Rate-dependent serrated flow and plastic deformation in Ti45Zr16Be20Cu10Ni9 bulk amorphous alloy during nanoindentation

Abstract

The plastic deformation of Ti₄₅Zr₁₆Be₂₀Cu₁₀Ni₉ bulk metallic glass has been investigated by nanoindentation performed with loads ranging from 10 to 200 mN in a wide range of loading rates. The plastic flow in the alloy exhibited conspicuous serrations at low loading rates. The serrations, however, became less prominent as the rate of indentation increased. Atomic force microscopy showed a significant pile-up of materials around the indents, indicating that a highly localized plastic deformation occurred under nanoindentation. The possible mechanism governing the plastic deformation in bulk metallic glass specimens is tentatively discussed in terms of strain-induced free volume.     

Rate-dependent serrated flow and plastic deformation in Ti45Zr16Be20Cu10Ni9 bulk amorphous alloy during nanoindentation

The plastic deformation of Ti₄₅Zr₁₆Be₂₀Cu₁₀Ni₉ bulk metallic glass has been investigated by nanoindentation performed with loads ranging from 10 to 200 mN in a wide range of loading rates. The plastic flow in the alloy exhibited conspicuous serrations at low loading rates. The serrations, however, became less prominent as the rate of indentation increased. Atomic force microscopy showed a significant pile-up of materials around the indents, indicating that a highly localized plastic deformation occurred under nanoindentation. The possible mechanism governing the plastic deformation in bulk metallic glass specimens is tentatively discussed in terms of strain-induced free volume.     

Plastic deformation of Zr-based bulk metallic glasses under nanoindentation

The plastic deformation of two Zr-based bulk metallic glasses (BMGs), Zr₅₅Cu₃₀Ni₅Al₁₀ and Zr_69.5Cu₁₂Ni₁₁Al_7.5, was investigated by instrumented nanoindentation over a broad range of loading rates. It was found that the plastic flow of the two BMGs exhibited conspicuous serrations at low loading rates. The serrations, however, became less prominent as the rate of indentation increased. AFM showed a significant pile-up of materials around the indents, indicating that a highly localized plastic deformation occurred under nanoindentation. The mechanism governing the plastic deformation in BMGs was tentatively discussed in terms of the mode of strain-induced free volume.     

Plastic deformation behaviors of Ni- and Zr-based bulk metallic glasses subjected to nanoindentation

Plastic deformation behaviors of Ni₄₂Ti₂₀Zr_21.5Al₈Cu₅Si_3.5 and Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉ bulk metallic glasses at room temperature were studied by nanoindentation testing and atomic force microscopy under equivalent indentation experimental conditions. The different chemical composition of these two bulk metallic glasses produced variant tendencies for displacement serrated flow to occur during the loading process. The nanoindentation strain rate was calculated as a function of indentation displacement in order to verify the occurrence of displacement serrated flow at different loading rates. Atomic force microscopy revealed decreasing numbers of discrete shear bands around the indentation sites as loading rates increased from 0.025 to 2.5 mNs^− 1. Variations in plastic deformation behaviors between Ni and Zr-based glasses materials can be explained by the different metastable microstructures and thermal stabilities of the two materials. The mechanism governing plastic deformation of these metallic glasses was analyzed in terms of an established model of the shear transformation zone.     

Acoustic emission inspection of Portevin-Le Chatelier effect and deformation mechanisms of two Mg–Li–Al alloys

In this paper, Portevin-Le Chatelier (PLC) effect of LA41 magnesium alloy is reported. With increasing strain rate, both ultimate tensile stresses (σ_b) and 0.2% proof (σ_0.2) stresses increase, while serrations become less severe. Major acoustic emission (AE) peaks appear at the end of elastic region, and transform to burst type signals in the plastic range. In another alloy containing less Li (LA11), serrations are absent and major AE peak values are considerably lowered. Additionally, PLC effect disappears in LA41 after annealing. The role of twins is discussed in details. Actual causes for AE activity during deformation are also proposed.     

Macro‐ and Nanomechanical Properties and Strain Rate Sensitivity of Accumulative Roll Bonded and Equal Channel Angular Pressed Ultrafine‐Grained Materials

Several processes of severe plastic deformation are suitable for the production of materials with ultrafine‐grained microstructures which are known to exhibit high strength and often good ductility as well as strain rate sensitive behavior. The most promising ones are equal channel angular pressing (ECAP) for bulk material and accumulative roll bonding (ARB) for the production of sheet material. In order to evaluate the influence of the process on these mechanical properties and the strain rate sensitivity, tensile tests, and nanoindentation tests were performed on material produced up to similar effective plastic strains of ε_ARB = 6.4 and ε_ECAP = 6.3. It could be shown that the macroscopic strength is slightly higher for ARB than for ECAP material and vice versa in nanoindentation. Independent of the testing method, the strain rate sensitivities and activation volumes are similar for both materials. Thus, both processes performed up to similar effective plastic strains lead to comparable improvements in the mechanical properties. Additionally it could be shown, that this comparison allows the identification of the dominant deformation mechanism which is responsible for the observed strain rate sensitivity.     

Depth dependent hardness variation in Ni–P amorphous film under nanoindentation

Abstract

The deformation behaviour and the depth dependent hardness variation in Ni–P amorphous alloy were investigated by nanoindentation. It was found that in addition to circular shear bands around the indent and on the indent surface, which have been previously observed, straight shear bands on the indent surface were also formed during nanoindentation. The indentation depth dependent hardness in the metallic glass is not modelled by the conventional dislocation based strain gradient theory for crystalline materials; instead it can be well described by a function of the inverse square root of the indentation depth. The structure evolution beneath the indentor is proposed to be the probable cause for the length scale dependent properties in metallic glass.     

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part II: The deformation of Al-based particulate reinforced composites at 473 K to 833 K

Constant-load pyramidal indentation tests were performed from 473 K to 833 K on P/M fabricated 2024 aluminum reinforced with either SiO₂, SiC, or Al₂O₃ particles to investigate the influence of particulate reinforcement on the high-temperature plastic deformation process during indentation. The composites all displayed larger apparent threshold stress σ_th than the previously reported unreinforced P/M 2024 aluminum alloy. SEM investigation of the indentations indicated that the indentation process is accompanied by considerable cracking and interfacial debonding of the reinforcing particles, the extent of which increases with increasing temperature. The magnitude of σ^th was largest for the Al₂O₃ reinforced composite and this is attributed to the load-transfer that occurs when the indenter contacts the reinforcing particles and the superior high-temperature interfacial strength of this composite. The apparent activation energy ΔG₀ of the indentation strain rate increased from 0.25μb³ at 473 K to 0.60μb³ at 833 K. These values are within the expected range for weak particles and dislocation-dislocation interactions but are lower than the previously reported Δ₀ of the unreinforced P/M 2024 alloy. We conclude that the low indentation strain rate of the particulate reinforced composites is the result of the load transfer due to the presence of the reinforcements and its affect on increasing the σ_th. The low values of ΔG₀ are consistent with our observation that extensive particle cracking and interfacial debonding occur in the reinforced material during indentation.     

An investigation on the onset of plastic deformation of nanocrystalline solid solution Ti-Al-N films

The nanoscale deformation behavior of the solid solution Ti_0.5Al_0.5N thin film was systematically investigated by nanoindentation measurements. The effect of the tip radius of the indenter on the behavior of elastic-plastic deformation was also evaluated. The Hertzian stress analysis was used to determine the distribution of resolved shear stress at the initiate plastic deformation, and the obtained critical resolved shear stress was compared to the theoretical shear strength to establish correlations and differences. Comparison of the calculated critical shear stress and theoretical shear strength also indicated that new complete dislocation nucleation during nanoindentation was not the prerequisite of the onset of plastic deformation, even at very shallow indentation depth.     

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.     

A multiscale-indentation study of deformation and fracture in 6H polycrystalline silicon carbide

ABSTRACT

The mechanical behaviour of a polycrystalline silicon carbide across length scales was studied using Vickers indentation, focusing on the hardness, fracture toughness and failure mechanism of the material. For macroscopic and microscopic indentations, the hardness decreased with an increase in load, which was associated with the well-known indentation size effect as well as the internal flaws. For nanoindentation, severe plastic deformation was discovered beneath the imprints on the basal plane (0001) which is the most favourable crystallographic plane for dislocation movement. Alternative sources of plastic deformation, including deformation twinning and stacking faults, were found for nanoindentations with an increased load. Also, cracking was observed for indents made at 100 mN and above, which was used to study the fracture toughness.     

Vickers indentation tests in a Zr62.55Cu17.55Ni9.9Al10 bulk amorphous alloy

Vickers indentation tests were conducted on a Zr_62.55Cu_17.55Ni_9.9Al₁₀ bulk amorphous alloy to investigate the evolution of shear bands and its plastic deformation dimension via a bonded interface technique. Under all indentation loads, the plastic deformation is accommodated through semi-circular and radial shear bands. The plastic deformation dimension increases with increasing the indentation loads. A simplified λ = C(P)^0.5 model was put forward to predict and estimate the plastic deformation dimension characterized by shear bands in the subsurface. For the Zr_62.55Cu_17.55Ni_9.9Al₁₀ amorphous alloy, C is about 15.314 µm/N^0.5. The normalized shear band zone is independent to the indentation load.     

Study on dynamic strain aging phenomenon of 3004 aluminum alloy

3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10⁻⁵ to 5.56 × 10⁻³ s⁻¹) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, T_t, was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than T_t. According to the activation energy, it is believed that the process at the temperature region lower than T_t is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.     

Superplastic behavior of Zn–Al eutectoid alloy with 2?% Cu

The effects of deformation temperature and strain rate on the superplastic behavior of the Zn–21Al–2Cu alloy (Zinalco alloy) were investigated by uniaxial tensile tests. Results were compared with those of the Zn–22Al eutectoid alloy without Cu. It was observed that additions of 2 % Cu leads to a decrease of the maximum strain attainable from 2600 % to 1000 %. The maximum strain in Zinalco alloy is obtained at lower strain rates. The presence of Cu increases the values of flow stress up to 600 % compared with those reported in the Zn-22Al alloy. Grain size sensitivity (p), true activation energy (Q _t ), and constant A of the constitutive equation were not affected by presence of Cu unlike the stress exponent (n) which increased from 2.5 to 3.9. The main effect of Cu was to decrease the plastic flow stability of the Zn–22Al alloy. The results indicate that presence of Cu in the Zinalco alloy causes a hardening effect at low strain rates leading to a decrease in the strain rate sensitivity which promotes the formation and growth of sharp necks. Microstructural characterization suggests that the large deformations at necking could possibly be due to the substantial elongation capability of the Zn-rich phase (η).     

In-plane anisotropy in low cycle fatigue properties of and bilinearity in Coffin-Manson plots for quaternary AI-Li-Cu-Mg 8090 alloy plate

Abstract

Low cycle fatigue (LCF) behaviour as a function of test direction was studied for a quaternary Al–Li–Cu–Mg alloy. The analysis of the variation in fatigue life with plastic strain amplitude ??_p/2 or with average stress amplitude ?σ/2 or with plastic strain energy per cycle ?W_p, revealed bilinear power law relationships in all the test directions. The transition strain values in the Coffin–Manson plots were seen to match closely with those obtained in the cyclic stress response as well as with the cyclic stress–strain relationships of the alloy. The observed bilinear behaviour in these LCF properties was attributable to a change in deformation as well as to the deformation assisted fracture mode. The alloy revealed significant in plane anisotropy in the LCF properties. The observed anisotropy was found to result from the combined effects of strong crystallographic texture and grain fibering.

MST/3016     

Plastic deformation — its role in fatigue crack propagation

Recognizing the fact that the effective driving force (ΔK _eff) determines the fatigue crack propagation (FCP) rate and that the shear strain, which is considered to develop due to an occurrence of crack closure, primarily contributes to the plastic deformation, an effort is made here to elucidate the role of plastic deformation in FCP by developing a correlation between the ΔK _eff and the applied driving force (ΔK) with shear strain as variable. The effect of the degree of plastic deformation (i.e. shear strain level) on the FCP rates at higher values of ΔK, where ΔK _eff approaches ΔK, approaching the Paris regime, appears minimal. On the other hand, the disparity between ΔK _eff and ΔK, which apparently increases with shear strain level, persists at lower values of ΔK. This suggests a strong influence of the degree of localized deformation on the FCP rates in the near threshold level. Hence, an improvement of FCP rates in the near threshold level should follow an effort that promotes the plastic deformation near the crack tip to a greater degree. This approach could, therefore, form the basis to explain the effect of the grain size, microstructure, environment,R-ratio and crack size on the near-threshold FCP rates.     

Hot Working behaviour of AI-AI2O3 particulate reinforced metal matrix composite

Abstract

The hot deformation behaviour of a particulate reinforced metal matrix composite, manufactured via a casting route and consisting of a 2000 series matrix reinforced with 20 vol.-%Al₂O₃ particles, was investigated over a range of temperatures and strain rates. The behaviour was compared with the unreinforced alloy deformed under the same conditions. Both materials exhibited similar hot working behaviour. However, under all deformation conditions the composite exhibited flow stresses higher than that of the alloy, but as the deformation temperature increased and the strain rate decreased, this difference became negligible. The activation energy for deformation was determined using constitutive equations. The value determined for the composite was slightly higher than that for the alloy. This suggested that the ceramic particles in the composite force the matrix to undergo additional strain hardening during deformation. Dynamic recovery was the sole restoration process in both materials. No evidence of dynamic recrystallisation was found.     

Effect of randomization on ductility of brittle Fe-Si-B glass

Using the Vickers indentation method, a critical plastic deformation energy, E _c, of collapse was proposed to be a convenient measurement to determine the ductility. It was found that the randomization increases the E _c value of a brittle Fe-Si-B alloy glass which is cooled slowly. Namely, the randomization enhances the ductility of the metallic glass.     

Investigation of serrated flow behaviour in Al–10Mg alloy

Abstract

In view of reported anomalies in the serrated flow behaviour of aged Al–8.6Mg alloy, characteristics of serrated flow were investigated in an Al–10Mg alloy after solution treatment as well as after aging. The material was prepared by melting and casting, and then it was extruded, solution treated, and aged at either 150 or 200°C. Strain rate sensitivity, types of serration, onset strain of serrated flow, magnitude of serrations, and frequency of serrations were studied as a function of aging and strain rate. It was found that the alloy exhibited all the usual features of serrated flow except one, i.e. the magnitude of serration increased in the overaged condition after decreasing up to peak aging.     

Thermomechanical response of 50.7 at.-%Ni-Ti alloy in the pseudoelastic regime

Abstract

The thermomechanical response of 50.7 at.-%Ni-Ti superelastic alloy has been investigated for a range of applied strains and strain rates. Parameters of interest include the critical stresses for nucleation and completion (σ _ms and σ _mf respectively) of the stress induced martensite (SIM) transformation, as well as the magnitude of the endothermal and exothermal reactions as a function of the applied strain and rate of deformation. Novel techniques including infrared thermometry and laser extensometry have been utilised, and are found to be particularly suited to experimentation with nitinol. The transformation temperatures and values of entropy and enthalpy of formation have been determined using differential scanning calorimetry.