Dynamic recrystallization during high temperature deformation of magnesium

As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in the deformation of magnesium, particularly at a deformation temperature of 200 °C, where a transition from brittle to ductile behavior is observed. Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperatures and strain rates to examine the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains (  −1.4) seemed to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain growth at elevated deformation temperatures, i.e. 400 °C, even at a low strain rate of 10⁻⁴ s⁻¹. Certain deformation conditions gave rise to a desired fully recrystallized microstructure with an average grain size of 18 μm and an almost random crystallographic texture. For samples deformed at 200 °C/10⁻² s⁻¹, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.     

Hot deformation behavior of SiCp/AZ91 magnesium matrix composite fabricated by stir casting

The uniaxial compressive deformation behavior of a 10 vol.% SiC particulate reinforced AZ91 magnesium matrix composite (SiCp/AZ91) fabricated by stir casting is investigated at elevated temperature (250–400 °C). Peak stresses and flow stresses decrease as temperatures increase and strain rates decrease. The extent of dynamic recrystallization (DRX) becomes less as temperatures decrease at 250–350 °C or strain rates increase, and recrystallization occurs mainly within the intergranular regions rich of particles. Dynamic recrystallization accomplishes at 400 °C even at the strain rate of 1 s⁻¹. An analysis of the effective stress dependence on strain rate and temperature gives a stress exponent of n = 5 and a true activation energy of Q = 99 kJ/kJ. The value of Q is close to the value for grain boundary diffusion in Mg. It is concluded that the deformation mechanism of SiCp/AZ91 composite during hot compression is controlled by the dislocation climb.     

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s⁻¹ was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary {} tensile twins and {} compression twins, and then {}–{} double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the {}–{} double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg₁₇Al₁₂ phases at grain boundaries occurs together with the precipitation of discontinuous Mg₁₇Al₁₂ phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.     

Hot deformation behavior and microstructure evolution of twin-roll-cast Mg–2.9Al–0.9Zn alloy: A study with processing map

The hot deformation behavior and microstructure evolution of twin-roll-cast of Mg–2.9Al–0.9Zn–0.4Mn (AZ31) alloy has been studied using the processing map. The tensile tests were conducted in the temperature range of 150–400 °C and the strain rate range of 0.0004–4 s⁻¹ to establish the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (i) the continuous dynamic recrystallization (CDRX) domain in the range of 200–280 °C/≤0.004 s⁻¹ with fine grains which provides a potential for warm deformation such as deep drawing; (ii) the discontinuous dynamic recrystallization (DDRX) domain around 400 °C at high strain rate (0.4 s⁻¹ and above) with excellent elongation which can be utilized for forging, extrusion and rolling; (iii) the grain boundary sliding (GBS) domain at slow strain rate (below 0.004 s⁻¹) above 350 °C appears abundant of cavities, which result in fracture and reduce the ductility of the adopted material; and (iv) the flow instability region which locates at the upper left of the processing map shows the metallographic feature of flow localization.     

Electrical and optical properties of 12CaO·7Al2O3 electride doped indium tin oxide thin film deposited by RF magnetron co-sputtering

12CaO·7Al₂O₃ electride (C12A7:e⁻) doped indium tin oxide (ITO) (ITO:C12A7:e⁻) thin films were fabricated on a glass substrate by an RF magnetron co-sputtering system with increasing number of C12A7:e⁻ chips (from 1 to 7) and at various oxygen partial pressure ratios. The optical transmittance of the ITO:C12A7:e⁻ thin film was higher than 70% in the visible wavelength region. In the electrical properties of the thin film, a decrease of the carrier concentration from 2.6 × 10²⁰ cm^− 3 to 2.1 × 10¹⁸ cm^− 3 and increase of the resistivity from 1.4 × 10^− 3 Ω cm to 4.1 × 10^− 1 Ω cm were observed with increasing number of C12A7:e⁻ chips and oxygen partial pressure ratios. It was also observed that the Hall mobility was decreased from 17.27 cm²·V^− 1·s^− 1 to 5.13 cm²·V^− 1·s^− 1. The work function of the ITO thin film was reduced by doping it with C12A7:e⁻.     

Microstructures and properties of biomedical TiNbZrFe β-titanium alloy under aging conditions

A metastable β-titanium alloy Ti–28Nb–13Zr–0.5Fe (TNZF alloy for short) was designed for implant biomedical application. The forged specimens were solute-treated at 850 °C followed by water quenching and then aged at 350 °C, 450 °C, and 550 °C for 2–6 h in order to evaluate the effect of phase transformation during ageing on the biomechanical compatibility of the alloy. The quenched microstructure consists of lath α″ martensite and β phase. A large quantities of shuttle-like ω phase precipitate at 350 °C, leading to the drastic increase of strength and elastic modulus and the decrease of plasticity. Ageing at 450 °C for 4 h, small amount of elliptic ω phase and dot α phase precipitate from β matrix. With increasing ageing time α precipitations begin to coarsen and precipitation free zones (PFZs) form around prior β grain boundaries. Needle-like α phase precipitates on grain boundaries and intra-grains when aged at 550 °C. Both PFZs and grain boundary α precipitates are prone to bring about the intergranular fracture and thus have adverse effects on the tensile strength and fracture plasticity. The quenched microstructure has good combination properties of high strength, high plasticity and low elastic modulus.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

The influence of dynamic strain aging on the low cycle fatigue behavior of near alpha titanium alloy IMI 834

Total strain controlled low cycle fatigue tests on IMI 834 have been conducted in air in the temperature range between 375 and 500 °C at a temperature interval of 25 °C at the nominal strain rate of 6.67 × 10⁻⁴ s⁻¹. The observed maximum peak stress ratio, minimum half-life plastic strain range and lower fatigue life at 425 °C indicates the occurrence of dynamic strain aging (DSA). Pronounced deformation bands, increased dislocation density and non-uniform dispersion of dislocations inside primary α grains observed by the study of transmission electron microscopy supports the occurrence of dynamic strain aging. Initial cyclic softening was attributed to shearing of Ti₃Al precipitates as revealed by TEM evidences.     

High temperature deformation behavior of a near alpha Ti600 titanium alloy

The high temperature deformation behavior of a near alpha Ti600 titanium alloy was investigated with isothermal compression tests at temperatures ranging from 800 to 1000 °C and strain rates ranging from 0.001 to 10.0 s⁻¹. The apparent activation energy of deformation was calculated to be 620.0 kJ mol⁻¹, and constitutive equation that described the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of Ti600 titanium alloy in the α + β phase region. The processing map was calculated to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. High efficiency values of power dissipation over 55% obtained under the conditions of strain rate lower than 0.01 s⁻¹ and temperature about 920 °C was identified to represent superplastic deformation in this region. Plasticity instability was expected in the regime of strain rate higher than 1 s⁻¹ and the entire temperature range investigated.     

Effect of temperature and strain rate on tensile behavior of ultrafine-grained aluminum alloys

Ultrafine-grained Al–4Y–4Ni and Al–4Y–4Ni–0.9Fe (at.%) alloys were synthesized by the consolidation of atomized powders and subsequent hot extrusion. The mechanical behavior of these two alloys has been studied by performing uniaxial tension tests ranging from room temperature to 350 °C. These alloys, with high volume fraction of second-phase particles, exhibited ambient temperature tensile strength ranging from 473 to 608 MPa and plastic elongation ranging from 6.7 to 9.6% at an initial strain rate of 1 × 10⁻³ s⁻¹. However, lower ductility was observed with decreasing strain rate at the intermediate temperature ranging from 150 to 250 °C for Al–Y–Ni–Fe alloys due to limited work hardening.     

Printed circuit heat exchanger thermal–hydraulic performance in supercritical CO2 experimental loop

Heat transfer and pressure drop characteristics of the Printed Circuit Heat Exchanger (PCHE) were investigated in an experimental supercritical CO₂ loop. The inlet temperature and pressure were varied from 280 to 300 °C/2.2 to 3.2 MPa in the hot side and from 90 to 108 °C/6.5 to 10.5 MPa in the cold side while the mass flow rate was varied from 40 to 80 kg h⁻¹. The overall heat transfer coefficient range is 300–650 W m⁻² K⁻¹ while the compactness with respect to the heat exchanger core is approximately 1050 m² m⁻³. The empirical correlations to predict the local heat transfer coefficient and pressure drop factor as a function of the Reynolds number have been proposed for the tested PCHE.     

Effects of Sn on microstructure and mechanical properties of as-rolled Mg–5Zn–1Mn alloy

Effects of Sn on microstructure and mechanical properties of Mg–5Zn–1Mn alloy subjected to high strain rate rolling (9.1?s^-1), 300°C and 80% pass reduction are investigated. With higher Sn content, the dynamic recrystallisation (DRX) grain size gradually decreases due to the stronger pinning of nano-scale precipitates at grain boundaries and the DRX fraction first increases due to the enhanced effect on DRX by decreasing stacking fault energy and then decreases due to more precipitates at grain boundaries. Ultimate tensile strength (UTS) and elongation to rupture (Er) of as-rolled alloys increase and then decrease. Alloy with 0.9 mass% Sn exhibits the highest DRX fraction (95?vol.-%), the finer DRX grain size (1.22?µm), UTS of 358?MPa and Er of 20.4%.     

Development of high-strength,low-cost wrought Mg–2.5 mass% Zn alloy through micro-alloying with Ca and La

A high-strength low-cost Mg–2.5Zn–0.3Ca–0.4La (mass%) alloy was fabricated by hot extrusion following direct-chill casting. Yield strength (YS), ultimate tensile strength (UTS) and elongation to failure of the alloy are 325 MPa, 341 MPa and 15%, respectively. The high strength of the extruded Mg–2.5Zn–0.3Ca–0.4La alloy is mainly due to grain refinement, dense precipitation and high density of dislocations. The extruded alloy exhibits a bimodal microstructure containing fine dynamic recrystallized (DRXed) grains and deformed regions. High density of dislocations is stored in the deformed regions while dense precipitates are homogeneously distributed in both the DRXed grains and the deformed regions. However, precipitates in the DRXed regions show in spherical shape only, while they are in rod-like shape and spherical shape in the deformed regions.     

Investigation of hot ductility in Al-killed boron steels

The influence of boron to nitrogen ratio, strain rate and cooling rate on hot ductility of aluminium-killed, low carbon, boron microalloyed steel was investigated. Hot tensile testing was performed on steel samples reheated in argon to 1300 °C, cooled at rates of 0.3, 1.2 and 3.0 °C s⁻¹ to temperatures in the range 750–1050 °C, and then strained to failure at initial strain rates of 1 × 10⁻⁴ or 1 × 10⁻³ s⁻¹. It was found that the steel with a B:N ratio of 0.19 showed deep hot ductility troughs for all tested conditions; the steel with a B:N ratio of 0.47 showed a deep ductility trough for a high cooling rate of 3.0 °C s⁻¹ and the steel with a near-stoichiometric B:N ratio of 0.75 showed no ductility troughs for the tested conditions. The ductility troughs extended from 900 °C (near the Ae₃ temperature) to 1000 or 1050 °C in the single-phase austenite region. The proposed mechanism of hot ductility improvement with increase in B:N ratio in these steels is that the B removes N from solution, thus reducing the strain-induced precipitation of AlN. Additionally, BN co-precipitates with sulphides, preventing precipitation of fine MnS, CuS and FeS, and forming large, complex precipitates that have no effect on hot ductility.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.     

Rapid densification and reaction sequences in self-reinforced Y-α-SiAlON ceramics with barium aluminosilicate as an additive

Y-α-SiAlON (Y_1/3Si₁₀Al₂ON₁₅) ceramics with 5 wt.%BaAl₂Si₂O₈ (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600 °C without holding and using a heating rate of 100 °C min⁻¹, but the transformation from α-Si₃N₄ to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800 °C for 5 min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2 GPa and fracture toughness of 6.8 MPa m^1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300 °C.     

Effects of strain on the workability of a high strength low alloy steel in hot compression

Based on the experimental results from the hot compression tests of 42CrMo steel, the efficiencies of power dissipation and instability parameter were evaluated. The effects of strain on the efficiency of power dissipation and instability parameter of 42CrMo steel have been discussed in detail. Processing maps were constructed by superimposition of the instability map over the power dissipation map. The dynamic recrystallization domains and instable zones were identified in the processing map. The effects of strain on microstructural evolutions were correlated with the processing maps. According to the 3D processing maps, the optimum domain of hot deformation is in the temperature range of 1050–1150 °C and strain rate range of 0.01–3 s⁻¹, with its peak efficiency of 32% at about 1140 °C and 0.23 s⁻¹, which are the optimum hot working parameters.     

Optimization of hot workability of an Al-Mg-Si alloy using processing maps

The hot workability of an Al-Mg-Si alloy has been studied by conducting constant strain-rate compression tests. The temperature range and strain-rate regime selected for the present study were 300–550 °C and 0.001–1 s^–1, respectively. On the basis of true stress data, the strain-rate sensitivity values were calculated and used for establishing processing maps following the dynamic materials model. These maps delineate characteristic domains of different dissipative mechanisms. Two domains of dynamic recrystallization (DRX) have been identified which are associated with the peak efficiency of power dissipation (34%) and complete reconstitution of as-cast microstructure. As a result, optimum hot ductility is achieved in the DRX domains. The strain rates at which DRX domains occur are determined by the second-phase particles such as Mg₂Si precipitates and intermetallic compounds. The alloy also exhibits microstructural instability in the form of localized plastic deformation in the temperature range 300–350 °C and at strain rate 1 s^–1.