Modeling of hot deformation behavior and prediction of flow stress in a magnesium alloy using constitutive equation and artificial neural network(ANN)model

The aim of the present study was to investigate the modeling and prediction of the high temperature flow characteristics of a cast magnesium(Mg-Al-Ca)alloy by both constitutive equation and ANN model.Toward this end,hot compression experiments were performed in 250-450℃and in strain rates of 0.001-1 s^(−1).The true stress of alloy was first and foremost described by the hyperbolic sine function in an Arrhenius-type of constitutive equation taking the effects of strain,strain rate and temperature into account.Predictions indicated that unlike low strain rates and high temperature with dominant DRX activation,in relatively high strain rate and low temperature values,the precision of the models become decreased due to activation of twinning phenomenon.At that moment and for a better evaluation of twinning effect during deformation,a feed-forward back propagation ANN was developed to study the flow behavior of the investigated alloy.Then,the performance of the two suggested models has been assessed using a statistical criterion.The comparative assessment of the gained results specifies that the well-trained ANN is much more precise and accurate than the constitutive equations in predicting the hot flow behavior.

Hot deformation behavior and dynamic recrystallization kinetics of AZ61 and AZ61+Sr magnesium alloys

In this study,the effect of strontium addition on hot deformation of AZ61 alloy was investigated by hot compression tests.A reference alloy(AZ61)and an Sr-containing alloy(AZ61+Sr)was cast while their average initial grain size were supposed to be about 140 and 40μm,respectively.In AZ61+Sr alloy,the Sr-containing precipitations were stable at homogenization temperature.Analysing the hot compression curves,it was revealed that dynamic recrystallization phenomenon had occurred and controlled the thermomechanical behaviour of the alloys.The derived constitutive equations showed that the hot deformation parameters(n and Q)in AZ61+Sr alloy is smaller than those of AZ61 alloy;this can be related to the small initial grain size and the lower amounts of solute aluminium atoms.The analysis of DRX kinetics along with the micrographs of the deformed microstructures showed that at the same condition the development of DRXed microstructure in AZ61+Sr alloy was faster than AZ61 alloy.The increased recrystallized microstructure was interpretated to be attributed to(1)the more grain boundaries present and(2)the existance of the Al-Mg-Sr precipitations assisted the PSN mechanism.Also,the attenuated intensity of the basal texture of AZ61+Sr was related to the DRX fraction of microstructure.

Microstructure refinement,mechanical and biocorrosion properties of Mg–Zn–Ca–Mn alloy improved by a new severe plastic deformation process

In this study,the microstructural evolution,mechanical properties and biocorrosion performance of a Mg–Zn–Ca–Mn alloy were investigated under different conditions of heat treatment,extrusion,one pass and two passes of half equal channel angular pressing(HECAP)process.The results showed significant grain refinement of the homogenized alloy after two passes of HECAP process from 345μm to 2μm.Field emission scanning electron microscopy(FESEM)revealed the presence of finer Mg_(6)Zn_(3)Ca_(2)phase as well asα-Mn phase after HECAP process.The results also showed that mechanical characteristics such as yield strength,ultimate tensile strength and elongation of the HECAPed samples improved by~208%,~144%and~100%compared to the homogenized one,respectively.Crystallographic texture analysis indicated that most of the grains at the surface were reoriented parallel to the(0001)basal plane after HECAP process.Electrochemical corrosion tests and immersion results indicated that the sample with two passes of HEACP had the highest biocorrosion resistance confirming that the basal planes had the lowest corrosion rate compared to the non-basal ones.The mechanical behavior and bio-corrosion evaluation demonstrated that the HECAPed Mg–Zn–Ca–Mn alloy has great potential for biomedical applications and a mechanism was proposed to explain the interrelations between the thermomechanical processing and bio-corrosion behavior.

Effect of Aging Time and Temperature on Mechanical Properties and Microstructural Evolution of 2205 Ferritic-Austenitic Stainless Steel

Duplex stainless steels （DSS） with ferritic-austenitic microstructures offer good combination of resistance to pitting corrosion and high strength that are not concomitantly attainable using conventional single phase austenitic or ferritic stainless steels, The DSS used in this investigation was 2205 alloy having a stable microstructure consisting of about 45% ferrite and 55% austenite at ambient temperature. In order to investigate aging behavior of this steel and the influences on mechanical properties, different aging treatments were conducted at temperatures of 350-950℃ for various aging time of 15, 30, 60 and 180 min. The aged specimens were subjected to impact testing and hardness measurements. Finally, the changes in microstructure due to aging were studied by optical and scanning electron microscopy. The results showed that aging at temperatures lower than 550℃ for different time had negligible effects on mechanical properties. Besides, no considerable changes in term of precipitation of harmful intermetallic particles were observed in microstructure below this temperature. However, a critical temperature range, 550-650 ℃, was introduced here. Aging in this range led to a significant decrease in toughness and notable increase in hardness. The formation of intermetallic phases such as CT was recognized as the major reason for the observed changes.

Texture evaluation in warm deformation of an extruded Mg-6Al-3Zn alloy

To assess the effect of strain and strain rate on texture evolution of an extruded Mg–6Al–3Zn alloy,compression tests were carried out.Samples were prepared in the extrusion direction(ED)and normal direction(ND).The compression tests were performed at 250℃ and with different strain rates of 0.01 sec−1 and 1 sec−1 and different strains.Microstructural observation and texture investigation show that at early stages of deformation,extension twins lead to the development of strong basal texture intensity along rolling direction(RD)in ED samples and contraction twins result in texture evolution along transverse direction(TD)in ND samples.Also,microstructural investigation at high strains reveals that dynamic recrystallization occurs in both samples and consequently the basal texture intensity has been decreased.

Processing map and microstructure evaluation of AA6061/Al_2O_3 nanocomposite at different temperatures

Hot compression behavior of Al6061/Al2O3nanocomposite was investigated in the temperature range of350-500°C andthe strain rate range of0.0005-0.5s-1,in order to determine the optimum conditions for the hot workability of nanocomposite.Theactivation energy of285kJ/mol for the hot compression test is obtained by using hyperbolic sine function.By means of dynamicmaterial model(DMM)and the corresponding processing map,safe zone for the hot workability of AA6061/Al2O3is recognized attemperature of450°C and strain rate of0.0005s-1and at temperature of500°C and the strain rate range of0.0005-0.5s-1,with themaximum power dissipation efficiency of38%.Elongated and kinked grains are observed at400°C and strain rate of0.5s-1due tothe severe deformation.

Deformation behavior and processing maps of Mg-Zn-Y alloy containing Ⅰ phase at elevated temperatures

The microstructure and mechanical properties of extruded Mg-Zn alloy containing Y element were investigated in temperature range of 300-450°C and strain rate range of 0.001-1 s-1 through hot compression tests.Processing maps were used to indicate optimum conditions and instability zones for hot deformation of alloys.For Mg-Zn and Mg-Zn-Y alloys,peak stress,temperature and strain rate were related by hyperbolic sine function,and activation energies were obtained to be 177 and 236 kJ/mol,respectively.Flow curves showed that the addition of Y element led to increase in peak stress and decrease in peak strain,and indicated that DRX started at lower strains in Mg-Zn-Y alloy than in Mg-Zn alloy.The stability domains of Mg-Zn-Y alloy were indicated in two domains as 1)300°C,0.001 s-1;350°C,0.01-0.1 s-1 and 400°C,0.01 s-1 and 2)450°C,0.01-0.1 s-1.Microstructural observations showed that DRX was the main restoration mechanism for alloys,and fully dynamic recrystallization of Mg-Zn-Y alloy was observed at 450°C.The instability domain in Mg-Zn-Y alloy was located significantly at high strain rates.In addition,the instability zone width of Mg-Zn and Mg-Zn-Y alloys increased with increasing strain,and cracks,twins and severe deformation were considered in these regions.

Microstructural evolution of a superaustenitic stainless steel during a two-step deformation process

Single-and two-step hot compression experiments were carried out on 16Cr25Ni6Mo superaustenitic stainless steel in the temperature range from 950 to 1150°C and at a strain rate of 0.1 s^（-1）. In the two-step tests, the first pass was interrupted at a strain of 0.2; after an interpass time of 5, 20, 40, 60, or 80 s, the test was resumed. The progress of dynamic recrystallization at the interruption strain was less than 10%. The static softening in the interpass period increased with increasing deformation temperature and increasing interpass time. The static recrystallization was found to be responsible for fast static softening in the temperature range from 950 to 1050°C. However, the gentle static softening at 1100 and 1150°C was attributed to the combination of static and metadynamic recrystallizations. The correlation between calculated fractional softening and microstructural observations showed that approximately 30% of interpass softening could be attributed to the static recovery. The microstructural observations illustrated the formation of fine recrystallized grains at the grain boundaries at longer interpass time. The Avrami kinetics equation was used to establish a relationship between the fractional softening and the interpass period. The activation energy for static softening was determined as 276 kJ/mol.

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.