THE EFFECT OF GRAIN SIZE, STRAIN AND TEMPERATURE ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR OF POLYCRYSTALLINE ALUMINIUM AND AL ALLOYS

Abstract The tensile yield and flow stresses of aluminium, A1-2.63Mg alloy and A1-2.07Li alloy at room temperature are shown to depend on the inverse square root of the polycrystal grain size and are described empirically by the Hall-Petch relation. The same relation describes the flow stress-grain size dependence for A1-2.07Li alloy at temperatures ranging from - 196°C to 400°C. The strain hardening in the friction stress of each material at 20°C is independent of the grain size, is approximately parabolic and is greatest for the precipitation strengthened A1-2.07Li alloy. The grain size contribution to the tensile flow stress is dependent on both the tensile strain and composition. The friction stress, σ₀, and slip band stress intensity parameter, k_ε, at yield, k_y, are both dependent on temperature. Low temperature suppresses dislocation annihilation and recovery processes, leading to planar pile-ups at grain boundaries and a hardening that is linear with strain. Weak hardening is observed at 250°C and 400°C due to extensive annihilation and recovery. The value of k_ε, at all temperatures falls following initial yielding with the generation of freshly unlocked sources.     

Influence of grain size on tensile properties of Al-Mg alloys

Abstract

Grain size refinement is an important strengthening mechanism in Al-Mg 5000 series alloys, which have a relatively large Hall-Petch slope compared with other Al alloys. In addition, the high work hardening rate exhibited by Al-Mg alloys provides excellent formability. This paper investigates the influence of grain size on the flow stress over a range of strains, and in several different Al-Mg alloys. It is found that the Hall-Petch slope decreases after yield, indicating that the large grain size effect is primarily associated with initiating plasticity in these alloys. Beyond yield the slope decreases to a value equivalent to other, non-Mg containing alloys, and shows no clear dependence on strain. The intercept stress from the Hall-Petch plots at different strains is non-linear with ? 1/2 for alloys containing up to 3 wt-%Mg, which indicates that the free slip distance is strain dependent in these alloys. In an Al-5 wt-%Mg alloy the intercept stress is linear with ? 1/2, indicating that solute atoms are controlling the free slip distance. If Mn is added to the Al-5 wt-%Mg alloy, as it is in commercial alloys, it has little influence on the grain size dependence, but it does increase the frictional stress at the highest Mn level of 0.7 wt-%.     

Changes in texture and grain size in hot rolled magnesium and effect on yield strength

Variations in the grain size and basal texture in magnesium hot-rolled in the temperature range 570–770 K have been studied and compared with those observed in specimens hot-rolled at 570 K and annealed at different temperatures in the range 570–770 K. In specimens hot-rolled temperatures higher than 670 K, a rapid increase in grain size and a weakening in basal texture have been observed. The yield stress of the specimen is lower than that predicted by the Hall-Petch relation and this result is interpreted in terms of the reduced basal texture. The dependence of yield stress on grain size in magnesium hot-rolled at different temperatures lower than 670 K matches with that observed in specimens annealed at different temperatures.     

Effects of aging temperature and grain size on the formation of serrated grain boundaries in an AISI 316 stainless steel

The effects of aging temperature and grain size on the formation of serrated grain boundaries have been investigated in an AISI 316 stainless steel. Grain size increased slightly over aging temperature ranges of 650–870 °C, resulting in predominantly serrated grain boundaries. However, when the temperature exceeded 880 °C, the grain size significantly increased, and grain boundary serration was not observed. The initial grain size also had an influence on the occurrence of grain boundary serration. For specimen having a large initial grain size of about 200 μm, no serrated grain boundary formed after aging treatment at 760 °C. Serrated grain boundaries were observed when “normal” initial grain sizes 55 μm were employed. It was found that the frequency of low angle boundaries markedly increased as the initial grain size increased from 55 to 200 μm. From the results obtained, it is possible to describe that the grain boundary serration could be considered as a spontaneous reaction that aims to reduce the total free energy of the system, and form a new interface of lower free energy. We proposed that the competition between grain growth and grain boundary serration during aging treatment reduces the total free energy of the alloy system: at temperatures exceeding 880 °C, the dominant process is the grain growth, while grain boundary serration predominates over the range of 650–870 °C.     

Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

Hall-Petch behaviour of 316L austenitic stainless steel at elevated temperatures

Abstract

The plastic deformation behaviour of two different batches (having differences in chemical composition) of 316L austenitic stainless steel has been explored in the 200-800°C temperature range as a function of grain size. The plastic behaviour is correlated with microstructural observations of annealed and deformed samples. The microstructural parameters measured in this study are grain size, grain size and shape distribution, grain aspect ratio, and the distribution of dihedral angles. Hardness measurements were also performed to assess the hardness profile across the grains. The applicability of Hall-Petch relationship was tested in the 200-800°C temperature range. It is observed that the Hall-Petch relationship is applicable in the coarse grain regime (d≥6 μm) and Kocks composite relationship (σ versus d^-1) in the fine grain regime (d≤6 μm) of batch 1 samples in the 200-600^°C temperature range. At 800°C, the Hall-Petch data is widely scattered and the scatter increases with increasing strain. The variation of Hall-Petch parameters and Kocks parameters with strain and temperature are analysed on the basis of changes in the microstructural parameters. The operating deformation mechanisms in different temperature and strain ranges are discussed on the basis of variation of microstructural parameters with strain and temperature.     

Plastic behaviour of CuAl2

The plastic behaviour of CuAl₂ was studied by compression testing of single crystals and polycrystals in the temperature range 300–575 °C. While single crystals were grown from the melt by the Bridgeman technique, ingot and powder metallurgy routes were adopted for polycrystalline specimens. In addition to exploring their flow behaviour, the deformation mechanism was assessed through thermal activation analysis. It was observed that CuAl₂ failed in a brittle manner in compression below 375 °C and its ductility improved progressively with temperature. The brittle-ductile transition (BDT) temperature was influenced by the initial dislocation density but not by the grain size. The strong temperature dependence of flow stress and grain size strengthening effect as per the Hall-Petch relation, were dominant up to nearly the melting temperature of CuAl₂. The measured activation parameters for deformation suggest that the Peierls mechanism is rate controlling over the investigated temperature range.     

Effect of grain size and indium addition on tensile characteristics of Al–1Si alloy

Abstract

The effect of grain size and indium addition on the workhardening characteristics of Al–1Si (wt-%) alloy has been investigated at room temperature (RT). The samples were preaged at different temperatures in the range 523–623 K. The yield stress, the fracture stress, the fracture time and the linear workhardening coefficient generally decreased with increasing temperature and/or grain size, while the fracture strain and dislocation slip distance increased. The yield and fracture stresses for different grain sizes at different temperatures were found to be linearly related to grain diameters. Indium addition caused general increase for all the measured strength parameters. As concluded from transmission electron microscope (TEM) investigations, In addition to Al–Si alloy may retard the coarsening of Si particles. The energies activating the operating fracture mechanisms were found to be 79·6±0·4 and 32·4±0·4 kJ mol^?1 for alloys Al–1Si and Al–1Si–0·2In respectively. This suggests a value of 47·2 kJ mol^?1 as a binding energy between Si and In atoms in Al matrix.     

The effect of grain size on the flow stress determined from spherical microindentation of die-cast magnesium AM60B alloy

Spherical microindentation tests were performed on samples cut from an AM60B magnesium alloy die casting to determine the effects of grain size on the local flow stress. Five samples were indented in the skin region (finer grain sizes), two samples in the core region (larger grain sizes and dendrites), and one sample was indented in both the skin and core region of the die-casting. It was determined that the Hall-Petch equation is applicable for predicting the initial yield point and the flow stress at several levels of plastic strain only in the skin region, and not the core region, of the die-casting. The Hall-Petch slope and intercept stress, determined from spherical indentation in the skin region, compare accurately with previously published results. Possible reasons for indentation results from the core region deviating from the Hall-Petch relationship are discussed. The Hall-Petch slope follows a linearly increasing relation with the square root of plastic strain; however, no conclusions can be drawn concerning this because of the small range of plastic strain tested.     

Effect of grain size on the tensile deformation of wrought Mg-MM-Al-Zn-Sn alloy

The effect of grain size on the tensile deformation of Mg-MM-Al-Zn-Sn (EAZT211) alloy sheet has been investigated. Specimens with grain size varying from ~ 10 to ~ 20 μm have been obtained by altering the annealing conditions after rolling. The yield strength of EAZT211 sheet exhibits grain size dependence according to Hall-Petch relationship from room temperature to 200 °C. Occurrence of yield phenomenon and decrease of Hall-Petch slope (k) with increasing strain suggest that non-basal slip system operates during deformation. In addition, deformation mechanism changes from slip mechanism to grain boundary sliding mechanism at ~ 150 °C.     

On the Influence of Microstructure on the Fracture Behavior of Gamma-Based Titanium Aluminides. I. Effects of Alloying with Mn

The results of a study of the effects of grain size on the tensile and fracture properties of Mn-containing gamma alloys are presented in part I of this paper. Ductility and yield/ultimate tensile strengths at room- and elevated-temperature are shown to exhibit a simple Hall-Petch dependence on the average grain/lamellar packet size. The observed Hall-Petch behavior is found to be independent of lamellar volume fraction and fracture mechanism at room - and elevated-temperature. The implications of the results are discussed for future alloy/process development.     

Localized plastic flow autowaves and the Hall-Petch relation in aluminum

Characteristics of the plastic strain macrolocalization are compared to parameters of the Hall-Petch relation for the flow stress in polycrystalline aluminum samples with grain sizes ranging from 0.008 to 5 mm. It is established that, in the range of brain sizes studied, there are two possible types of the dependence of the length of localized strain autowave on the grain size and two variants of the Hall-Petch relation. It is shown that the boundary between the two variants in both cases corresponds to d ≈ 0.1 mm. Interconnection of the patterns of plastic flow localization and the Hall-Petch relation is traced.     

Computer modelling of phase diagrams

Abstract

An investigation has been made of the tensile behaviour between 20 and 600°C of two ultrahigh boron steels (Fe–2·2B and Fe–4·9B), consolidated by hot isostatic pressing at temperatures ranging from 700 to 1100°C. Tensile tests showed plastic deformation only in the Fe–2·2B alloy. A decrease in yield and ultimate tensile stresses occurred when the consolidation temperature was increased. This was accompanied by an increase in the elongation to failure. This alloy satisfies the Hall–Petch relation for all testing temperatures. The slope of the yield stress versus d^?1/2 curve (d is grain size) decreases as the temperature increases, indicating that the mechanism controlling plastic deformation becomes independent of grain size at high testing temperatures. The fracture mode observed was brittle at room temperature and ductile, shown by the presence of dimples, at temperatures above 400°C.

MST/2050     

Microstructural evolution of AZ61 magnesium alloy during hot deformation

Abstract

The microstructural evolution of AZ61 magnesium alloy during hot compression at various temperatures was investigated. The experimental results show that dynamic recrystallisation occurs over a wide temperature range. Grains can be greatly refined through dynamic recrystallisation. The mean size of the recrystallised grains increases with a decrease of temperature or value of Z (Zener – Hollomon parameter), while the reciprocal of the recrystallised grain size has a good linear relationship with the natural logarithm of the Z value, as well as the hyperbolic term of the flow stress. Basal and non-basal segments have been found in both recrystallised grains and primary grains, whereas dislocation pileups exist only in recrystallised grains when the temperature is lower than 673 K. The occurrence of twins is dependent on temperature and strain. When the strain increases, primary twins evolve into secondary twins. However, secondary twins grow with an increase of temperature; some secondary twins evolve into subgrains.     

Analysis of high flow stress and microstructural evolution of TC6 titanium alloy during isothermal forging

Abstract

Elevated temperature true stress – strain curves have been determined for the isothermal deformation of a TC6 titanium alloy using hot compression testing in the deformation temperature range 800 – 1040°C, strain rate range 0.001 – 50 s^-1 and reduction in height of 30 – 50%. The experimental results show that the flow stress of TC6 titanium alloy is strongly dependent on process parameters, especially on the deformation temperature and strain rate. The peak stress and steady stress of such an alloy have the same characterisation, which increases with higher strain rate and lower deformation temperature. During isothermal forging, microstructural characterisation, including volume fraction, grain size, and grain pattern of prior α phase, varies with different temperatures, height reductions, and strain rates.     

Mechanical properties of Fe-Co soft magnets

The tensile behavior of two magnetically soft alloys, Fe-49Co-2V and Fe-27Co, has been characterized as a function of testing temperature, grain size, and degree of long-range order. Several trends in the yield strength of the two alloys have been noted and possible mechanisms for their occurrence discussed. Ordering is found to markedly lower the yield strength of the Fe-49Co-2V. Both alloys exhibit three distinct regions in their yield strength vs. temperature curves. At lower and higher temperatures, i.e. regions I and III, the yield strength shows the normal drop with increasing temperature. In the intermediate temperature range (region II), however, Fe-49Co-2V with a B2 ordered structure demonstrates an anomalous strengthening with increasing temperature while the yield strength remains constant in the disordered Fe-27Co. Both alloys exhibit a Hall-Petch type relationship in their yield strength as a function of grain size and show a decrease in the strain hardening coefficient with increasing grain size.     

Mechanical behaviour and texture of annealed AZ31 Mg alloy deformed by ECAP

Abstract

AZ31 Mg alloy samples were processed by equal channel angular pressing (ECAP) at 220°C for four passes. An average grain size of ～1·9 μm with reasonable homogeneity was obtained. The ECAP process imparted large plastic shear strains and strong deformation textures to the material. Subsequent annealing of the equal channel angular pressed samples produced interesting mechanical behaviours. While yield strength increased and ductility decreased immediately after undergoing ECAP, annealing at temperatures <250°C restored ductility significantly at a small decrease in of yield strength. Annealing at temperatures >250°C reduced yield strength without additional improvement in ductility. It is believed that the combination of stress relief via dislocation elimination, refined microstructure and the retention of a strong ECAP texture at low annealing temperatures enhance ductility. High temperature annealing breaks down the ECAP texture resulting in no further improvement in ductility. The results show that the mechanical properties of the alloy can be positively influenced by annealing after ECAP to achieve a combination of strength and ductility.     

Mechanics of a nanocrystalline coating and grain-size dependence of its plastic strength

From the mechanics perspective the most critical properties of a coating material are likely its yield strength, hardness, and wear resistance. These properties are intimately related to each other as a material with high yield strength usually also possesses superior hardness and strong wear resistance. Our objective in this study is to seek for the strongest material state in terms of its plastic strength, and the critical grain size at which this strength can be attained. Based on the cross-sectional morphology of a nanocrystalline coating we first conceive a composite plate model consisting of the columnar grains and the grain boundary affiliated region (broadly called the GB zone), and present a plane-stress theory to investigate its in-plane behavior. We then make use of a linear comparison composite and a field fluctuation approach to explore the competition between the grain interior and the plastically softer GB zone as the grain size decreases from the coarse grain to the nanometer range. Based on the developed model the anisotropic stress-strain relations of a ZrN coating are illustrated as a function of grain size in both in-plane and out-of-plane directions. It is demonstrated that, as the grain size decreases, the variation of plastic strength in the Hall-Petch plot undergoes a transition from a positive to a negative slope, exhibiting the Hall-Petch and the inverse Hall-Petch effects. The strength-differential effect between tension and compression is also observed. The critical grain sizes at which the maximum strength develops are found to be lower along the out-of-plane direction than along the in-plane ones, and also lower under compression than under tension. Based on Tabor’s law, our calculated critical grain size for the hardness was about 16 nm, while a recent indentation test indicated a range of 14.2-19 nm. The existence of Hall-Petch, inverse Hall-Petch, and strength-differential effects, and of an optimal grain size, is thus theoretically proven for the first time for nanocrystalline coatings.     

Modelling recovery and recrystallisation during annealing of AA 5754 aluminium alloy

Abstract

A microstructure model taking into account recovery and recrystallisation has been developed to predict the yield stress and the recrystallised grain size during continuous annealing of cold rolled AA 5754 sheet alloy. Using isothermal annealing tests, recovery and recrystallisation kinetics were quantified as a function of temperature and cold reduction. The model was formulated employing the internal state variable approach with the following three state variables: dislocation density, volume fraction recrystallised, and grain size. A rule of mixtures is adopted to separate the effect of recovery and recrystallisation in the overall softening. Model validation has been carried out by comparing the predicted softening curves with those obtained in continuous heating tests replicating heating rates of industrial continuous annealing lines. The model can be applied to non-isothermal processing routes of industrial cold rolled AA 5754 with thickness reduction in the range 40-80%.     

The Portevin-Le Châtelier effect of gradient nanostructured 5182 aluminum alloy by surface mechanical attrition treatment

Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment (SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Châtelier (PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.