Microstructure and Properties of ZrC-Cu Composites by Zr-Cu Melt Infiltration Carbides at Relatively Low Temperature

Author:Wang Dong

Supervisor:wang yu jin

Database:Doctor

Degree Year:2018

Download:13

Pages:180

Size:12796K

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The prepared self-generated ZrC-Cu based composites were prepared by reactive melt infiltration at relatively low tempwerature.The Zr-Cu binary melt was used as the infiltrant and the IV-VIB carbide was the porous preforms.Compared with ZrC-MeC-Cu composite from IVB and VB carbides,the reaction of Zr-Cu melt with VIB Mo and W carbides is mild and easy to control.The as-prepared ZrC-Mo-Cu and ZrC-W-Cu composites have uniform microstructure.Compared with W+WC and WC preformss,ZrC-W-Cu composites prepared by partially carbonized W powder have the advantages of no WC residue,uniform structure and short reaction time.The calcination of the green body can stabilize the material properties.A porous preform with a porosity of 47.0–53.6%was obtained by cold isostatic pressing at 100 MPa.ZrC-W-Cu composite with a W content of 48–73 vol%was obtained after 1 h of Zr2Cu melt at 1300℃.The microstructure of ZrCx-TiCy-Cu,intragranular ZrC-TiC/W-Cu,ZrCx-NbCy-Cu,ZrCx-TaCy-Cu and ZrC-Mo-Cu composites showed delamination along the infiltration direction.When the Zr content increased,the reaction and mass transfer rate are obviously accelerated,the composite grains are coarse;when the Zr content is low,the reaction is moderate,and the material structure is uniform.The W-rich nanoparticles and layered nano-phases,Ta2C,Nb2C and other plate-like crystals formed in the composite material can effectively improve the fracture toughness of the composite by deflection and bridging cracks.The fracture toughness of ZrCx-TiCy-Cu,intragranular ZrC-TiC/W-Cu,ZrCx-NbCy-Cu and ZrCx-TaCy-Cu composites reached 9.67±0.96,10.21±0.31,12.1±4.4 and 5.34±1.70 MPa·m1/2,respectively.The flexural strength and fracture toughness of ZrC-Mo-Cu composites reached 569 MPa and 8.2 MPa·m1/2,respectively.The ZrC-W-Cu material consists of W,ZrC and intercrystalline Cu-rich phases,with substantially no WC residue.There are metal(W,Cu)and intermetallic(W3Cu,Cu5Zr)nano phases in the ZrC grain.When the W content is reduced from 73vol%to48vol%,the microhardness of ZrC-W-Cu composite increases from 4.71 GPa to 8.93GPa,the flexural strength increases from 519 MPa to 657 MPa,and the fracture toughness increases to 9.1 MPa·m1/2 to 10.6 MPa·m1/2.The linaer ablation rate and mass ablation rate of ZrC-73vol%W-Cu composites were only 0.8μm·s-1 and3.5mg·s-11 after ablation for 60–120 s under oxyacetylene flame.When the W content in the material is less than 50 vol%,the material is prone to thermal shock damage.The ablation resistance of the material is mainly the formation of a viscous ZrO2protective layer and the sweating cooling of WO3 and Cu.Using Mo as an indicator,Zr-Cu was revealed by analyzing the element distribution,phase and microstructure of the(W0.9Mo0.1)C solid solution and Zr-Cu melt reaction interface.The law of the reaction between the melt and the WC solid.(W0.9Mo0.1)C dense and bulk pre-formed at 1200–1600℃ and Zr-Cu melt reaction for 15 min,respectively,W,Mo elements are segregated.A W segregation zone was found inside the Zr-Cu melt and ZrC grains.It indicates the presence of WC dissolution into the Zr-Cu melt during the solid solution reaction.As the reaction temperature increases,the Zr content in the melt increases,and the WC particle size decreases,the dissolution rate and the amount of dissolution increase.When the activity of Zr in the Zr-Cu melt is less than 0.3,the dissolved W precipitates as a simple substance;when the Zr content is high,it precipitates as a W-Zr intermetallic compound;when the reaction temperature exceeds 1600℃,The reaction interface forms a continuous ZrC layer to block the contact of the WC with the melt,and subsequent reactions are controlled by the diffusion of C atoms in the ZrC/W product layer.The segregations of W and Mo elements in the Zr-Cu melts and W rich inclusions in ZrC grains after reaction show both W and C in WC solid both dissolve into the Zr-Cu melts at the initial contact of the melt and WC solid.The dissolution rate and amount of W and C increase with decreasing radius of the WC particles and increasing Zr content in the Zr-Cu melts and the reaction temperature.The nucleation rate and growth rate both improve a lot with raising temperature.The dissolution of WC is restrained by the formation of a continuous ZrC layer,when the reaction temperature is 1600℃.However,there is always a dissolution process of WC causing incorporation of W atoms into Zr-Cu melt in the initial contact between the melt and solid.At 1300℃,the ZrC-W-Cu composite prepared by Zr2Cu/WC(Zr/C=1)mixed powder extended the reaction time from 10 min to 60 min,and a ZrC increased from0.4665 nm to 0.4671 nm.The solid solution of W and Cu atoms reduces aZrC.When the W and Cu atoms are dissolved in the ZrC lattice with C vacancies,the formation energy of the structure is low;the solubility decreases with the decrease of the C vacancy concentration.The formation mechanism of Cu/W-rich nanophase in ZrC grains is:the solid solubility of Cu/W in ZrCx decreases with the decrease of C vacancy concentration;with the extension of time,the solid solutes of Cu/W atoms precipitate in the form of elemental or intermetallic compounds.