Sastry S. Indrakanti

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.

Influence of microstructures and crystalline defects on the superconductivity of MgB2

This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB2, with the objective to improve its flux pinning. A MgB2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (Jc) at higher fields than its un-HIPed counterpart. The HIPed sample had a Jc of 10 000 A/cm2 in 50 000 Oe (5 T) at 5 K. This was 20 times higher than that of the un-HIPed sample, and the same as the best Jc reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep Jc drop with magnetic field H that occurred in the un-HIPed MgB2 pellet at H>45 000 Oe(4.5 T) and T=5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB2...

Hot isostatic pressing of bulk magnesium diboride: Mechanical and superconducting properties

Hot isostatic pressing with glass encapsulation (pressure, 200MPa; temperature, 1000C over 200min; cooling under pressure) was used to synthesize bulk MgB 2 with diameters up to 20mm and thicknesses up to 10mm from MgB 2 powder. A sharp superconducting transition was measured at 38.5K. This method can be scaled to larger sample sizes and complex shapes. The data for density, microhardness, fracture toughness and sound speed for bulk magnesium diboride are presented. Ball milling the powder results in a more homogeneous final microstructure with a higher resistivity and with a similar superconducting transition.

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.

Degradation of MgB2 under ambient environment

The superconductivities of samples prepared by several procedures were found to degrade under ambient environment. The degradation mechanism was studied by measuring the change of surface chemical composition of dense MgB2 pellets (prepared by hot-isostatic pressure, HIPed) under atmospheric exposure using x-ray photoelectron spectroscopy (XPS). Results showed that samples with poor connectivity between grains and with smaller grain sizes degrade with time when exposed to ambient conditions. In these samples, the TC did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. In contrast, our well-sintered and the HIPed samples remained stable for several months under ambient condition. The degradation was found to be related to surface decomposition as observed by XPS. We observed the formation of oxidized Mg, primarily in the form of a Mg hydroxide, the increase of C and O contents, and the reduction of B concentration in the surface layer of MgB2 samples.

Critical scaling and flux dynamics in bulk MgB2 and high-purity YBa2Cu3O7−δ single crystals

Abstract A dense sample of MgB 2 ( T c =38.5 K) was synthesized under 200 MPa pressure using a novel hot isostatic pressing (HIPing) technique, and a high-purity single crystal of YBa 2 Cu 3 O 7− δ ( T c =93.5 K) was grown in a home-made BaZrO 3 crucible. Electric field vs current density ( E–J ) isotherms of the MgB 2 and YBa 2 Cu 3 O 7− δ high-temperature superconductors were measured and analyzed in terms of the critical scaling model. E–J isotherm sets were taken at fields ranging from 2 to 90 kOe. Scaling and resistivity data reveal a smooth transition from a vortex glass to a vortex liquid state for the bulk MgB 2 sample and the existence of both a vortex lattice melting line and a vortex glass to vortex liquid line in the YBa 2 Cu 3 O 7− δ single crystal. The critical scaling exponents for both systems are presented. A more complete ( H–T ) phase diagram, containing the upper critical field H c2 ( T ), the vortex-glass transition line H g ( T ), and the magnetic irreversibility line H irr ( T ) was established for bulk MgB 2 .

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.

Mixed-state flux dynamics in bulk MgB2

Electric field vs. current density (E–J) isotherms in the mixed-state of a bulk sample of the high-temperature superconductor MgB2 (Tc=38.5 K), synthesized under 200 MPa pressure by hot isostatic pressing (HIPing), have been measured and analyzed in terms of the critical scaling model. Magnetization data reveal distinctly different critical current density (Jc) behaviors in high and low magnetic field critical scaling regions. E–J isotherm sets at fields ranging from 2 to 90 kOe conform to the vortex-glass (VG) scaling anzatz. Scaling analysis, resistivity data and Jc data suggest that a Bragg-glass state may exist for H<8 kOe. The critical scaling exponents depend on field and range from ν∼0.6, z∼3.1 in low fields (H<8 kOe) to ν∼0.9, z∼3.4 in high fields. A more complete magnetic field–temperature (H–T) phase diagram, containing the upper critical field Hc2(T), the VG transition line Hg(T), and the magnetic irreversibility line Hirr(T) has been established for bulk MgB2.

Vortex- and Bragg-glass phases in bulk MgB2

Abstract A more complete magnetic field–temperature (H–T) phase diagram, containing the upper critical field Hc2(T), the vortex-glass melting line Hg(T), and the magnetic irreversibility line Hirr(T) has been established for bulk MgB2 (Tc=38.5 K), synthesized under 200 MPa pressure by hot isostatic pressing (HIPing). Scaling analysis of resistivity data, Jc data, and electric field vs current density (E–J) isotherms in the mixed-state suggest that a Bragg-glass state exists for H

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.