Zr-modified Alloy Research Articles

Dispersoid evolution in Al–Zn–Mg alloys by combined addition of Hf and Zr: A mechanistic approach

Coherent Al3X-type L12-structured dispersoids have the potential of effectively stabilizing the grain structure and increasing strength. This concept has been successfully demonstrated for non-hardenable and rapidly solidified Al alloys. In precipitation-hardened Al alloys, effective dispersoid addition requires both controlling their high-temperature stability and minimizing their impact on precipitation hardening. The current study focuses on dispersoid-modified AlZn5.0Mg1.2 alloys, which exhibit MgZn precipitation upon age-hardening and include less than 1 wt% of Zr and Hf for dispersoid formation. Heat treatments between 350 °C and 500 °C for varying times were applied to evaluate dispersoid formation, thermal stability and the related strengthening potential. The microstructure was assessed using transmission electron microscopy (TEM) and atom probe tomography (APT), and the mechanical response was evaluated by hardness testing. TEM after heating at 500 °C reveals Ostwald ripening for the dispersoids. APT results on the dispersoids reveal a core–shell structure development upon longer annealing times. The Zr–Hf-modified alloy exhibits a higher initial strength than the Zr-modified alloy but the latter displays greater strength retention even after prolonged exposure to 500 °C. This effect is attributed to a destabilization of the mixed Zr–Hf dispersoids that arises from lower enthalpic benefits of Al3Hf formation over Al3Zr.

Effect of Zr on the high cycle fatigue and mechanical properties of Al–Si–Cu–Mg alloys at elevated temperatures

The Zr-modified Al–Si–Cu–Mg alloy with 0.14 wt%Zr addition was studied against the counterparts of commercially used EN-AC-42000 (Al7Si0.5Cu) baseline alloy for the effect of Zr on the high cycle fatigue (HCF) and mechanical properties at elevated temperatures of 150, 200, 250 °C. It was found that the fatigue life was significantly improved by 8–10 times at the high stress amplitude of 140 MPa in the Zr-modified alloy at all different temperatures. The fatigue strength coefficient, σf', of the baseline alloy was 574.9, 589.8, and 514.8 MPa at 150, 200, and 250 °C, respectively, which was greatly increased to 1412.3, 620.1, and 821.6 MPa for the Zr-modified alloy. The tensile strength was considerably improved by 34%–50%, dependant on the testing temperatures. The improved fatigue and tensile properties in the Zr-modified alloy could be mainly ascribed to: (1) the refined microstructure, with α-Al grain size decreasing from 335 to 253 μm and the secondary dendrite arm spacing (SDAS) dropping from 39 to 28 μm; (2) the reduced porosity; and (3) the additional precipitates strengthening effect by the nano-sized Al–Si–Zr–Ti dispersoids.

Effects of Zr, Ti and Sc additions on the microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe alloys

The evolution of microstructure and mechanical properties of Al-0.4Cu-0.14Si-0.05Mg-0.2Fe (wt.%) alloys, micro-alloyed with Zr, Ti and Sc, were investigated. The addition of 0.2%Zr to base alloy accelerates the precipitation of Si-rich nano-phase in α-Al matrix, which plays an important role in improving the mechanical properties of an alloy. The tensile strength increases from 102MPa for the base alloy to 113MPa for the Zr-modified alloy. Adding 0.2%Zr+0.2%Ti to base alloy effectively refines α-Al grain size and accelerates the precipitation of Si and Cu elements, leading to heavy segregation at grain boundary. By further adding 0.2%Sc to Zr+Ti modified alloy, the segregation of Si and Cu elements is suppressed and more Si and Cu precipitates appeared in α-Al matrix. Accompanied with the formation of coherent Al3Sc phase, the tensile strength increases from 108MPa for the Zr+Ti modified alloy to 152MPa for the Sc-modified alloy. Due to excellent thermal stability of Al3Sc phase, the Sc-modified alloy exhibits obvious precipitation hardening behavior at 350°C, and the tensile strength increases to 203MPa after holding at 350°C for 200h.

Development of Ultra-high Purity (UHP) Fe-Based Alloys with High Creep and Oxidation Resistance for A-USC Technology

The design of ultra-high purity (UHP) Fe-based model alloys for advanced ultra-supercritical (A-USC) technology is attempted in this work. Creep testing has been performed in air at 700 °C and a stress level of 150 MPa. Analysis of the fracture surface and cross section of the crept specimen was performed. To evaluate the oxidation resistance in A-USC conditions, oxidation testing was performed in supercritical water (SCW) at 700 °C and 25 MPa. Weight gain (WG) measurements and meticulous characterization of the oxide scale were carried out. Based on thermodynamics and density functional theory calculations, some reactive elements in the Fe-Cr-Ni system were designated to promote precipitation strengthening and to improve the hydrogen-accelerated oxidation resistance. The addition of a 2 wt pct Mo into Fe-22Cr-22Ni-0.6Nb wt pct-based matrix did not significantly improve the creep resistance. The addition of 0.26 wt pct Zr coupled with cold working was effective for improving creep properties. The Mo-modified model alloy showed almost the same WG value as SUS310, while the Zr-modified alloy showed a higher WG value. Meanwhile, a Cr-enriched continuous oxide layer was formed at the oxidation front of the Zr-modified alloy and SUS310S after exposure to SCW conditions.

Alloying effects on creep and oxidation resistance of austenitic stainless steel alloys employing intermetallic precipitates

Microstructure evolution during creep testing at 750 °C and 100 MPa of Fe–20Cr–30Ni–2Nb (at.%) alloys with and without 0.4 Si, 0.2 Zr or 5.0 Al additions has been studied, in order to explore the viability of Fe-rich austenitic stainless alloys strengthened by intermetallic phases. Fine Fe 2Nb Laves phase dispersions with the size of less than 1 μm within the γ-Fe matrix were obtained in the base and Si-modified alloys after aging at 800 °C. The addition of Si helped to refine and stabilize the size of particles, resulting in finer and denser Fe 2Nb dispersion than that in the base alloy. The alloys with only solution heat-treatment exhibited superior creep resistance to the aged ones, which is due to dynamic precipitation of the Fe 2Nb Laves phase during creep testing with a size of 300–400 nm, resulting in more effective pinning of dislocations. The base alloy also showed the meta-stable γ″-Ni 3Nb phase with a size of 50 nm during the transient state of the creep testing. The Zr-modified alloy achieved significant improvement of creep properties. This is hypothesized to be due to the stabilization of δ-Ni 3Nb phase relative to Fe 2Nb, resulting in the formation of multiple fine dispersions of stable intermetallic compounds of δ and Fe 2Nb within the γ-Fe matrix. A small amount of a {Ni, Zr and Nb}-enriched unidentified phase was also observed. The addition of Al significantly improved the oxidation resistance because of the formation of protective alumina scales on the surface. The alloy also showed superior creep resistance to that of the base alloy due to the formation of a dense dispersion of spherical Ni 3Al, typically 30 nm in diameter. Collectively, these findings provide the alloy design basis for creep and oxidation-resistant austenitic stainless steel alloys via intermetallic precipitates.

Mechanical properties and microstructure of Al–Mg–Mn–Zr alloy processed by equal channel angular pressing at elevated temperature

An Al–Mg–Mn alloy containing Zr, and a standard 5083Al alloy were fabricated. The as-cast alloys were homogenized, hot extruded and hot ECAP (equal channel angular pressing). After hot extrusion, the ultimate strength of the Al–Mg–Mn–Zr alloy was 30.0 MPa higher than that of 5083Al, but the elongation was 10.1% lower. This was because the Al 3Zr dispersoids in the Al–Mg–Mn–Zr alloy enhanced the strength and inhibited the recovery process during the hot extrusion, but simultaneously decreased the ductility. After hot ECAP, the strength of the Al–Mg–Mn–Zr alloy was still 32.6 MPa higher than that of 5083Al but without significant loss of the elongation. This implied that the fine-grained, high-angle boundaries microstructure produced by hot ECAP can effectively enhance the strength with little compromise of the ductility of the Zr-modified alloy.

Phenomenological observations on thixoformability of a zinc alloy ZA27 and the resulting microstructures

The thixoformability to cylindrical rods of a Zr-modified zinc alloy ZA27 and the resulting microstructures were observed with different reheating temperatures, reheating durations and mould temperatures. The results were compared with those of an unmodified ZA27 alloy formed by semi-solid die-casting. It was found that the formability of the Zr-modified alloy was significantly superior to that of the unmodified one. The microstructures of the thixoformed rods were basically uniform along their axial direction and the primary particle fraction obviously decreased along radial direction. The primary particle size slightly increased and the primary particle fraction decreased with increasing reheating duration. Both the size and fraction decreased with the increase of reheating temperature, while both of them increased as the die temperature increased. The mould-filling process and subsequent secondary solidification were also discussed.

A study of the initial stages of oxidation of Fe-Cr-Ni-Zr alloysin situ using a high-voltage electron microscope

Structural Fe-Cr-Ni alloys rely upon a thermally formed, protective, surface oxide scale to prevent rapid corrosive degradation. The protective capacity of the surface scale may be strongly influenced by the alloy composition, with minor additions of reactive elements playing an important role in the initial stages of scale formation. The influence of an addition of Zr on initial scale growth on an Fe-Cr-Ni alloy has been investigated in situ utilizing an environmental cell incorporated into a high-voltage electron microscope. Oxidation experiments were conducted on a pure ternary Fe-Cr-Ni alloy and one containing 6 wt.% Zr for durations up to 1800 s. At 500°C in a low oxygen-partialpressure environment, a continuous surface oxide layer formed more quickly on the Zr-free alloy than on the Zr-modified alloy. Also, on the Zr-modified alloy, the scale was richer in Cr, and the rate of increase in oxide grain size was also greater.

An exploratory study of a zirconium-modified, precipitation-strengthened nickel-30 copper alloy

A precipitation-strengthened alloy has been produced through minor additions of zirconium to a base Ni-30 Cu alloy. The results of this exploratory study indicate that thermomechanical processing of a solution-treated Ni-30Cu-0.2 Zr alloy produced a dispersion of precipitates. The precipitates have been tentatively identified as a Ni5Zr compound. Comparison of the mechanical properties, as determined by testing in air, of the Zr-modified alloy to those of a Ni-30 Cu alloy reveals that the precipitation-strengthened alloy has improved tensile properties to 1200 K and improved stress-rupture properties to 1100K. The oxidation characteristics of the modified alloy appeared to be equivalent to those of the base Ni-30 Cu alloy.