| BackgroundThe extreme conditions created by multi-bubble cavitation in liquids are physical foundation of ultrasound applications(such as sonochemistry).The theoretical and experimental research on single bubble cavitation is very mature.The multi-bubble cavitation dynamics and collapse mechanism are extremely complex,and difficult to be cleared through methods of numerical simulation and experiment.Cavitation bubbles has been modeled as a spherically symmetric bubble using a HYADES hydrocode.The center pressure and temperature of a collapsing bubble at 30 MPa static pressure are calculated as 4,400 GPa and 200,000 K,respectively,which are an order of magnitude than those of 20 MPa.High hydrostatic pressure is conducive to realization of intense cavitation,but the complex physical mechanism of multi-bubble cavitation at high hydrostatic pressure is still inexplicable.In our previous study,0.2 mm-diameter tungsten wire with melting point of 3410? and yield strength in GPa order could be served by intense cavitation in 0.1 s at 10 MPa.It suggests a new form of material failure,which is expected to provide a new approach of material preparation and characterization,and an inversion study on multi-bubble cavitation physics mechanism at high hydrostatic pressure through material failure,and deserves further study.ObjectiveThe fracture process of tungsten wire at focus in focused ultrasound field are combined with fractographic analysis to define the micro-fracture mechanism of tungsten wire under different hydrostatic pressures and driving powers.The cavitation damages of metal materials with different melting points and hardness are compared for inversion and speculation of extreme conditions resulting from cavitation cluster collapsing at 10 MPa hydrostatic pressure and 2 k W driving power.Methods1.Study on tungsten wire fracture caused by acoustic cavitation at different hydrostatic pressures and driving powersFor determining the internal microstructure of tungsten wire,laser scanning confocal microscopy(LSCM)and electron back-scattered diffraction(EBSD)technology were adopted to examine the microstructure and crystal orientation of tungsten wire's side and roll,respectively.Tensile test,shear test and tensile fatigue test were conducted to study the mechanical properties of tungsten wire at normal temperature and pressure.In degassed water with oxygen capacity of 0.8 mg/L,a spherical cavity transducer with two open ends,with frequency of 0.6 MHz,inner diameter of 500 mm,and aperture height of 474 mm,was used in this study.At different hydrostatic pressures(0.1 MPa,5 MPa,10 MPa)and driving powers(0.5 k W,1 k W,2 k W),multi-bubble distribution and multi-bubble sonoluminescence(MBSL)in focused ultrasound field were observed by digital camera and electron multiplying charge coupled devices(EMCCD),respectively.The luminous intensity of MBSL was measured to characterize cavitation intensity.The fracture process of 0.2 mm-diameter tungsten wire at focus was recorded by high-speed camera system to explore the relationship between the fracture time of tungsten wire and cavitation intensity.Three time points in each group of tungsten wire at certain time intervals(5 MPa,1 k W: 1 s,2 s,3 s;10 MPa,1 k W: 0.1 s,0.3 s,0.5 s;10 MPa,2 k W: 0.05 s,0.1 s,0.15 s)were selected to perform cavitation erosion.Combined with the micromorphology changes of surface damage on tungsten wire at different fracture stages,four types of fractures' morphology characteristics from macro and micro were compared and analyzed to explain the micro-fracture mechanism of tungsten wire under acoustic cavitation.2.Damages of aluminum,copper and nickel with different melting points and hardness under intense cavitationAluminum(Al),copper(Cu),nickel(Ni)were selected as the experimental materials in this chapter.Heat treatment processes were conducted to obtain copper sheets and nickel sheets with different hardness,and then surface grinding and polishing treatments were also performed.The transducer and degassed water used in this chapter were same as chapter one.At 10 MPa hydrostatic pressure and 2 k W driving power,the processes of cavitation damage on metals' surfaces were recorded by high-speed camera system during ultrasonic irradiation of 1 s.Cu and Ni with similar hardness were treated by acoustic cavitation for 0.5 s,1 s,and 2 s,respectively.The diameter and depth of cavitation pits of Cu and Ni were measured by LSCM to analyze the effect of hardness,melting point and ultrasonic irradiation time on cavitation damage.The micromorphology of pits on Al,Cu and Ni surface were observed using field emission scanning electron microscope(FESEM),and to estimate the effective transient temperature created by collapsing bubble cluster at 10 MPa and 2 k W.Results1.Study on fracture of tungsten wire at normal temperature and pressureThe metallographic examination results showed that tungsten wire consisted of long and narrow fibers.EBSD showed obvious fibrous texture phenomenon inside tungsten wire.The yield strength,tensile strength and shear strength of 0.2 mm-diameter tungsten wire were about 1280 MPa,2625 MPa and 1584 MPa,respectively.The tensile fatigue life of tungsten wire was greater than 106,which belonged to high stress and high cycle fatigue.The fracture analysis results showed that tensile and shear fracture were transgranular cleavage fracture.There were typical three-zone structures for fatigue fracture: fatigue source zone,crack propagation zone,and instantaneous fracture zone.2.Study on tungsten wire fracture caused by acoustic cavitation at different hydrostatic pressures and driving powersWith the increase of hydrostatic pressure,the range of bubble cluster gradually decreased,and the intensity of MBSL gradually increased.With the increase of driving power,the range of bubble cluster and the intensity of MBSL both increased slightly.The contribution of driving power to increasing luminous intensity was less than hydrostatic pressure.The transient cavitation at 10 MPa and 2 k W could fracture 0.2 mm-diameter tungsten wire in 0.1 s,and the fracture time of tungsten wire decreased with the increase of hydrostatic pressure or driving power.Obvious delamination phenomenon was found in tungsten wire fractures created by acoustic cavitation.According to crack propagation direction,tungsten wire fracture was divided into three zones,namely fiber split zone,mixed fracture zone and tensile fracture zone.The fracture morphology of tungsten wire with different fracture time was different,and the shorter the fracture time,the greater the proportion of fiber split zone.It was noteworthy that fractured fiber tracts were softening,shaped like "ball" or "flower" at 10 MPa.The micromorphology changes of surface damage on tungsten wire at different fracture stages suggested that with the increase of hydrostatic pressure or driving power,the extent of damage gradually decreased,and the velocity of damage gradually increased.With the increase of ultrasonic irradiation time,the fiber amount at both ends and the erosion degree in the middle of the damage area gradually increased.3.Damages of aluminum,copper and nickel with different melting points and hardness under intense cavitationThe cavitation damage results of Al,Cu and Ni with different melting points and hardness at 10 MPa,2 k W showed that the diameter-to-depth ratio of pits on Cu and Ni surface were about 2.2 ~ 3.2 and 1.7 ~ 2.6,respectively.For the same metal,the longer the irradiation time,the greater the depth of pits.The fitting line between the depth of Cu pits and irradiation time was basically parallel to that of Ni.For the same kind of metal,the depth of pits decreased with the increase of hardness.For different metals with similar hardness,the lower the melting point,the greater the depth of pits.In addition,we found that all metal surface damages were characterized by a large number of goblet craters,and obvious melting phenomenon could be observed on grain surface under FESEM.The melting depth and degree decreased with the increase of melting point of metals.ConclusionWith the increase of hydrostatic pressure and driving power,the cavitation intensity at focus gradually increased.The fracture time of tungsten wire was inversely proportional to cavitation intensity.The possible fracture mechanism of tungsten wire was as follows.The cavitation bubbles collapsed non-spherically,and micro-jets directed towards tungsten wire surface were formed,leading to tungsten wire surface tearing.Then,the cracks propagated along both the radial and axial directions of tungsten wire.The higher the cavitation intensity,the more likely the cracks extended along the radial direction,namely transgranular fracture,the shorter the axial crack propagation path,which mainly appeared as intergranular fracture.In addition,the degree of cavitation erosion was inversely proportional to melting point and hardness of metals,and was linearly proportional to irradiation time.At 10 MPa and 2 k W,we speculated that the effective transient temperature generated by collapsing bubble cluster was above 1453?. |
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