Effect of stacking fault energy on the dynamic recrystallization during hot working of FCC metals: A study using processing maps

The influence of stacking fault energy (SFE) on the mechanism of dynamic recrystallization (DRX) during hot deformation of FCC metals is examined in the light of results from the power dissipation maps. The DRX domain for high SFE metals like Al and Ni occurred at homologous temperature below 0·7 and strain rates of 0·001 s⁻¹ while for low SFE metals like Cu and Pb the corresponding values are higher than 0·8 and 100 s⁻¹. The peak efficiencies of power dissipation are 50% and below 40% respectively. A simple model which considers the rate of interface formation (nucleation) involving dislocation generation and simultaneous recovery and the rate of interface migration (growth) occurring with the reduction in interface energy as the driving force, has been proposed to account for the effect of SFE on DRX. The calculations reveal that in high SFE metals, interface migration controls DRX while the interface formation is the controlling factor in low SFE metals. In the latter case, the occurrence of flow softening and oscillations could be accounted for by this model.     

Constitutive behaviour in as quenched Al–5Cu–0·4Mn alloy during hot deformation

The overburning temperature of the ZL205A (Al–5Cu–0·4Mn) alloy is first determined by differential scanning calorimetry analysis. Then, the solid solution temperature of ZL205A was determined by metallurgical microstructure observation. Isothermal compression tests of the as quenched ZL205A were conducted in temperature from 25 to 500°C and the strain rate from 0·001 to 1 s^?1. The deformation behaviour of the as quenched ZL205A was investigated. The prediction of the flow stresses were studied using artificial neural network. The average absolute relative error between the predicted flow stresses and experimental results is 4·4%, which demonstrates that the network proposed in the present paper has high precision. Therefore, it can be chosen as a thermomechanical model to treat the distortion problems of components during quenching process.     

Damping properties of open cell microcellular pure Al foams

Abstract

Damping behaviours of the open cell microcellular pure Al foams fabricated by sintering and dissolution process with the relative density of 0·31–0·42 and the pore size of 112–325 μm were investigated. The damping characterisation was conducted on a multifunction internal friction apparatus. The internal friction (IF) was measured at frequencies of 1·0, 3·0 and 6·0 Hz over the temperature range of 298–725 K. The measured IF shows that the open cell pure Al foam has a damping capacity that is enhanced in comparison with pure Al. At a lower temperature (～400 K), the IF of the open cell pure Al foams increases with decreasing relative density, with decreasing pore size and with increasing frequency. The IF peak was found at the temperature range of 433–593 K in the IF curves. It is clear that the IF peak is relaxational type and the activation energy associated with the IF peak is about 1·60 ± 0·02 eV. Defect effects can be used to interpret the damping mechanisms.     

Influence of strain rate on recrystallisation– precipitation interaction in V,Nb, and V–Ti microalloyed steels

Abstract

A study has been made of the recrystallisation–precipitation interaction in three microalloyed steels, containing respectively V, Nb, and V–Ti, applying two different strain rates. Recrystallised fraction v.time curves were determined and used to draw recrystallisation–precipitation–time–temperature (RPTT) diagrams. The influence of strain rate has been shown to be similar in the three steels. On the basis of the results the value of }}SB0·19 has been found for the exponent of the strain rate, following Dutta and Sellars'model for the parameter t _0·05 , which differs from the value }}SB0·5 proposed by these authors. Simultaneously, the influence of strain rate on the static recrystallis ation critical temperature has been determined, it being observed that an increase in the former leads to a drop in the latter. Furthermore, strain rate is shown to have an influence on the recrystallisation–precipitation interaction, acting on those parameters that best contribute to defining RPTT diagrams In this sense, it was found that an increase in the strain rate led to a drop in the curve nose temperature T _N and a reduction in the time necessary for precipitation to finalise t _0·95 , as well as an increase in the recrystallisation rate.     

Low temperature enhanced ductility of friction stir processed 5083 aluminum alloy

Commercial 5083 Al rolled plates were subjected to friction stir processing (FSP) with two different processing parameters, having 430 and 850 rpm tool rotational speed with a single traverse feed rate of 90 mm/min. These FSP conditions resulted in two fine grained microstructures of 0·95 μm (430 rpm) and 2·6 μm (850 rpm). Tensile elongations were measured at a relatively low temperature of 250°C at three strain rates, and demonstrated that a decrease in grain size resulted in significantly enhanced ductility and lower forming loads. The occurrence of a relatively high value of strain rate sensitivity, m of 0·45 for a grain size of 0·95 μm, suggests the operation of superplastic deformation under these present experimental conditions.     

Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Joint formation mechanism of high depth-to-width ratio friction stir welding

High depth-to-width ratio friction stir welding is an attractive method for the joining demands of aluminum profiles, which is sparked with its extremely low heat input and high mechanical performance. In this study, the joint formation mechanism was studied by a numerical model of plastic flow combined with experimental approaches. A fluid-solid-interaction algorithm was proposed to establish the coupling model, and the material to be welded was treated as non-Newtonian fluid. The thread structure and the milling facets on tool pin promoted drastic turbulence of material. The thread structure converged the plasticized material by its inclined plane, and then drove the attached material to refill the welds. The milling facets brought about the periodic dynamic material flow. The thread structure and the milling facets increased the strain rate greatly under the extremely low heat input, which avoided the welding defects. The condition of the peak temperature of 648 K and the strain rate of 151 s⁻¹ attributed to the lowest coarsening degree of precipitate. The tensile strength of the joint reached 265 MPa, equivalent to 86% of base material. The amelioration via the material flow model inhibits the welding defects and optimizes the parameter intervals, providing references to extracting process-structure-property linkages for friction stir welding.     

Hot deformation behaviour of precipitation hardening stainless steel

Abstract

The behaviour of 17-4 precipitation hardening (PH) stainless steel was studied using the hot compression test at temperatures of 950–1150°C with strain rates of 0·001–10 s^?1. The stress–strain curves were plotted by considering the effect of friction. The work hardening rate versus stress curves were used to reveal whether or not dynamic recrystallisation (DRX) occurred. Using the constitutive equations, the activation energy of hot working for 17-4 PH stainless steel was determined as 337 kJ mol^?1. The effect of Zener–Hollomon parameter Z on the peak stress and strain was studied using the power law relation. The normalised critical stress and strain for initiation of DRX were found to be 0·89 and 0·47 respectively. Moreover, these behaviours were compared to other steels.     

Investigation on hot deformation of 20Cr–25Ni superaustenitic stainless steel with starting columnar dendritic microstructure based on kinetic analysis and processing map

Abstract

The deformation behaviour of a 20Cr–25Ni superaustenitic stainless steel (SASS) with initial microstructure of columnar dendrites was investigated using the hot compression method at temperatures of 1000–1200°C and strain rates of 0·01–10 s^?1. It was found that the flow stress was strongly dependent on the applied temperature and strain rate. The constitutive equation relating to the flow stress, temperature and stain rate was proposed for hot deformation of this material, and the apparent activation energy of deformation was calculated to be 516·7 kJ mol^?1. Based on the dynamic materials model and the Murty’s instability criterion, the variations of dissipation efficiency and instability factor with processing parameters were studied. The processing map, combined with the instability map and the dissipation map, was constructed to demonstrate the relationship between hot workability and microstructural evolution. The stability region for hot processing was inferred accurately from the map. The optimum hot working domains were identified in the respective ranges of the temperature and the strain rate of 1025–1120°C and 0·01–0·03 s^?1 or 1140–1200°C and 0·08–1 s^?1, where the material produced many more equiaxed recrystallised grains. Moreover, instability regimes that should be avoided in the actual working were also identified by the processing map. The corresponding instability was associated with localised flow, adiabatic shear band, microcracking and free surface cracks.     

New insight to the oxidation kinetics of silicon-containing steel at high temperature

This study investigated the oxidation kinetics of silicon-containing steel at 1150–1300 °C using a Simultaneous Thermal Analyzer under atmospheric conditions similar to that of an industry reheating furnace. There is a critical time point for the oxidation kinetics at an oxygen concentration of 4·0 vol.-%., following which the oxidation rate constant increases with the increasing oxidation temperature. The model coefficient A in the kinetic oxidation equation was found to be constant. However, before the critical time point, the oxidation rate constant remained unchanged; the model coefficient A decreased with the increasing temperature. Therefore, the kinetic model of silicon-containing steel for isothermal oxidation was observed to be a modified one on the basis of the experimental data. In addition, the critical time point was prolonged with the increasing isothermal oxidation temperature. Moreover, the oxidation activation energy of the tested silicon-containing steel was 366·16 kJ mol^?1.     

Tribological performance of polymer composites used in electrical engineering applications

Sliding wear performance of 20% mica-filled polyamide 6 (PA6 + 20% mica) and 20% short glass fibre-reinforced polysulphone (PSU + 20 GFR) polymer composites used in electrical applications were investigated using a pin-on-disc wear test apparatus. Two different disc materials were used in this study. These are AISI 316 L stainless steel and 30% glass fibre-reinforced polyphenylenesulphide (PPS + 30%GFR) polymer composite. Wear test was carried out at 10, 20 and 30 N applied load values and 0·5 m/s sliding speed and at ambient temperature and humidity. Different combinations of rubbing surfaces were examined and friction coefficient and specific wear rate values were obtained and compared. For two material combinations used in this investigation, the coefficient of friction shows insignificant sensitivity to applied load values and large sensitivity to material combinations. For specific wear rate, PA6 + 20% mica composite has shown insensitivity to change in load, speed and materials combination while PSU + 20% glass fibre composite has shown high sensitivity to the change in load and material combinations. The friction coefficient of PA6 + 20% mica and PSU + 20 glass fibre rubbing against the AISI 4140 steel disc is between 0·35 and 0·40. In rubbing against PPS + 30% glass fibre their values were between 0·25 and 0·30. Specific wear rate for PA6 + 20% mica and PSU + 20% glass fibre composites are in the order of 10^???13 to 10^???14 m²/N. Finally, the wear mechanisms are a combination of adhesive and abrasive wear processes. In terms of application, especially in electrical systems, a substantial contribution was provided to extend switch life. Thus, besides robustness, this also ensured safety for the system and the users against undesirable situations.     

Growth kinetics of (θ′– and θ-phases in Al–3Cu alloy

Abstract

The precipitation of θ′ and θ in Al–3Cu alloy has been investigated, using a resistivity and a transmission electron microscopy (TEM) method. Both stages of the transformation obey the Avrami equation, ξ = 1? exp (?ktⁿ), where ξ is the precipitated fraction of excess solute. The growth parameters are n = 1·68±0·10 for θ′ precipitation and n = 0·63 ± 0·04 for θ precipitation, and the corresponding activation energies are 85·3 ± 10·9 and 111·1±5·2 kJ, respectively. The n-value for the θ′ precipitation stage corresponds to the model proposed by Ham for diffusion-limited growth of disc-like particles, and is in agreement with the TEM results of this investigation. The activation energy for θ′ precipitation is in the range for θ′ boundary migration reported by Aaronson and Laird. The n-value and activation energy for θ precipitation suggests a growth model in which concurrent dissolution of θ′ supplies solute to θ particles via dislocation paths.

MST/480     

Measurement of activation energy values for sintering of active urania powder compacts

Abstract

Wide variations in sintering behaviour have been observed for uranium dioxide powder compacts prepared from ammonium diuranate (ADU) powder precursor and calcined at various temperatures. In general, it is observed using X-ray diffraction that powders calcined below 800°C are non-crystalline and they are also highly sinterable (active), whereas powders calcined above 800°C have poor sinterability and are designated inactive. A model has been proposed for measurement of the activation energy values for sintering of compacts of urania powder based upon a modification derived from Frenkel's sintering model as applicable to non-crystalline powders, from which the shrinkage rate at zero time (t → 0) can be obtained. Moreover, the associated effect on sintering due to the non-crystalline to crystalline transition with progressive thermal treatment is eliminated using this model. The activation energy values for sintering of the non-crystalline powders at various calcination temperatures are in good agreement and, below 800°C, values of ~167·5 kJ mol^?1 are obtained. For powders calcined above 800#x00B0;C, values of ~314·1 kJ mol^?1 are obtained.

MST/812     

Influence of process parameters on superplasticity of friction stir processed nugget in high strength Al – Cu – Li alloy

Abstract

A parametric study was carried out to evaluate the influence of friction stir processing (FSP) parameters (tool rotation speed and feed rate) on the superplasticity of the weld nugget. Dynamically recrystallised AA 2095 thin sheets with a fine grain size of 2 μm were welded using four feed rates and three rotational speeds. High temperature tensile testing was employed to understand the significance of the FSP parameters and to optimise the parameters for maximum elongation. The tool rotation speed was found to be the most decisive parameter for controlling superplastic behaviour. A strain rate sensitivity of 0·68 was measured for the highest rotational speed at the optimum superplastic forming (SPF) temperature of 495°C. A maximum percentage 'elongation to failure' of 550% was achieved for the sheets subjected to FSP at 1000 rev min^?1 and 4·2 mm s^?1, compared with 475% obtained for the base metal at the optimum SPF temperature and strain rate of 495°C and 10^?3s^?1, respectively.     

Relaxor behaviour of (Ba0·5Sr0·5)(Ti0·6Zr0·4)O3 ceramics

Ba_0·5Sr_0·5Ti_0·6Zr_0·4O₃ ceramic has been prepared through solid state reaction route. X-ray diffraction shows that the sample has cubic perovskite structure with space group Pm-3m at room temperature. Temperature dependent dielectric study of the ceramic has been investigated in the frequency range 50 Hz-1 MHz. The density of the sample is determined using Archimedes’ principle and is found to be ∼99% of X-ray density. The dielectric study revealed diffuse phase transition of second order. A broad dielectric anomaly coupled with the shift of dielectric maxima toward a higher temperature with increasing frequency indicates the relaxor-type behaviour in the ceramics. The index of relaxation (γ) and the broadening parameter (Δ) were estimated from a linear fit of the modified Curie-Weiss law. The value of γ ∼ 1·72 indicates the strong relaxor nature of the ceramic. A remarkably good fit to the Vogel-Fulcher relation further supports such a relaxor nature.     

Conduction mechanism in anthracene as a function of temperature

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.     

Mapping microstructure inhomogeneity using electron backscatter diffraction in 316L stainless steel subjected to hot plane strain compression tests

Abstract

The microstructure inhomogeneity in 316L stainless steel subjected to hot plane strain compression tests has been assessed using electron backscatter diffraction (EBSD). Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed isothermally at 950°C at nominal equivalent tensile strain rates of 0·01 and 1 s^?1. Low and high friction conditions have been simulated by applying both a glass based lubricant and a boron nitride spray respectively. Results suggest that friction causes a variation in microstructure from the surface to the midplane of the deformed specimen. Several methods used to quantify and represent this inhomogeneity are presented in the present paper. Electron backscatter diffraction measurement issues are discussed. A grain size mapping method using a two-dimensional moving average has been developed to overcome the difficulties associated with the visualisation of measurement results over large areas on EBSD maps. It has proved to be a powerful tool for the spatial statistics of large quantity data obtained by EBSD.     

Factors influencing long-term creep of type 316 steel at 600°C

Abstract

Creep tests have been carried out at 600°C on type 316 pipe and bar material, some of which was pre-aged to accelerate metallurgical changes. Solute concentrations were assessed by determining the lattice parameters and analyzing extract solutions. A transient strain-induced strengthening of ~3 × 10⁴ h duration was observed for material containing >0·025 wt-% dissolved carbon before testing; this was ascribed to a Mo–C interaction which progressively pinned dislocations. When the transient had decayed, a steady creep rate was achieved. The transient strain induced strength could be eliminated by pre-aging to reduce the dissolved carbon, and this occurred more rapidly in finer-grained material. In the steady creep regime viscous drag was rate controlling, but the effect of dissolved molybdenum was observed to become saturated in the range 0·8–2·0%. There is some evidence that this was because molybdenum acts in conjunction with nitrogen in the steady creep regime.

MST/489     

Experimental study of friction stress on the wall of a cylindrical tube in an oscillating turbulent flow of a liquid

Results of experiments where the nonstationary friction stress in an oscillating flow in a cylindrical tube was measured by a mechanical sensor within rather wide ranges of frequency (1–160Hz) and Reynolds number (1.7·10⁴–1.2·10⁵) are presented. A technique is suggested for calculating dynamic characteristics for the friction stress within three ranges of frequency, for which quasistationary and quasilaminar models of flow and a model for the intermediate frequency range are applicable. On the basis of experimental data, an approximation relation for the transmission ratio of the friction stress as a function of the dimensionless frequency is suggested. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 4, pp. 704–711, July–August, 2000.     

Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract

The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature T_KM and rate parameter α_m. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction f?=?0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.