2024-T351, 8090-T81, and 2091 CPHK-T8X. The alloys 2091 T8X and 6061-T4 are found to be susceptible to intergranular stress corrosion cracking. At strain rates below 4 · 10^?7 s^?1, the slow strain rate testing technique indicates a slight SCC sensitivity with alloy 6013-T6. Fractography reveals transgranular stress corrosion cracking. Transgranular stress corrosion cracking is also observed with 6061-T4 specimens which are dynamically strained at strain rates below 5 · 10^?7 s^?1. Aqueous 3% NaCl solution with hydrogen peroxide addition promotes pitting and intergranular corrosion. The loss of ductility caused by these corrosion processes interferes with the evaluation of the results of the slow strain rate testing technique.