Plastically deforming amorphous ZrO2-Al2O3

We report for the first time non-viscous, plastic deformation in an amorphous oxide: ZrO₂-Al₂O₃. Dense samples of amorphous ZrO₂-Al₂O₃ made by hot pressing spray-pyrolysed powder were deformed in uniaxial compression at 600–700°C at strain rates from 6×10⁻⁵ s⁻¹ to 10⁻³ s⁻¹. A transition from elastic to plastic deformation occurred at a critical stress ~360 MPa. The onset of plastic deformation was associated with a drop in the stress by 20–25%. Little influence of strain and strain rate on the flow stress was observed. The non-viscous, plastic deformation is related to the open structure of the amorphous phase as indicated by its low true density.     

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.     

Effect of atomic-level stresses on local dynamic and mechanical properties in CuxZr100−x metallic glasses: A molecular dynamics study

In this article, molecular dynamics simulations of Cu_xZr_100−x metallic glasses are performed to study the dynamic and mechanical behaviors of structural defects. Based on atomic-level stress theory, atomic strain energy and von Mises shear stress were correlated with the local dynamic/mechanical properties of MGs. Atoms with high atomic strain energy (n-type/p-type defects) are found to exhibit larger mean square displacements than solid-like atoms during structural relaxation process, indicating that these atoms have faster dynamics than solid-like entities. Additionally, simulated shear loading was conducted to investigate the relationship between von Mises shear stress and local shear properties in MGs. By monitoring the von Mises shear strain for each atom, γ-defects (atoms with excessive shear stress) are proven to be fertile regions for shear transformations.     

Effect of temperature,strain rate and grain size on the mechanical response of Ti3SiC2 in tension

Tensile, relaxation and cycling loading–unloading tests indicate that the mechanical response of Ti₃SiC₂ has a strong dependence on temperature and strain rate, but a weak dependence on grain size. Loading at low temperatures, and/or high strain rates, results in elastic and anelastic deformation, followed by brittle fracture. Anelastic deformation in this regime can be attributed to the easy glide of dislocation into pileups during loading, and their run back during unloading. At high temperatures (≈1100–1200°C), and/or low (<10⁻⁵ s⁻¹) strain rates, the response is plastic. The resulting strain is elastic, anelastic and plastic. Even at 1200°C, intense stress-relaxation processes are observed, and a sizable fraction (≈13%) of the strain is anelastic. At intermediate temperatures and strain rates (transition regime) the mechanical response is controlled by simultaneous damage formation (microcracking) and localized plastic deformation. Combining the results obtained in this work with previous results, viz. tensile creep and strain transient dip tests, a deformation map that takes into account temperature, grain size and strain rate is defined.     

Serration behavior and negative strain rate sensitivity of Al0.1CoCrFeNi high entropy alloy

Temperature dependent deformation mechanism of Al_0.1CoCrFeNi high entropy alloy (HEA) was studied using monotonic and strain rate jump tests at various test temperatures in a coarse-grained single phase FCC HEA. The tensile properties of Al_0.1CoCrFeNi HEA exhibited a modest temperature dependence in the tested range of 300–673 K. At an initial strain rate of 10⁻⁵ s⁻¹, the serration type was a function of the test temperature. Furthermore, the strain rate sensitivity of the flow stress changed from positive to zero to negative once the unstable plastic deformation region due to dynamic strain aging was attained.     

Hot deformation behaviour and microstructure control in AlCrCuNiFeCo high entropy alloy

An AlCrCuNiFeCo high entropy alloy (HEA), which has simple face centered cubic (FCC) and body centered cubic (BCC) solid solution phases as the microstructural constituents, was processed and its high temperature deformation behaviour was examined as a function of temperature (700–1030 °C) and strain rate (10⁻³–10⁻¹ s⁻¹), so as to identify the optimum thermo-mechanical processing (TMP) conditions for hot working of this alloy. For this purpose, power dissipation efficiency and deformation instability maps utilizing that the dynamic materials model pioneered by Prasad and co-workers have been generated and examined. Various deformation mechanisms, which operate in different temperature–strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate two distinct deformation domains within the range of experimental conditions examined, with the combination of 1000 °C/10⁻³ s⁻¹ and 1030 °C/10⁻² s⁻¹ being the optimum for hot working. Flow instabilities associated with adiabatic shear banding, or localized plastic flow, and or cracking were found for 700–730 °C/10⁻³–10⁻¹ s⁻¹ and 750–860 °C/10^−1.4–10⁻¹ s⁻¹ combinations. A constitutive equation that describes the flow stress of AlCrCuNiFeCo alloy as a function of strain rate and deformation temperature was also determined.     

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.     

Hot working behavior of near alpha titanium alloy analyzed by mechanical testing and processing map

The hot deformation characteristics of the Ti−5.7Al−2.1Sn−3.9Zr−2Mo−0.1Si (Ti-6242S) alloy with an acicular starting microstructure were analyzed using processing map. The uniaxial hot compression tests were performed at temperatures ranging from 850 to 1000 °C and at strain rates of 0.001−1 s⁻¹. The developed processing map was used to determine the safe and unsafe deformation conditions of the alloy in association with the microstructural evolution by SEM and OM. It was recognized that the flow stress revealed differences in flow softening behavior by deformation at 1000 °C compared to the lower deformation temperatures, which was attributed to microstructural changes. The processing map developed for typical strain of 0.7 in two-phase field exhibited high efficiency value of power dissipation of about 55% at 950 °C and 0.001 s⁻¹, basically due to extensive globularization. An increase in strain rate and a decrease in temperature resulted in a decrease in globularization of α lamellae, while α lamellar kinking increased. Eventually, the instability domain of flow behavior was identified in the temperature range of 850−900 °C and at the strain rate higher than 0.01 s⁻¹ reflecting the flow localization and adiabatic shear banding. By considering the power efficiency domains and the microstructural observations, the deformation in the temperature range of 950−1000 °C and strain rate range of 0.001−0.01 s⁻¹ was desirable leading to high efficiencies. It was realized that (950 °C, 0.001 s⁻¹) was the optimum deformation condition for the alloy.     

Correlation between the elastic modulus and the intrinsic plastic behavior of metallic glasses: The roles of atomic configuration and alloy composition

Recent reports suggest that Poisson’s ratio (ν), or the related ratio of shear modulus G to bulk modulus B, indicates the potential of metallic glasses (MGs) to sustain plastic strain. Using molecular dynamics simulations of the Cu₆₄Zr₃₆ MG as a representative, we demonstrate why and how these elastic and plastic properties are correlated, in terms of the common structural origin underlying these mechanical behaviors in MGs. The full icosahedral ordering has been identified as the key structural feature in the Cu–Zr MGs that controls not only the G and the G/B (or ν), but also the initiation of shear localization and the intrinsic plasticity. Additional analysis of the Cu–Zr MGs of different compositions and MGs in different alloy systems reveals a general correlation of the plasticity with the G/B ratio, as the latter is able to represent and couple the effects of both the atomic configuration and the alloy composition.     

Dislocation and Structural Studies at Metal–Metallic Glass Interface at Low Temperature

In this paper, molecular dynamics (MD) simulation deformation studies on the Al (metal)–Cu₅₀Zr₅₀ (metallic glass) model interface is carried out based on cohesive zone model. The interface is subjected to mode-I loading at a strain rate of 10⁹ s^?1 and temperature of 100 K. The dislocations reactions and evolution of dislocation densities during the deformation have been investigated. Atomic interactions between Al, Cu and Zr atoms are modeled using EAM (embedded atom method) potential, and a timestep of 0.002 ps is used for performing the MD simulations. A circular crack and rectangular notch are introduced at the interface to investigate the effect on the deformation behavior and fracture. Further, scale size effect is also investigated. The structural changes and evolution of dislocation density are also examined. It is found that the dominant deformation mechanism is by Shockley partial dislocation nucleation. Amorphization is observed in the Al regions close to the interface and occurs at a lower strain in the presence of a crack. The total dislocation density is found to be maximum after the first yield in both the perfect and defect interface models and is highest in the case of perfect interface with a density of 6.31 × 10¹⁷ m^?2. In the perfect and circular crack defect interface models, it is observed that the fraction of Shockley partial dislocation density decreases, whereas that of strain rod dislocations increases with increase in strain.     

Flow behavior and microstructures of large-grained Fe3Si during high temperature deformation

Fe₃Si polycrystals having a large initial grain size of about 92 μm exhibit a large tensile elongation exceeding 200% at 1173 K and at 4.68×10⁻⁴ s⁻¹. A stress–strain curve corresponding to the large tensile elongation is characterized by a steady state flow after an initial work hardening at a small plastic strain until final fracture. The apparent activation energy and strain rate sensitivity index is estimated to be 120 kJ/mol and 0.30, respectively. The deformation microstructure responsible for the large elongation consists of a well-defined subgrain microstructure with a low dislocation density within dynamic recrystallization (DRX) grains evolved during high temperature deformation. Large elongation is achieved by glide motion of 〈001〉 type dislocations. It is suggested that glide and climb motion of 〈001〉 type dislocations leads to simultaneous and/or alternate occurrence of DRX and dynamic recovery (DRV), which retards the initiation of plastic instability and results in large elongation.     

Characterization of mechanisms of hot deformation of as-cast nickel aluminide alloy

The hot deformation behavior of as-cast Ni₃Al alloy has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression testing in the temperature range 1100–1250°C and strain rate range 0.001–10 s⁻¹. The mechanisms of hot working have been evaluated using four generations of materials modeling techniques, which included shape of stress–strain curves, kinetic analysis, processing maps and dynamical systems approach. The material exhibited a steady-state flow behavior at slower strain rates but flow softening associated sometimes with broad oscillations, was observed at higher strain rates. The flow stress data did not obey the kinetic rate equation over the entire regime of testing but a good fit has been obtained in the intermediate range of temperatures (1150–1200°C). In this range, a stress exponent value of 6.5 and an apparent activation energy of about 750 kJ/mol have been evaluated. Microstructural investigations have shown that the matrix γ′ phase undergoes dynamic recovery in the presence of harder γ colonies The processing maps revealed four different domains out of which three are interpreted to represent cracking processes. The fourth domain, which has a peak efficiency of about 44%, occurred at 1250°C/0.001 s⁻¹. Microstructural observations revealed that this domain represents dynamic recrystallization (DRX) of γ phase and is desirable for hot working the material. The material exhibits flow instabilities when deformed in the intermediate temperature regime at strain rates higher than 1 s⁻¹ and these are manifested as shear localization.     

Plastic deformation properties of the orthorhombic complex metallic alloy phase Al13Co4

Plastic deformation experiments were performed on single crystals of the orthorhombic Al₁₃Co₄ complex metallic alloy phase. The material was compression tested in a temperature range between 873 and 1073 K at a strain rate of 10⁻⁵ s⁻¹. The stress–strain curves show a pronounced yield-point effect followed by a regime of weak work hardening. Incremental tests, i.e. stress-relaxation tests and temperature changes, were conducted in order to determine thermodynamic activation parameters of the deformation process. After deformation the samples exhibit localised zones of inhomogeneous deformation. Microstructural investigations by means of transmission electron microscopy reveal a high density of dislocation with [010] Burgers vector terminating (001) planar defects. Contrast-extinction experiments show that the Burgers vectors of dislocations as well as the displacement vectors of the planar defects are parallel to the [010] direction. The basic mechanism of plastic deformation is dislocation glide on (001) planes.     

Formation of nanocrystalline structure in steels by ball drop test

Formation of nanocrystalline structure in a eutectoid steel by severe plastic deformation has been studied by a `ball drop test'. The microstructures and hardness similar to those of nanocrystalline structures produced by ball milling have been obtained near the surface of specimens. The high strain rate of around 10⁴ s⁻¹ is proposed to be an essential condition to produce nanostructure by deformation.     

Superplastic behavior of a TiAl-based alloy

Superplastic behavior under the conditions of a temperature range from 850 to 1075°C and strain rates varying from 8×10⁻⁵ to 1×10⁻³ s⁻¹ was investigated for Ti–33Al–3Cr–0.5Mo (wt%) alloy with a very fine grain size obtained by the multi-step thermal mechanical treatment. The results show that the TiAl-based alloy with a hot-deformed fine grain size possesses good superplasticity. It exhibits a strain rate sensitivity coefficient of 0.9 at a strain rate of 3×10⁻⁵ s⁻¹ and temperature from 1000 to 1075°C. Moreover, the strain rate sensitivity coefficient is stable during the hot deformation, and a tensile elongation of 517% was obtained at 1075°C and a strain rate of 8×10⁻⁵ s⁻¹. The superplastic behavior of the present fine-grained TiAl-based alloy can be explained by the local strain hardening and high m value during the tensile deformation. Microstructure evolution in the superplastic deformation was also discussed.     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.     

Superplastic forming at high strain rates after severe plastic deformation

An Al-3% Mg-0.2% Sc alloy was fabricated by casting and subjected to severe plastic deformation through equal-channel angular pressing to a strain of ∼8. The grain size after pressing was ∼0.2 μm and increased to −1.1 μm when holding at 673 K for 10 min. Very high tensile elongations were recorded at 673 K with a maximum elongation of ∼2280% when testing with an initial strain rate of 3.3 × 10⁻² s⁻¹. The strain rate sensitivity was measured as ∼0.5 at strain rates in the vicinity of 10⁻² s⁻¹. Small disks were cut from the rods after pressing and these disks were successfully formed into domes at 673 K using a biaxial gas-pressure forming facility and forming times up to a maximum of 60 s. Measurements of the local thicknesses at selected points around the domes revealed reasonably uniform thinning which is consistent with the high strain rate sensitivity of this alloy.     

SuperplaSticity and superplastic instability of AZ31B magnesium alloy sheet

1 Introduction Due to its light mass, high specific strength, good damping characteristics, strong thermo-conductivity and electromagnetic shielding, magnesium alloys have been regarded as “the green material” with the greatest application potential in …     

铸态粗晶Ti-5553钛合金高温变形行为与机理研究

本文借助Gleeble-3800热模拟试验机系统地研究了铸态粗晶Ti-5553合金在温度700 ℃~1100 ℃、应变速率为0.001 s_^-1~10 s_^-1条件下的高温变形行为。研究结果表明合金的流变应力对变形温度和速率都有强敏感性,流变软化过程也随变形参数的改变呈现出不同的模式。通过经典的动力学模型,建立了合金高温变形的本构关系和激活能分布图,进一步基于动态材料模型构建了合金的热加工图并实现了对不同加工区间变形机制的识别。合金在低温区(700 ℃)和高速率区( 1 s_^-1)均展现出失稳变形的特征,包括外部开裂、绝热剪切带、局部流变等机制,在实际加工中应对这些加工区域进行规避。合金在800 ℃及中低速率( 0.1 s_^-1)变形下的主导机制为α相的动态析出,在中高温(900 ℃-1100 ℃)及中低速率变形下的主导机制为动态回复与动态再结晶的结合。此外,合金在高温较低应变速率(1100 ℃/0.01 s_^-1)条件的变形中表现出大范围动态再结晶的行为特点并伴随稳定的流变软化,因此此条件附近的参数区间被认定为该合金的最优加工窗口,应在实际加工中给予优先考虑。     

The positive strain-rate dependence of ductility in a 50Mo-50Re alloy

This paper is a review of the recent studies of deformation and fracture behaviors, especially the anomalous strain-rate effect on plasticity of a 50Mo-50Re alloy. The ductility of this alloy was found to increase with increase in strain rate within the range of 10⁻⁶ s⁻¹ to 1 s⁻¹ at room temperature in air. The fracture surfaces in the alloy changed from brittle to ductile in nature with increasing strain rate. A damage-toughening phenomenon was also observed in this alloy.