Ultrafine-grained nickel refined by dislocation activities at intermediate strain rate impact: deformation microstructure and mechanical properties

This article presents an application of the impact-induced deformation in effective grain refinement in polycrystalline nickel. Ultrafine-grained microstructure was processed by means of Dynamic Plastic Deformation at room temperature using a falling impactor with a maximum impact velocity of 10 m s⁻¹. The commercially pure (98.4 wt%) starting material was characterised by a coarse-grained (~25 μm) microstructure. Electron backscattering diffraction and transmission electron microscopy studies showed that the initial equiaxed grains evolved into a laminar structure of submicron size narrow domains delineated by high-angle grain boundaries. The texture after deformation exhibits preferential orientations including a strong 〈220〉 fibre texture. The mechanical behaviour under quasi-static compression at room temperature and at a strain rate of 2 × 10⁻³ s⁻¹ was investigated in directions parallel and perpendicular to the impact axis. Stress–strain responses showed an increased yield strength (440–520 MPa) compared with the initial state (90 MPa). The strain-hardening behaviour was found to strongly depend on the orientation of the compression axis.     

Rate-dependent deformation of Sn–3.5Ag lead-free solder

Compression experiments on bulk Sn-3.5Ag lead-free solder specimens have been carried out to help formulate the material constitutive behaviour of this alloy using the concept of an evolving internal stress. Tests covered the temperature range 0–125 °C and fixed strain rates between 3 × 10⁻⁷–3 × 10⁻³ s⁻¹. Flow behaviour was found to be compatible with that for a deforming a tin-rich matrix (stress exponent n = 7, activation energy Q = 46.7 kJ/mol) in which the external applied stress is reduced by an internal back stress due to the presence of precipitate phase particles. Stress–strain curves have been satisfactorily modelled using rate equations incorporating linear hardening and diffusion-controlled recovery. Comparison with supplementary tension and creep experiments, and with data from other researchers, indicates that inconsistencies in reported flow behaviour is most likely to be due to variations in initial microstructure rather than the nature of the applied loading.     

The evolution of delta-phase in a superplastic Inconel 718 alloy

An Inconel 718 sheet alloy was tested in tension at a temperature of 965°C and an initial strain rate of 10⁻⁴ s⁻¹ corresponding to the conditions for optimum superplastic deformation. Detailed observations and quantitative measurements record the evolution of the δ-phase during tensile deformation. The experiments show that the total precipitation of the δ-phase increases with strain but there is a decrease with strain in the number density of the needle/plate δ-phase particles and a corresponding increase with strain in the number density of the blocky/globular δ-phase particles.     

Experimental study of the quasi-static and dynamic behaviour of cork under compressive loading

Cork is a natural cellular material with increasing industrial applications due to its remarkable combination of properties. Its mechanical behaviour explains why it is often used for applications like sealing, packaging, insulation, vibration control, weight reduction, flotation, sound damping, etc. However, the mechanical behaviour of cork when subjected to impact has not been well investigated yet since the studies described in the literature generally focus strain rates below 10⁻¹ s⁻¹. Understanding the behaviour of cork at high rates of deformation becomes imperative when considering applications such as crash protection. Hence, in the present work, the authors compare the quasi-static and dynamic response of four types of cork when compressed axially at strain rates from 10⁻³ s⁻¹ to 600 s⁻¹. Data from the Split-Hopkinson Pressure Bars are used to generate stress–strain curves for natural and agglomerate cork samples, and the results are discussed in terms of the cellular structure of cork.     

Effect of joint stiffness on peel strength of diffusion bonded joints between Al–Li 8090 alloy sheet

Abstract

The peel behaviour of diffusion bonded joints between Al–Li 8090 alloy sheet depends upon joint geometry, sheet thickness, and the local stiffness of the bonded joint. The local stiffness was increased by bonding 8090 metal matrix composite onto the faces of the joint. At the superplastic forming temperature of 530°C the peel strengths of solid state or liquid phase diffusion bonded joints at peak load were increased from 5–7 N mm^?1 to >8 N mm^?1. This led to superplastic deformation of the sheet without peel fracture at the bonded joint. After air cooling and aging, the corresponding room temperature peel strengths were 174–252 N mm^?1, compared with 30–54 N mm^?1 for an unstiffened joint, an increase by a factor of 3·2–8·4. It was concluded that stiffened bonded joints would enable multiple thin sheet structures to be manufactured in Al–Li 8090 alloy via a diffusion bonding/superplastic forming (DB/SPF) technique. A DB/SPF technique for a three sheet structure is described.

MST/1687     

Effect of short-time thermal oxidation on the surface of Al−Li−Cu−Mg alloy (8090) sheet

The oxidation behaviour of 8090 Al−Li alloy sheet was studied after exposure for short times (<5 min) in laboratory air, moist air, moist argon and hydrogen at 530°C. Oxidation was greater at grain boundaries in hydrogen but not in air. Sites of rapid local oxidation (number density 9 to 8 × 10³ cm⁻²) were associated with insoluble particles. Four types of growth morphology were identified with these sites. Some growths believed to be spinel, were 10 to 25 μm diameter and grew to ≈1 mm in height at 2μm sec⁻¹. Pits beneath these growths were ≈25 μm diameter and 10 to 20 μm deep. A qualitative model is proposed for rapid oxidation in Al−Li alloys and the effect of these sites on weight gain measurements, surface contamination and mechanical properties is discussed.     

Superplasticity and associated activation energy in Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The high temperature deformation characteristics of a commercial β -titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr have been studied in the temperature range 830–925∘C. The alloy exhibited superplasticity in a narrow temperature and strain rate range i.e. 850–865∘C and 5× 10^{− 5}–3× 10^{− 3} s^{− 1} respectively, with a maximum elongation of 634% at 855∘C. The superplastic behaviour in the alloy is considered to arise as a result of subgrain formation at the higher strain rates (region III) which enhances diffusional creep at lower strain rates (region II). The activation energy values for regions II and III were found to be close to the lower of the two activation energy values (129.2 KJ/mole) proposed to describe self diffusion in β -phase suggesting that the rate controlling mechanism during high temperature deformation of the alloy was that for lattice diffusion.     

Deformation behaviour and microstructure of a 20% Al2O3 reinforced 6061 Al composite

Deformation and microstructural behaviours of a 20% (volume percent) particle reinforced 6061 Al matrix composite have been studied by torsion from 25 to 540°C with strain rates of 0.1, 1 and 5 s⁻¹. The logarithmic stress versus reciprocal temperature relationship exhibits two slopes indicating different deformation mechanisms. The 20% Al₂O₃/6061 Al composite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal much smaller subgrain size and higher volume of non-cellular substructures, as well as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the 10% Al₂O₃/6061 Al composite and matrix alloy the same test condition. The torsion fracture surface was studied and compared to the three point bending failure specimens.     

Effect of transient change in strain rate on plastic flow behaviour of low carbon steel

Plastic flow behaviour of low carbon steel has been studied at room temperature during tensile deformation by varying the initial strain rate of 3·3 × 10⁻⁴s⁻¹ to a final strain rate ranging from 1·33 × 10⁻³s⁻¹ to 2 × 10⁻³s⁻¹ at a fixed engineering strain of 12%. Haasen plot revealed that the mobile dislocation density remained almost invariant at the juncture where there was a sudden increase in stress with a change in strain rate and the plastic flow was solely dependent on the velocity of mobile dislocations. In that critical regime, the variation of stress with time was fitted with a Boltzmann type Sigmoid function. The increase in stress was found to increase with final strain rate and the time elapsed in attaining these stress values showed a decreasing trend. Both of these parameters saturated asymptotically at a higher final strain rate.     

Hot deformation behavior of Ti-15-3 titanium alloy: a study using processing maps,activation energy map,and Zener–Hollomon parameter map

The hot deformation behavior of Ti-15-3 titanium alloy was investigated by hot compression tests conducted in the temperature range 850–1150 °C and strain rate range 0.001–10 s⁻¹. Using the flow stress data corrected for deformation heating, the activation energy map, processing maps and Zener–Hollomon parameter map were developed to determine the optimum hot-working parameters and to investigate the effects of strain rate and temperature on microstructural evolution of this material. The results show that the safe region for hot deformation occurs in the strain rate range 0.001–0.1 s⁻¹ over the entire temperature range investigated. In this region, the activation energy is ~240 ± 5 kJ/mol and the ln Z values vary in range of 13.9–21 s⁻¹. Stable flow is associated with dynamic recovery and dynamic recrystallization. Also, flow instabilities are observed in the form of localized slip bands and flow localization at strain rates higher than 0.1 s⁻¹ over a wide temperature range. The corresponding ln Z values are larger than 21 s⁻¹. The hot deformation characteristic of Ti-15-3 alloy predicted from the processing maps, activation energy map, and Zener–Hollomon parameter map agrees well with the results of microstructural observations.     

High-temperature deformation of Al-Li-Cu-Mg-Zr alloys 8090 and 8091

Specimens were prestretched in the range 0 to 7% plastic deformation prior to artificial ageing, or were duplex aged, to investigate the effect of dislocation substructure and of S(Al₂CuMg) particles on plastic flow during room and elevated temperature tensile tests. The yield stresses increased in Al-Li-Cu-Mg-Zr alloys 8090 and 8091 after stretching, due to the dislocation cells introduced by the stretch and also to the nucleation and growth of the S-phase particles on these dislocations. Up to 400 K the modulus-normalized proof stresses for the variously treated materials were constant, but they fell at higher temperatures, as dislocation climb mechanisms operated. The proof stresses of alloy 8090 fell at a temperature 50 K lower than alloy 8091, and this difference is considered to arise from the differing volume fraction of S-phase in the two alloys. The work hardening rate (ϑ) at a strain of 0.2% was measured between 300 and 500 K. In alloy 8090, the temperature at which dynamic recovery occurs was influenced by the degree of stretch, but this was not so in alloy 8091. In alloy 8090, the substructure introduced by stretching was able to act as a dislocation sink during dynamic recovery, whereas in alloy 8091 the higher copper content brought about more S-phase precipitation which was sufficient to inhibit this effect. The athermal hardening rate (ϑ_o) was obtained by extrapolation of the ϑ-σ_t curves, giving ϑ_o=10000 GPa, for alloy 8090 and ϑ_o=8500 G Pa, for alloy 8091. This difference may reflect a higher mobile dislocation density in alloy 8090 due to the lower volume fraction of S-phase particles in that material.     

The strain rate and temperature dependence of microstructural evolution of Ti–15Mo–5Zr–3Al alloy

A compressive split-Hopkinson pressure bar apparatus and transmission electron microscopy (TEM) are used to investigate the deformation behaviour and microstructural evolution of Ti–15Mo–5Zr–3Al alloy deformed at strain rates ranging from 8 × 10² s⁻¹ to 8 × 10³ s⁻¹ and temperatures between 25 °C and 900 °C. In general, it is observed that the flow stress increases with increasing strain rate, but decreases with increasing temperature. The microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result, primarily, of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate, but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. The amount of α phase increases with increasing temperature below the β transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain under the current loading conditions.     

Ageing response and mechanical properties of a SiCp/Al-Li (8090) composite

The ageing kinetics of a silicon carbide particle-reinforced Al-Li (8090) matrix composite and unreinforced alloy, both made by spray forming, were investigated. Ageing treatments, without any straining after solutionizing, and with a 2% plastic strain after solutionizing, were employed. The peak ageing times of the matrix in the composite was shorter than that of the unreinforced alloy. The enhanced hardening rate of the matrix in the composite was attributed to the higher dislocation density induced as a result of the plastic deformation occurring at the particle/matrix interface. This plastic deformation is a result of the large difference in the coefficient of thermal expansion between the particles and matrix. Subjecting the samples to a 2% plastic strain reduced the peak ageing times even further. The tensile strength of the composite samples was marginally higher than that of the unreinforced alloy. Samples subjected to 2% plastic straining prior to ageing also exhibited higher strength values. The strain to failure of all the samples did not recover in the over-aged state.     

Compressive deformation behavior of Al 2024 alloy using 2D and 4D processing maps

The hot deformation behavior of Al 2024 was studied by isothermal hot compression tests in the temperature range of 250–500 °C and strain rate range of 10⁻³ to 10² s⁻¹ in a computer-controlled 50 kN servo-hydraulic universal testing machine (UTM). The results show that the flow stress of Al 2024 alloy increases with strain rate and decreases after a peak value, indicating dynamic recovery and recrystallization. The processing map exhibits two domains of optimum efficiency for hot deformation at different strains, including the low strain rate domain at 500 °C and between 10⁻² and 10⁻¹ s⁻¹ and the high strain rate domain in 250 and 300 °C in the strain rate range of 10¹ to 10² s⁻¹. An attempt has been made in this article to generate a new hybrid 4D process map which illustrates contours of power dissipation and instability in the 3D space of strain rate, temperature, and strain.     

Effect of sillimanite particle reinforcement on dry sliding wear behaviour of aluminium alloy composite

Abstract

The effect of sillimanite reinforcement on the dry sliding wear behaviour of aluminium silicon alloy (BS LM6) composite was investigated using a pin-on-disc sliding wear test machine. The composite specimens were prepared using the liquid metallurgy technique and 10 wt-% of sillimanite particles were incorporated in the matrix alloy. Sliding wear tests were conducted at applied pressures between 0.2 and 1.6 MPa and speeds of 1.89, 3.96 and 5.55 m s^-1. The matrix alloy was also prepared and tested under identical conditions in order to enable comparison. It was observed that the sillimanite reinforced composite exhibited a lower wear rate than the matrix alloy. Increase in applied load increased the wear rate while increase in speed exhibited the reverse effect. The seizure pressure of the composite was significantly higher than that of the matrix alloy. The temperature rise near the contacting surface and the coefficient of friction were less in the composite than in the matrix alloy. SEM micrographs of the worn surface and subsurface were used to predict the nature of the wear mechanism.     

Polymeric foam behavior under dynamic compressive loading

Polymeric foams are commonly used in many impact-absorbing applications and thermal-acoustic insulated devices. To improve their mechanical performances, these structures have to be modeled. Constitutive equations (for their macroscopic behavior) have to be identified and then determined by appropriate tests.Tests were carried out on polypropylene foams under high strain rate compression. In this work, the material behaviour has been determined as a function of two parameters, density and strain rate. Foams (at several densities) were tested on a uniaxial compression for initial strain rates equal to 0.34 s⁻¹ and on a new device installed on a flywheel for higher strain rates. This apparatus was designed in order to do stopped dynamic compression tests on foam. With this testing equipment, the dynamic compressive behaviour of the polymeric foam has been identified in the strain rate range [6.7.10⁻⁴s⁻¹, 100s⁻¹].Furthermore, the sample compression was filmed with a high speed camera monitored by the fly wheel software. To complete this work, picture-analysis techniques were used to obtain displacement and strain fields of the sample during its compression. Comparisons between these results and stress-strain responses of polypropylene foam allow a better understanding of its behaviour. The multiscale damage mechanism, by buckling of the foam structure, was emphasised from the image analysis.     

Effects of heat treatments on the serrated flow in a Ni–Co–Cr-base superalloy

Serrated flow in a Ni–Co–Cr-base superalloy was studied in three microstructural conditions (SUB, SUBA, and SUPER) from 25 to 750 °C by tensile test at initial strain rates ranging from 8 × 10⁻⁵ to 3 × 10⁻³ s⁻¹. The results showed that the SUB and SUBA samples had fine grain size of about 9 μm, whereas the SUPER samples had coarse grain size of about 600 μm. The tertiary γ′ fraction was about 0 in the SUB, 5% in the SUBA, and 15% in the SUPER samples, respectively. The types and temperature ranges of serration were different in the alloy with SUB, SUBA, and SUPER microstructures. It is proposed that the tertiary γ′ fraction and size had great effects on the serrated flow of the alloy with different microstructures.     

Viscoplastic behaviour of metals deformed at high-temperature, application to tension–compression cycles

A mechanical modelling is proposed in order to describe viscoplastic behaviour without hardening of a nickel-base super alloy loaded at high temperature (900 °C) with strain rates varying within a wide range (from 10⁻¹ to 10⁻⁴ s⁻¹). A mathematical law is associated to the viscoplastic model; the parameters of the law are identified from monotonic biaxial tests of membranes loaded by pressure of inert gas (disk pressure testing under helium). The viscoplastic law provides calculated stresses with accuracy better than 1% at the highest strain rates and 4% at the lowest strain rates; the identified yield stress is a logarithmical function of strain rate as for other metallic alloys studied in the bibliography. The parameters identified from biaxial tensile tests of disks have been successfully used to calculate the stresses during stabilized tension–compression loops of cylindrical specimens. The proposed experimental method and behaviour model are interesting because the disk biaxial testing is much more easily performed at high temperature than the tension–compression testing of cylindrical specimens.     

Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy

Compression properties of a refractory multi-component alloy, Ta₂₀Nb₂₀Hf₂₀Zr₂₀Ti₂₀, were determined in the temperature range of 296–1473 K and strain rate range of 10⁻¹–10⁻⁵ s⁻¹. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473 K for 24 h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter a = 340.4 pm. The grain size was in the range of 100–200 μm. During compression at a strain rate of έ = 10⁻³ s⁻¹, the alloy had the yield strength of 929 MPa at 296 K, 790 MPa at 673 K, 675 MPa at 873 K, 535 MPa at 1073 K, 295 MPa at 1273 K and 92 MPa at 1473 K. Continuous strain hardening and good ductility (ε ≥ 50%) were observed in the temperature range from 296 to 873 K. Deformation at T = 1073 K and έ ≥ 10⁻³ s⁻¹ was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473 K or a decrease in the strain rate to ~10⁻⁵ s⁻¹. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273 K and completed dynamic recrystallization was observed at 1473 K.     

Hydrogen permeation through internally oxidized Pd alloys

Hydrogen diffusivity and solubility were determined by electrochemical hydrogen permeation tests in samples of Pd_0.97 Al_0.03 and Pd_0.97 Ce_0.03 in the as received and internally oxidized conditions. Internal oxidization caused the appearance of nanosized oxide precipitates in the Pd matrix. The shape and size of precipitates and also the coherence between the precipitates and the matrix were observed by transmission electron microscopy (TEM). Electrochemical hydrogen permeation tests revealed that the presence of oxides increases the apparent hydrogen solubility, S _app, but decreases the hydrogen diffusivity, D _app. The values of D _app were 2.0 × 10⁻¹¹ m² s⁻¹ for vacuum heat treated Pd_0.97 Al_0.03 alloy and 4.0 × 10⁻¹² m² s⁻¹ for internally oxidized Pd_0.97 Al_0.03 alloy. However, the concentration of trapped hydrogen was 49 mol H.m⁻³ for the Pd_0.97 Al_0.03 alloy in the vacuum heat treated condition and 403 mol H.m⁻³ for the Pd_0.97 Al_0.03 alloy in the internally oxidized condition. Both heat treatments were performed at 1073 K for 24 h. The influence of the nature, size and distribution of the precipitates on the hydrogen permeation parameters are discussed in this paper.