Zr-containing Alloys Research Articles

Interfacial Solute-Atom Segregation: An Atomic-scale Ahenomenon with Implications for the Oxidation Kinetics of Pt-Al Alloys

Abstract Interfacial solute-atom segregation is known to have profound effects on macroscopic physical properties (e.g. electronic transport or fracture toughness) in interface-influenced or controlled materials. It has recently been shown that solute-atom segregation also can play an influential role in the dynamics of phase transformations, in particular the oxidation kinetics of alumina-forming alloys such as PtAl. By coupling analytical electron microscopy (AEM) with conventional macroscopic oxidation studies, we have demonstrated that oxidation kinetics are affected by atomic-scale segregation of dopant species to the grain boundaries and metal-oxide boundaries in the system. Furthermore, these studies illustrate the utility of AEM techniques in elucidating the atomic-scale aspects of macroscopic physical phenomena such as phase transformations. Oxidation studies were carried out on PtAl alloys with and without small additions of Zr to the alloy. While the pure PtAl alloys exhibited oxide scale spallation and very fast oxidation rates, the Zrcontaining alloys maintained a well-adhered oxide scale and significantly lower oxidation rates.

Atomic size effects in Ni–Al based solid solutions

We have determined the atomic volumes of Ni, Al and Ni vacancies in NiAl by analyzing the variations of lattice parameter with composition. We have also attempted to determine the atomic size factors for the solid-solution strengthening elements: Ti, Hf, and Zr, through lattice parameter measurements of solid-solution alloys. The atomic size factor for Ti is found to be +0.34, indicating a sufficiently large size mismatch to account for significant solid-solution strengthening in this system. We calculated a similar atomic size factor of +0.27 for Ti using lattice parameters that had been computed from first-principles. Lattice contractions and negative atomic size factors were observed for Hf and Zr-containing alloys, suggesting the presence of trapped vacancies. Annealing and slow-cooling experiments minimized these lattice contractions and supported the trapped vacancy hypothesis. Using the approximation that the NiAl(Hf, Zr) lattice parameters vary linearly with composition out to the Heusler phase boundary, we determined size factors of Hf and Zr to be +0.57 and +0.65, respectively.

鋳造-HIPプロセスによって作製したTiAl基合金のミクロ組織と機械的性質に及ぼすZr添加の影響

The effects of Zr on tensile strength and ductility of cast-HIP’ed γ-TiAl were investigated. A Ti-48 at%Al alloy was chosen as the base alloy and Zr was added to the base alloy up to 4 at% with Al/(Ti+Al) of 0.48 atomic ratio. In the tensile test of the HIP’ed alloys at room temperature, as the Zr content increased, the yield strength rose rapidly and the elongation decreased. However, the alloys containing 1 at% to 2 at%Zr showed a marked strength improvement with little decreasing elongation, which indicated the excellent strength-ductility balance that was not observed in other ternary alloys we investigated previously. All microstructures of the HIP’ed alloys studied consisted of grains in lamellar form and equiaxed γ grains. The volume fraction of grains in lamellar form increased as the Zr content increased. The yield strength of all the HIP’ed alloys showed a linear relationship with the volume fraction of grains in lamellar form. Thus the yield strength of the HIP’ed alloys could be analyzed on the basis of the rule of mixture in which these alloys were assumed to be composed of grains in lamellar form and equiaxed γ grains. The equiaxed γ region had the Zr segregated zone that enclosed lamellar colonies forming like a network. It was considered that superior strength-ductility balance of HIP’ed Zr-containing alloys was caused by strength improvement resulted from increasing volume fraction of grains in lamellar form and by ductility improvement of equiaxed γ grains due to the Zr segregated zone and of its network-like morphology. The annealing of the HIP’ed alloys at 1603 K resulted in a disappearance of the superior strength-ductility balance and the Zr segregated zone. The relationship between microstructure and tensile property of Zr-containing γ-TiAl was studied in detail.

Superplastic behaviour of a rapidly solidified 7475 aluminium alloy

To investigate possible superplastic behaviour, tensile tests were performed on conventionally cast and extruded aluminium alloy AA 7475, rapidly solidified and extruded AA 7475, and rapidly solidified and extruded AA 7475, which contained 0.45 wt%. Zr. The rapid solidification process used was melt spinning. The extruded materials were not subjected to thermomechanical processing, but the effects of various simple heat treatments were investigated. The rapidly solidified Zr-containing alloy had the smallest grain size (1 μm), the highest elongation, and the lowest flow stress. Heating at 300 °C for 4 h improved the elongation and lowered the flow stress for this alloy. After this treatment, an elongation of 390% was observed at 450 °C and a strain rate of 8 × 10 −2 s −1. The strain rate sensitivity exponent was 0.58. It was concluded that the rapidly solidified Zr-containing alloy was superplastic and that a full thermomechanical treatment after rapid solidification is not needed to achieve such an effect.

Coarsening of precipitates and dispersoids in aluminium alloy matrices: a consolidation of the available experimental data

Experimental data on the coarsening of precipitates and dispersoids in aluminium-based matrices are reviewed. Available data are tabulated as K=(r 3−r 0 3 )/t where r 0 is the initial particle radius and r is its value after time t at temperature T, and then plotted as log (KT) against 1/T for consolidation and assessment. The considerable body of data for δ′-A3Li in Li-containing alloys is well represented by K=(K 0/T) exp (−Q/RT) with K 0=(1.3 −0.5 +3.0 ) × 10−13m3Ks−1 and Q=115±4kJ mol−1. The relatively limited data for θ′ and θ″ in Cucontaining alloys are representable by the same relationship with K 0∼4 × 10−8 and — 4 × 10−10 m3 Ks−1, respectively, and Q — 140 kJ mol−1. Available data for coarsening of L12 − Al3(Zr, V) and related phases in Zr-containing alloys and of Al12Fe3Si and related phases in Al-Fe based alloys indicate (i) rates of coarsening at 375 to 475 °C (0.7 to 0.8Tm) five to eight orders of magnitude less than would be expected for δ′, θ′ and θ″ in this temperature range, and (ii) high activation energies of ∼300 and 180 kJ mol−1, respectively.

Cast magnesium alloys for elevated temperature applications

The alloy development, microstructure, properties and uses of cast magnesium alloys for elevated temperature applications are reviewed. The alloying principles and strengthening mechanisms of magnesium are discussed to identify the potential alloy systems for elevated temperature applications in automotive and aerospace industries. It is concluded that the Mg-Zr family of sand cast alloys exhibit adequate mechanical properties at both ambient and elevated temperatures for aerospace applications, and Ca-modified sand cast AS41 alloy might provide a cost-effective alternative for the Zr-containing alloys. For diecasting applications, no current alloy systems meets the requirements of good high temperature properties, acceptable castability and low cost for critical automotive components, future development is especially needed in this area. Development of dispersion strengthened magnesium alloys and improvement of current Mg-Al-RE and Mg-Al-Si systems are the potential routes to expand diecast magnesium alloys to elevated temperature applications.

Effect of third elements on the microhardness of Ni 3Al

The room-temperature microhardness of Ni 3Al with and without third elements has been studied using the Vicker's indentation technique. In pure Ni 3Al phase alloys, the hardness value increased with decreasing Al concentration from stoichiometric composition. This increase is attributed to the defects present at off-stoichiometric compositions. The addition of third elements Hf, Zr, Ti and Si increased the hardness value of Ni 3Al suggesting the solid-solution strengthening effect. The presence of 0.20 at% boron in Hf- and Zr-containing alloys shows a softening effect at small concentrations of Hf and Zr.

Mössbauer spectra and magnetic properties of Sm2 (Co, Fe)17 alloys

Sm2(Co1−xFex)17 alloys form the basis of a new class of permanent magnets. We have investigated these materials with57Fe Mossbauer spectroscopy. Alloys withx≤0.5 have easy-axis anisotropy at room temperature; their spectra can be fitted with sextets corresponding to the four nonequivalent sites of the Th2Zn17 structure. Fe hyperfine fields increase with increasing Fe concentration. An average Fe moment of ≈2.0 μB is estimated. Fe atoms show a strong preference for occupying the 2c dumbbell sites. This preference is reduced in Zr-containing alloys used for magnet preparation.

Texture and Plastic Anisotropy in Model Aluminium - Lithium Alloys

The specific roles of Li, Zr and Cu alloying elements in Al-Li alloys have been investigated in terms of their influence on texture evolution during rolling and subsequent plastic anisotropy. Three model alloys; Al-2.3% Li, Al-2.3% Li-0.1% Zr and Al-2.3% Li-1.2% Cu-0.1% Zr were hot and cold rolled to reductions up to 92% and their texture evolution systematically characterised by X-ray pole figures and complete ODFs. Hot rolling of the Zr containing alloys, which do not recrystallize, led to a strong Bs texture component. Cold rolling of all alloys gave the usual Taylor-type β fibre components, with certain variations according to the precipitation state which can favour or inhibit shear banding.Pronounced plastic anisotropies, R(α), were measured on the rolled sheets with Rmin at α = 0° and Rmax at α ≈ 60°. Actual values depend upon rolling temperature and strain but typically Rmin ≈ 0.3 and Rmax ≈ 1.5 to 2. The quantitative texture data was used together with the CMTP and Taylor models to predict the anisotropy; in general the CMTP model gave a reasonable correlation with the experimental values.

Tin-based reactive solders for ceramic/metal joints

Contact angle measurements on silicon-nitride substrates were conducted on tin-based alloys, containing titanium and zirconium, to determine the suitability of these alloys as filler metals for low-temperature joining of ceramics. Titanium-containing alloys exhibited excellent wettability characterized by contact angles less than 20 deg, whereas the Zr-containing alloys exhibited contact angles around 50 deg. The superior wettability of the Sn−Ti alloys is attributed to the higher activity coefficient of Ti in Sn−Ti alloys. The liquidus temperature of the Sn−Ti alloys is in the 400°C to 600°C range. Hence, these alloys are expected to reduce the residual stress problem.

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form α-Al2O3 scales in a flow of pure O2 at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H2 −10% H2S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO2 particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.

Microstructure and tensile properties of Fe-40 At. pct Al alloys with C, Zr, Hf, and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe6Al6Zr and Fe6Al6Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.

Ｚｒを添加したＨＫ４０およびＨＰ耐熱遠心鋳造管のクリープ・疲労特性

Creep-fatigue and oxidation properties of Zr-free and Zr-containing HK40 and HP centrifugal heat-resisting alloys were studied. The mechanism of improving these properties by Zr addition was discussed through microstructural examination of materials. The following results were obtained;(1) A small amount of Zr addition markedly impoves the low-cycle-fatigue life under creep conditions, creep rupture strength and ductility, and oxidation resistance of HK40 and HP.(2) The improvement of creep-fatigue properties is thought to be brought about by the following two effects;(i) Zirconium gives an excellent resistance to the localized oxidation of eutectic carbides and/or grain-boundaries, which can be fatigue crack initiation sites.(ii) A number of small Zr-carbides (ZrC) are precipitated homogeneously in grains at elevated temperatures. These carbides suppress the formation of sub-boundaries and the precipitation of coarse carbides (M23C6) on sub-boundaries, which can be fatigue crack propagation paths.

Elevated temperature tensile properties of powder metallurgy Ni3Al alloyed with chromium and zirconium

The tensile properties of hot extruded powders of Ni-24.1Al, Ni-19.1Al-8.5Cr, and Ni-17.4Al-7.9Cr-0.5Zr have been evaluated from room temperature to 1000° C. These powder metallurgy materials have a fine grain size that results in relatively little increase in yield stress with increasing temperature compared to coarse-grained or single-crystal materials. The alloy containing chromium and zirconium shows greatly reduced dynamic embrittlement in the temperature range 600 to 800° C where the unalloyed aluminide exhibits brittle behaviour. The Cr- and Cr + Zr-containing alloys deform superplastically above 900° C. The mechanism of superplastic deformation appears to be predominantly grain-boundary sliding.

熱間圧延したＡｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ合金の再結晶に及ぼすＺｒとＣｒ添加の影響の差

Two sorts of Al-5.6%Zn-2.4%Mg-1.3%Cu alloys with 0.12%Zr and 0.26%Cr, respectively, were hot-rolled by 86% at 400°C and then solution-treated at 470°C. Transmission electron microstructures of both alloys in hot-rolled state are nearly the same. Subgrain structures of both alloys developed after disappearance of the η-particles are quite different. Zr-containing alloy shows an almost perfect subgrain structure, reflecting that the particle size of Al3Zr is much smaller than that of E phase. In Zr-containing alloy, recrystallized grains have naturally appeared faster than in Cr-containing alloy and the grain growth is retained. Elongated grains in the rolling direction are commonly observed in both recrystallized alloys, and are attributed to the layered distribution of Al3Zr or E particles in the rolled sheet.

Microstructures of Rapidly Solidified Fe-Al-Zr Alloys

ABSTRACTRapidly quenched short fibers of Fe–8Al and Fe– 8Al–XZr (X = 0.1, 0.2, 0.4, and 1) alloys were produced by the Pendant Drop Melt Extraction Process, (PDME) to explore the feasibility of producing intermediate temperature, oxidation resistant iron base alloys. The fibers had a diameter of 20 to 50 μm and ranged in length from 2–5 mm. The cooling rate was insufficient to retain zirconium in solution in the Zr-containing alloys, resulting in precipitates of the type Fex(Zr,Al) having differing compositions at the grain boundaries and within the matrix. No precipitates were observed in the FeAl alloys. The precipitates were uniform in distribution and size (nominally 0.1 to 0.3 μm diameter). Grain boundary precipitation and precipitate size both decreased with decreasing Zr contents.

Observations on the distribution of iron and silicon particles in a high-strength aluminum alloy

Iron- and silicon-containing impurity particles have been shown to be detrimental to the mechanical properties of high-strength aluminum alloys. This work studies the relationships between area percent of impurities and stringer spacings in 7×75 (Cr containing) and Zr-modified 7×75 type aluminum plate with five iron and silicon contents and two tempers. Total iron and silicon in the 5 8 -inch-thick plate varied between 0.02 and 0.31 wt%. The commercial T651 temper and a thermomechanical processing (TMP) temper were applied to the plates resulting in 20 combinations of composition and processing. Area percent of impurities varied considerably among the three perpendicular planes of polish in the T651 plates. The TMP plates, with their warm rolling step, exhibited both smaller particle sizes and a more homogeneous distribution of particles. There were more identifiable stringers in the T651 plates than in the TMP plates. An empirical relationship was found to exist between area percent of impurities and stringer spacing in the T651 plates. The TMP plates containing Zr displayed a similar trend; however, the TMP plates containing Cr did not. This difference in behavior was attributed to the fact that the TMP treatment used was developed using a Zr-containing alloy.