The structure of adiabatic shear bands in metals: A critical review

A review of the structure of adiabatic shear bands in metals is presented. Shear bands are redefined as being either “transformed” or “deformed” according to how the prior shear deformation is partitioned between two discrete zones in metallographic section. Metals are then classified by their general tendency to form these two types of shear zone during adiabatic shear deformation, based on available literature. Metals of low thermal diffusivity and of low resistance to adiabatic shear localization tend more readily to form “transformed” shear bands; these metals are also capable of transforming to other phases at elevated temperature (and pressure), and forming martensite on rapid cooling to room température. Shear bands of “transformed” appearance can also form in other metals during extremely localized adiabatic shear deformation resulting from the effect of localized plastic flow and elevated temperature alone. The role of phase transformations themselves in promoting the formation of “transformed” shear bands cannot be isolated using the arguments presented in this work, and may even by incidental.     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

The structure of adiabatic shear bands in a titanium alloy

A study has been made of the structure of adiabatic shear zones in Ti-6% Al-4% V with different parent microstructures, resulting from ballistic impact of steel spheres. Metallographic examination of well-developed shear bands showed that they consisted of zones of intense shearing distortion of the original microstructure, modified by the effects of elevated temperature. An analogy is made between their structure and that of the white-etching shear zones observed in steels. Unlike steels, however, there was no clear evidence in this alloy to suggest that the shear bands in the α + β microstructures had undergone a martensitic phase transformation. The structure of the shear zones in an α' martensite parent alloy appeared to be a tempered form of the original microstructure.     

Microscopic observations of adiabatic shear bands in three different steels

Microscopic observations are made of the shear band material in three different steels: (1) an AISI 1018 cold-rolled steel (CRS), (2) a structural steel (HY-100), and (3) an AISI 4340 vacuum arc remelted (VAR) steel tempered to either of two hardnesses, RHC 44 or 55. To produce the shear bands, specimens were subjected to large shear strains at relatively high strain rates, ≈10³/s, resulting in essentially adiabatic deformation conditions. It was found that whenever the shear band led to fracture of the specimen, the fracture occurred by a process of void nucleation and coalescence; no cleavage was observed on any fracture surface, including the most brittle of the steels tested (RHC = 55). This is presumably due to the softening of the shear band material that results from the local temperature rise occurring during dynamic deformation. Differences in shear band behavior between the various microstructures are also described. Formerly Research Assistant, Brown University     

Development of adiabatic shear bands in annealed 316L stainless steel: Part II. TEM studies of the evolution of microstructure during deformation localization

The evolution of adiabatic shear localization in an annealed AISI 316L stainless steel has been investigated and was reported in Part I of this paper (Met. Trans. A, 2006, Vol. 37A, pp. 2435–446). In the present research (Part II), a comprehensive transmission electron microscopy (TEM) examination was conducted on the microstructural evolution of shear localization in this material at different loading stages. The TEM results indicate that elongated subgrain laths and an avalanche of dislocation cells are the major characteristics in an initiated band. Development of the substructures within shear bands is controlled by dynamic recovery and continuous dynamic recrystallization. The core of shear bands was found to consist of fine equiaxed subgrains. Well-developed shear bands are filled with a mixture of equiaxed, rectangular, and elongated subgrains. The equiaxed subgrains, with a typical size less than 100 nm, are postulated to result from either the breakdown and splitting of subgrain laths or the reconstruction of subcells.     

Formation of adiabatic shear bands in eutectoid steels in high strain rate compression

A detailed study has been made of the localized adiabatic shear band formation in a plain carbon and a low alloy eutectoid rail steel subjected to high strain rate compression at initial deformation temperature of 298, 453 and 623 K. Localized adiabatic shearing due to impact is found to be favored with alloy steels and decreasing temperatures of deformation. It is shown that the deformed and transformed shear bands rather than being two separate phenomena are only an outcome of the extent of adiabatic strain localization occuring during deformation; the deformed bands forming with lesser localized flow and the transformed bands forming with extensive localized flow. This study explains convincingly the formation of the white phase on the surface of the rail heads during wheel-rail contact as due to the coalescence of the numerous adiabatic shear bands.     

Development of adiabatic shear bands in annealed 316L stainless steel: Part I. Correlation between evolving microstructure and mechanical behavior

Adiabatic shear localization in an annealed AISI 316L stainless steel was examined through a forced shear technique using a split Hopkinson pressure bar and hat-shaped specimens. A well-controlled forced shear technique provided the possibility of correlating the microstructural evolution of adiabatic shear localization to its transient mechanical behavior. The initiation of adiabatic shear bands occurred when the shear stress peaked after substantial work hardening. The work-hardening rate was found to play a dominant role in the formation of adiabatic shear localization. The stress drop presupposed the development of the localized deformation. A core structure of shear bands was generated within the shear band, which characterized a narrow-down process in the early stage of the shear band evolution. The continuous expansion of the shear band core to the entire width of the band was seen to correlate with the full development of shear localization.     

Microstructural development of adiabatic shear bands formed by ballistic impact in a WELDALITE 049 alloy

The object of the present study is to investigate the microstructural development of the adiabatic shear band formed by ballistic impact in a WELDALITE 049 alloy. The microstructure of the shear band was examined by optical microscopy and transmission electron microscopy. The results indicated that the adiabatic shear band consisted of fine recrystallized grains with a high dislocation density. This microstructure was considered to be formed in an extremely short time by the combined effects of the highly localized shear deformation and the high-temperature rise that occurred within the shear band. However, no precipitates could be observed in the interior of the grains, since the temperature rise in the shear band formation process was inferred to be above 460 °C and below the solidus temperature. Dynamic recrystallization was suggested as a possible mechanism to explain the microstructural development of the adiabatic shear band formed in the WELDALITE alloy.     

A process model for the heat-affected zone microstructure evolution in Al-Zn-Mg weldments

In the present investigation, process modeling techniques have been applied to unravel the sequence of reactions occurring during welding and subsequent natural aging of Al-Zn-Mg extrusions. The model uses a combination of chemical thermodynamics and diffusion theory to capture the heat-affected zone (HAZ) dissolution and aging kinetics, with the particular feature of writing the constitutive evolution equation in a differential form. Separate response equations are then developed to convert the calculated values for the particle volume fraction and the matrix solute content into engineering quantities such as hardness or strength, for a direct comparison to experiments. The model indicates that particle dissolution is the main factor contributing to strength loss during welding. At the same time, growth of the remaining particles occurs during cooling, leading to solute depletion within the aluminium matrix. This, in turn, reduces the precipitation potential and contributes to the development of a permanent soft zone within the partly reverted region after prolonged room-temperature aging. It is concluded that the combination of a microstructure model with an appropriate heat-flow model creates a powerful tool for alloy design and optimization of welding conditions for Al-Zn-Mg extrusions, and an illustration of this is given toward the end of the article.     

A process model for the microstructure evolution in ductile cast iron: Part I. the model

In the present investigation, the multiple phase changes occurring during solidification and subsequent cooling of near-eutectic ductile cast iron have been modeled using the internal state variable approach. According to this formalism, the microstructure evolution is captured mathematically in terms of differential variation of the primary state variables with time for each of the relevant mechanisms. Separate response equations have then been developed to convert the current values of the state variables into equivalent volume fractions of constituent phases utilizing the constraints provided by the phase diagram. The results may conveniently be represented in the form of C curves and process diagrams to illuminate how changes in alloy composition, graphite nucleation potential, and thermal program affect the microstructure evolution at various stages of the process. The model can readily be implemented in a dedicated numerical code for the thermal field in real castings and used as a guiding tool in design of new treatment alloys for ductile cast irons. An illustration of this is given in an accompanying article (Part II).     

几种变形方式对钨合金组织性能及绝热剪切敏感性的影响

讨论了变形方式、变形量、变形材料的受力状态及微观组织结构等方面因素对材料性能和绝热剪切敏感性的影响。钨合金的受力状态、颗粒形状、微观组织取向对材料的变形、破坏和变形局域化机制有重要影响。X射线分析表明，旋锻后的钨合金组织有织构存在。力学性能的各向异性导致了材料绝热剪切破坏难易程度上的差异。     

Adiabatic shear bands on the titanium side in the titanium/mild steel explosive cladding interface: Experiments, numerical simulation, and microstructure evolution

The microstructure and microtexture in adiabatic shear bands (ASBs) on the titanium side in the titanium/mild steel explosive cladding interface are investigated by means of optical microscopy, scanning electron microscopy/electron backscattered diffraction (SEM/EBSD), and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Microtextures (25 deg, 75 deg, 0 deg), (70 deg, 45 deg, 0 deg), and (0 deg, 15 deg, 30 deg) formed within the ASBs suggest the occurrence of the recrystallization. The grain boundaries within ASBs are geometrically necessary boundaries (GNBs) with high angles. Finite element computations are performed to obtain the effective strain and temperature distributions within the ASBs under the measured boundary conditions. The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nanograins’ formation and the recrystallized process within ASBs. During the deformation time (about 5 to 10 μs), the following processes take place: dislocations accumulate to form elongated cell structures, cell structures break up to form subgrains, and subgrains rotate and finally form recrystallized grains. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Microstructural study of adiabatic shear band

This article presents a study of the microstructural development of the adiabatic shear band in an HY-100 steel. The steel was deformed at a high strain rate by ballistic impact, and subsequent metallographic observations along with electron microscopy were performed. A number of white- etched shear bands were found near the perforated region, and three typical microstructural features of the adiabatic shear band were observed: elongated grain structure at the boundary between the shear band and matrix, fine equiaxed grain structure with high dislocation densities in the middle of the shear band, and relatively coarse-grained structure located between the above two structures. These microstructures might be formed in an extremely short time by the combined effects of the large temperature rise and the highly localized deformation. Since very complex phenomena might occur within the shear band, possible mechanisms, such as dynamic recovery and strain-induced dynamic phase transformation, are suggested to explain the micro- structural development of the adiabatic shear band.     

A process model for the microstructure evolution in ductile cast iron: Part II. Applications of the model

In the present investigation, the process model developed in Part I has been implemented in a dedicated numerical code to reveal the evolution of the coupled thermal and microstructural fields during directional solidification of ductile iron. In a calibrated from, the model predicts adequately both the variation in the graphite nodule count and the resulting microstructural profiles (i.e., graphite, iron carbide, ferrite, and pearlite) in the length direction of the bar. At the same time, the model has the required flexibility to serve as a research tool and predict behavior under conditions that have not yet been explored experimentally. In this article, the aptness of the model to alloy design and optimization of melt treatment practice for ductile iron is illustrated in different case studies and numerical examples.     

Reverse shear of shear bands and annealing embrittlement in amorphous alloy

Shear bands produced by bending before annealing could be reversed if isothermally annealed or continuously heated before the onset of embrittlement. The activation energy for the onset of embrittlement is 69–72 kcal/mol similar to that of crystallization, 67–68 kcal/mol, if the latter is evaluated at the same X-ray first peak height. The activation energy of crystallization based on Kissinger plots of DSC data is erroneous, since samples at the onset or peak of crystallization in the thermogram do not have similar X-ray first peak heights. It seems that whatever structural modification is inside the shear bands is stable during annealing until embrittlement and that the embrittlement is related to the incipience of crystallization.     

A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part I. the model

The present investigation is concerned with modeling of the microstructure evolution in duplex stainless steels under thermal conditions applicable to welding. The important reactions that have been modeled are the dissolution of austenite during heating, subsequent grain growth in the delta ferrite regime, and finally, the decomposition of the delta ferrite to austenite during cooling. As a starting point, a differential formulation of the underlying diffusion problem is presented, based on the internal-state variable approach. These solutions are later manipulated and expressed in terms of the Scheil integral in the cases where the evolution equation is separable or can be made separable by a simple change of variables. The models have then been applied to describe the heat-affected zone microstructure evolution during both thick-plate and thin-plate welding of three commercial duplex stainless steel grades: 2205, 2304, and 2507. The results may conveniently be presented in the form of novel process diagrams, which display contours of constant delta ferrite grain size along with information about dissolution and reprecipitation of austenite for different combinations of weld input energy and peak temperature. These diagrams are well suited for quantitative readings and illustrate, in a condensed manner, the competition between the different variables that lead to structural changes during welding of duplex stainless steels.     

细晶钨合金的绝热剪切敏感性

采用粉末冶金法制备平均晶粒度<5μm细晶90W-Ni-Fe含金.利用HOPKINSON压杆装置,分别在0.9 MPa和1.4 MPa的冲击气压条件下对该合金进行一维应力冲击实验,并对冲击后的样品进行金相组织观测,考察其在一维应力冲击条件下的绝热剪切性能,分析细晶钨合金的绝热剪切敏感性.研究表明:晶粒细化有助于绝热剪切带的扩展,可以提高钨合金绝热剪切敏感性,使得烧结态细晶钨合金在一维冲击应力加载条件下就可以产生绝热剪切带.随着冲击(加载)气压的加大,绝热剪切现象更明显,冲击气压为1.4 MPa时剪切带宽度约为10μm,从而有助于材料在动态压缩条件下产生绝热剪切破坏,提高材料在穿甲过程中的"自锐"能力.