Research On The Deformation Mechanism And Corrosion Resistance Of WC-Ni₃Al Cemented Carbide

WC cemented carbides have been widely used in cutting and wear applications due to their high hardness,fracture strength and wear resistance.However,WC with Co binder has been shown to have many limitations such as high density,low corrosion and poor oxidation resistance.Moreover,the mechanical properties of WC-Co show a sharp decline with increasing temperature because of the rapid softening of the Co phase at high temperature.Significant effort has been devoted to replacing Co with other binders,with the intermetallic compound Ni₃Al being considered one of the most promising alternatives.In this dissertation,a new type of Ni₃Al alloy powder has been designed and prepared by powder metallurgy methods based on the composition of classic Ni₃Al superalloy IC221.The effects of different ball milling suspensions on the microstructure and mechanical properties of WC-Ni₃Al composites were studied,and the deformation mechanisms and corrosion behavior of WC-Ni₃Al cemented carbides were analyzed.First,mechanical alloying?MA?through dry milling was used to synthesize Ni₃Al powders with a composition of Ni-8.25Al-7.85Cr-1.45Mo-0.01B.The results show that the intermetallic compound Ni₃Al is formed after 20 h of ball milling.With increasing milling time,the crystallite size of Ni₃Al decreases,the lattice strain increases slightly and the lattice parameter remains stable.After ball milling for 50 h,the particle size was refined and the chemical structure was stable.The as-milled Ni₃Al powder was mixed with WC powder with percentages of 2,6,10,15,or 20 wt.%by low energy ball milling under wet milling conditions.After milling,the mixed powders were sintered using spark plasma sintering.It is found that the oxygen content,phase composition and mechanical properties of bulk samples are directly related to whether oxygen is contained in the suspension used in the wet milling.The effects of ball milling powders with two different organic solvents on the microstructure and mechanical properties of cemented carbides were studied.We show that the oxygen in the organic solvent can be absorbed into the mixed powders by ball milling when ethanol?CH₃CH₂OH?is used as a ball milling suspension.This oxygen leads to the formation of?-Al₂O₃ during sintering,which improves the fracture toughness due to crack deflection and bridging,and the formation of?-phase?Ni₃W₃C?which inhibits the grain growth and increases the hardness.the samples prepared by the same sintering process with cyclohexane(C₆H₁₂)as the ball milling medium show no obvious oxide aggregation,while some WC grains grow abnormally and their hardness is relatively low.Second,based on the study of properties and microstructure of WC-Ni₃Al cemented carbide,the strain-rate dependent deformation behavior of WC-Ni₃Al cemented carbide at room temperature was studied with in-situ compression.The results show that Ni₃Al can maintain the synergic interaction of deformation phenomena taking place within the constitutive phases better than Co,and the micro-compressive strength of cemented carbides with different Ni₃Al contents?2²0 wt.%?are more than 6.5 GPa.A stress burst occurs in the lower stress state of the stress-strain curve due to plastic flow and deformation in the Ni₃Al binder,while a stress burst in the higher stress state is due to the deformation of WC grains under the synergistic effect of the binder.The deformation mechanisms can be explained by the competitive relationship between dislocation increment and dislocation annihilation.At high strain rates(?1×10^-2 s^-1),the dislocation density increases rapidly,and the interface can not absorb the dislocations in time,resulting in grain boundary slip and brittle fracture.The stress-strain response is stable and the dislocation annihilation mechanism is dominant at low strain rate(5×10^-3 s^-1),which leads to high plasticity and stress bursts in the stress-strain response The internal interface and free surface play a key role in the process of thermally activated deformation.The mutation of the strain rate sensitivity index validates the transformation of the deformation mechanisms.Third,the high temperature mechanical properties and high temperature strengthening mechanisms in WC-Ni₃Al cemented carbide were studied at 650⁹00°C.A large number of dislocations were found in WC-Ni₃Al cemented carbides at various temperatures under compression.At these temperatures,antiphase boundaries were generated in the Ni₃Al binder phase.Due to this mechanism,the WC-10%Ni₃Al sample exhibits a high compressive strength of 1303¹648 MPa at elevated temperatures.With the increasing temperature,WC-Ni₃Al cemented carbide exhibits a transition from linear elastic behavior to plastic yield.TEM analysis indicates that plastic accommodation during the deformation process is primarily due to deformation of WC.Yield loci slip mode analysis was used to confirm that dislocations,stacking faults,and even twins can exist on the prismatic plane{011?0}in WC at high temperatures.The deformation mechanism at high temperature is similar to but different from that of dislocation multiplication and annihilation at room temperature.This type of combined strengthening from both the matrix and binder provides a new strategy to improve the high-temperature mechanical properties of cemented carbides.Finally,the corrosion resistance of WC-10Ni₃Al cemented carbide was studied by comparison with that of WC-8Co cemented carbide.The corrosion resistance of WC-10Ni₃Al cemented carbide in acidic solutions?HCl,H₂SO₄?and alkaline solution?KOH?are better than that of WC-8Co cemented carbide.The presence of authigenic?-Al₂O₃ in WC-10Ni₃Al cemented carbide protects the contact between cemented carbide surface and corrosion solution.NiO formed in acid solution can effectively inhibit the dissolution of Ni₃Al in the acidic solutions,while Ni₂O₃ and Ni?OH?₂ were produced by Ni passivation in alkaline solution,while further corrosion in alkaline solution was inhibited by Ni passivation in coordination with Al₂O₃.In a neutral solution?NaCl?,WC-10Ni₃Al cemented carbide and WC-8Co cemented carbide have almost no corrosion behavior.