YaBei Gu

Dynamic response of conventional and hot isostatically pressed Ti–6Al–4V alloys: experiments and modeling

Abstract This paper presents the results of a systematic comparative study of the dynamic thermomechanical response of Ti–6Al–4V alloys with three different microstructures. Two of the alloys are produced by the hot isostatically pressed technique using rapidly solidified granules, with one alloy milled prior to hot pressing. Experiments are performed over a broad range of strain rates, 10−3– 7000 s −1 , and initial temperatures, 77–1000 K. Depending on the test temperature, compressive strains of 10–60% are achieved. The microstructure of the undeformed and deformed specimens is investigated, using optical microscopy. The dependence of the flow stress on the temperature and the strain rate is examined for various strains and it is related to the corresponding material microstructure. The results show that adiabatic shearbands develop at high strain rates, as well as at low strain rates and high temperatures. Depending on the test temperature, shearbands initiate once a sample is deformed to suitably large strains. The flow stress is more sensitive to temperature than to the strain rate. Based on these results and other published work, the thermally activated mechanisms associated with the dislocation motion are identified. The physically based model proposed by Nemat-Nasser and Li (1997) for OFHC copper, is suitably modified and applied to this class of titanium alloys. In the absence of dynamic strain aging, the model predictions are in good accord with the experimental results. Comparing the results for the three considered Ti–6Al–4V alloys, with different microstructures, it is found that the initial microstructural features affect only the magnitude of the threshold stress and the athermal part of the flow stress, but not the functional dependence of the thermally activated part of the flow stress on the temperature and the strain rate.

Hot isostatic pressing of powder in tube MgB2 wires

The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by dc transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr∼17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of subgrains within MgB2 crystallites.

Defect structures in MgB2 wires introduced by hot isostatic pressing

The microstructures of MgB2 wires prepared by the powder-in-tube technique and subsequent hot isostatic pressing were investigated using transmission electron microscopy. A large amount of crystalline defects including small-angle twisting, tilting and bending boundaries, in which high densities of dislocations reside, was found forming sub-grains within MgB2 grains. It is believed that these defects resulted from particle deformation during the hot isostatic pressing process and are effective flux pinning centres that contribute to the high critical current densities of the wires at high temperatures and at high fields.

Response of hot isostatically pressed Ti–6Al–4V targets to normal impact by conical and blunt projectiles

First experimental results are reportedon ballistic performance of hot isostatically pressed(HIPed ) texture-free targets using rapidly solidified powders of Ti–6Al–4V alloy (PREP and ELI-PREP). Plastic deformation by ball milling of these powders was performed to modify the microstructure of the materials. HIPed samples of 40 mm diameter and a thickness of 10–30 mm were shrinkfit into holes in larger diameter steel plates andstruck by 50 caliber 60 1 cylindro-conical projectiles of hardness Rc 60 with contact prod ucedeither by the conical or blunt surface. The mass of the projectile was about 31 g andthe initial velocities rangedfrom 300 to 450 m/s for flat-end edandfrom 900 to 950 m/s for the conical impacts. Comparative behavior of HIPed samples and standard samples made from commercially available Ti–6Al– 4V alloy MIL-T-9047G (bar, forged, annealed)—baseline material after impact with the same geometry was investigated. As a rule the final velocity of the plug for HIPed alloy was smaller (or no penetration was observed) than that of the baseline material for impact with the conical projectile. Slightly sloped cylindrical shear plugs were characteristic for both materials upon impact by flat projectiles andtheir velocities were usedto evaluate the shear resistance of HIPedmaterial to plugging. Comparative features of fracture in both cases are presented. The texture-free HIPed materials from powders demonstrated better ballistic performance than the baseline material andcouldbe successfully usedfor ballistic applications. This is the first step in the development of high-gradient composite materials using a Ti–6Al–4V matrix from powder. r 2002 Elsevier Science Ltd. All rights reserved.

Elastic properties of hot-isostatically-pressed magnesium diboride

Magnesium diboride was hot-isostatically pressed using three qualitatively different cycles: dense material cooled under pressure (DMCUP), “standard” cycle with pressure and temperature simultaneously reduced, and isothermal pressure release. Elastic properties of dense MgB2 were measured at normal conditions using resonant ultrasound spectroscopy method. The highest values of elastic moduli correspond to the sample processed using DMCUP cycle. The data for fully dense samples are in satisfactory agreement with theoretical predictions based on quantum mechanics calculations. The effect of lower density on elastic constants is consistent with a theoretical approach based on elasticity theory taking into account effect of porosity.

Long rod penetration test of hot isostatically pressed Ti-based targets

Hot Isostatic Pressing (HIP) is one of the most efficient techniques to produce high quality materials from powders. Nevertheless there is a shortage of data on high-strain-rate behavior and penetration resistance of such materials. In this paper the results of penetration test with tungsten (93%) heavy alloy penetrators of solid and porous composite samples of Ti-6Al-4V alloy with different microstructures (Widmanstatten pattern and equiaxed) are presented. Penetration depth for HIPed materials is smaller than in baseline samples of Ti-6Al-4V alloy (forged rod MIL-T-9047G). Composite materials with alumina rods and tubes filled with B4C powders demonstrated a new features of penetration: projectile deflection with self sealing of hole and forced shear localization caused by tubes fracture. The results demonstrate the applicability of HIPing for Ti-based armor materials.

Microwave performance of high-density bulk MgB2

We have performed microwave measurements on superconducting hot-isostatically pressed (HIPed) bulk MgB2 using a parallel-plate resonator technique. The high density and strength of the HIPed material allowed preparation of samples with mirror-like surfaces for microwave measurements. The microwave effective surface resistance Rs decreased by about 40% at 20 K when the root-mean-square surface roughness was reduced from 220 to 110 nm through surface polishing and ion milling. The Rs was independent of surface microwave magnetic field at least up to 4 Oe and below 30 K. We attribute this behavior, and the overall low Rs (∼0.8 mΩ at 10 GHz and 20 K), to the high density of our samples and the absence of weak links between grains.

Design and Ballistic Testing of Ti-6Al-4V Matrix Composites

Ti—6Al—4V matrix composites enhanced by alumina tubes filled with boron carbide, alumina rods, and alumina plate is termed high gradient composite because of the big density differences among Ti—6Al—4V, alumina, and boron carbide materials. This high gradient composite has been developed for a theoretical study to help determine the best configuration for armor protection. This is the first attempt where materials are prepared using a powder processing method with the specific goal to address the ballistic performance. Hot isostatic pressing (HIP) was first used to manufacture ballistic grade Ti—6Al—4V alloy and high gradient composites incorporating Al2O3 rods, plates, and tubes filled with B4C powders. Processing parameters were investigated and optimized based on materials properties. An important feature of powder-based materials is the lack of texture in comparison with traditional material. Several configurations of this high gradient composite were designed and tested by long rod penetration tests. High gradient composite materials demonstrated new damage patterning features during the long rod projectile penetration process. These features include projectile deflection, self-sealing of the hole, and forced shear localization in the direction of 45° to the impact line caused by fracture of Al2O3 tubes on the initial stage of penetration process. Powder filled voids and rods induced volume distributed, highly heterogeneous pattern of damage initiated by cavities and their interactions. The test results are shown and analyzed in depth in this paper. The results prove that the powder-based approach can be used for processing of materials suitable for ballistic applications.

Ballistic Testing and High‐Strain‐Rate Properties of Hot Isostatically Pressed Ti‐6Al‐4V

Hot isostatically pressed (HIPed) Ti‐6Al‐4V powder based targets (including composites) have a good ballistic performance against long rod, conical and flat projectiles impact (velocity range ∼ 0.4 – 1km/s). Compared to baseline material (MIL‐T‐9047G), new features such as different shape of craters in long rod penetration tests were observed. The results of compression Hopkinson bar tests, cut from tested targets (final strain controlled tests and hat‐shaped specimen tests) are presented with a goal to establish relations between ballistic performance and high strain rate properties of HIPed materials.

Shear Localization and Patterning of Shear Bands in PTFE and Its Mixtures with Metals

Shear instability and patterning of shear bands in PTFE and in its mixtures with Tin particles were studied using explosively driven and newly developed small scale Hopkinson bar based thick walled cylinder (TWC) methods. Currently available theoretical models of shear instability and spacing between shear bands are based on a thermal softening mechanism which apparently does not account for the beginning of shear instability in PTFE. Critical effective strain at strain rate 104 sec−1 for propagation of shear bands in PTFE under plane strain conditions was equal to 0.4 and shear band spacing for the initial stage of shear localization was 0.5 mm. In the Hopkinson bar based TWC test shear instability, in a form of a group of small number of shear bands, starts at overall strain on inner surface above 0.22 and causes a break of cylindrically symmetrical motion of material. Contrary to the behavior of solid PTFE, its mixture with Tin particles is more stable with respect to shear localization.