An anodic tool polarity is generally adopted in sink electrical discharge machining (EDM) to maximize material removal relative to tool wear. Sink EDM of Ti and Ti6Al4V is however atypical in that these materials necessitate a cathodic tool polarity. Adding to the intrigue is γ-TiAl, which machines better under the conventional anodic polarity. This research focused on clarifying the phenomena behind this interesting behavior by investigating removal mechanisms over a range of relevant process conditions. The anomaly is demonstrated to arise from the polarity-dependent nature and extent of TiC formation on the work surface, which significantly affects material removal. 相似文献
The diffusion anisotropy of intrinsic point defects is an important factor governing the behavior of the HCP metals bombarded
by energetic particles. The effects of stress on the diffusion and its anisotropy, although known to be important, have not
been well understood. In this paper, we use a combination of molecular dynamics and molecular statics methods to investigate
energy states of a self-interstitial in α-titanium, a typical HCP metal. Our calculation shows that the most stable configuration
of the self-interstitial is the basal-split dumbbell configuration on the basal plane. Compression along the [0001] or the
[1ˉ100] directions leads to an insignificant change in the migration energies, while compression along the [11ˉ20] direction
leads to a larger migration energy. A significant change of the diffusion anisotropy is observed when a uni-axial compressive
stress of 200 MPa is applied along the [11ˉ20] direction. Similar stress along the other two directions does not produce substantial
changes of the anisotropy. We also show that an applied hydrostatic stress can significantly change the diffusion anisotropy
of HCP metals and alloys. Thus, under irradiation, a hydrostatic stress can produce a significant creep-like deformation (i.e.,
with a deviatoric strain rate) through a stress-dependent change of the growth rate.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
Titanium is an important candidate in the search for lighter weight armors. Increasingly, it is being considered as a replacement for steel components. It is also an important component in the application of ceramics to armor systems, especially in armor modules that are capable of defeating kinetic energy penetrators while sustaining little or no penetration of the ceramic element. The best alloy available today for ballistic applications is Ti-6Al-4V, an aerospace grade titanium alloy. The principal deterrent to widespread use of this alloy as an armor material is cost, and a significant portion of the cost is in processing. Consequently, the U.S. Army Research Laboratory undertook a program to study a particular lower cost processing technique [1].
The objectives of this work are to characterize the low-cost titanium alloy by generating constants for the Johnson-Cook (JC) and Zerilli-Armstrong (ZA) strength models, and to use and compare these two models in simulations of ballistic experiments. High strain rate strength data for the low-cost titanium alloy are used to generate parameters for the two models. The approach to fitting the JC parameters follows one previously used successfully to model 2-in thick rolled homogeneous armor (RHA) [2]. The approach to fitting the ZA parameters is based on a method described by Gray et al. [3]. The resulting model parameters are used in the shock physics code CTH [4] to model a Ti-6Al-4V penetrator penetrating a Ti-6Al-4V semi-infinite block at impact velocities up to 2,000 m/s. Similar experiments are performed, and the predictions of the two models are compared to each other and to the experimental results. 相似文献
Ohmic contacts to n-GaN using Ag, Au, TiN, Au/Ti, Au/Mo/Ti, and Au/Si/Ti have been studied. The Fermi level of GaN appears
to be unpinned, and metals and compounds with work functions less than the electron affinity resulted in ohmic contacts. Reactively
sputter deposited TiN was ohmic as deposited. However, Au/Ti, Au/Mo/Ti, and Au/Si/Ti required heat treatments to form ohmic
contacts, with the best being an RTA at 900°C. Ag and Au were shown to diffuse across the GaN surface at T>500°C; therefore,
they are unstable, poor ohmic contact metallizations as single metals. The other contact schemes were thermally stable up
to 500°C for times of 30 min. 相似文献