Woon Hyung Baek

Microstructure and mechanical properties of mechanically alloyed and solid-state sintered tungsten heavy alloys

The mechanical properties of solid-state sintered 93W‐5.6Ni‐1.4Fe tungsten heavy alloys fabricated by mechanical alloying were investigated. Blended W, Ni and Fe powders were mechanically alloyed in a tumbler ball mill at a milling speed of 75 rpm employing a ball-to-powder ratio of 20:1 and a ball filling ratio of 15%. A nanocrystalline size of 16 nm and fine lamellar spacings of 0.2 mm were obtained in mechanically alloyed powders at a steady state milling stage. Mechanically alloyed powders were consolidated into green compacts and solid-state sintered at 1300°C fo r1hi n ahydrogen atmosphere. The alloys sintered from mechanically alloyed powders showed fine tungsten particles (about 3 mm in diameter) and a relative density above 99%. The volume fraction of the matrix phase was 11% and the tungsten:tungsten contiguity was determined to be 0.74. The alloys exhibited high yield strengths (about 1100 MPa) due to their fine microstructures, but exhibited reduced elongation and impact energy due to a large area fraction of tungsten:tungsten boundaries and the low volume fraction of matrix phase.

Mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy

Abstract The mechanical alloying process of 93W-5.6Ni-1.4Fe tungsten heavy alloy from the elemental powders of W, Ni and Fe by a high energy ball mill in argon atmosphere was investigated. The mechanical alloying process parameters such as milling speed, milling time, ball-to-powder ratio and ball filling ratio were varied in order to investigate their influence on the microstructural evolution of mechanically alloyed powders. The mechanical alloying process proceeded following five distinct stages such as flattening stage, welding dominant stage, equiaxed particle fprming stage, random lamellar forming stage and steady state stage with increasing the milling time. The steady state stage of mechanical alloying was reached after milling for 48 hours with milling speed of 75 rpm, ball-to-powder ratio of 20:1 and ball filling ratio of 15%. Nanocrystalline grain size of 16 nm was obtained at the steady state stage of mechanical alloying. Mechanically alloyed powders were consolidated by cold isostatic pressing and followed by sintering at temperature ranged 1300–1485°C for 1 hour in hydrogen atmosphere. When liquid phase sintered at 1485°C, tungsten heavy alloy from mechanically alloyed powders showed finer tungsten particles about 27μm than that from conventionally blended powders. The density of liquid phase sintered tungsten heavy alloy decreased with increasing the milling time due to the swelling during sintering. When solid state sintered below 1430°C, tungsten heavy alloy from mechanically alloyed powders showed ultra-fine tungsten particles about 3μm and showed high relative density above 97% insensitive to the milling time.

Matrix pools in a partially mechanically alloyed tungsten heavy alloy for localized shear deformation

A fabrication process for a tungsten heavy alloy with matrix pools was suggested in order to increase the susceptibility to localized shear deformation. A partial mechanical alloying process was introduced to form matrix pools intentionally within a tungsten heavy alloy. The partially mechanically alloyed powders were sintered by solid-state sintering, followed by liquid-phase sintering. The volume fraction of matrix pools was decreased with an increase in the secondary liquid-phase sintering time. Penetration test showed that the matrix pools were effective in enhancing the triggering of localized shear deformation.

Microstructural study of adiabatic shear bands formed by high-speed impact in a tungsten heavy alloy penetrator

Abstract The microstructures of adiabatic shear bands formed by high-speed impact in a tungsten heavy alloy penetrator were investigated in the present study. The penetrator was highly deformed at high strain rate by high-speed impact, and microstructural observation of the remained penetrator and the debris was conducted after the impact test. Heavily elongated tungsten particles and reaction products such as tungsten oxides were observed on the surface region of the debris presumably due to the local temperature rise occurring upon high-speed impact. A few adiabatic shear bands were observed in the regions near the surface cracks of the remaining penetrator, and their width was wide in comparison to the shear band formed in armor plates. The crack had general trends to propagate along the shear band, but sometimes changed its propagation path along the interfaces between adhering tungsten particles. It was suggested from the microstructural observation of the shear bands that in order to improve the penetration performance of the tungsten heavy alloy, the minimization of the tungsten–tungsten particle interfaces and the optimization of the fabricating process were required.

Microstructural control of and mechanical properties of mechanically alloyed tungsten heavy alloys

Abstract93W-5.6Ni-l.4Fe tungsten heavy alloys with controlled microstructures were fabricated by mechanically alloying of elemental powders of tungsten, nickel and iron by two different process routes. One was the full mechanical alloying of blended powders with a composition of 93W-5.6Ni-l.4Fe, and the other was the partial mechanical alloying of blended powders with a composition of 30W-56Ni-14Fe followed by blending with tungsten powders to form a final composition of 93W-5.6Ni-l.4Fe. The raw powders were consolidated by die compaction followed by solid state sintering at 1300°C for 1 hour in a hydrogen atmosphere. The solid state sintered tungsten heavy alloys were subsequently liquid phase sintered at 1445∼1485°C for 4-90 min. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed tungsten particles of about 6-15 μm much finer than those of 40 um in a conventional liquid phase sintered tungsten heavy alloy. An inhomogeneous distribution of the solid solution matrix phase was obtained in the two-step sintered tungsten heavy alloy using partially mechanically alloyed powders. The two-step sintered tungsten heavy alloy using mechanically alloyed 93W-5.6Ni-l.4Fe powders showed larger elongation of 16% than that of 1% in the solid state sintered tungsten heavy alloy due to the increase in matrix volume fraction and decrease in W/W contiguity. Dynamic torsional tests of the two-step sintered tungsten heavy alloys showed reduced shear strain at maximum shear stress than did the sintered tungsten heavy alloys using the conventional liquid phase sintering.

Heat Treatment Behavior of Tungsten Heavy Alloy

This paper focuses on the variations of static and dynamic properties of tungsten heavy alloy with heat treatment. The matrix phase of 93W-4.9Ni-2.1Fe (weight percent) has been penetrated into W/W grain boundaries during a cyclic heat treatment which consists of repeated isothermal holdings at 1150 °C and water quenching between them. By applying the cyclic heat treatment, the impact energy of tungsten heavy alloy is increased about three times from 57 to 170 J. When the tungsten heavy alloy is cyclically heat treated at 1150 °C and then re-sintered at 1485 °C, W/matrix interface is changed from round to undulated shape. The irregularity of the interface is increased with increasing the number of heat treatment cycles. From the measurement of the residual stress of W grains by X-ray diffraction, it is found that the irregularity of the interface is closely related with strain energy stemmed from the difference of thermal expansion coefficient between W particles and matrix phase. From dynamic ballistic test, it is found that the tungsten heavy alloy with undulated W grains forms many narrow fracture bands which are preferential for the self sharpening effect, thus, for the improvement of the penetration performance.