Precipitation behavior and its effect on strengthening of an HSLA-Nb/Ti steel

The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 μm, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needle-shaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650°C to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.     

Modeling early failure in integrated circuit interconnect

A model based on the random electron–atom scattering is developed to characterize the effects of defects and grain sizes on electromigration caused failure in confined sub-micron metal interconnect lines. Our study shows that lines at sub-micron widths with a more uniform microstructure exhibit a greater consistency in time to failure. Taking mean time to failure and dispersion in time to failure as criteria, the simulator predicts that grain sizes in the 0.03–0.05 μm range are optimal for 0.125 μm wide Al alloy lines. We also argue that the early failure mechanism associated with the missing metal defects is eliminated by using a homogeneous, fine-grained material. The uniformity of the structure results in a mono-modal failure distribution and contributes to increasing the built-in reliability of the interconnect lines.     

Layered Al2O3-TiO2 compositedielectric resonators

Aluminium oxide displays a very low tanδ at microwave frequencies. It also possesses a remarkably high thermal conductivity, ideal for heat dissipation in high power satellite filters. However, its temperature coefficient of the resonant frequency (τ_f) is approximately 60 ppm/K. It is shown that the application of a film of titanium oxide which has a T_f of opposite sign (45O ppm/K) produces a composite in which the τ_f can be tuned to be zero over a wide temperature range. The tanδ of the composite at zero T_f is 3.3×10⁵ (Q=30000) at room temperature and at 10 GHz     

Early-life reliability prediction methodology for integrated circuits

A methodology for predicting early failures due to random process flaws in integrated circuits is proposed. Early failures are not intrinsic failures, and therefore the current practice of extrapolating intrinsic life-test distributions to estimate early-life reliability is incorrect and yields optimistic results. Early failure mechanisms are classified into three categories based on the physical understanding and statistical data available for the mechanism. Subpopulations with defect-related failure distributions are characterized by a knowledge of the effects of defects in category one and using past field return data in category two. The third category, associated with failures due to applied overstress or misuse, iis characterized by field return, technology and design data. Modeling early failures at the ‘micro’ level (subsystem level) is an improvement over the existing practice of characterizing infant mortality based on field returns at the ‘macro’ level (chip level). Using the proposed methodology, process and design improvements can be incorporated in the early failure predictions. Examples showing the application of this methodology are included.     

Observation of the Stages of Homogeneous Nucleation in Isotopic Solid Helium Mixtures

We report a study of nucleation and phase separation in solid helium isotopic mixtures using a combination of pressure and NMR measurements. The experiments were performed both at pressures where the phase-separated ³He inclusions formed as solid droplets and as liquid droplets. Our observations clearly indicate the three stages of the nucleation process: creation of nucleation sites; growth of the new-phase component at these nucleation sites; and finally dissolution of sub-critical droplets with consequent further growth of super-critical droplets or ‘Ostwald ripening’.     

The Effect of Bridge Geometry on Microstructure and Texture Evolution During Porthole Die Extrusion of an Al–Mg–Si–Mn–Cr Alloy

Porthole die extrusion is used to produce complex hollow aluminum cross-sections for automotive applications. In a porthole die, the material is first divided into multiple streams which are separated by a bridge, before rejoining in the weld chamber and finally passing through the die orifice. The rejoining of the material in the weld chamber produces lines known as weld lines in the final extruded product. The microstructure along the weld line and its associated quality are strongly influenced by the thermal-mechanical history the material experiences as it passes through the portholes, the weld chamber, and the die orifice, which can be altered by die design and, in particular, the bridge geometry. To study the influence of bridge geometry on weld line microstructure and final quality, a series of porthole die extrusion experiments was conducted using an Al–Mg–Si–Mn–Cr alloy and two different types of bridge geometry (streamlined and flat). The experimental results showed that bridge geometry had a significant effect on the local microstructure and crystallographic texture at the weld line. Specifically, EBSD analysis indicated that the weld line texture associated with a streamlined bridge geometry consisted of a deformation texture (mainly the copper component), while the local texture produced by a flat bridge was a recrystallization texture consisting of Cube, Goss, and Cube_RD texture components. Simulation of the extrusion process, using DEFORM 3D, indicated that the weld line produced using a flat bridge experienced a slightly higher temperature, but much higher equivalent strains than the streamlined case. Material away from the weld line was very similar for both cases, indicating that the effect of the die bridge geometry is localized to the region close to the weld line.

     

Intercritical Austenite Formation and Decomposition in the Coarse Grain Heat-Affected Zone of an X80 Line Pipe Steel

Metallurgical and Materials Transactions A - Martensite–austenite constituents (M/A) decomposed from intercritical austenite in the coarse grain heat-affected zone (CGHAZ) affect the...     

A mathematical model coupled to CALPHAD to predict precipitation kinetics for multicomponent aluminum alloys

A mathematical model was developed to simulate the precipitation kinetics during heat treatment of multicomponent aluminum alloys. The model is based on the general numerical framework proposed by Kampmann and Wagner, and features a full coupling with CALPHAD software for the evaluation of the Gibbs–Thomson effect. It also does not rely on the assumption that precipitate phase composition is stoichiometric or uniform, and is therefore applicable for predicting complex precipitation kinetics encountered in industrial practices. Applications of the model to various aging treatments of binary Al–Sc alloys and a ternary Al–Sc–Zr alloy were conducted. It was found that the model predictions for extended time coarsening kinetics are in good agreement with the analytical Lifshitz–Slyozov–Wagner coarsening theory. Its ability to reproduce the complex precipitation pathways in multicomponent alloys was demonstrated by simulation of the precipitation kinetics for an Al–0.09 at.% Sc–0.03 at.% Zr alloy. Comparison of the simulation results with experimental measurement has also highlighted research directions that require further effort.     

Evidence for altered synthesis of type II collagen in patients with osteoarthritis

There is evidence to suggest that the synthesis of type II collagen is increased in osteoarthritis (OA). Using an immunoassay, we show that the content of the C-propeptide of type II procollagen (CPII), released extracellularly from the newly synthesized molecule, is directly related to the synthesis of this molecule in healthy and osteoarthritic articular cartilages. In OA cartilage, CPII content is often markedly elevated (mean 7.6-fold), particularly in the mid and deep zones, reaching 29.6% of the content in newborn. Synthesis is also directly related to total collagen II content in OA, suggesting its importance in maintaining collagen content and cartilage structure. The release of CPII from cartilage is correlated directly with cartilage content. However, the increase in CPII in OA cartilage is not reflected in serum, where a significant reduction is observed. Together these studies provide evidence for alterations in procollagen II synthesis in vivo in patients with OA.