Needle-shaped Precipitates Research Articles

Precipitation in dilute Cu-Cr alloys: The effects of phosphorus impurities and aging procedure

This paper reports that precipitation in dilute Cu-Cr alloys has been studied extensively in part because this alloy can be used as a model system for the investigation of the crystallography and interfaces in FCC-BCC phase transformations. Hall et al. first reported needle- or lath-shaped Cr-rich precipitates with a {l brace}335{r brace}{sub f} habit plane and a variable orientation relationship ranging from Nishiyama-Wasserman (N-W) to Kurdjumov-Sachs (K-S). Hall and Aaronson later confirmed their early findings. Weatherly et al. however, found a constant K-S orientation relationship for this alloy system and a preferred growth direction of {l angle}651{r angle}{sub f} for the needle-shaped precipitates. The variation of the orientation relationship and its potential effect on the precipitate morphology and interface structure have become key points in studying the precipitate crystallography of this alloy system. Dahmen et al. attributed the variation of the orientation relationship to the different quenching and aging conditions applied to the alloy; a direct quench from the solutionizing to the aging temperature employed by Hall et al. would result in a heterogeneous nucleation and hence a variation in the precipitation behavior, while the water quench and aging procedure utilized by Weatherly et al, would facilitate homogeneous nucleation andmore » produce a constant crysallography.« less

The coprecipitation of Cr3P and Cr in Cu

Cu-Cr age-hardening alloys are of interest as a model system for the investigation of fcc/bcc interface structures. Several past studies have investigated the morphology and interface structure of Cr precipitates in a Cu matrix (1-3) and good success has been achieved in understanding the crystallography and strain contrast of small needle-shaped precipitates. The present study investigates the effect of small amounts of phosphorous on the precipitation behavior of Cu-Cr alloys.The same Cu-0.3% Cr alloy as was used in earlier work was rolled to a thickness of 150 μm, solution treated in vacuum at 1050°C for 1h followed by quenching and annealing for various times at 820 and 863°C.Two laths and their corresponding diffraction patterns in an alloy aged 2h at 820°C are shown in correct relative orientation in Fig. 1. To within the limit of accuracy of the diffraction patterns the orientation relationship was that of Kurdjumov-Sachs (KS), i.e. parallel close-packed planes and directions.

Substructure Of needle-shaped precipitates in An Al-Ge alloy

Dilute Al-Ge alloys provide a good model for metal-semiconductor interface studies because clean interfaces in different orientations can be grown by precipitating Ge from the A1 terminal solid solution. A great variety of orientation relationships and morphologies have been observed in this alloy, including plates, laths, needles and tetrahedral Needle precipitates along <100> A1 which are invariably twinned form in abundance during low-temperature aging. Since <110> Ge is exactly parallel to the needle axis, high resolution microscopy allows direct and accurate analysis of the precipitate orientation relationship, morphology, interface and defect structure.It was reported earlier that the particle morphology usually conformed to the symmetry of its orientation relationship with the matrix. This rule was broken only by some of the twinning defects observed. In the present contribution we focus attention on the substructure formed by these twins and their relationship to the particle morphology.

Effects of 600 MeV proton irradiation on nucleation and growth of precipitates and helium bubbles in a high-purity Al-Mg-Si alloy

Solution treated specimens of a high-purity Al-0.75%Mg-0.42%Si alloy were irradiated with 600 MeV protons at 150 and 240°C to a dose level of 0.47 and 0.55 dpa, respectively. Mg 2Si-type precipitates formed during irradiation at 150 and 240°C; at 240°C, however, a large number of precipitates seem to have dissolved during the later stages of irradiation. Thermally aged reference specimens have also been investigated. The needle-shaped precipitates in the aged and the irradiated specimens lie along the 〈100〉 matrix directions. At 150°C bubbles were observed only at grain boundaries whereas at 240°C bubbles were seen in the grain interior as well as at the grain boundaries. Long rows of bubbles were observed with the same orientation in the matrix as the precipitate needles. Grain boundary bubbles were found to grow faster in the Al-Mg-Si alloy than in high-purity aluminium.

Creep Rupture Testing of Aluminum Alloys: Metallographic Studies of Fractured Test Specimens

Abstract Specimens of 1100, 5454, and 6061 aluminum alloys were examined after failure in creep rupture tests of up to 20 000 h duration. No microstructural changes occurred in 1100-O and 5454-O alloys during testing other than slight grain growth at the highest test temperatures. Such growth had only a small effect on creep-rupture properties. Cold-worked alloys 1100-H14 and 5454-H34 were unstable during creep, passing through stages of recovery, recrystallization, and grain growth. These changes invalidate extrapolation of short-time data. The precipitation-hardened 6061 alloy underwent drastic changes in microstructure during testing, from thin needle-shaped precipitates to larger β′ rods to coarse β-Mg2Si plates. These changes produced several different slopes in the log stress-log rupture time plots and long-time rupture stresses considerably below those predicted by extrapolation.

The dynamic observation of the formation of defects in silicon under electron and proton irradiation

Abstract Silicon single crystal samples have been irradiated in a high voltage (1 MeV) transmission electron microscope. The defects formed are described as a function of specimen temperature and compared with similar defects formed by proton irradiation in an accelerator-electron microscope link. The defects are contrasted with those obtained with heavy ions with particular regard to possible mechanisms for the crystalline to amorphous transition which was not seen for electron and proton bombardments. For irradiations at 450°C needle-shaped precipitates were formed on {113} planes.

Precipitation and Hardening Behavior of Czochralski Star Sapphire

Sapphire with 0.11-wt% titania in solution (mainly as TiO1.5) was aged isothermally in air up to 21 days at 1100°–1600°C. Optical studies showed that the solubility limit of the titania was exceeded at 1400°C in air, and consequently needle-shaped precipitates were developed. For aging temperature between 1100° and 1300°C, precipitation initiated at grown-in dislocations and at subboundaries. This phenomenon was attributed to rapid oxidation along these defects. Following this initial stage, precipitates formed uniformly throughout the crystals, and they coarsened with time. The variation of average needle length with time was measured. Knoop microhardness measurements showed that Ti4+ ions plus the attendant defects in solution caused measurable hardening of sapphire, but the greatest changes resulted from precipitation. Significant hardening resulted from aging at 1300°C for 13 days. The hardness anisotropy in the basal plane of sapphire, i.e., KH〈101̄0〉>KH〈112̄0〉, was not altered by precipitation.