The mechanical properties of nanostructured materials

In this paper, the state-of-the-art progress in research on novel mechanical properties of nanocrystalline materials and carbon nanotubes is reviewed. There is evidence that the relation between the strength of nanocrystalline materials and grain size does not observe the classic Hall-Petch plot. Lowtemperature and high-strain rate superplasticity have been found in some nanocrystalline materials. Theoretical prediction and experimental data indicate that carbon nanotubes are materials with high stiffness, high strength, great toughness, and low density. There are already some application examples for novel mechanical properties of nanocrystalline materials and carbon nanotubes. For more information, contact P.K. Liaw, University of Tennessee at Knoxville, Materials Science and Engineering Department, 427-B Dougherty Engineering Building, Knoxville, Tennessee 37996; (865) 974-6356; fax (865) 974-4115; e-mail pliaw@utk.edu.     

Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass

In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional high-strength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum.     

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part II. Theoretical modeling of fatigue behavior

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) metal matrix composites reinforced with cross-ply, continuous silicon carbide (SiC) fibers. The titanium composites had an eightply (0, 90, +45, -45 deg) symmetric layup. Mechanistic investigation of the fatigue behavior is presented in Part I of this series. In Part II, theoretical modeling of the fatigue behavior was performed using finite element techniques to predict the four stages of fatigue deflection behavior. On the basis of the mechanistic understanding, the fiber and matrix fracture sequence was simulated from ply to ply in finite element modeling. The predicted fatigue deflection behavior was found to be in good agreement with the experimental results. Furthermore, it has been shown that the matrix crack initiation starts in the 90 deg ply first, which is in agreement with the experimental observation. Under the same loading condition, the stress in the 90 deg ply of the transverse specimen is greater than that of the longitudinal specimen. This trend explains why the longitudinal specimen has a longer fatigue life than the transverse specimen, as observed in Part I. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TSM/SMD/ASM-MDS Composite Materials Committee.     

On the SiO2-based gate-dielectric scaling limit forlow-standby power applications in the context of a 0.13 μm CMOS logictechnology

This paper describes a leading-edge 0.13 μm low-leakage CMOS logic technology. To achieve competitive off-state leakage current (I _off) and gate delay (T_d) performance at operating voltages (V_cc) of 1.5 V and 1.2 V, devices with 0.11 μm nominal gate length (L_g-nom) and various gate-oxide thicknesses (T_ox) were fabricated and studied. The results show that low power and memory applications are limited to oxides not thinner than 21.4 Å in order to keep acceptable off-state power consumption at V_cc=1.2 V. Specifically, two different device designs are introduced here. One design named LP (T_ox=26 Å) is targeted for V_cc=1.5 V with worst case I_off <10 pA/μm and nominal gate delay 24 ps/gate. Another design, named LP1 (T_ox=22 Å) is targeted for V_cc =1.2 V with worst case I_off<20 pA/μm and nominal gate delay 27 ps/gate. This work demonstrates n/pMOSFETs with excellent 520/210 and 390/160 μA/μm nominal drive currents at V_cc for LP and LP1, respectively. Process capability for low-power applications is demonstrated using a CMOS 6T-SRAM with 2.43 μm² cell size. In addition, intrinsic gate-oxide TDDB tests of LP1 (T _ox=22 Å) demonstrate that gate oxide reliability far exceeding 10 years is achieved for both n/pMOSFETs at T=125°C and V _cc=1.5 V     

Anode region design and focusing properties of STAR silicon drift detectors

Electron collection at the anodes of large-area silicon drift detectors was studied with STAR2 prototypes. Results of measurements of anode leakage currents, signal dependence on focusing voltages, anode uniformity, and model simulations are reported. A design of the anode region is presented. The design optimization is discussed.     

Fracture toughness behavior of ex-service 2-1/4Cr-1Mo steels from a 22-year-old fossil power plant

Elevated-temperature fracture toughness properties were developed on ex-service 2-l/4Cr-1Mo steel weldments. Fracture toughness was measured on both base and heat-affected zone (HAZ) metals. A composite specimen consisting of base, HAZ, and weld metals was used to develop fracture toughness properties in the HAZ area. It was observed that the J-R curve of the HAZ was significantly lower than that of the base metal. Increasing crack extension increased the difference between theJ-R curves of the base metal and the HAZ. Dimpled fracture was the prime fracture mode in the base metal specimen, and a mixed-mode (ductile and “granular”) fracture was found in the HAZ specimens. Scanning transmission electron microscopy (STEM) examination revealed significant intergranular carbide precipitation and agglomeration within the HAZ. The lower fracture toughness of the HAZ, as compared to the base metal, was attributed to the large accumulation of carbides in the grain boundaries of the HAZ, which weakened the grain boundaries and caused “granular” fracture.     

Development of a universal modeling tool for rechargeable lithium batteries

An interesting universal modeling tool for rechargeable lithium batteries is presented in this paper. The generic model is based on an equivalent circuit technique commonly used in electrochemical impedance characterization. Therefore, the parameters used in the model can be easily parameterized from the electrochemical impedance derivations, which provide a convenient integration with experimental cell characterizations. Such integration offers the universality in this modeling approach.     

Inductive modeling of lithium-ion cells

Sandia National Laboratories has conducted a sequence of studies on the performance of lithium ion and other types of electrochemical cells using inductive models. The objectives of some of these investigations are: (1) to develop procedures to rapidly determine performance degradation rates while these cells undergo life tests; (2) to model cell voltage and capacity in order to simulate cell output under variable load and temperature conditions; (3) to model rechargeable battery degradation under conditions of cyclic charge/discharge, and many others. Among the uses for the models are: (1) to enable efficient predictions of battery life; (2) to characterize system behavior.

Inductive models seek to characterize system behavior using experimentally or analytically obtained data in an efficient and robust framework that does not require phenomenological development. There are certain advantages to this. Among these advantages is the ability to avoid making measurements of hard to determine physical parameters or having to understand cell processes sufficiently to write mathematical functions describing their behavior. We have used artificial neural networks (ANNs) for inductive modeling, along with ancillary mathematical tools to improve their accuracy.

This paper summarizes efforts to use inductive tools for cell and battery modeling. Examples of numerical results are presented.     

The strain-controlled fatigue behavior and modeling of Haynes HASTELLOY C-2000 superalloy

The strain-controlled fatigue behavior of the new nickel-chromium-molybdenum superalloy, HASTELLOY^® C-2000^® was investigated. Low-cycle fatigue specimens were tested in strain-controlled fatigue conditions under axial strain range control at 24 °C, 204 °C and 427 °C. The results indicated that at total strain ranges below 0.8%, temperature does not significantly influence fatigue life, whereas at high total strain ranges, there is a significant drop in fatigue life at 427 °C. At room temperature and 204 °C, the cyclic stress response was cyclic hardening, followed by cyclic stability, cyclic hardening, or cyclic softening, depending on the total strain range. At 427 °C, only cyclic hardening was observed. Fractographic and metallographic analyses were also conducted. The strain-life and Holloman relation parameters are calculated. A theoretical model that predicts the life of this alloy under the tested conditions using the plastic strain energy method is also presented. The results of the theoretical model are in good agreement with the experimental results.