Characteristics of steady state deformation of an Al–0.1 Mg alloy at low temperatures

The present work aims to address the characteristics of steady state deformation, which determines the limit of grain refinement for a given material by severe plastic deformation. The focus is on low temperatures at which most deformation processing is conducted. Submicron grained Al–0.1 Mg alloy prepared by equal channel angular pressing was deformed by plane strain compression in a channel-die and rolling at a constant strain rate of 10^?2 s^?1 and at a range of temperatures from 77 to 473 K to various strains. Microstructures were characterized by electron backscatter imaging and EBSD in a FEGSEM. Grain refinement to the ECAP submicron structure occurred during deformation at cryogenic temperatures of 77–213 K, whereas coarsening took place during deformation at elevated temperatures. A steady state deformation was observed at all temperatures where a constant grain structure was developed and maintained upon further straining. The microstructural characteristics of steady state deformation and mechanism responsible for the establishment of the steady state are discussed.     

Work-hardening effect and strain-rate sensitivity behavior during hot deformation of Ti–5Al–5Mo–5V–1Cr–1Fe alloy

The work-hardening effect and strain-rate sensitivity behavior during hot deformation have been quantitatively investigated in this present paper. Isothermal compression experiment of Ti–5Al–5Mo–5V–1Cr–1Fe titanium alloy has been conducted for verification. Linear relationship between work-hardening rate and true strain/stress has been derived from Kocks–Mecking dislocation relation. The work-hardening effect shows two obvious stages with strain: steady fluctuations and linear decreasing. Obvious work-hardening effect could be demonstrated under lower temperatures and higher strain rates. The work-hardening decrease at linear-decreasing regime becomes more stronger with temperature elevated and rate lowered, reverse-proportional to Zener–Hollomon parameters. Strain-rate sensitivity coefficient for hot deformation was decomposed into three parts from JMAK recrystallization kinetics. The influence of strain rate on DRX evolution has been termed as the major factor determining strain-rate sensitivity. Strain-rate sensitivity coefficients for steady-state deformation (ɛ = 0.7) of Ti–5Al–5Mo–5V–1Cr–1Fe alloy have been characterized as a function of deformation parameters and strain-rate sensitivity has been identified more obvious with temperature elevated and rate lowered.     

Phase-field modelling on effect of pressure on growth kinetics of Mg–Al–Sn alloy

The effects of pressure on dendritic growth kinetics of Mg–Al–Sn alloy were investigated using a ternary phase-field model coupled with thermodynamics with pressure effects and experiments. The results showed that the improved growth velocity and nucleation rate caused by pressure had the opposite effects on grain size. For one-grain growth, the dendrite was larger and more developed under pressure of 85?MPa than that under ambient pressure owing to larger thermodynamic driving force, and thus higher growth velocity. However, when nucleation was considered in multigrain growth case, the average grain size under pressure was smaller owing to less growth space. Growth velocity decreased with the increase in Sn content, on which pressure had no great influence.     

Workability of Ti–6Al–4V alloy at high temperatures and strain rates

Hot workability of Ti–6Al–4V has been investigated by means of hot compression tests carried out in the 880–950 °C temperature range and 1–50 s⁻¹ strain rate range. The effect of microstructural characteristics of the deformed specimens have been studied and correlated with the test temperature, total strain and strain rate. A constitutive equation for the flow stress has been defined and the test conditions for a homogeneous deformation evaluated. The machine employed for testing allowed to reach very high strain rates by means of a uniform compression for long strains (until 0.9), whereas data extracted from the scientific literature are significantly limited in comparison. In this way, a higher accuracy could be obtained in material behaviour modelling for forging process simulation.     

Grain size effect on deformation twinning in Mg–Al–Zn alloy

Abstract

Specimens of Mg–3Al–1Zn alloy with a wide range of grain size distribution were compressed along different directions. The compressed microstructure was examined to clarify the grain size effect on deformation twinning in magnesium alloys. Small strains were used to reveal the twinning behaviour. The results show that the grain size affects the formation of deformation twins in an Mg–Al–Zn alloy. The reason for a different result being previously reported is given. This study also reports the different deformation microstructures in specimens compressed along different directions.     

Some mechanical properties of Sn–3.5 Ag eutectic alloy at different temperatures

The eutectic alloy Sn–3.5 wt % Ag has been examined as one of the lead-free solder alloys. Microhardness tests as a function of temperature were performed to calculate the effective activation energy of the solder alloy Sn–Ag and compared with the pure elements Sn and Ag. Various creep parameters such as, exponent n_tr and the parameter in the transient creep stage and the values of the stress exponent n from the steady-state stage were calculated under different constant applied stresses at different working temperatures. The structure changes of the alloy were reported before and after creep test.     

Microstructure study and constitutive modeling of Ti–6Al–4V alloy at elevated temperatures

A reliable and accurate prediction of flow behavior of metals in industrial forming process considering the coupled effects of strain, strain rate and temperature is crucial in understanding the workability of the metal and optimizing parameters for hot forming process. In this study, the tensile fracture behavior of the Ti–6Al–4V alloy is examined with scanning electron microscope (SEM) over the range of magnifications. SEM study revealed that microvoids and shallow dimples are observed at the fracture surface which indicates the fracture is predominately ductile in nature. Also, an investigation on flow behavior of Ti–6Al–4V alloy is done using constitutive models. Four constitutive models; modified Johnson-Cook (m-JC), modified Arrhenius type equations (m-Arr), modified Zerilli–Armstrong (m-ZA) and Rusinek–Klepaczko (RK) models are developed to predict the flow stress. The predictions of these constitutive models are compared with each other using statistical measures like correlation coefficient, average absolute error and its standard deviation. Comparing the statistical measures, m-Arr model is a better model for predicting the flow stress, but considering the fact that m-ZA model is a physical based model, m-ZA model is preferred over the m-Arr model.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

The effect of cooling rate on α-phase ordering in Cu-12.4wt% Al alloy

The formation of a superlattice in the-phase of Cu-12.4wt% Al alloy was studied during cooling. Specimens cooled at different rates were examined using electron microscopy and differential thermal analysis. The superlattice structure formed was described by means of theD-parameter which determines the position of superspots in the reciprocal lattice. Variation of theD-parameter with cooling rate has a linear form, hence it may be concluded that the superlattice in the-phase is formed as a transitional structure. The relationship between theD-parameter and enthalpy suggests that the latter can be taken as a measure of superstructure development.     

Constitutive modeling and the effects of strain-rate and temperature on the formability of Ti–6Al–4V alloy sheet

The constitutive model considering the strain-rate and temperature effects was presented by fitting the true stress–strain curves of Ti–6Al–4V alloy over a wide range of strain-rates (0.0005–0.05 s⁻¹) and temperatures (923–1023 K). The Forming Limit Curve (FLC) of Ti–6Al–4V alloy at 973 K was measured by conducting the hemispherical dome test with specimens of different widths. The forming limit prediction model of Ti–6Al–4V alloy, which takes strain-rate and temperature sensitivity into account, was predicted based on Marciniak and Kuczynski (M–K) theory along with Von Mises yield criterion. The comparison shows that the limit strain decreases with temperature lowering but strain-rate increasing. The comparison between theoretical analysis and experiment of FLC verifies the accuracy and reliability of the proposed methodology, which considers the strain-rate and temperature effects, to predict limit strains in the positive minor strain region of Forming Limit Diagram (FLD).     

Evolution of secondary phase particles during deformation of Al-5Ti-1B master alloy and their effect on α-Al grain refinement

Addition of Al-5Ti-1B alloy to molten aluminum alloys can refine α-Al grains effectively and thereby improve their strength and toughness. TiAl₃ and TiB₂ in Al-5Ti-1B alloy are the main secondary-phase particles for refinement, while the understanding on the effect of their sizes on α-Al grain refinement continues to be fragmented. Therefore, Al-5Ti-1B alloys with various sizes and morphologies of the secondary-phase particles were prepared by equal channel angular pressing (ECAP). Evolution of the secondary-phase particles during ECAP process and their impact on α-Al grain refinement were studied by X-ray diffraction and scanning electron microscope (SEM). Results show that during the ECAP process, micro-cracks firstly appeared inside TiAl₃ particles and then gradually expanded, which resulted in continuous refinement of TiAl₃ particles. In addition, micro-distribution uniformity of TiB₂ particles was improved due to the impingement of TiAl₃ particles to TiB₂ clusters during deformation. Excessively large sizes of TiAl₃ particles would reduce the number of effective heterogeneous nucleus and thus resulted in poor grain refinement effectiveness. Moreover, excessively small TiAl₃ particles would reduce inhibitory factors for grain growth Q and weaken grain refinement effectiveness. Therefore, an optimal size range of 18–22 μm for TiAl₃ particles was suggested.     

Compressive failure of alumina continuous fibre reinforced Al–4·5 wt-%Cu alloy at elevated temperatures

Abstract

Alumina continuous fibre reinforced Al–4·5 wt-%Cu alloy composite specimens were compressed in the axis direction at room temperature, 200°C, 300°C, and 400°C. The compressive stress–end shortening relationships at all test temperatures were similar to the elastic response, but with some non-linearity shortly before the macro failures. Composite compressive stress declined at elevated temperatures. The difference between the failure strength and the onset failure strength decreased with increase in temperature. The dominant failure mode at room temperature and 200°C was that of buckling, but it changed to kinking at elevated temperatures. Composite compressive behaviour at all test temperatures conformed to plastic buckling theory.     

Effect of reverse modification of Al–5Ti–B master alloy on hypoeutectic ZnAl4Y alloy

The present work focuses on reverse modification of Al–5 wt.%Ti–1 wt.%B master alloy on the structural characteristics and mechanical properties of hypoeutectic ZnAl4Y alloy. The results shows that with the increase of the adding amount of master alloy to ZnAl4Y alloy, the morphology and the kind of primary phase as well as the amount of eutectic structure of the modified alloy vary considerably. With the increase of the adding amount of Al–5 wt.%Ti–1 wt.%B, the tensile strength and the hardness of modified ZnAl4Y alloy increase. When the adding amount is 0.5 wt.%, the impact toughness and the elongation of the alloy reach the maximum.     

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 μm in the rolled material and 80 μm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 10⁶ cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 10⁶ cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.     

High-cycle fatigue properties at cryogenic temperatures in forged- and rolled-Ti–5% Al–2.5% Sn ELI alloys

High-cycle fatigue properties were investigated at 4, 77 and 293 K in Ti–5% Al–2.5% Sn ELI alloys, in which mean alpha grain sizes were about 30 mm in the rolled material and 80 mm in the forged material. The ultimate tensile strengths of both materials were almost same and increased with decreasing temperature. The fatigue strength of each material also tended to increase with decreasing temperature. At 293 K, the fatigue strength of each material was almost equivalent. At 4 and 77 K, however, the fatigue strength of the rolled material was higher than that of the forged material. Concerning the rolled material, the fatigue strengths at 10⁶ cycles at 4 and 77 K were about 1.6 and 1.5 times higher than that at 293 K, respectively. On the other hand, in the forged material, it should be noted that the fatigue strengths in longer-life region (over 10⁶ cycles) were almost equivalent not depending on test temperatures. Fatigue cracks initiated in the specimen interior independently of test temperatures and materials (we call this type of crack initiation ‘sub-surface crack initiation’) and formed facet-like structures at the sub-surface crack initiation sites at 4 and 77 K. The size of each facet-like structure corresponded closely to the grain size itself. The sizes of crack initiation sites were smaller in the rolled material than in the forged material. Since sub-surface cracks, which form facets or crack initiation sites, are supposed to act as defects, it is concluded that grain refinement leads to reduce the size of crack initiation site and this contributes effectively to improve the fatigue strength in high-cycle region at cryogenic temperatures.     

Dynamic observations of deformation in an ultrafine-grained Al–Mg alloy with bimodal grain structure

The tensile properties and deformation response of an ultrafine-grained (UFG) Al–Mg alloy with bimodal grain structure were investigated using a micro-straining unit and a strain mapping technique. Atomized Al 5083 powder was ball-milled in liquid N₂ to obtain a nanocrystalline (NC) structure, then blended with 50 wt.% unmilled coarse-grained (CG) powder, and consolidated to produce a bimodal grain structure. The blended powder was hot vacuum degassed to remove residual contaminants, consolidated by cold isostatic pressing (CIP), and then quasi-isostatic (QI) forged twice. The resultant material consisted of a UFG matrix and CG regions. The dynamic response during tensile deformation was observed using a light microscope, and the surface displacements were mapped and visualized using a digital image correlation (DIC) technique. The DIC results showed inhomogeneous strain between the UFG and CG regions after yielding, and the strain was localized primarily in the CG regions. Strain hardening in the CG regions accompanied the localization and was confirmed by variations in Vickers hardness.