Adiabatic shear band formation in Al-SiCw composites

The objective of this study is to investigate the adiabatic shear band formation in 2124-T6 aluminum composites reinforced with SiC whiskers. The composites were deformed at high strain rates by ballistic impact. Adiabatic shear bands initiated from cracks near the impacted region were observed. The shear bands tended to propagate along the extrusion direction, since they were blocked by SiC whiskers. Within the shear bands, microvoids and microcracks formed, presumably by the temperature rise. Shear bands, together with microvoids and microcracks, deteriorated the impact resistance of target materials. Finally, performance of the composites against ballistic impact loading was discussed by comparing the behavior of shear banding with dynamic fracture toughness.     

Dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy having equiaxed and bimodal microstructures

Effects of microstructural morphology on dynamic deformation behavior and ballistic impact properties of Ti-6Al-4V alloy plates were investigated in this study. Dynamic torsional and ballistic impact tests were conducted on equiaxed and bimodal microstructures, which were processed by different heat treatments, and then the test data were analyzed in relation to microstructures and tensile properties. According to the dynamic torsional test data, maximum shear stress and fracture shear strain of the bimodal microstructure were higher than those of the equiaxed microstructure, and the possibility of the adiabatic shear band formation was more likely in the equiaxed microstructure than in the bimodal microstructure. In the ballistically impacted region of the equiaxed microstructure, a number of adiabatic shear bands and cracks were observed to be formed along plastic flow lines, and delamination occurred because of cracking along the flow lines or shear bands. In the case of the bimodal microstructure, shear bands were found in limited areas near the penetrated surface without occurring delamination, and their number was smaller than that of the equiaxed microstructure. Thus, ballistic performance of the bimodal microstructure was better than that of the equiaxed microstructure, which was consistent with the dynamic torsional test results.     

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.     

Effect of tungsten particle shape on dynamic deformation and fracture behavior of tungsten heavy alloys

The effect of the tungsten particle shape on the dynamic deformation and fracture behavior of tungsten heavy alloys was investigated. Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregularly shaped, cleavage fracture occurred in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

Effect of tungsten particle shape on dynamic deformation and fracture behavior of tungsten heavy alloys

The effect of the tungsten particle shape on the dynamic deformation and fracture behavior of tungsten heavy alloys was investigated. Dynamic torsional tests were conducted using a torsional Kolsky bar for five alloys, one of which was fabricated by the double-cycled sintering process, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. The dynamic torsional test results indicated that in the double-sintered tungsten alloy whose tungsten particles were very coarse and irregularly shaped, cleavage fracture occurred in the central area of the gage section with little shear deformation, whereas shear deformation was concentrated in the central area of the gage section in the other alloys. The deformation and fracture behavior of the double-sintered alloy correlated well with the observation of the impacted penetrator specimen and the in situ fracture test results, i.e., microcrack initiation at coarse tungsten particles and cleavage crack propagation through tungsten particles. These findings suggested that the cleavage fracture mode would be beneficial for the self-sharpening effect, and, thus, the improvement of the penetration performance of the double-sintered tungsten heavy alloy would be expected.     

Effect of surface carburization on dynamic deformation and fracture of tungsten heavy alloys

Effects of surface carburization on dynamic deformation and fracture behavior of tungsten heavy alloys were investigated in order to improve the penetration performance. Dynamic torsional tests using a torsional Kolsky bar were conducted on four specimens, three of which were carburized by the case carburization process. The test data were then compared with hardness, Charpy impact energy, adiabatic shear banding, deformation and fracture mode, and penetration performance. With increasing carburization temperature and time, surface hardness increased, but impact energy decreased. The dynamic torsional test results indicated that for the carburized tungsten specimens, cleavage fracture occurred in the center of the gage section with little shear deformation, whereas shear deformation was concentrated at the center of the gage section for the conventionally processed specimen without carburization. The deformation and fracture behavior of the carburized specimens correlated well with the observation of the impacted penetrator specimens, i.e., microcrack initiation at tungsten particles and cleavage crack propagation. Since the cleavage fracture mode is thought to be beneficial for self-sharpening, these findings suggest the beneficial effect of the surface carburization on the penetration performance.     

Analysis and prevention of cracking phenomenon occurring during cold forging of two AISI 1010 steel pulleys

This study is concerned with the effects of microstructural parameters on the cracking phenomenon occurring during cold forging of two AISI 1010 steels that were fabricated by converter steel making and electric furnace steel making, respectively. This allowed a comparison between microstructures that contained a small or large amount of nitrogen. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks initiated and propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using a torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. From the dynamic shear stress-strain curves, the steel containing a considerable amount of nitrogen showed a smaller shear strain of 0.2 at the maximum shear stress point, after which the shear stress decreased rapidly prior to fracture, whereas the other steel containing a smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with the yield-point phenomenon occurring in the steel containing more nitrogen. Because the cracking occurring during cold forging was associated with the adiabatic shear banding and the yield-point phenomenon, the minimization of nitrogen and the fast cooling rate after hot rolling were suggested to prevent the cracking.     

高强度低成本Ti5322合金的抗弹性能及其抗弹机理研究

采用底推式105 mm长杆穿甲模拟弹,对厚度为45 mm的高强度低成本Ti5322合金靶板开展了终点弹道侵彻实验,研究了该合金的抗弹性能与抗弹机理,并探究了合金的典型抗弹效应——倾角效应。结果表明,Ti5322合金的质量防护系数为1.80,空间防护系数为1.02。Ti5322合金的损伤机制和抗弹机理是通过自身的绝热剪切局域化行为实现的。这种绝热剪切局域化行为,一方面通过靶板的变形破碎协调了弹体侵入过程的挤凿作用;另一方面,弹靶作用过程中发生的弹靶互侵蚀行为有效地消耗了弹体动能。     

Effect of test temperature on the dynamic torsional deformation behavior of two aluminum-lithium alloys

The objective of the present study is to investigate the effect of test temperature on the dynamic torsional deformation behavior of two Al-Li alloys, i.e., 2090 and 8090 alloys. Dynamic torsional tests were conducted using a torsional Kolsky bar at room temperature and a low temperature (−196 °C), and the torsionally deformed regions and the fracture surfaces of the tested specimens were examined. The dynamic properties of the two Al-Li alloys at the low temperature were improved, owing to the modification of the deformation behavior. The dynamic deformation behavior at room temperature was dominated by intergranular cracks due to planar slips and by crack propagation along the grain boundaries. At the low temperature, plastic deformation proceeded more homogeneously as planar slip was prevented. These results indicated that the overall deformation mode of both the Al-Li alloys changed from planar slip to homogeneous deformation with decreasing temperature, resulting in the improvement of cryogenic properties under dynamic torsional loading.     

Effects of microstructural morphology on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy

The effects of microstructural morphology on quasi-static and dynamic deformation behavior of a Ti-6Al-4V alloy were investigated in this study. Quasi-static and dynamic torsional tests were conducted using a torsional Kolsky bar for Widmanstätten, equiaxed, and bimodal microstructures, which were processed by different heat treatments, and then, the test data were analyzed in relation to microstructures, tensile properties, and fracture mode. Quasi-static torsional properties showed a tendency similar to tensile properties and ductile fracture occurred in all three microstructures. Under dynamic torsional loading, maximum shear stress of the three microstructures was higher and fracture shear strain was lower than those under quasi-static loading, but the overall tendency was similar. In the Widmanstätten and equiaxed microstructures, adiabatic shear bands were found in the deformed region of the fractured specimens. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed, depending on how plastic deformation energy was distributed to either void initiation or adiabatic shear banding. It was found to be most likely in the equiaxed microstructure, whereas it was least likely in the bimodal microstructure.     

Effects of Cerium on Texture and Mechanical Property of 2090 Aluminium-Lithium Alloys

Conventional processing of Al-Li alloys asstructural materials used in aerospace industries such as rolling, prestretching, solutioningand aging will produce various textures such asrolling texture, recrystallization texture whichcan lead to mechanical anisotropy. Adding Ceto Al-Li alloys by means of rare earth microalloying and purification, the mechanical properties will be improved[' ~3). It is significant tostudy the actions of Ce on Al-Li alloys from theviewpoint of texture, hilt resea…     

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。     

Effect of size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys

The effect of the size and shape of tungsten particles on dynamic torsional properties in tungsten heavy alloys was investigated. Dynamic torsional tests were conducted on seven tungsten alloy specimens, four of which were fabricated by repeated sintering, using a torsional Kolsky bar, and then the test results were compared via microstructure, mechanical properties, adiabatic shear banding, and deformation and fracture mode. The size of tungsten particles and their hardness were increased as sintering temperature and time were increased, thereby deteriorating fracture toughness. The dynamic torsional test results indicated that in the specimens whose tungsten particles were coarse and irregularly shaped, cleavage fracture occurred predominantly with little shear deformation, whereas shear deformation was concentrated into the center of the gage section in the conventionally fabricated specimens. The deformation and fracture behavior of the specimens having coarse tungsten particles correlated well with the observation of the in situ fracture test results, i.e., cleavage crack initiation and propagation. These findings suggested that there would be an appropriate tungsten particle size because the cleavage fracture mode would be beneficial for the “self-sharpening” of the tungsten heavy alloys.     

Metallographic features associated with the penetration of titanium alloy targets

Ballistic tests on Ti-6A1-4V alloy targets have produced a number of interesting features. There was direct evidence of melting at the projectile-target interface. The targets failed by adiabatic shear and evidence for cavity growth in the shear bands is presented. Observed directionality in the ballistic performance of the target material is associated with anisotropy of the target material.     

Correlation of microstructure with dynamic deformation behavior and penetration performance of tungsten heavy alloys fabricated by mechanical alloying

In this study, tungsten heavy alloy specimens were fabricated by mechanical alloying (MA), and their dynamic torsional properties and penetration performance were investigated. Dynamic torsional tests were conducted on the specimens fabricated with different sintering temperatures after MA, and then the test data were compared with those of a conventionally processed specimen. Refinement of tungsten particles was obtained after MA, but contiguity was seriously increased, thereby leading to low ductility and impact energy. Specimens in which both particle size and contiguity were simultaneously reduced by MA and two-step sintering and those having higher matrix fraction by partial MA were successfully fabricated. The dynamic test results indicated that the formation of adiabatic shear bands was expected because of the plastic localization at the central area of the gage section. Upon highspeed impact testing of these specimens, self-sharpening was promoted by the adiabatic shear band formation, but their penetration performance did not improve since much of kinetic energy of the penetrators was consumed for the microcrack formation due to interfacial debonding and cleavage fracture of tungsten particles. In order to improve penetration performance as well as to achieve selfsharpening by applying MA, conditions of MA and sintering process should be established so that alloy densification, particle refinement, and contiguity reduction can be simultaneously achieved.     

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.     

弹丸高速冲击条件下高强度钢板抗弹性能

摘要：为了研究弹丸高速冲击条件下不同洁净度的40CrNi2Mo钢板的抗弹性能，利用12.7mm穿甲燃烧弹对抗拉强度分别为800和1200MPa级的钢板进行抗弹性能测试。通过观察不同强度钢板出现的损伤形貌，评定背面强度极限，分析了穿甲机制。结果表明：抗拉强度为800MPa级的钢板在弹丸冲击过程中以塑性扩孔方式侵彻，抗弹性能随着强度升高而提高，与洁净度关系不大。抗拉强度为1200MPa级的钢板，弹丸冲击过程中因钢板较低的绝热剪切临界失稳应变而出现绝热剪切；由于塑韧性较低，低洁净度钢板阻止绝热剪切引发裂纹扩展的能力较弱，因此形成与绝热剪切相关的裂纹，导致抗弹性能降低；高洁净度钢板抗弹性能相对较高，因背面出现剪切裂纹而失效，此裂纹与绝热剪切无关。     

Dynamic deformation and fracture behavior of ultrafine-grained aluminum alloy fabricates by equal-channel angular pressing

In the present study, ultrafine-grained microstructures of a conventional 5083 aluminum alloy were fabricated by equal-channel angular pressing, and their dynamic deformation and fracture behavior were investigated. Dynamic torsional tests were conducted on four aluminum alloy specimens using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-banding behavior. The equal-channel angular-pressed (ECAP) specimens consisted of ultrafine grains and contained a considerable amount of second-phase particles, which were refined and distributed homogeneously in the matrix as the equal-channel angular pressing pass number increased. The dynamic torsional test results indicated that the maximum shear stress increased, while the fracture shear strain remained constant, with increasing equal-channel angular pressing pass number. Observation of the deformed area beneath the dynamically fractured surface showed that a number of voids initiated mainly at second-phase particle/matrix interfaces and that the number of voids increased with increasing pass number. Adiabatic shear bands of 200 to <300 μm in width were formed in the as-extruded and 1-pass ECAP specimens having coarser particles, whereas they were hardly formed in the four-pass and eight-pass ECAP specimens having finer particles. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.     

Shear Localization in Dynamic Deformation: Microstructural Evolution

Investigations made by the authors and collaborators into the microstructural aspects of adiabatic shear localization are critically reviewed. The materials analyzed are low-carbon steels, 304 stainless steel, monocrystalline Fe-Ni-Cr, Ti and its alloys, Al-Li alloys, Zircaloy, copper, and Al/SiC_p composites. The principal findings are the following: (a) there is a strain-rate-dependent critical strain for the development of shear bands; (b) deformed bands and white-etching bands correspond to different stages of deformation; (c) different slip activities occur in different stages of band development; (d) grain refinement and amorphization occur in shear bands; (e) loss of stress-carrying capability is more closely associated with microdefects rather than with localization of strain; (f) both crystalline rotation and slip play important roles; and (g) band development and band structures are material dependent. Additionally, avenues for new research directions are suggested. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.     

Mechanical behavior of aluminum-lithium alloys at cryogenic temperatures

The cryogenic mechanical properties of aluminum-lithium alloys are of interest because these alloys are attractive candidate materials for cryogenic tankage. Previous work indicates that the strength-toughness relationship for alloy 2090-T81 (Al-2.7Cu-2.2Li-0.12Zr by weight) improves significantly as temperature decreases. The subject of this investigation is the mechanism of this improvement. Deformation behavior was studied since the fracture morphology did not change with temperature. Tensile failures in 2090-T81 and -T4 occur at plastic instability. In contrast, in the binary aluminum-lithium alloy studied here they occur well before plastic instability. For all three materials, the strain hardening rate in the longitudinal direction increases as temperature decreases. This increase is associated with an improvement in tensile elongation at low temperatures. In alloy 2090-T4, these results correlate with a decrease in planar slip at low temperatures. The improved toughness at low temperatures is believed to be due to increased stable deformation prior to fracture.