Phase Transformation Behavior Of Superlattice Ordered Phase And Mechanical Properties Of C2000 Alloy

Ni-Cr-Mo-based alloys are a class of solution strengthening and corrosion resistancing superalloys,particularly for Hastelloy C2000 alloy with exceptional corrosion resistance under both oxidizing and reducing medium,which is a promising candidate as strainer in high temperature.It is equivalent to a long-range aging treatment for C2000 alloy during the alloy component long-term service under high temperature corrosion environment of 450-700 oC,which meet to precipitation of superlattice phase in C2000 alloy with a low Mo/Cr ratio.Precipitation of superlattice phase has a significantly effect on the mechanical and corrosion resistance properties of the alloy.However,these have not yet been fully studied that phase transformation mechanism,characteristic of superlattice phase and effect its on mechanical properties in Ni-based superalloy with low Mo/Cr ratio.The solving of these problems has an important implication for service life of the alloy and its application in nuclear power industy.In the paper,C2000 alloy supplied by Haynes International Inc.USA was chosen as the research object,precipitation behavior of short-range order（SRO）was discussed,phase transformation,characteristic,thermostability of long-range order（LRO）and effect of derformation on LRO were studied systemically,formation mechanism of serrated tensile stress-strain curve at room temperature induced by precipitating of Pt₂Mo-type superlattice phase was revealed,formation reason of serrated tensile stress-strain curve at high temperature was analysised.In this work,the main research contents and the innovative products are as follows:Diffraction spot with diffuse intensity maxima is exhibited at {1?0} positions after solid solution followed by water quenching,i.e.formation rate of {1?0}SRO is very slow in the C2000 alloy with low Mo/Cr ratio.The spherical Pt₂Mo-type superlattice phase with dizzy margin can be precipitated in C2000 alloy after aging during 500-600 oC,and its precipitation rate up to peak value at 600 oC（t=100 h）.Both size and volume fraction of the superlattice phase increase with increasing of aging time and aging temperature.Pt₂Mo-type superlattice phase is roughly spherical and its edge is poorly defined,and no specific orientation distribution in the matrix.Orientation relationship between the γ phase and the six unique variants of Pt₂Mo-type superlattice phase along [100]fcc,[110]fcc and [112]fcc is: [001]γ//[010]Pt₂Mo,[110]γ//[100]Pt₂Mo and [1—12]γ//[11—1]Pt₂Mo//[111]Pt₂Mo//[110]Pt₂Mo,respectively.Moreover,stoichiometric ratio of Pt₂Mo-type superlattice phase is Ni1.68（Cr0.81Mo0.19）,which is a Cr-rich and Mo-deplet order phase.The alloying elements with Cr,Cu,Fe and Mn mainly segregated on the outer layer of the roughly spherical Pt₂Mo-type superlattice phase.There is nothing other types of superlattice phase in the C2000 matrix after aging at 600-800 oC/100 h,but Mo-rich phases and carbides are precipitated at 800 oC/100 h.The effect of cold deformation on precipitation,evolution behavior of the precipitated phase was analyzed.Cold deformation before treatment of 705 oC/16 h to 605 oC/32 h promote precipitation of σ-FeCr phase.Cold deformation of 50% before aging at 600 oC /500 h induce precipitation of M6 C carbide besides affect distribution of Pt₂Mo-type superlattice phase.Moreover,cold deformation before aging at 550 oC and 600 oC promote and restrain precipitation of Pt₂Mo-type superlattice phase,respectively resulting from rising of aging temperature due to cold deformation.Cold deformation before aging lead to variation of location of isothermal TTT（Temperature-Time-Transformation）curve for Pt₂Mo-type superlattice phase.As a result,the nose temperature close to 550 oC,while deviate from 600 oC.Pt₂Mo-type superlattice phase has a good thermostability and keep good coherant interface with fcc matrix during long-term thermal exposure at 600 oC.Crystal structure of superlattice phase keep unchenged and its size increaese with increasing of thermal exposure time.There is a linear relationship between major axis cube of the superlattice phase and thermal exposure time.Morphology of superlattice phase change with increasing of thermal exposure time of 100-340 h,i.e.morphology of superlattice phase evolve gradually from spherical to ellipsoidal.Meanwhile,three variants of superlattice phase along [112]fcc exhibit reciprocal link with increasing of thermal exposure time,and variants of superlattice phase with reciprocal link from to 1 to 3 in turn,which attribute the evolution of morphology and appearance of reciprocal link for Pt₂Mo-type superlattice phase to decrease of strain energy.However,ellipsoidal Pt₂Mo-type superlattice phase and reciprocal link of variants disappear when thermal exposure time up to 600 h,which resulting from overlapping of superlattice phase.The viewpoint was also confirmed by appearance of additional diffraction spots in 1/3（200）and 1/3（020）position.On the other hand,Pt₂Mo-type superlattice phase display a snowflake-like morphology after cold deformation and short-range thermal exposure.Both heat and compress stress do not change crystal structure of Pt₂Mo-type superlattice phase.Plastic deformation of the alloy will carry though shearing of superlattice phase by dislocations under heat and compress stress.The nanoscale Pt₂Mo-type superlattice phase can significantly improve tensile yield strength and tensile strength of the C2000 alloy at room temperature and high temperature of 200-600 oC,and keep high elongation（>40%）.Pt₂Mo-type superlattice phase induce formation of serrated stress-strain curve during the room temperature tensile.The serrated stress-strain curve exhibit an embedded feature that a small serration is embedded in two adjacent large serration.Large serration and small serration were attributed to twinning of the alloy matrix induced by Pt₂Mo-type superlattice phase and twinning of superlattice phase oneself,respectively.During high temperature tensile,at 200 oC,the stress-strain curve of sample with and without Pt₂Mo-type superlattice phase display a normal smooth behavior characteristic.However,during 400-700 oC,the serrated stress-strain curves were obsserved for both sample with and without Pt₂Mo-type superlattice phase,and the type of serration change with changing of tensile temperature.In the tensile temperature range,dynamic interaction between solute atoms and dislocations contribute to formation of the Portevin-Le Chatelier（PLC）effect for sample without Pt₂Mo-type superlattice phase.While for sample with the superlattice phase,factors induced by formation of the PLC effect change with changing of strain rate,i.e.strain rate increase from 10-3 to 10-2 to 10-1 s-1,the factors were simple slip→simple slip→simple slip+deformation twin.On the other hand,Pt₂Mo-type superlattice phase can decrease creep rate of the alloy during second stage after high temperature creep at 600 oC and σ=300 MPa,i.e.creep rate decrease from 0.01176%/h of solid solution state to 0.01053%/h of aging state.