Adiabatic Shear Instability Research Articles

Microstructural study of adiabatic shear bands formed in serrated chips during high-speed machining of hardened steel

The characterization of the microstructure and phase transformation in adiabatic shear bands (ASBs) within the serrated chips generated during high-speed machining of hardened 30CrNi3MoV steel has been performed. The observations showed that the microstructure gradually changes from the center of the ASB to the matrix of the chip, the fine equiaxed grains appear with size of about 0.4–0.6 μm in the center of the ASBs, the transitional region adjacent to the ASB is characterized by the broken and elongated martensite laths in shear direction. The analysis indicated that the serrated chip formation was likely due to adiabatic shear instability that occurred in the primary shear zones and the transformation to martensite within the ASB. Dynamic recovery and recrystallization are the dominant metallurgical processes during microstructural evolution of ASB.

유한요소 시뮬레이션을 통한 저온 분사 코팅의 계면 접합에 대한 연구

A finite element modeling approach has been described for the simulation and analysis of the micron-scaled solid particle impact behavior in kinetic spraying process, using an explicit code (ABAQUS 6.7-2). High-strain-rate plastic deformation and interface bonding features of the copper, nickel, aluminum, and titanium were investigated via FEM in conjunction with the Johnson-Cook plasticity model. Different aspects of adiabatic shear instabilities of the materials were characterized as a concept of thermal boost-up zone (TBZ), and also discussed based upon energy balance concept with respect to relative recovery energy (RRE) for the purpose of optimizing the bonding process.

General aspects of interface bonding in kinetic sprayed coatings

In this study, different engineering materials are classified into four impact cases according to their physical and mechanical properties, i.e., soft/soft, hard/hard, soft/hard, and hard/soft (particle/substrate). Based on finite-element modeling, impact behaviors of the four cases were numerically analyzed. For soft/soft and hard/hard cases, the size variation of the thermal boost-up zone (TBZ), accompanied with the different aspects of adiabatic shear instability, was numerically estimated and is theoretically discussed. Meanwhile, for soft/hard and hard/soft cases, the specific aspect of shear instability, which has a very high heat-up rate, is always observed on the relatively soft impact counterpart where a thin molten layer is expected as well. Based on these phenomenological characteristics, bonding aspects are characterized, and a database for numerically estimated critical velocities of different particle/substrate combinations was developed for kinetic spraying process.

Grain refinement in a single titanium powder particle impacted at high velocity

Titanium powder particles were deposited at high velocity on a steel substrate by a process called warm spraying. Extremely fine grains, several tens of nanometers in size, were observed along the interfacial boundary of the deposited particles known as splats and the substrate, where the most severe deformation had taken place. A schematic model to explain the mechanism of grain refinement of titanium powder is presented, in which dynamic recrystallization caused by adiabatic shear instability plays the major role.

Dynamic amorphization and recrystallization of metals in kinetic spray process

We reported dynamic amorphization and recrystallization processes of metals upon impact of micron-scaled particles at a high strain rate (109s−1) combining adiabatic heating with rapid cooling (1010Ks−1) in a kinetic spray process. At the interface of the particle/substrate, an amorphous zone with a thickness of about 3nm was observed after individual particle impact. It is consistent with the mechanism of amorphous shear lamella and adiabatic shear instability characteristics in kinetic spray process. At the interface of coating/substrate, a rapid phase transition from unstable amorphous to crystalline helps the formation of ductile joints of coatings.

비행입자의 열 에너지에 따른 NiTiZrSiSn 벌크 비정질 분말의 적층 거동

Mechanical and thermomechanical properties of the bulk metallic glass (BMG) are so unique that the deformation behavior is largely dependent on the temperature and the strain rate. Impacting behavior of NiTiZrSiSn bulk metallic glass powder during kinetic spraying was investigated in this study. Considering the impact behavior of the BMG, the kinetic spraying system was modified and attached the powder preheating system to make the transition from the inhomogeneous deformation to the homogeneous deformation of impacting BMG particle easy BMG splat formation is considered from the viewpoint of the adiabatic shear instability. It is suggested that the impact behavior of bulk metallic glass particle is determined by the competition between fracture and deformation. The bonding of the impacting NiTiZrSiSn bulk amorphous particle was primarily caused by the temperature-dependent deformation and fracture (local liquid formation) behavior.

Impacting behavior of bulk metallic glass powder at an abnormally high strain rate during kinetic spraying

Impacting behavior and deposition formation of Ni–Ti–Zr–Si–Sn bulk metallic glass during kinetic spraying were investigated in this study. Bulk metallic glass particle deposition was primarily dependent on the impacting particle velocity. Secondly, the impacting particle temperature could transfer the deposition mode from fracture-induced local melting to severe viscous flow, especially for the splat on previously deposited splat. It is suggested that the deposition behavior of bulk metallic glass particles is determined by the competition between fracture and deformation. Fracture and deformation are controlled by adiabatic shear instability at the faying interface during impacting, with causal factors including temperature-dependent superplasticity of the bulk metallic glass and an abnormally high strain rate of the individual particle impact during kinetic spraying.

Examination of the Critical Velocity for Deposition of Particles in Cold Spraying

The critical velocity of copper (Cu) particles for deposition in cold spraying was estimated both experimentally and theoretically. An experimental method is proposed to measure the critical velocity based on the theoretical relationship between deposition efficiency and critical velocity at different spray angles. A numerical simulation of particle impact deformation is used to estmate the critical velocity. The theoretical estimation is based on the critical velocity corresponding to the particle velocity at which impact begins to cause adiabatic shear instability. The experimental deposition was conducted using Cu particles of different particle sizes, velocities, oxygen contents, and temperatures. The dependency of the critical velocity on particle temperature was examined. Results show that the critical velocity can be reasonably measured by the proposed test method, which detects the change of critical velocity with particle temperature and oxygen content. The Cu particles of oxygen content 0.01 wt.% yielded a critical velocity of about 327 m/s. Experiments show that the oxygen content of powder significantly influences the critical velocity. Variations in oxygen content can explain the large discrepancies in critical velocity that have been reported by different investigators. Critical velocity is also found to be influenced by particle temperature as well as types of materials. High particle temperature causes a decrease in critical velocity. This effect is attributed to the thermal softening at elevated temperatures.

Development of a generalized parameter window for cold spray deposition

In cold spraying, bonding occurs when the impact velocities of particles exceed a critical value. This critical velocity depends not only on the type of spray material, but also on the powder quality, the particle size and the particle impact temperature. Bonding in cold spraying is associated with adiabatic shear instabilities caused by high strain rate deformation during impact. Numerical and experimental methods are developed to investigate the influence of impact conditions and related phenomena on the coating quality. For a quantitative analysis, the materials behaviour was investigated through spray experiments, which were complemented by ballistic impact tests and explosive powder compaction. In this way, impact dynamics, bonding mechanism and critical velocities are linked to develop a general formulation, incorporating material properties and particle size, for the prediction of required impact conditions for cold spray deposition.

Cold gas dynamic spraying of a high temperature Al alloy

The present paper describes a study of the influence of cold spraying on the microstructure, heat treatment response and mechanical behavior of a high temperature 2618 aluminum alloy (Al–Cu–Mg–Fe–Ni) modified with 0.16 wt.% Sc. The microstructure of the starting powder consisted of a network of Al2CuMg phase on the grain boundaries of the Al matrix, along with Al9FeNi precipitates for atomized powder particles larger than 20 μm, and a cellular solidification structure for particles smaller than 20 μm. The coating after deposition was composed of the same microstructure features as observed in the feedstock powder but highly deformed. The observed overall deformation suggests the occurrence of localized adiabatic shear instability at the particle/particle boundary and adiabatic deformation softening at the powder interior during splat formation. The coating hardness was similar to the one obtained at the peak aged condition (T6). The coating aging response was influenced by the presence of Al7FeCu2 particles that depleted the matrix from Cu, reducing the overall hardness and overcoming the benefits of Sc addition.

Evidence of scuffing initiation by adiabatic shear instability

A mechanism for the initiation of scuffing based on adiabatic shear instability was assessed experimentally through the measurement of dislocation densities by X-ray diffraction. Scuffing was predicted to occur when the rate of local thermal softening exceeded that of work hardening in the tribological contact. Since these rates were dependent on microstructure, the theory was tested by performing scuffing tests on SAE 4340 steel subjected to five different heat treatments. The scuffing resistances were consistent with the theory, and thermal events during the tests and the resulting microstructural morphologies were consistent with a locally adiabatic environment. The dislocation densities measured in samples from tests stopped before, during, and after scuffing were consistent with the theory. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy under Contract W-31-109-ENG-38.

Microstructural evolution during scuffing of hardened 4340 steel—implication for scuffing mechanism

Microstructural changes occurring during scuffing failure in hardened 4340 steel were studied with a block-on-ring test rig using a step-loading test protocol. Tests were interrupted before, during, and after scuffing. Both surface and sub-surface changes in the original tempered microstructure were characterized by scanning electron microscopy. Results of our study showed that scuffing occurred by severe and sudden plastic deformation of near-surface material in a sub-second time frame. Based on this observation and other previously observed microstructural changes, i.e. formation of a large fraction of retained austenite during scuffing, a scuffing mechanism based on adiabatic shear instability is proposed. The proposed scuffing mechanism can effectively explain most of the phenomenological observations associated with scuffing failure.

Adiabatic shear instability based mechanism for particles/substrate bonding in the cold-gas dynamic-spray process

Particles/substrate interactions during the cold-gas dynamic-spray deposition process are studied using a dynamic axisymmetric thermo-mechanical finite element analysis. In addition, the particles/substrate bonding mechanism has been investigated using a one-dimensional thermo-mechanical model for adiabatic strain softening and the accompanying adiabatic shear localization. The results obtained show that the minimal impact particles velocity needed to produce shear localization at the particles/substrate interface correlates quite well with the critical velocity for particles deposition by the cold-gas dynamic-spray process in a number of metallic materials. This finding suggests that the onset of adiabatic shear instability in the particles/substrate interfacial region plays an important role in promoting particle/substrate adhesion and, thus, particles/substrate bonding during the cold-gas dynamic-spray process.

Bonding mechanism in cold gas spraying

Cold gas spraying is a relatively new coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to the well-known thermal spray processes such as flame, arc, and plasma spraying, in cold spraying there is no melting of particles prior to impact on the substrate. The adhesion of particles in this process is due solely to their kinetic energy upon impact. Experimental investigations show that successful bonding is achieved only above a critical particle velocity, whose value depends on the temperature and the thermomechanical properties of the sprayed material. This paper supplies a hypothesis for the bonding of particles in cold gas spraying, by making use of numerical modelling of the deformation during particle impact. The results of modelling are assessed with respect to the experimentally evaluated critical velocities, impact morphologies and strengths of coatings. The analysis demonstrates that bonding can be attributed to adiabatic shear instabilities which occur at the particle surface at or beyond the critical velocity. On the basis of this criterion, critical velocities can be predicted and used to optimise process parameters for various materials.

Influence of specimen geometry on adiabatic shear instability of tungsten heavy alloys

Finite element simulations are carried out for the deformation and instability of truncated-conic tungsten heavy-alloy specimens subjected to impact loading. A two-dimensional axis-symmetric adiabatic model is introduced in the simulations and the Johnson–Cook model is employed to describe the thermo-viscoplastic behavior of the material. In order to obtain detailed information of the deformation and instability of specimens in different scales, two meshes are used in the simulations. A coarse mesh is used to gain a general picture of the global deformation. Then, a locally refined mesh with a minimum element dimension of 10 μm is used to analyze the initiation and propagation of the adiabatic shear band (ASB). The results obtained by the present numerical simulations agree well with experimental observations. Finally, three truncated-conic specimens with different geometry are simulated. It is found that the specimen geometry as well as the surface friction has significant effect on the sensitivity of ASB formation.

Onset of Adiabatic Shear Instability in Strain Gradient Dependent Metal Matrix Composites

Article Onset of Adiabatic Shear Instability in Strain Gradient Dependent Metal Matrix Composites was published on December 1, 2002 in the journal International Journal of Nonlinear Sciences and Numerical Simulation (volume 3, issue 3-4).

Rheological structure and deformation of subducted slabs in the mantle transition zone: implications for mantle circulation and deep earthquakes

Rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain-size, stress, temperature and pressure dependence of rheology. It is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain-size of spinel (ringwoodite) and the magnitude of forces acting on a slab. We use a theoretical model of grain-size evolution associated with the olivine–spinel transformation, plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated. Three deformation mechanisms for olivine and spinel (diffusional creep, power-law (dislocation) creep and the Peierls mechanism) are considered. Due to the large variation in temperature, stress and grain-size, a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth. A cold slab in the deep transition zone is characterized by a weak, fine-grained spinel region surrounded by narrow but strong regions. The flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress-dependent rheology is incorporated in a self-consistent fashion. Although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large, the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained. Slabs with very low thermal parameters (warm slabs) are weak, but slabs with large thermal parameters (cold slabs) are also weak due to small spinel grain-size and large external force (bending moment). Slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation. Thus the 660 km discontinuity may work as a rheological filter for mantle convection. This prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western Pacific where temperatures of the slabs are considered to be low. Our slab rheology models also have important implications for deep earthquakes. Overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability (adiabatic shear instability). Our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology: energy dissipation in deep, cold slabs is concentrated in high strength regions surrounding a weak, fine-grained spinel core. The regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic. The width of this seismogenic region is well constrained by our model and is predicted to be ∼40–60 km which is in excellent agreement with seismological observations. Other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model.

Implications of slab mineralogy for subduction dynamics

Phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs, both under equilibrium and disequilibrium (metastable) conditions. Such perturbations yield anomalies not only in seismic velocities but also in density. The buoyancy forces arising from these density anomalies may exert several important effects. They contribute to the stress field within the slab, in a fashion consistent with observed patterns of seismicity. They may affect subduction rates, both by inducing time-dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions. They may affect slab morphology, possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs. Latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating, which may generate adiabatic shear instabilities capable of triggering deep seismicity.

Formation of adiabatic shear bands and instability of plastic flow in Zr and Zr-Nb alloys in spherical stress waves

Formation of adiabatic shear bands (ASBs) in different metals and alloys occurs at high strain rates and levels of stresses. The method of loading by spherical converging shock waves is the most perspective method for the investigation of the formation and evolution of the ASBs and other instabilities of the plastic flow. Spherical shock-wave recovery experiments allow to obtain extremely high stresses, substantially exceeding dynamic yield strength, as well as high strain rate at the fronts of spherical converging and diverging shock waves. Such experiments ensure the spherically-symmetric motion during the whole time of the wave processes progress up to the shut-down of the recovery device, exclude the distorting influence of the lateral and face-end unloading wave (in contrast to plane and cylindrical recovery devices), as well as the secondary effects associated with the deceleration and the entrapping of the compressed and recovered samples. Although a number of investigations has been made concerning the nature of the ASBs, the understanding of the ASBs formation and flow localization as modes of deformation is still insufficient. The purpose of the present work is to study the formation and evolution of the ASBs and the instability of plastic flow in zirconium and itsmore » alloys with 1 and 2.5wt% niobium under loading by spherical converging stress waves of different intensity.« less

Analysis of energy absorption of adiabatic shear plugging in thermoviscoplastic targets

A mechanical model, based on the theory of adiabatic shear instability and fracture of thermoviscoplastic materials, is proposed to analyse the energy absorption of plugging. An important relationship between energy absorption and initial impact velocity is obtained. When the initial velocity is slightly greater than the critical impact velocity, the energy absorption decreases with increasing initial velocity; however, the energy absorption increases for initial velocity greater than the critical impact velocity. This analytical result is the same as that observed experimentally. This phenomena is explained with the adiabatic shear instability and fracture criterion controlled by both the strain and strain-rate.