Effect of Ti and Ta content on the oxidation resistance of Co-Ni-based superalloys

Co-Ni-based superalloys are known for their capability to function at elevated temperatures and superior hot corrosion and thermal fatigue resistance.Therefore,these alloys show potential as crucial high-temperature structural materials for aeroengine and gas turbine hot-end components.Our previous work elucidated the influence of Ti and Ta on the high-temperature mechanical properties of alloys.However,the intricate interaction among elements considerably affects the oxidation resistance of alloys.In this paper,Co-35Ni-10Al-2W-5Cr-2Mo-1Nb-xTi-(5−x)Ta alloys(x=1,2,3,4)with varying Ti and Ta contents were designed and compounded,and their oxidation resistance was investigated at the temperature range from 800 to 1000℃.After oxidation at three test conditions,namely,800℃for 200 h,900℃for 200 h,and 1000℃for 50 h,the main structure of the oxide layer of the alloy consisted of spinel,Cr_(2)O_(3),and Al_(2)O_(3)from outside to inside.Oxides consisting of Ta,W,and Mo formed below the Cr_(2)O_(3)layer.The interaction of Ti and Ta imparted the highest oxidation resistance to 3Ti2Ta alloy.Conversely,an excessive amount of Ti or Ta resulted in an adverse effect on the oxidation resistance of the alloys.This study reports the volatilization of W and Mo oxides during the oxidation process of Co-Ni-based cast superalloys with a high Al content for the first time and explains the formation mechanism of holes in the oxide layer.The results provide a basis for gaining insights into the effects of the interaction of alloying elements on the oxidation resistance of the alloys they form.

Grain refinement and weak-textured structures based on the dynamic recrystallization of Mg–9.80Gd–3.78Y–1.12Sm–0.48Zr alloy

We utilized electron backscatter diffraction to investigate the microstructure evolutions of a newly developed magnesium-rare earth alloy(Mg–9.80 Gd–3.78 Y–1.12 Sm–0.48 Zr)during instantaneous hot indirect extrusion.An equiaxed fine-grained(average grain size of 3.4±0.2μm)microstructure with a weak texture was obtained.The grain refinement was mainly attributed to the discontinuous dynamic recrystallization(DDRX)and continuous DRX(CDRX)processes during the hot indirect extrusion process.The twin boundaries formed during the initial deformation stage effectively increased the number of high angle grain boundaries(HAGBs),which provided sites for new grain nuclei,and hence,resulted in an improved DDRX process.Along with DDRX,CDRX processes characterized by low angle grain boundary(LAGB)networks were also observed in the grain interior due to effective dynamic recovery(DRV)at a relatively high temperature of 773 K and high strain rates.Thereafter,LAGB networks were transformed into HAGB networks by the progressive rotation of subgrains during the CDRX process.

Twin-twin geometric structure effect on the twinning behavior of an Mg-4Y-3Nd-2Sm-0.5Zr alloy traced by quasi-in-situ EBSD

We studied the microstructure evolution of Mg-4Y-3Nd-2Sm-0.5Zr alloy by quasi-in-situ electron backscatter diffraction(EBSD)along with several strains under compression tests,which provided direct evidence for the influence of different twin-twin geometric structure on the twinning behavior.The results showed that the mechanical properties of the alloy were higher than traditional magnesium alloys(the maximum compressive strength reaches 402.5 MPa)due to the strengthening effect of Sm and Nd elements addition on solution strengthening,precipitation strengthening,and grain refinement.Combined with the quasi-in-situ EBSD technique,two different twin-twin geometric structures,‘parallel structure’and‘cross structure’,were observed directly in the alloy.In the later stage of deformation,for‘parallel structure’,residual stress and a large number of dislocations mainly existed in the twin boundary and tip position.For the‘cross structure’,there was a lot of dislocation density in the interior of twins after fusion.The twin growth rate of‘parallel structure’was much faster than that of‘cross structure’because the stress of twins was mainly concentrated on the tip of twin.When the movement for the tip of twin was blocked,the growth rate of twin would be obviously decreased.Moreover,the‘cross structure’was easy to produce closed space.Due to the constraints of surrounding twins,the confined space was easy to stress concentration,thus inhibiting the growth of twins.At the same time,the‘cross structure’of twins needed a more external force to continue to deform,which also served as a strengthening structure.

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth(Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5 Y-2 Nd-3 Sm-0.5 Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal Ⅱ dislocations in the later stage of deformation;these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors:(i) single twin variant 'parallel' structure,(ii) single twin variant 'cross' structure, and(iii) multi twin variant 'cross' structure. In addition, three types of twin-grain boundary interaction behaviors were summarized:(i) twin 'refracting through' grain boundary,(ii) twin'parallel through' grain boundary, and(iii) twin 'fusing through' grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

High-temperature ultra-strength of dual-phase Re_(0.5)MoNbW(TaC)_(0.5) high-entropy alloy matrix composite

The dual-phase Re_(0.5)MoNbW(TaC)_(0.5) composite,consisting of refractory body-centered cubic(BCC)highentropy alloy and carbide with many fine eutectic structures,was successfully synthesized by arc melting.The phase stability,high-temperature mechanical properties and strengthening mechanism of the ascast composite were studied.The microstructure of the composite remained stable after annealing at 1300℃for 168 h.It exhibited remarkably high-temperature strength,yield strength~901 MPa,and true ultimate compressive strength~1186 MPa at 1200℃.The BCC phase and carbide exhibited a semi-coherent interface with good bonding after severe deformation at 1200℃.The dipolar dislocation walls in BCC phase,restricted dynamic interaction between defects in carbide,and the pinning effect of semi-coherent interface offered effective strengthening effects.

Effect of thermo-mechanical processing parameters on the dynamic restoration mechanism in an Mg-4Y-2Nd-1Sm-0.5Zr alloy during hot compression

Microstructure evolution and dynamic restoration mechanism of solution-treated Mg-4Y-2Nd-1Sm-0.5Zr alloy have been studied under three TMP parameters consisting of deformation temperatures(350-500℃),strain rates(0.01-5 s^(-1)),and strains(0.2,0.4,and 0.8).A strong dynamic softening is observed in all stress-strain curves,even at higher strain rates(1 and 5 s^(-1))due to an adiabatic heating effect.Various stress-strain curves are applied to construct a processing map and develop an Arrhenius-type constitutive equation.With the prediction of the processing map,an optimal processing domain has been determined to be the temperature range 450-500℃and strain rate range 0.01-0.1 s^(-1)at a strain of 0.8.The volume fraction of DRX grains is the largest in the corresponding domain of high temperature and low strain rate.For the effect of TMP parameters on the dynamic restoration,the discontinuous dynamic recrystallization(DDRX)and continuous DRX(CDRX)synergistic effect occur throughout the whole process at high temperature and high strain rate.In terms of high temperature and low strain rate,DDRX characteristics at a low strain and then the DDRX+CDRX synergistic effect is observed at a higher strain.Although the DRX process is weak at low temperature and low strain rate,deformation twins have occurred and provided nucleation sites for DRX grains.