Effect of initial microstructure on high velocity and hypervelocity impact cratering and crater-related microstructures in thick copper targets: Part II Stainless steel projectiles

Three different, thick copper targets (an as-received, 98 m grain size containing 1010 dislocations/cm2 (Vickers hardness of 0.89 GPa); an annealed, 124 m grain size containing 109 dislocations/cm2 (Vicker's hardness of 0.69 GPa); and a 763 m grain size containing 109 dislocations/cm2 (Vickers hardness of 0.67 GPa) were impacted with 3.18 mm diameter ferritic stainless steel projectiles at nominal velocities of 0.7, 2 and 5 km s-1. Like companion experiments utilizing soda-lime glass projectiles (Part I), absolute grain size of the target was observed to be less important than the dislocation density in the cratering process. At low impact velocity, depth/diameter ratios were observed to increase dramatically in contrast to less dense soda-lime glass impactors, and the impactor behaviours were also very different. The ferritic stainless steel impactors spalled into small fragments at or above 2 km s-1 impact velocity and a significant fraction of these fragments remained in the craters. No significant melt phenomena were observed either in connection with projectile fragmentation or in the crater-related, residual microstructures. Dynamic recrystallization, dislocation cell structures and microbands were significant microstructural features in the targets. They extended from the crater walls and contributed to hardness profiles within the cratered targets. These hardness profiles and actual hardness zones generally increased in extent from the crater wall with both impact velocity and projectile density.     

Micrometer size grains of hot isostatically pressed alumina and its characterization

Alumina samples were prepared from two different particle size powders. Finer particle compacts when heated along with coarser particle compacts at same processing temperatures produce bigger grain microstructures due to higher grain growth. An unconventional method of etching by molten V₂O₅ was adopted to look at the microstructure for accuracy in reported data. On an average starting with finer particles give microstructure with a grain size of 5.5 μm and starting with coarser particles, give microstructure with 2.2 μm average grain size. The flexural strength is around 400 MPa for alumina samples prepared from finer powder in comparison with about 550 MPa for alumina samples prepared from coarser powder. The Vickers hardness in 5.5 μm grain microstructure is around 20 GPa in comparison to about 18 GPa in microstructure with smaller grains of 2.2 μm size.     

Evolution of microstructure and microtexture in fcc metals during high-pressure torsion

Pure nickel and commercially pure (CP) aluminium were selected as model fcc materials for a detailed investigation of the experimental parameters influencing grain refinement and evolution of microstructure and microtexture during processing by high-pressure torsion (HPT). Samples were examined after HPT using microhardness measurements, transmission electron microscopy and orientation imaging microscopy. Processing by HPT produces a grain size of ∼170 nm in pure Ni and ∼1 μm in CP aluminium. It is shown that homogeneous and equiaxed microstructures can be attained throughout the samples of nickel when using applied pressures of at least ∼6 GPa after 5 whole revolution. In CP aluminium, a homogeneous and equiaxed microstructure was achieved after 2 whole revolutions under an applied pressure of 1 GPa. For these conditions, the distributions of grain boundary misorientations are similar in the centre and at the periphery of the samples. It is shown that simple shear texture develops in fcc metals subjected to high-pressure torsion. Some grain growth was detected at the periphery of the Al disk after 8 revolutions. The factors influencing the development of homogeneous microstructures in processing by HPT are discussed.     

Microstructural aspects of hypervelocity impact cratering and jetting in copper

Light and transmission electron microscopy techniques have been applied in observations of hypervelocity impact craters in two different copper targets: a 38 m grain size mill-processed target, and a 763 m grain size annealed target, the smaller grained target being impacted with a 1100 aluminium sphere and the larger grained target being impacted with a soda-lime glass sphere, at velocities near 6 km s^–1. Both target craters exhibited dynamic recrystallization near the crater wall. The jetting associated with these two craters was very different. Considerably more plastic flow and a larger rim characterized the larger grained target. No significant melt-related phenomena were observed either near the crater wall or in the jetted rim for either crater. Consequently, the principal features of crater formation involve extreme plastic flow in the solid state. Microbands were observed to occur profusely in a zone below the smaller grained mill-processed target crater while more profuse and extremely long, unidirectional bundles of microbands (which were coincident with traces of {1 1 1} planes) occurred below the annealed larger grained target crater. These observations attest to the dominant and unique role played by deformation microbands in cratering in copper, because essentially no deformation twins were observed in either target.     

Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB<Subscript>2</Subscript>–SiC

Structure–property relations were examined for ZrB₂ containing 30 volume percent SiC particulates. Two grades of ZrB₂ with initial particle sizes of 2 and 6 μm were used. Billets of ZrB₂–SiC were produced by hot pressing at 1850, 1950 or 2050 °C for 45 min. In addition, the material prepared from ZrB₂ with an initial particle size of 2 μm was hot pressed at 2050 °C for 90 and 180 min. Microstructures and mechanical properties were characterized to determine the effects of the initial particle size, hot pressing time, and hot pressing temperature on the final grain size and morphology. The average grain size of the ZrB₂ phase ranged from 2.2 to 4.7 μm. Similarly, the average grain size of the SiC phase ranged from 1.2 to 2.7 μm. Hardness and modulus of elasticity were not affected by the processing conditions with average values of 22 and 505 GPa, respectively. However, flexural strength decreased as grain size increased from a maximum of ∼1050 MPa for the finest grain sizes to ∼700 MPa for the largest grain sizes. Analysis suggested that the strength of ZrB₂–SiC was limited by the size of the SiC inclusions in the ZrB₂ matrix.     

High Preformation SiC-AlN Composites

High performance SiC-AlN composites were fabricated by hot-pressing with Y2O3 as additive via liquid phase sintering. The SiC-AlN composites containing 5 vol. pct AlN exhibit superior mechanical properties with flexural strength, fracture toughness and Vickers hardness of 1131 MPa,6.1 MPa·m1/2 and 28.6 GPa respectively Microstructure observations indicate that the grain size of the composites is obviously inhibited due to the formation of solid solution. TEM-EDS analysis demonstrates the existence of the solid solution. In addition, subgrain boundaries induced by dislocations in the matrix SiC grains, seem to divide a large grain into numerous nano-sized small grains, which significantly increase the mechanical properties of the composites     

High-rate deformation and spall fracture of hadfield steel under action of high-current nanosecond relativistic electron beam

Features of the plastic deformation and dynamic spall fracture of Hadfield steel under conditions of shock wave loading at a straining rate of ∼10⁶ s⁻¹ have been studied. The shock load (∼30 GPa, ∼0.2 μs) was produced by pulses of a SINUS-7 electron accelerator, which generated relativistic electron bunches with an electron energy of up to 1.35 MeV, a duration of 45 ns, and a peak power on the target of 3.4 × 10¹⁰ W/cm². It is established that the spalling proceeds via mixed viscous-brittle intergranular fracture, unlike the cases of quasi-static tensile and impact loading, where viscous transgranular fracture is typical. It is shown that the intergranular character of the spall fracture is caused by the localization of plastic deformation at grain boundaries containing precipitated carbide inclusions.     

Microstructure and scaling effects in the damage of WC-Co alloys by single impacts of hard particles

The size and nature of the craters produced by single impacts of angular Al₂O₃ particles on WC-Co alloys (6 to 50 wt% Co) were investigated as a function of microstructure, impacting particle size (63 to 405 m), velocity (35 to 93 m sec^–1) and angle of impact (30 to 90°). In general, the size of the craters increased with increase in particle size and velocity and with cobalt content of the target. Two distinct types of behaviour occurred depending on the number of WC grains encompassed by the projected area of the crater. When the number was less than 10, the crater was formed mainly by cracking of WC grains (brittle regime); when it was more than 100, the crater resulted mainly from the plastic deformation of the binder phase (ductile regime). The cracking of the WC grains in the brittle regime did not have the typical appearance of either cone or lateral cracks. The behaviour in the ductile regime correlated reasonably well with the model for plastic indentation; the derived work of plastic deformation was, however, only about 10% of the initial kinetic energy of the impacting particle. Besides the usual elastic effects, fragmentation of particles was considered to be responsible for a significant fraction of the kinetic energy loss, The effects of microstructure of the target on the crater size in the ductile regime was deduced to be mainly through its effect on the hardness, which was given reasonably well by the Lee and Gurland relation.     

Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD^TM technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100–500 K/s, 20–50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with 3 m in width and 50 m in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 m width and 200 m length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from 50 m to 200 m and from about 15 to 200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy.     

Strength and toughness of sintered plus forged T1 high speed steel

The stresses for macroscopic plastic flow and critical stages of fracture, fracture toughness and hardness of sintered plus forged T1 high speed steel were determined. The results are compared to similar data for sintered, sintered to closed porosity plus hot isostatically pressed and electroflux refined (EFR) alloys of comparable composition. EFR meltstock, with addition of 0.6 wt% Mo, was water-atomized in a 200 kg unit which incorporated ceramic filters and an argon shroud to ensure maximum cleanliness. The powder was sieved, <125 μm, vacuum annealed, blended, isostatically compacted and vacuum sintered and hot forged to produce a 300 kg billet. Mechanical properties were determined in four-point bending of heat-treated beam specimens. Most samples showed evidence of macroscopic plastic flow, up to ∼1%, beyond a stress of ∼1.8 GPa, σY. Using surface replica microscopy, crack nucleation was detected at stresses σN, between 0.5 and 0.9 σY, and subcritical short crack growth, at stresses generally larger than σY. Fracture, from crack nuclei associated (only) with fractured M6C carbides, took place at stresses, σF, in the range 1.4 to 3.0 GPa Macroscopic fracture toughness, KIC, was in the range 17–24 MPa m1/2 and, like σN and σF, appeared to depend sensitively on the tempering temperature. The most attractive combination of properties, for the overtempered, 580°C, structure at HV50 ∼750 appears to be: σY≈1.9 GPa, σF≈2.8 GPa, KIC≈23 MPa m1/2. These values are comparable to those for EFR aerospace quality T1 high speed steel. This revised version was published online in November 2006 with corrections to the Cover Date.     

The Cu matrix cermets remarkably strengthened by TiB2 “in situ” synthesized via self-propagating high temperature synthesis

The TiB₂–Cu cermets with predominant concentration of superhard TiB₂ (from 45 to 90 vol.%) were fabricated using elemental powders by means of SHS (self-propagating high-temperature synthesis) process and simultaneously densified by p-HIP (pseudo-isostatic pressing technique). The heat released during highly exothermic SHS reaction was “in situ” utilized for sintering. The combustion occurred even for 50 vol.% Cu dilution. According to XRD metallic copper binder was formed in those cermets in whole range of investigated compositions. The TiB₂ volume fraction significantly influenced the properties of fabricated materials, especially grain size and hardness. Both the average grain size and hardness significantly increased with TiB₂ content, so the maximum value of 18 GPa was measured for TiB₂–5 vol.%Cu composite. Coarse grains of 6.4 μm in size were observed for this composite while TiB₂-based submicro-composites were formed for 40–50% of Cu where the average grain size did not exceed 0.6 μm. The Vickers hardness of 16–18 GPa obtained for cermets containing from 85 to 90 vol.% of TiB₂ and no radial cracks in Vickers hardness test proved that in term of hardness and fracture toughness the composites might be competitive to WC–Co cermets.     

Grain growth and mechanical properties in bulk polycrystalline silicon

The grain structure of bulk polycrystalline silicon has been examined as a function of annealing temperatures and times between 1000 and 1400°C and 6 to 24 hours, respectively. The initial high aspect grain structure decomposes into roughly equiaxed grains at 1000°C over the course of 24 hours. The grains proceed to grow via Oswald ripening with an activation energy for grain growth of 1.49 eV. The hardness increases slightly during annealing and the subsequent transformation to an equiaxed grain structure, from a Vickers hardness of 964 to 1160 kg/mm². The fracture toughness is 0.8 MPa(m)^1/2in the as grown structure, and increases to 1 MPa(m)^1/2in annealed samples. The hardness and fracture toughness are independent of grain size for grain diameters between 2.7 and 4 m.     

Effect of annealing on precipitation, microstructural stability, and mechanical properties of cryorolled Al 6063 alloy

The effect of annealing on precipitation, microstructural stability, and mechanical properties of cryorolled Al 6063 alloy has been investigated in the present work employing hardness measurements, tensile test, XRD, DSC, EBSD, and TEM. The solution-treated bulk Al 6063 alloy was subjected to cryorolling to produce ultrafine grain structures and subsequently annealing treatment to investigate its thermal stability. The CR Al 6063 alloys with ultrafine-grained microstructure are thermally stable up to 250 °C as observed in the present work. Within the range of 150–225 °C, the size of small precipitate particles is <1 μm. These small precipitate particles pin the grain boundaries due to Zener drag effect, due to which the grain growth is retarded. The hardness and tensile strength of the cryorolled Al 6063 alloys have decreased upon subjecting it to annealing treatment (150–250 °C).     

Influence of stacking fault energy on defect structures and microhardness of Cu and Cu alloys

Nano-structured Cu, Cu-10 wt%Zn and Cu-2 wt%Al with stacking fault energies (SFE) of 78, 35 and 37 mJ/m2, respectively, were preprared through high energy ball milling. X-ray diffraction and Vickers microharness test were used to investigate the microstructure and microhardness of all the samples after ball milling. X-ray diffraction measurements indicate that lower SFEs lead both to decrease in grain size and increase in microstrain, dislocation and twin densities for Cu-10 wt%Zn and Cu-2 wt%Al after 5 h of ball milling. The microhardnesses of Cu-10 wt%Zn and Cu-2 wt%Al reach to nearly the same values of 2.5 GPa after 5 h of ball milling, which is higher than that of Cu of 2.0 GPa. Two factors are considered to contribute to the finer grian size and higher microhardness of Cu-10 wt%Zn and Cu-2 wt%Al: (1) the effect of solid solution strengthening, which result in the interaction of solute atoms with screw dislocations; (2) the introduction of deformation twins during ball milling process by the decreasing of SFE, which results in the grain refinement.     

TiC含量对WC-TiC-TaC硬质合金材料微观组织及力学性能的影响

本研究采用真空热压烧结技术, 在1600℃下制备了WC-TiC-TaC硬质合金材料, 研究了TiC含量对其微观组织及力学性能的影响。结果表明, 随着TiC含量的增多, 硬质合金材料的晶粒显著增大。当TiC的含量从10wt% 增加到25wt%时, 硬质合金材料的硬度逐渐增大, 最高可达19.81 GPa, 这是由于TiC的硬度高于基体WC的硬度; 与此同时, 硬质合金材料的抗弯强度和断裂韧度逐渐减小。当TiC的含量为10wt%时, 材料的抗弯强度有最大值, 其值为1147.24 MPa, 这是由于在材料内部形成了均匀、细小的晶粒组织; 在此含量下, 复合材料的增韧机理为细晶增韧、裂纹偏转、裂纹分支、裂纹桥接和韧窝增韧, 其断裂韧度有最大值, 为14.60 MPa·m^1/2。     

Fast-sintering of hydrothermally synthesized BaTiO3 powders and their dielectric properties

Hydrothermally synthesized barium titanate (BaTiO₃) powders with a submicrometre particle size have been fast-sintered with a heating rate of ∼ 200 °C min^-1 at various temperatures (1250–1350 °C) for short times (5 and 15 min). The microstructures and dielectric properties of the sintered samples are studied and compared with those sintered conventionally. The sample fast-sintered at 1250 °C for 5 min had the highest dielectric constant value of approximately 3700 with an average grain size of about 1 μm. Both the dielectric constant and the Curie–Weiss temperature are found to be dependent on the grain size of the sintered samples, particularly when the average grain size is less than 5 μm. This has been attributed to the presence of internal stress in the fine-grained BaTiO₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

Properties of lead zirconate-alumina ‘nanocomposites’

Microstructures, Vickers hardness and dielectric properties of PbZrO₃ ceramics with co-additions of 0.5-5 vol% Al₂O₃ nanoparticles have been investigated. The additive inhibited grain growth, with average grain size decreasing from ∼13 μm for PbZrO₃ to ∼1 μm for the nanocomposites. The mode of fracture also changed, from predominantly inter-granular in PbZrO₃ to a mixed-mode of intra- and inter-granular fracture in the composite samples. Vickers hardness values increased from 2.9 GPa for PbZrO₃ to 4.1 GPa for the sample with 1 vol% Al₂O₃, but there was a more gradual increase for higher Al₂O₃ contents. Plots of relative permittivity versus temperature indicated subtle differences which were attributed to a chemical reaction between the additive and matrix during sintering. X-ray powder diffraction showed that lead aluminium oxides were the principal products of this reaction.     

Convection–diffusion derived gradient films on porous substrates and their microstructural characteristics

By virtue of the convection–diffusion effect of solution in porous solid media, an alumina substrate-supported electrolyte film (YSZ) with gradient microstructure has been successfully fabricated for the first time and structurally characterized by SEM and XRD–ADA (angular dispersion analysis). This novel fabrication technique is simple, controllable, economical and potentially applicable to fabricating electrode-supported gradient electrolyte films for SOFCs. The so-prepared YSZ-film/substrate structures are featured with a dense YSZ film of ∼10 μm, a uniform filling layer of ∼50 μm just beneath the interface and a successive diffuse layer stretching as deep as ∼250 μm within the porous substrate matrix.     

Direct observations and comparison of crater cross-section microstructures in copper targets for aluminium projectiles impacting at 1.4 and 6.7 km s−1

Light and transmission electron microscopy observations of impact crater-related microstructures in copper targets have revealed dramatic differences in the extent and type of microstructures. For a crater formed by a 6.4 mm diameter aluminium (1100) spherical projectile impacting at 1.4 km s^–1, a narrow (20 m) recrystallized zone extended axially outward from the crater wall, with dislocation cells which increased in size extending from this zone. By comparison, a crater formed by a 3.2 mm diameter aluminium (1100) spherical projectile impacting at 6.7 km s^–1 exhibited a recrystallization zone extending more than 200 m axially from the crater wall, a connecting zone of increasingly dense microbands, having an axial width of about 2000 m. This zone converged upon a region of dislocation cells which increased in size away from the crater wall. These observations highlight important microstructural differences in cratered metal targets in the hypervelocity impact regime in contrast to the lower-velocity regimes where shock-wave and related ultra-high-strain-rate effects are unimportant.