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.     

Corrosion Behavior of Mg-Zn-Y Alloy with Long-period Stacking Ordered Structures

Mg-Zn-Y alloys with long-period stacking ordered structures were prepared by an ingot casting method. The corrosion performance of Mg-Zn-Y alloys was studied by combining gas-collecting test, immersion test and electrochemical measurements in order to determine the corrosion rate and mechanism of the alloys. The results showed that the volume fraction of Mg(12)YZn phase increased and the shape of the Mg(12)YZn phase changed from discontinuous to continuous net-like with increasing Zn and Y content. The corrosion rate of the alloys greatly depended on the distribution and volume fraction of the Mg(12)YZn phase. Corrosion products appeared at the junction of Mg phase and Mg(12)YZn phase, indicating that the Mg(12)YZn phase accelerated galvanic corrosion of Mg matrix. Mg(97)Zn1Y2 alloy shows the lowest corrosion rate due to the continuous distribution of Mg(12)YZn phase.     

On Temperature and Strain Rate Dependent Strain Localization Behavior in Ti-6.5Al-3.5Mo-1.5Zr-0.3Si Alloy

Deformation behaviors of Ti-6.5AI-3.5Mo- 1.5Zr-0.3Si alloys withα/βlamellar structure were investigated systematically at different temperatures from room temperature to 950℃and different strain rates.Results reveal that when the deformation temperature is higher than a critical temperature of 600℃,an evident transition of deformation behavior from localized shear banding toα/βlamella kinking,flow softening and temperature/strain rate-dependent peak flow stress occurred in the alloy.The critical conditions for the occurrence of internal cracking and strain localization behavior associated with temperature and strain rate were determined.     

Mechanism of Bainite Nucleation in Steel, Iron and Copper Alloys

During the incubation period of isothermal treatment(or aging) within the bainitic transformation temperature range in a salt bath (or quenching in water) immediately after solution treatment, not only are the defects formed at high temperatures maintained, but new defects can also be generated in alloys, iron alloys and steels. Due to the segregation of the solute atoms near defects through diffusion, this leads to non-uniform distributions of solute atoms in the parent phase with distinct regions of both solute enrichment and solute depletion. It is proposed that when the Ms temperature at the solute depleted regions is equal to or higher than the isothermal (or aged) temperature, nucleation of bainite occurs within these solute depleted regions in the manner of martensitic shear. Therefore it is considered that, at least in steel, iron and copper alloy systems, bainite is formed through a shear mechanism within solute depleted regions, which is controlled and formed by the solute atoms diffusion in the parent phase.     

Phase Stability, Kinetic Diagrams and Diffusion Path in High Temperature Oxidation of Binary Solid-Solution Alloys

The phase diagrams of ternary systems involving two metal components and one oxidant are considered first, the limitations to their use is discussed in relation to the high temperature oxidation of binary alloys. Kinetic diagrams,which are useful to predict the conditions for the stability of the two mutually insoluble oxides as the external scale, are then calculated on the basis of thermodynamic and kinetic data concerning both the alloys and the oxides, assuming the validity of the parabolic rate law. A combination of the two types of diagrams provides a more detail information about the oxidation behavior of binary alloys. The calculation of the diffusion paths, which relate the oxidant pressure to the composition of the system in terms of the alloy components both in the alloy and in the scale during an initial stage of the reaction in the presence of the parabolic rate law, is finally developed.     

Phase decomposition behavior and its effects on mechanical properties of TiNbTa0.5ZrAl0.5 refractory high entropy alloy

The mechanical properties of refractory high entropy alloys(RHEAs) strongly depend on their phase structures. In this work, the phase stability of a BCC TiNbTa0.5ZrAl0.5 refractory high entropy alloy subjected to thermomechanical processing was evaluated, and the effects of phase decomposition on room/high temperature mechanical properties were quantitatively studied. It was found that, the thermomechanical processing at 800℃and 1200℃ leads to phase decomposition in the TiNbTa0.5ZrAl0.5 alloy. The phase decomposition is caused by the rapid rising of free energy of the primary BCC phase. The effect of the precipitates on room temperature strength is determined by the competition between the increasing in precipitation strengthening and the decreasing in solid solution strengthening. But at high temperatures(800-1200℃), the phase decomposition causes significant reduction in strength, mainly due to the grain boundary sliding and the decreasing in solid solution strengthening.     

Mechanism for the multi-stage precipitation of Fe-Ni based alloy

This work lays great emphasis on the distribution evolution of sigma(σ) phase in Fe-Ni based N08028 alloy during aging process,the result of which could provide new insights into phase change and precipitation mechanism.It is found that the σ phase,in any case,tends to separate out with a granular shape at grain boundaries(GBs) primarily,and with the increment of time,it is obliged to precipitate in grain interiors(GIs) with a lamellar structure.The mechanisms for the evolving volume fraction and morphology of σ phase are discussed,and a model appropriate for the multi-stage behavior transforming from intergranular to intragranular precipitation is derived,as well as revealing the thermodynamics and transformation kinetics corresponding to this process.The results reveal that the occurrence of multi-stage precipitation is correlated with the redistribution of solute atoms and with the difference in coupling effect of thermodynamic driving force and kinetic activation energy between the intergranular and intragranular precipitation.     

Predicting the Transition between Upper and Lower Bainite via a Gibbs Energy Balance Approach

The transition temperature between upper bainite and lower bainite is calculated with an extended Gibbs energy balance model, which is able to quantitatively describe the evolution of carbon supersaturation within bainitic ferrite sheaves during the entire thickening process. The nucleation rate of intra-lath cementite precipitation on a dislocation is calculated based on of the degree of carbon supersaturation.Upper bainite and lower bainite are thus distinguished by the effective nucleation density and therefore a numerical criterion can be set to define the transition. The model is applied to Fe-xC-1Mn/2Mn/1 Mo ternary alloys. Results show that the transition temperature increases with bulk carbon content at lower carbon concentration but decreases in the higher carbon region. This prediction agrees very well with the experimental observations in Mn and Mo alloyed systems. Moreover, the highest transition temperature and the carbon content at which it occurs in the Fe-xC-2Mn system are in good agreement with reported experimental data. The inverse "V" shaped character of the carbon concentration–transition temperature curve indicates two opposite physical mechanisms operating at the same time. An analysis is carried out to provide an explanation.     

Formation Mechanism of Chunky Graphite and Its Preventive Measures

The formation mechanism of chunky graphite has been reviewed and studied. The study consisted of a unidirectional solidification method, a small droplet method and a furnace cooling method. Four kinds of iron samples were prepared, namely, the pure Fe-C, Fe-C-S, Fe-C-Ce and Fe-C-Si-Ce alloys, and three kinds of nickel samples, namely the Ni-C, Ni-C-S and Ni-C-Mg alloys. The results of the unidirectional solidification of the Ni-C alloys showed that spheroidal graphite is not observed in the continuous solidified region, in which only flake-like graphite is observed, while spheroidal graphite is usually observed in the quenched liquid region. The existence of spheroidal graphite in the solidified phase is recognized only in the discontinuous growth mode of the Ni-C-Mg alloy solidified at 150 mm/h. This means that the spheroidal graphite is directly crystallized from the melt and entrapped by the flake-like chunky graphite that is formed by the continuous growth mode. In the small droplet method, a small piece of the Fe-C or Fe-C-Ce sample was melted on a pure graphite plate then cooled at a different cooling rate in a He-3%H2 atmosphere. The graphite in the Fe-C-Ce alloy is usually spherical. Nevertheless, the graphite morphology of the final solidified area changed from spherical to chunky and chunky to ledeburite with an increase in the cooling rate. This means that the chunky graphite is formed in the residual liquid region by the solidification into Fe-graphite system. The sample was cooled in a furnace, and the graphite morphology changes from spherical to chunky and chunky to ledeburite with the decrease in the Si content. These phenomena can be confirmed by the cooling curves of these samples.     

Microstructure evolution and mechanical properties at high temperature of selective laser melted AlSi10Mg

In this study,the microstructure and tensile properties of selective laser melted AlSilOMg at elevated temperature were investigated with focus on the interfacial region.In-situ SEM and in-situ EBSD analysis were proposed to characterize the microstructural evolution with temperature.The as-fabricated AlSilOMg sample presents high tensile strength with the ultimate tensile strength(UTS)of~450 MPa and yield strength(YS)of~300 MPa,which results from the mixed strengthening mechanism among grain boundary,solid solution,dislocation and Orowan looping mechanism.When holding at the temperature below 200℃for 30 min,the micro structure presents little change,and only a slight decrement of yield strength appears due to the relief of the residual stress.However,when the holding temperature further increases to 300℃and 400℃,the coarsening and precipitation of Si particles inα-Al matrix occur obviously,which leads to an obvious decrease of solid solution strength.At the same time,matrix softening and the weakness of dislocation strengthening also play important roles.When the holding temperature reaches to 400℃,the yield strength decreases significantly to about 25 MPa which is very similar to the as-cast Al alloy.This might be concluded that the YS is dominated by the matrix materials.Because the softening mechanism counteracts work hardening,the extremely high elongation occurs.     

Microstructure and mechanical properties at elevated temperature of Mg-Al-Ni alloys prepared through powder metallurgy

Mg-Al-Ni alloys were prepared by powder metallurgy, and their microstructure and elevated temperature mechanical properties were investigated. Results indicate that, in addition to α-Mg matrix, both coarse Al_3Ni_2 particles and fine Al Ni nano-particles exist in the Mg-Al-Ni alloys. The strength at 150?C is improved with the increase in Ni content. Mg-18.3Al-8Ni alloy possesses a compressive strength of234.7 MPa and a yield strength of 146.5 MPa. Plasticity is also improved with a low concentration of Ni. Mg-11.3Al-2Ni alloy possesses a compression ratio of 17.3%. The phases of Al_3Ni_2 and Al Ni in the alloys block the movements of grain boundaries and dislocations during the deformation at elevated temperature. The existence of Al Ni phase provides a non-basal slip system, leading to the improvement in plasticity. Finally, the formation mechanism of Al-Ni phases in the process is discussed with thermodynamics and kinetics.     

Effect of Cu on Discontinuous Precipitation of a Symmetrical AlZn Alloy

The effect of Cu additions on discontinuous precipitation of AlZn alloy were studied with optical microscopy, scanning electron microscopy and X-ray diffraction. It is found that the effect of addition of 2 at. pct Cu on cellular nucleation site is not remarkable, while the effect of aging temperature on cellular nucleation site of AlZn and AIZn-2Cu alloys is fairly obvious. The cell growth rate of discontinuous precipitation of AlZn alloy is remarkably accelerated with the addition of 2 at. pct Cu. The discontinuous precipitation microstructure of AlZn alloy is not apparently affected with the addition of 2 at. pct Cu and the cellular front is still fine microstructure of spinodal decomposition. The phase constituents consist of a fcc Al-rich phase, a hcp Zn-rich phase and a CuZn4 phase. The transformation rate of discontinuous precipitation of AlZn alloy is remarkably accelerated with the addition of 2 at. pct Cu.     

Effect of environments and normal loads on tribological properties of nitrided Ni_(45)(FeCoCr)_(40)(AlTi)_(15) high-entropy alloys

The tribological properties of nitrided Ni_(45)(FeCoCr)_(40)(AlTi)_(15) high-entropy alloys(HEAs) were investigated in air and simulated acid rain under different normal loads(5,7,10,and 12 N) at ambient temperature.The results revealed that as-cast HEAs were only composed of FCC phase,while the volume fraction of FCC phase in the nitrided alloys was significantly reduced.Moreover,the hard phases of AIN,CrN,Fe4 N,and TiN phases were formed in the nitrided alloys.The thickness of the nitriding layer was about 8.4 μm.The hardness increased from 8.7 GPa in as-cast alloys to 14.5 GPa in the nitrided alloys.In addition,under the same conditions,the friction coefficient of the nitrided alloys was higher than that of as-cast alloys,but the wear rate was generally lower than that of as-cast alloys.Furthermore,the wear rate of the nitrided alloys was the lowest in acid rain due to the lubrication,cleaning,and cooling in the liquid environment.In air,dominating wear mechanisms in as-cast and nitrided alloys were abrasive,delamination,and adhesive wears.And,the wear mechanism of as-cast and nitrided alloys in acid rain was mainly abrasive and corrosion wears.     

Thermal Stability of a New Ni-Fe-Cr Base Alloy with Different Ti/Al Ratios

Influence of Ti/Al ratios on the thermal stability of a new low cost Ni-Fe-Cr base wrought alloy,designed for application at 700℃ in advanced ultra-supercritical coal-fired power plants(700℃A-USC),was investigated both experimentally and thermodynamically. After standard heat treatment,the alloys with different Ti/Al ratios had the same microstructural characteristics. However,compared with the alloys with high Ti/Al ratio,the low Ti/Al ratio can increase the γ'-solvus temperature,decrease γ' coarsening rate and reduce the temperature range of η phase precipitation. For the alloys with low Ti/Al ratio,the yield strength has no obvious decrease during long-term thermal exposure at 700 and 750℃,but after thermal exposure at 750℃ for 5000 h,the yield strength of the alloys with high Ti/Al ratio obviously decreases due to the η phase precipitation. The influence of h phase on mechanical properties is related with its size. When the h phase is small,it has no obvious influence on mechanical properties,but h phase becomes the crack initiation site with the further growth of η phase. It can be concluded that the decrease in Ti/Al ratio can improve the thermal stability to meet the requirement of 700℃ A-USC coal-fired power plants.     

Mechanism and Kinetics of Phase Transformation in Two-phase TiAl-based Alloys

The aged and quenched microstructures of both alloys, Ti-42at-%Al and Ti-45at -%Al,homogenized in the disordered single phase field. were investigated And the results show that the quinched microstructure is a supersaturated single phase of ordered 22. When the supersaturated phase is aged in the two phase range at 1273 and 1373 K, it will transform to a lamellar microstructure of γ+α2. with a discontinuous decomposition mechanism in Ti-42at-%Al alloy and a semicontinuous decomposition mechanism in T1-45at-%Al alloy. With the methods of quantitative metallograph examination and X-ray diffraction analysis. the relationship between the amount of γ, phase precipitation and the time of isothermal transformation is agreed     

Influence of Temperature on Stacking Fault Energy and Creep Mechanism of a Single Crystal Nickel-based Superalloy

The influence of temperatures on the stacking fault energies and deformation mechanism of a Recontaining single crystal nickel-based superalloy during creep at elevated temperatures was investigated by means of calculating the stacking fault energy of alloy, measuring creep properties and performing contrast analysis of dislocation configuration. The results show that the alloy at 760 °C possesses lower stacking fault energy, and the stacking fault of alloy increases with increasing temperature. The deformation mechanism of alloy during creep at 760 °C is γ′ phase sheared by 110 super-dislocations, which may be decomposed to form the configuration of Shockley partials plus super-lattice intrinsic stacking fault, while the deformation mechanism of alloy during creep at 1070 °C is the screw or edge superdislocations shearing into the rafted γ′ phase. But during creep at 760 and 980 °C, some superdislocations shearing into γ′ phase may cross-slip from the {111} to {100} planes to form the K–W locks with non-plane core structure, which may restrain the dislocations slipping to enhance the creep resistance of alloy at high temperature. The interaction between the Re and other elements may decrease the diffusion rate of atoms to improve the microstructure stability, which is thought to be the main reason why the K–W locks are to be kept in the Re-containing superalloy during creep at 980 °C.     

Microstructure and Mechanical Properties of an Extruded Mg-2Dy-0.5Zn Alloy

Microstructure and mechanical properties of an extruded Mg-2Dy-0.5Zn(at.%) alloy during isothermal ageing at 180 ℃ were investigated.Microstructure of the as-extruded alloy is mainly composed of α-Mg phase,14H long period stacking order(LPSO) phase and small amounts of(Mg,Zn)_xDy particle phases.During ageing,the 14H LPSO phase forms and develops and its volume fraction increases with increasing ageing time.Tensile test showed that the peak-aged alloy exhibits similar yield and ultimate tensile strengths and elongation to failure at room temperature,100 ℃ and 200 ℃,but excellent elevated temperature strengths at 300 ℃ as compared to the as-extruded and over-aged alloys.The analysis showed that the excellent elevated temperature strengths of the peak-aged alloy are attributed to the LPSO phase strengthening and the grain refinement strengthening,and the role of the LPSO strengthening is related to not only its amount,but also its morphology.     

Formation Mechanism of Microstructure of Melt Spun Al—In Immiscible Alloys

Immiscible alloys are attractive for their valuable physical and mechanical properties. In this paper, Al-ln immiscible alloy is prepared by melt spinning process and its morphological evolution is studied at various indium contents. The results show that the morphologies of the matrix phase depend on the indium content. Different morphologies lead to different distribution of the second phase particles. Due to a particular solidification mechanism of immiscible alloys, even under the melt spinning rapid solidification condition, it is still impossible to produce homogeneous Al-In hypomonotectic alloy ribbons. But for Al-In hypermonotectic alloys, there is almost no segregation of the second phase throughout the cross section of the ribbons.     

Surface Tension of Molten Ni and Ni-Co Alloys

Surface tension of molten Ni and Ni-Co (5 and 10 mass fraction) alloys was measured at the temperature range of 1773-1873 K using an improved sessile drop method with an alumina substrate in an Ar+3%H2 atmosphere. The error of the data obtained was analyzed. The surface tension of molten Ni and Ni-Co (5 and 10 mass fraction) alloys decreases with increasing temperature. The influence of Co on the surface tension of Ni-Co alloys is little in the studied Co concentration range.     

Massive Si Phase and Its Growth Mechanism in Al-Si Casting Alloy

Optical microscope and scanning electron microscope were used to observe the microstructure of the AI-11.6%Si and AI-11.6%Si-0.4%Mg alloys and the morphology of the massive silicon particles. It is found that the massive silicon phase, observed in the unfully modified alloys with 0.010%Sr, disappears completely in the alloys fully modified with 0.020%Sr. The serrations and reentrant edges shown in the massive silicon particles with the conventional casting indicate that the TPRE mechanism plays an important role in the growth of the massive silicon phase. The ripples and steps suggest that the "lateral microscopic growth" may be another operating mechanism.