Constitutive Modeling and Hot Deformation Behavior of Duplex Structured Mg–Li–Al–Sr Alloy

Hot deformation behavior of an as-extruded duplex structured Mg–9Li–3Al–2.5Sr alloy is investigated via hot compression tests conducted at 200–350 °C with strain rate of 0.001–1 s-1.The flow behavior of Mg–9Li–3Al–2.5Sr alloy can be described accurately by hyperbolic sine constitutive equation and the average activation energy for deformation is calculated as 143.5 k J/mol.Based on a dynamic materials model,the processing maps of Mg–9Li–3Al–2.5Sr alloy which describe the variation of power dissipation efficiency are constructed as a function of temperature and strain rate.The processing maps exhibit an area of discontinuous dynamic recrystallization occurring at 280–300 °C with strain rate of 0.001–0.01 s-1,which corresponds to the optimum hot working conditions.     

Compression Mechanical Behaviour of 7075 Aluminium Matrix Composite Reinforced with Nano-sized SiC Particles in Semisolid State

The 7075 aluminium matrix composite reinforced with nano-sized Si C particles was fabricated by ultrasonic assisted semisolid stirring method. The compression mechanical behaviour of the fabricated composite in semisolid state was investigated. The results show that the microstructure of the composite before semisolid compression consists of fine and spheroidal solid grains surrounded by liquid phase.Semisolid compression led to a nonuniform plastic deformation of solid grains. A slight plastic deformation occurred in the locations near the free surface due to the dependence of deformation on liquid flow and flow of liquid incorporating solid grains. However, obvious plastic deformation occurred in the central location and location contacting to die due to the contribution of plastic deformation of solid grains.The true stress–strain curve of the sample compressed at 500 °C consists of rapid increase of true stress and steady stage. However, rapid increase of true stress and decrease of true stress and steady stage are involved in the true stress–strain curves of the samples compressed at 550, 560, 570, 580 and 590 °C.The true stress–strain curve at 600 °C is similar to that at 500 °C. Apparent viscosity decreases with an increase of shear rate, indicating a shear thinning occurrence. When soaking time increases from 5 min to 15 min, the peak stress and steady stress decrease significantly. A further increase of the soaking time led to a slight change. Peak stress and steady stress increase with increasing volume fraction of Si C particles. A sudden increase or decrease of compression velocity led to a significant increase or decrease of the steady stress. The destruction of the samples compressed at solid state temperature mainly depends on cracks parallel to compression direction. However, the destruction forms of the samples compressed at semisolid temperatures consist of cracks parallel to compression direction and partial collapse. Increasing soaking time led to an obvious change of the destruction forms. Compression velocity affects slightly the macro appearance of the sample compressed at semisolid temperatures.     

Characterization of Austenite Dynamic Recrystallization under Different Z Parameters in a Microalloyed Steel

A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850-1100℃ and strain rates 0.01-1s-1 to study the influence of deformation conditions on the dynamic recrystallization characteristics of austenite.The results show that dynamic recrystallization occurs more easily with the decrease of strain rate and the increase of deformation temperature.The complete dynamically recrystallized grain size as a function of Zener-Hollomon parameter was established.It was found that dynamically recrystallized grain sizes decrease with increasing strain rate and decreasing deformation temperature.The effect of microalloying elements on peak strain was investigated and the solute drag corrected peak strain was determined.Also,the dynamic recrystallization map of austenite was obtained by using recrystallization critical parameters.     

Flow behavior and processing map for hot deformation of ATI425 titanium alloy

Flow behavior and processing map play important roles in the hot deformation process of titanium alloys. In this research, compression Gleeble tests have been carried out to investigate the stress–strain relationship at temperatures ranging from 700 to 1000℃ and strain rates ranging from 0.001 to 1 s~(-1) for ATI 425 titanium alloy. Arrhenius type constitutive equation was obtained to describe the compressive flow behavior with modification of additional deformation dead zone, friction model, temperature model and strain rate. The introduction of novel calculation method for value in Arrhenius equation gives more accurate fitting than traditional one. Processing maps were drawn based on the distribution of dissipator co-content, and optimized deformation temperature and strain rate range obtained. It is proven to be accurate and effective through the experimental results. The microstructure analysis shows that more dynamic recrystallization can be achieved in the area with larger value on the processing map.     

Constitutive Relationship and Hot Processing Maps of Mg-Gd-Y-Nb-Zr Alloy

The hot working behavior of Mg-Gd-Y-Nb-Zr alloy was investigated using constitutive model and hot processing maps in this work. Isothermal compression tests were conducted with temperature and strain rate range of 703–773 K and 0.01–5 s~(-1), respectively. Improved Arrhenius-type equation incorporated with strain compensations was used to predict flow behavior of the alloy, and the predictability was evaluated using correlation coefficient, root mean square error and absolute relative error. Processing maps were constructed at different strains for Mg-Gd-Y-Nb-Zr alloy based on dynamic materials model.The processing maps are divided into three domains and the corresponding microstructure evolutions are referred to the forming of straight grain boundaries, twinning, dynamic recrystallization and grain growth. Instability occurred mainly at the strain rate range of 0.3s~(-1)–0.5s~(-1). The optimum processing domain is mainly at the temperature range of 703–765 K with the strain rate range of 0.01–0.1 s~(-1).     

Hot Deformation Behavior of 2124 Al Alloy

The mechanical behavior of 2124 Al alloy produced by powder metallurgy was investigated with compression test at different temperatures and strain rates. The tests were performed in the temperature range of 300℃～500℃ and at strain rates from 0.001 s^-1 to 1.0 s^-1. The compression flow curves exhibited an initial sharp increase with strain, followed by monotonous hardening. The maximum stress decreased with decreasing strain rate and increasing temperature. The hot deformation characteristics of the material were studied using processing maps. The domain of safety and unsafe regime were identified and validated through microstructural examination.     

Deformation Behavior of the Zr_(53.5)Cu_(26.5)Ni_5Al_(12)Ag_3 Bulk Metallic Glass Over a Wide Range of Strain Rate and Temperatures

The stress–strain relations for the Zr53.5Cu26.5Ni5Al12Ag3 bulk metallic glass(BMG) over a broad range of temperatures(room temperature to its supercooled liquid region) and strain rates(10-4to 10-1s-1) were established in uniaxial compression using a thermal-mechanical simulation system. The superplastic flow was seen above its glass transition temperature(Tg = 694 K) and strain rates of up to 10-1s-1from the variation of stress–strain curves. A deformation map of strain rate vs temperature of Zr53.5Cu26.5Ni5Al12Ag3 was obtained, which was mainly composed of homogeneous and inhomogeneous deformation regions,the former featuring either Newtonian or non-Newtonian flow while the latter characterizing linear elastic behavior followed by shear localization, respectively. A phenomenological constitutive equation used to describe a master curve of viscosity with respect to the strain rate was obtained by fitting the experimental results, which determines the viscosity of the present alloy at the temperature near and above Tg. The results show the Zr53.5Cu26.5Ni5Al12Ag3 BMG is the subject suitable for net shape forming process at the supercooled liquid region.     

Microstructural Evolution of Nb–V–Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing

The microstructural evolution and precipitation behaviour of Nb–V–Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb–V–Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite.The TEM analysis showed that a signi?cant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the ?nishing stage in Nb–V–Mo steel(i.e. 830 °C). It was also found that the V(C,N)precipitation primarily occurred in both ferrite and deformed austenite below the ?nishing stage. The results suggested that Nb–Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a signi?cant precipitation strengthening in both Nb–V–Mo and V steels at room temperature.     

Hot Deformation and Work Hardening Behavior of an Extruded Mg–Zn–Mn–Y Alloy

The aim of this study was to evaluate the strain hardening and hot deformation behavior of asextruded Mg-Zn-Mn(ZM31) magnesium alloy with varying Y contents(0.3,3.2,and 6 wt%) via compression testing along the extrusion direction at room temperature,200 °C and 300 ℃.Texture and phases were identified by X-ray diffraction.Alloy ZM31 + 0.3Y consisted of a mixture of fine equiaxed grains and elongated grains with I-phase(Mg_3YZn_6);alloy ZM31 + 3.2Y contained I-phase and W-phase(Mg_3Y_2Zn_3);alloy ZM31 + 6Y had long-period stacking-ordered(LPSO) X-phase(Mg_(12)YZn) and Mg_(24)Y_5 particles.With increasing Y content the basal texture became weakened significantly.While alloys ZM31 + 0.3Y and ZM31 + 3.2Y exhibited a skewed true stress-true stain curve with a three-stage strain hardening feature caused by the occurrence of {1012} extension twinning,the true stress-true stain curve of alloy ZM31 + 6Y was normal due to the dislocation slip during compression.With increasing temperature the extent of skewness decreased.While the compressive yield stress,ultimate compressive stress,strain hardening exponent,and hardening capacity all decreased as the temperature increased,the retention of the hightemperature deformation resistance increased with increasing Y content mainly due to the presence of thermally-stable LPSO X-phase.     

Computer Simulation of Ferrite Transformation during Hot Working of Low Carbon Steel

On the basis of transformation kinetics and thermodynamics, the austenite-ferrite transformation start temperature during deformation was predicted for several grades of low-carbon steels under different processing conditions. Results indicate that Ar3d temperature mostly depended on alloying composition and processing parameters. Ar3d increased as strain rate or strain increased for the same steel grade. In view of enhancement of deformation on transformation, the basic kinetics model was established to simulate deformation induced transformation behavior, using which the influence of the deformation stored energy and effective deformation ledge on the nucleation and growth can be considered. The simulated results are in good agreement with experiment results.     

Dynamic Recrystallization Behavior of 13%Cr Martensitic Stainless Steel under Hot Working Condition

In this study,the effect of hot deformation on martensitic stainless steel was carried out in temperatures between 950 to 1100℃and strain rates of 0.001,0.01 and 0.1 s-1.Two important dynamic recrystallization parameters,the critical strain and the point of maximum dynamic softening,were derived from strain hardening rate vs stress curves.Then the calculated parameters were used to predict the dynamic recrystallized fraction.Our results show that critical stress and strain increase with decreasing deformation temperature and increasing strain rate.The hot deformation activation energy of the steel is also investigated in the present work with 413 kJ/mol.Our experimental flow curves are in fair agreement with the kinetics of dynamic recrystallization model.     

Hot Deformation Behaviour of SiC/AA6061 Composites Prepared by Spark Plasma Sintering

In this study, Si C/AA6061 composites with different Si C volume fractions(5%, 10%, 15% and 20%) were fabricated by spark plasma sintering. The deformation behaviour of the composites was studied by uniaxial compression test at temperatures from 573 K to 773 K and strain rates between 0.001 s(-)~1.and 1 s(-)~1..Results indicate that the flow stress of Si C/AA6061 composites increases with the increase of Si C volume fraction, with the decrease of deformation temperature and with the decrease of strain rate. The main deformation mechanism of the composites is dynamic recrystallisation(DRX), and the DRX degree depends on the processing parameters of deformation. Higher Si C volume fraction, higher deformation temperature and lower deformation strain rate promote the occurrence of DRX. The strain rate sensitivity and deformation activation energy of Si C/AA6061 composites are calculated. Results show that with the increase in deformation temperature and the decrease in Si C volume fraction, the strain rate sensitivity of the composites increases. From 573 K to 773 K, the average deformation activation energy of 5vol.%Si C/AA6061, 10 vol.%Si C/AA6061, 15 vol.%Si C/AA6061 and 20 vol.%Si C/AA6061 are 207.91, 230.88, 237.7 and249.87 k J mol(-)~1., respectively. The optimum hot working zone of the Si C/AA6061 composites is in the temperature range of 723 K to 773 K at strain rates from 0.1 s(-)~1.to 1 s(-)~1.     

Fractal Characteristics and Prediction of Ti-15-3 Alloy Recrystallized Microstructure

Grain shape of the hot deforming alloy is an important of material. The fractal theory was applied to analyze index to character the microstructure and performance the recrystallized microstructure of Ti-15-3 alloy after hot deformation and solution treatment. The fractal dimensions of recrystallized grains were calculated by slit island method. The influence of processing parameters on fractal dimension and grain size was studied, It has been shown that the shapes of recrystallized grain boundaries are self-similar, and the fractal dimension varies from 1 to 2. With increasing deformation degree and strain rate or decreasing deformation temperature, the fractal dimension of grain boundaries increased and the grain size decreased. So the fractal dimension could characterize the grain shape and size. A neural network model was trained to predict the fractal dimension of recrystallized microstructure and the result is in excellent agreement with the experimental data.     

Research on the Characteristics of Hot Deformation in BT20 Titanium alloy and Its Optimum Spinning Temperature Range

Hot compression tests were conducted on a Gleeble-1500 simulator to investigate the hot deformation behavior of BT20 Ti alloy (Ti-6Al-2Zr-1Mo-1V) in the temperature range from 550 to 1000℃ at constant strain rate in the range of 0.01～1 s-1, and then the optimum spinning temperature range was determined. Moreover, tube spinning experiments were executed to verify the reasonability of the optimum temperature range. The results show that the flow stress declines gradually with increasing deformation temperature and decreasing strain rate. In α β phase region the dynamic recrystallization is the main softening mechanism and in β phase region the hot deformation softening is controlled by dynamic recovery. In α βphase region with reducing strain rate dynamic recrystallization is fully developed. The optimum temperature of hot spinning is 850～900℃ and that of warm spinning is 600～650℃.Meanwhile, at the temperature above 600℃ tubular workpieces of BT20 Ti alloy have been spun without surface cracks and microstructure inhomogeneity, which proves that the optimum spinning temperature range obtained through hot compression experiments is reasonable.     

Hot deformation behavior and processing map development of AZ110 alloy with and without addition of La-rich Mish Metal

In order to compare the workability of AZ110 alloy with and without addition of La-rich Mish Metal(MM), hot compression tests were performed on a Gleeble-3500 D thermo-mechanical simulator at the deformation temperature range of 473-623 K and strain rate range of 0.001-1 s-1. The flow stress, constitutive relation, DRX kinetic model, processing map and microstructure characterization of the alloys were investigated. The results show that the flow stress is very sensitive to deformation temperature and strain rate, and the peak stress of AZ110 LC(LC = La-rich MM) alloy is higher than that of AZ110 alloy.The hot deformation behavior of the alloys can be accurately predicted by the constitutive relations. The derived constitutive equations show that the calculated activation energy Q and stress exponent n for AZ110 alloy are higher than the calculated values of AZ110 LC alloy. The analysis of DRX kinetic models show that the development of DRX in AZ110 LC alloy is earlier than AZ110 alloy at the same deformation condition. The processing maps show that the workability of AZ110 LC alloy is significantly more excellent than AZ110 alloy and the microstructures are in good agreement with the calculated results.The AZ110 LC alloys can obtain complete DRX microstructure at high strain rate due to its higher stored energy and weak basal texture.     

Processing Maps for Hot Working Behavior of a PM TiAl Alloy

The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B(at.%) titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature range from 950 C to 1300 C and at the strain rates between 10 3 s 1 and 10 s 1.The processing maps have been established to evaluate the optimum hot processing conditions and reveal the instability regions.It is found that the flow stress of the investigated alloy is a strong function of the temperature and the strain rate.The investigated alloy has the optimum hot-working condition at 950 C and 10 3 s 1,since the material undergoes dynamic recrystallization to produce a fine-grained microstructure.At 1250 C and 10 3 s 1,the alloy exhibits superplastic deformation.At 1300 C and 10 1 s 1,the cyclic dynamic recrystallization with high temperature grain coarsening takes place.The material undergoes flow instabilities at lower temperatures and higher strain rates,as predicted by the instability criterion.The processing maps demonstrate that the strain significantly affected the instability regions.The manifestations of the instabilities have been observed in the form of microvoids,wedge cracks,and surface fractures.     

Strain Softening and Hardening Behavior in AZ61 Magnesium Alloy

The deformation behavior of AZ61 Mg alloy during hot deformation has been investigated in wide temperature and strain rate range by a Gleeble simulator. Specimens are deformed in compression in the temperature range of 523～673 K and at strain rates of 0.001～1 s-1. It is found that the flow curves exhibit a peak and then decrease towards steady-state of classical DRX, which decrease with rising temperature and decreasing strain rate. The deformation behavior of the specimens can be attributed to the occurrence of strain hardening and softening. As stress decreases, the strain hardening rate declines at a fast rate when temperature rises or strain rate decreases. The shapes of θ-σ curves indicate some important features such as subgrain formation, the critical stress, the peak stress and steady stress. The onset of DRX can be determined by the point of inflection on θ-σ or Inθ-σ curves.     

Dynamic Recrystallization and Precipitation Behavior of Mn-Cu-V Weathering Steel

The hot deformation behavior of a Mn-Cu-V weathering steel was investigated at temperatures ranging from 850 to 1050℃ and strain rates ranging from 0.01 to 5 s-1 using MMS-300 thermal-mechanical simulator. The activation energy for dynamic recrystallization and stress exponent were calculated to be 551 kJ/mol and 7.73, respectively. The accurate values of critical strain were determined by the relationship between work hardening rate and flow stress (θ-σ) curves. The hyperbolic sine constitutive equation was employed to describe the relationship between the peak stress and Zener-Hollomon parameter during hot deformation. The interaction between dynamic recrystallization and dynamic precipitation of V(C,N) at a low strain rate was analyzed. The results showed that precipitation particles size of weathering steel increased with increasing strain at deformation temperature 950℃ and strain rate 0.1 s-1. The calculation results of the recrystallization driving force and pinning force showed that dynamic precipitation could retard the progress of dynamic recrystallization but not prevent it while the pinning forces is less than driving force. On the contrary, dynamic precipitation can effectively prevent the progress of dynamic recrystallization.     

Active metal brazing of SiO_2–BN ceramic and Ti plate with Ag–Cu–Ti+BN composite filler

SiO_2–BN ceramic and Ti plate were joined by active brazing in vacuum with Ag–Cu–Ti+BN composite filler.The effect of BN content,brazing temperature and time on the microstructure and mechanical properties of the brazed joints was investigated.The results showed that a continuous Ti N–Ti B_2reaction layer formed adjacent to the SiO_2–BN ceramic,whose thickness played a key role in the bonding properties.Four Ti–Cu compound layers,Ti_2Cu,Ti_3Cu_4,Ti Cu_2and Ti Cu_4,were observed to border Ti substrate due to the strong affinity of Ti and Cu compared with Ag.The central part of the joint was composed of Ag matrix,over which some fine-grains distributed.The added BN particles reacted with Ti in the liquid filler to form fine Ti B whiskers and Ti N particles with low coefficients of thermal expansion(CTE),leading to the reduction of detrimental residual stress in the joint,and thus improving the joint strength.The maximum shear strength of 31 MPa was obtained when 3 wt%BN was added in the composite filler,which was 158%higher than that brazed with single Ag–Cu–Ti filler metal.The morphology and thickness of the reaction layer adjacent to the parent materials changed correspondingly with the increase of BN content,brazing temperature and holding time.Based on the correlation between the microstructural evolution and brazing parameters,the bonding mechanism of SiO_2–BN and Ti was discussed.     

Deformation Localization and Shear Fracture of a Rapidly Solidified Al-Fe-V-Si Alloy at Elevated Temperature

The tensile and fatigue behavior of a dispersoid strengthened, powder metallurgy Al-Fe-V-Si alloy at ambient and elevated temperatures was investigated. The results show that the strength and ductility of the alloy decrease significantly with increasing temperature and decreasing strain rate. Micro-structural examinations reveal that this change in mechanical behavior with increasing temperature is related to the mode of deformation of the alloy. Further observations show that localized shear deformation is responsible for the losses in both strength and ductility of the alloy at elevated temperature.