Si-containing Alloy Research Articles

Effects of Ag or Si on precipitation in the alloy Al-2.5 mass% Cu-1.5 mass% Mg

Calorimetric measurements and electron microscopy observations were performed on Al-2.5 mass% Cu-1.5 mass% Mg alloys containing also 0.4, 1 or 2% Ag or 0.5% Si, in order to improve understanding of the relationships between precipitation processes and age hardening. The analogous behaviour of calorimetric and hardness data confirms that the first hardening stage is initiated in all alloys by GPB zone formation which occurs via a nucleation and growth controlled mechanism. The vacancy-trapping effect of Mg is increased by Ag and Si additions and leads to slower precipitation kinetics. Consequently, refined GPB zones sizes are obtained leading to an increase in hardness with respect to the ternary alloy. During the second hardening stage, the formation of the more stable S′ phase increases the total amount of strengthening precipitates in the ternary alloy. Phases typical for binary Al–Cu alloys form additionally in the Si-containing alloy. In the Ag-bearing alloys, precipitation of the hardening X′ phase occurs the earlier the higher the Ag content; it is followed by S′ precipitation. During heating of the ternary alloy, the S′ phase forms after substantial dissolution of GPB zones and of the S′′ phase identified by high resolution electron microscopy; this contradicts the concept of a continuous precipitation sequence.

Formation, Thermal Stability and Deformation Behavior of High‐Strength Cu‐Based Bulk Glassy and Nanostructured Alloys

Cu47Ti33Zr11Ni8X1 (X = Fe, Si, Sn) bulk glass‐forming alloys were prepared using arc melting and copper mold casting. Thermal analysis reveals an obvious glass transition and large supercooled liquid regions (ΔTx = 50 – 57 K) for all alloys. The bulk glassy alloys possess good mechanical properties, including high yield stress (1818 – 2102 MPa) and fracture (1943 – 2186 MPa) strengths, Young's moduli of 67 – 109 GPa and large elastic strains of about 2.0 – 2.7 %. Only the Si‐containing alloy exhibits distinct strain hardening and plasticity (fracture strain of 4.4. %). This is due to a unique nanocomposite microstructure consisting of a uniform distribution of Cu‐rich nanoparticles embedded in the bulk amorphous matrix. The nanocomposite microstructure facilitates the formation of multiple shear bands that make the bulk glassy alloy resistant to crack propagation.

Orientation Relationships of Icosahedral Quasicrystalline Phase and Laves Phase Precipitates in a Ferritic Alloy

The precipitation behavior and orientation relationships of icosahedral quasicrystalline phase (I-phase) and Laves phase precipitates in ferrite matrix have been investigated by transmission electron microscopy (TEM) in an Fe-10Cr-1.4W-4.5Co-0.3Si (at%) alloy. It is found that the precipitates of the alloy aged at 873 K are the I-phase but those of the alloy aged at 973 K are the Laves phase. Through a double aging experiment at both temperatures, it is shown that the transformation between the I-phase and the Laves phase occurs. Although a single orientation relationship is established between the I-phase and the ferrite matrix, three different types of the orientation relationships between the Laves phase and the ferrite matrix are obtained by the analysis of SAD patterns. The results can be explained by the coincidence between the five-fold symmetrical plane of the I-phase and the (11� 2) plane of the Laves phase on the phase transformation between them. High Cr ferritic heat resistant steels containing W have been developed for the power generating plants. These steels are composed generally of the tempered martensite matrix and the precipitates of MX carbonitride, M23C6 carbide and intermetallic compounds such as the Laves phase, � phase and z phase, etc. 1) It is suggested that fine precipitation of the Fe2W Laves phase under the service condition improves the creep resistance of the ferritic heat resistant steels. 2) On the other hand, the coarsening of the Laves phase reduces the creep strength and toughness of the steels particularly at ambient temperature. Then it is important to make it clear the precipitation behavior of the Laves phase in order to best utilize the Laves phase. The present authors have investigated the precipitation behavior of the intermetallic phases by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co alloys with and without 0.3 at%Si. 3) From a series of SAD patterns taken from the precipitates of the Si-containing alloy aged at 873 K, the two-, three- and five-fold symmetry and diffraction spots at the positions related to the golden section have been found to originate from the icosahedral quasicrystal, hereafter denoted as I-phase. It has been also found that the precipitates of the Si-containing alloy aged at 873 K are the I-phase but those of the Si-free alloy are the crystalline R-phase. However, the precipitates in both the Si-containing and Si- free alloys aged at 973 K have been found to be the Laves phase. Furthermore, it is interesting that the transformation between the Laves phase and the I-phase (or R-phase) occurs between the two aging temperatures. The objective of this work is to investigate the orientation relationships among the I-phase, the Laves phase and the � - Fe matrix which are related to the transformation between the I-phase and the Laves phase precipitates in an Fe-10Cr- 1.4W-4.5Co-0.3Si (at%) alloy. 2. Experimental Procedure

High-strength Cu–Ti-rich bulk metallic glasses and nano-composites

Abstract Cu47Ti34Zr11Ni8, Cu47Ti33Zr11Ni8Fe1 and Cu47Ti33Zr11Ni8Si1 bulk glassy alloys were prepared by injection copper mold casting. Mechanical properties, glass-forming ability, thermal stability and microstructural characteristics of as-cast rods were investigated. Calorimetric studies indicate a beneficial role of small Si or Fe addition on the thermal stability of Cu–Ti–Zr–Ni bulk glassy alloys. Compression tests reveal fracture strengths of 2040 to 2190 MPa, Young's moduli of 100 to 109 GPa and elastic strains up to 2.4%. The Si-containing glassy alloy exhibits a plastic elongation of 2.2%. The significant increase in plasticity observed for the Si-containing alloy is due to a special bimodal composite structure consisting of nano-scaled Cu crystals homogeneously dispersed in the bulk metallic glass matrix. The increase of the global plasticity can be explained by the formation of multiple shear bands that make the glassy alloy resistant to crack propagation.

Effects of Si and Nb additions on CuAINiTi rapidily solidified alloys microstructures

Intensive investigations have been carried out on CuAlNiTi shape memory alloys (SMAs) with 1wt.%Ti and Ni content from 5 to 6 wt%. In this present work, we used a rapid solidification technique to prepare alloys of two compositions: (wt%) Cu 79 Al 13 Ni 5 Ti 2 Si 1 , Cu 79 Al 13 Ni 5 Ti 1 Nb 2 . The ribbons obtained were characterised by means of XRD, DSC measurements and TEM observations. XRD patterns indicated that the first alloy has a two-phase structure with beta and martensite 18R phases, whereas the second one is only beta. DSC measurements gave characteristic transformations temperatures. The Si-containing alloy showed low temperature transformations with M, about -88°C and A f at -41°C in as-quenched state. The Nb-containing alloy exhibited high temperature transformations with A f about 170°C in the as-quenched state, which is quite higher for this alloy family. The transformation temperatures are stabilised after only one cycle at ΔT M =(M s -M f ) constant for both alloys. This stabilisation is likely to be generated, in the case of Cu 79 Al 13 Ni 5 Ti 1 Nb 2 , by nanocrystalline precipitates revealed on TEM micrographs. Narrow and faulted martensite plates have been observed in agreement with previous results on rapidly solidified CuAlNiTi SMAs.

Effect of Cr and Si additions on the continuous-cooling-transformation kinetics of γ-based Ti-45 at.% A1 alloy

Abstract The effect of Cr and Si additions on the continuous-cooling-transformation (CCT) kinetics of Ti45Al alloy has been studied at cooling rates from about 0.5 to 500°C s−1 using our recently constructed in-situ real-time resistivity–temperature measurement apparatus. The α → γL lamellar reaction occurs at slow cooling rates and only the α → α2 ordering reaction occurs at fast cooling rates; no massive reaction occurs at intermediate cooling rates. The addition of Cr results in a simple decrease in the reaction temperatures for the α → γL lamellar reaction without a change in the critical cooling rate for the ordering reaction only, in partial agreement with the depression of the α transus temperature. The ordering start temperature was measured to be slightly higher. The addition of Si, on the other hand, significantly accelerates the kinetics of α → γL lamellar reaction by shifting its CCT curve towards a higher temperature and a shorter time. This is because silicides formed at high temperatures on grain boundaries raise the γ-phase nucleation temperature and also because Si enhances its growth rate, probably by increasing the chemical diffusivity of α phase. The addition of Cr to a Si-containing Ti–45 at.% Al alloy significantly refines its lamellar spacing either by suppressing the Si segregation to grain boundaries or by reducing the α-γ interfacial energy. The antiphase domain size in the α2 phase is distinctively small in the Si-containing alloy probably owing to its interfacial segregation tendency.

Effect of Cr and Si additions on the continuous-cooling-transformation kinetics of γ-based Ti–45 at.% A1 alloy

Abstract The effect of Cr and Si additions on the continuous-cooling-transformation (CCT) kinetics of Ti45Al alloy has been studied at cooling rates from about 0.5 to 500°C s−1 using our recently constructed in-situ real-time resistivity–temperature measurement apparatus. The α → γL lamellar reaction occurs at slow cooling rates and only the α → α2 ordering reaction occurs at fast cooling rates; no massive reaction occurs at intermediate cooling rates. The addition of Cr results in a simple decrease in the reaction temperatures for the α → γL lamellar reaction without a change in the critical cooling rate for the ordering reaction only, in partial agreement with the depression of the α transus temperature. The ordering start temperature was measured to be slightly higher. The addition of Si, on the other hand, significantly accelerates the kinetics of α → γL lamellar reaction by shifting its CCT curve towards a higher temperature and a shorter time. This is because silicides formed at high temperatures on grain boundaries raise the γ-phase nucleation temperature and also because Si enhances its growth rate, probably by increasing the chemical diffusivity of α phase. The addition of Cr to a Si-containing Ti–45 at.% Al alloy significantly refines its lamellar spacing either by suppressing the Si segregation to grain boundaries or by reducing the α-γ interfacial energy. The antiphase domain size in the α2 phase is distinctively small in the Si-containing alloy probably owing to its interfacial segregation tendency.