Influence of zirconium content on microstructure and creep properties of Mo–9Si–8B alloys

Mo–Si–B alloys with a molybdenum solid solution accompanied by two intermetallic phases and Mo₅SiB₂ are a prominent example for a potential new high temperature structural material. In this study the influence of 1, 2 and 4 at.% zirconium on microstructure and creep properties of Mo–9Si–8B (at.%) alloys produced by spark plasma sintering is investigated. Creep experiments have been carried out at temperatures of 1100 °C up to 1250 °C in vacuum. The samples exhibit sub-micron grain sizes as small as 450 nm due to the chosen production route. With addition of 1 at.% zirconium, formation of SiO₂ on the grain boundaries can be prevented, thereby enhancing grain boundary strength and creep properties significantly. Moreover ZrO₂ particles also enhance creep resistance of the molybdenum solid solution. Creep deformation is a combination of dislocation creep in the grains including dislocation-particle interaction and grain boundary sliding leading to intergranular fracture surfaces. It is promising to use grain size adjustments in order to balance the creep and oxidation resistance of the investigated material.     

Effect of Phosphorous Inoculation on Creep Behavior of a Hypereutectic Al-Si Alloy

Creep behavior of Al-Si hypereutectic alloys inoculated with phosphorus was investigated using the impression creep testing. The results showed that at stress regimes of up to 400-450 MPa and temperatures up to 300 °C, no significant creep deformation occurred in both uninoculated and inoculated specimens; however, at temperatures above 300 °C, the inoculated alloys presented better creep properties. Creep data were used to calculate the stress exponent of steady-state creep rate, n, and creep activation energy, Q, for different additive conditions where n was found varied between 5 and 8. Owing to the fact that most alloys have lower values for n (4, 5), threshold stress was estimated for studied conditions. The creep governing mechanisms for different conditions are discussed here, with a particular attention to the effect of phosphorous addition on the microstructural features, including number of primary silicon particles, mean primary silicon spacing, and morphology and distribution of eutectic silicon.     

A new method to strengthen turbine disc superalloys at service temperatures

TMW® superalloys have superior mechanical properties up to 725 °C. Numerous annealing twins are present because these alloys have relatively low stacking fault energies. Nanoscale deformation twins are the dominant mechanism during creep and tensile testing at 725 °C. In addition to traditional strengthening mechanisms enhanced by higher Ti addition, twinning structures enhanced by higher Co addition also play an important role in strengthening coarse-grained alloys at high temperatures. The effects of twin strengthening are discussed.     

The creep of intermetallics and their composites

This article evaluates the creep behavior of nickel aluminides, titanium aluminides, and molybdenum disilicides and their composites as a function of stress and temperature. Significant improvements in creep strength were achieved in NiAl by the addition of HfC dispersoids, and in MoSi₂ and its alloys through the addition of SiC whiskers or particulates. On the basis of creep resistance, molybdenum disilicide alloys and their composites have a high potential for application at temperatures greater than 1,000°C, and they are also potential competitors to more brittle ceramic-ceramic composites.     

Role of the grain-boundary phase on the elevated-temperature strength,toughness, fatigue and creep resistance of silicon carbide sintered with Al,B and C

The high-temperature mechanical properties, specifically strength, fracture toughness, cyclic fatigue-crack growth and creep behavior, of an in situ toughened silicon carbide, with Al, B and C sintering additives (ABC-SiC), have been examined at temperatures from ambient to 1500°C with the objective of characterizing the role of the grain-boundary film/phase. It was found that the high strength, cyclic fatigue resistance and particularly the fracture toughness displayed by ABC-SiC at ambient temperatures was not severely compromised at elevated temperatures; indeed, the fatigue-crack growth properties up to 1300°C were essentially identical to those at 25°C, whereas resistance to creep deformation was superior to published results on silicon nitride ceramics. Mechanistically, the damage and shielding mechanisms governing cyclic fatigue-crack advance were essentially unchanged between ∼25°C and 1300°C, involving a mutual competition between intergranular cracking ahead of the crack tip and interlocking grain bridging in the crack wake. Moreover, creep deformation was not apparent below ∼1400°C, and involved grain-boundary sliding accommodated by diffusion along the interfaces between the grain-boundary film and SiC grains, with little evidence of cavitation. Such unusually good high-temperature properties in ABC-SiC are attributed to crystallization of the grain-boundary amorphous phase, which can occur either in situ, due to the prolonged thermal exposure associated with high-temperature fatigue and creep tests, or by prior heat treatment. Moreover, the presence of the crystallized grain-boundary phase did not degrade subsequent ambient-temperature mechanical properties; in fact, the strength, toughness and fatigue properties at 25°C were increased slightly.     

Intermediate-temperature mechanical properties of Ni–Si alloys: oxygen embrittlement and its remedies

With good corrosion resistance, reasonable room-temperature ductility, and excellent strength up to temperatures of 700 °C, Ni₃Si-based alloys show considerable potential for structural applications. The Ni–Si alloys used for acid-corrosion resistance suffer from a dynamic environmental embrittlement when tested at intermediate temperatures around 600 °C. To assess these Ni–Si alloys for elevated-temperature structural application, the mechanical properties of these alloys strengthened by Ni₃Si precipitates were systematically evaluated at different temperatures in various test environments. Oxygen was identified as the embrittling species responsible for the low ductility and premature fracture of the Ni–Si alloys. A strong dependence of elongation and fracture mode on strain rate was observed. Based on the understanding of the embrittlement mechanism, some unique approaches for improving the intermediate-temperature ductility, strength and fabricability of Ni–Ni₃Si alloys were identified: reactive element doping (such as Zr and Y) to change the grain boundary chemistry; preoxidation to form adherent oxide layers; and thermomechanical processing to tailor the grain structure/shape. Some other properties such as creep resistance and weldability of these alloys were also briefly evaluated and are discussed in this paper.     

The effect of thermal exposure on the mechanical properties of 2099-T6 die forgings, 2099-T83 extrusions, 7075-T7651 plate, 7085-T7452 die forgings, 7085-T7651 plate, and 2397-T87 plate aluminum alloys

Aluminum alloys 2099-T6 die forgings, 2099-T83 extrusions, 7075-T7651 plate, 7085-T7452 die forgings, 7085-T7651 plate, and 2397-T87 plate were thermally exposed at temperatures of 180 °C (350 °F), 230 °C (450 °F), and 290 °C (550 °F) for 0.1, 0.5, 2, 10, 100, and 1000 h. The purpose of this study was to determine the effect of thermal exposure on the mechanical properties and electrical conductivity of these alloys. The data shows that higher temperatures and longer exposure times generally resulted in decreased strength and hardness and increased percent elongation and electrical conductivity.     

Comparison of GRCop-84 to Other Cu Alloys with High Thermal Conductivities

The mechanical properties of six highly conductive copper alloys, GRCop-84, AMZIRC, GlidCop Al-15, Cu-1Cr-0.1Zr, Cu-0.9Cr, and NARloy-Z were compared. Tests were done on as-received hard drawn material, and after a heat treatment designed to simulate a brazing operation at 935 °C. In the as-received condition AMZIRC, GlidCop Al-15, Cu-1Cr-0.1Zr, and Cu-0.9Cr had excellent strengths at temperatures below 500 °C. However, the brazing heat treatment substantially decreased the mechanical properties of AMZIRC, Cu-1Cr-0.1Zr, Cu-0.9Cr, and NARloy-Z. The properties of GlidCop Al-15 and GRCop-84 were not significantly affected by the heat treatment. Thus there appear to be advantages to GRCop-84 over AMZIRC, Cu-1Cr-0.1Zr, Cu-0.9Cr, and NARloy-Z if use or processing temperatures greater than 500 °C are expected. Ductility was the lowest in GlidCop Al-15 and Cu-0.9Cr; reduction in area was particularly low in GlidCop Al-15 above 500 °C, and as-received Cu-0.9Cr was brittle between 500 and 650 °C. Tensile creep tests were done at 500 and 650 °C; the creep properties of GRCop-84 were superior to those of brazed AMZIRC, Cu-1Cr-0.1Zr, Cu-0.9Cr, and NARloy-Z. In the brazed condition, GRCop-84 was superior to the other alloys due to its greater strength and creep resistance (compared to AMZIRC, Cu-1Cr-0.1Zr, Cu-0.9Cr, and NARloy-Z) and ductility (compared to GlidCop Al-15).     

Microstructure and properties of Nb-Mo-Zr based refractory alloys

Microstructure and mechanical properties of five Nb alloys containing 8 to 17 at.% Mo, 8 or 35 at.% Zr, up to 7 at.% Ti, up to 2 at.% Al and up to 2 at.% Cr are reported. These alloys have been developed to replace heavy, expensive and difficult to process commercial Nb alloys, such as C-3009, for use at temperatures up to 1600 °C. The density of the alloys is in the range from 7.6 to 8.6 g/cm³. The alloys have a BCC matrix phase, and they also contain small amounts of secondary phases, which are rich in Zr and have BCC, FCC, hexagonal or monoclinic crystal structures depending on the concentration of other alloying elements, including oxygen and nitrogen. In the temperature range from 25 °C to 1600 °C, the alloys with a smaller amount of Zr are ductile and have higher specific strength than C-3009. The alloy containing 35 at.% Zr is stronger, but less ductile, than other alloys at temperatures up to 600 °C; however, it loses the strength rapidly at higher temperatures and becomes softer than other alloys at T > 1000 °C. The possible strengthening mechanisms responsible for the observed temperature dependence of the yield stress of the alloys are also discussed.     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Effect of thermal exposure on microstructure and mechanical properties of Al−Si−Cu−Ni−Mg alloy produced by different casting technologies

The effect of thermal exposure at 350 °C for 200 h on microstructure and mechanical properties was investigated for Al−Si−Cu−Ni−Mg alloy, which was produced by permanent mold casting (PMC) and high pressure die casting (HPDC). The SEM and IPP software were used to characterize the morphology of Si phase in the studied alloys. The results show that the thermal exposure provokes spheroidization and coarsening of eutectic Si particles. The ultimate tensile strength of the HPDC alloy after thermal exposure is higher than that of the PMC alloy at room temperature. However, the TEPMC and TEHPDC alloys have similar tensile strength around 67 MPa at 350 °C. Due to the coarsening of eutectic Si, the TEPMC alloy exhibits better creep resistance than the TEHPDC alloy under studied creep conditions. Therefore, the alloys with small size of eutectic Si are not suitably used at 350 °C.     

Effect of Ti content on creep properties of Ni-base single crystal superalloys

The effect of Ti content on the creep properties and microstructures of experimental Ni-base single crystal superalloys has been investigated. The experimental alloys were designed to provide better high temperature properties than the commercial single crystal alloy CMSX-4. The creep properties of the experimental alloys, Alloy 2 and Alloy 3, were superior to those of CMSX-4. Alloy 3 showed a longer creep life than Alloy 2 at 900 °C and 950 °C, while it has similar creep life with Alloy 2 at 982 °C. Transmission electron microscopy micrographs of the experimental alloys after the creep test showed distinct deformation features as a function of temperature and Ti content. The dissociation of dislocations into partial dislocations with stacking faults in Alloy 3 was found to improve resistance to creep deformation at 950 °C. The effect of Ti on the creep deformation mechanism was not evident at 982 °C, which resulted in similar creep properties in both experimental alloys. The transition of the γ′ cutting mechanism from dislocations coupled with stacking faults to anti-phase boundary coupled pairs occurred both in Alloy 2 and Alloy 3. However, the transition temperature was higher in Alloy 3 than in Alloy 2 because of the difference in Ti contents.     

Improving the oxidation resistance of γ‐titanium aluminides by halogen treatment

Intermetallic alloys based on TiAl are candidates for several structural high temperature applications but their oxidation resistance is limited to temperatures below 800 °C. In this paper the results of high temperature oxidation and creep tests will be presented and discussed. The treatment with halogens improves the oxidation resistance of these alloys up to 1050 °C. A thin protective Al₂O₃‐layer is formed after treatment with halogens instead of the mixed TiO₂/Al₂O₃/TiN scale typically grown on these alloys. This alumina layer protects the component under isothermal and thermocyclic conditions. The protective effect is stable up to at least 8760 h. Creep tests of halogen treated materials at high temperatures showed no effect on the creep behaviour. Automotive turbocharger rotors were exposed at 1050 °C in air with and without fluorine‐treatment for demonstration of real parts.     

Creep response and deformation processes in nanocluster-strengthened ferritic steels

There is increasing demand for oxide-dispersion-strengthened ferritic alloys that possess both high-temperature strength and irradiation resistance. Improvement of the high-temperature properties requires an understanding of the operative deformation mechanisms. In this study, the microstructures and creep properties of the oxide-dispersion-strengthened alloy 14YWT have been evaluated as a function of annealing at 1000 °C for 1 hour up to 32 days. The ultra-fine initial grain size (approx. 100 nm) is stable after the shortest annealing time, and even after subsequent creep at 800 °C. Longer annealing periods lead to anomalous grain growth that is further enhanced following creep. Remarkably, the minimum creep rate is relatively insensitive to this dramatic grain-coarsening. The creep strength is attributed to highly stable, Ti-rich nanoclusters that appear to pin the initial primary grains, and present strong obstacles to dislocation motion in the large, anomalously grown grains.     

The effect of carbon and silicon additions on the creep properties of Fe-40 at. % Al type alloys at elevated temperatures

The mechanical properties of Fe–Al alloys with 39–43 at.% Al, C contents up to 4.9 at.% and Si contents up to 1.2 at.% were studied using uniaxial compressive creep at temperatures from 600 to 800 °C. The stress and temperature dependence of the creep rate were determined by stepwise loading and evaluated in terms of the stress exponent n and the activation energy Q, respectively. These quantities can be interpreted by means of dislocation motion controlled by climb and by the presence of second-phase particles. The dislocation motion is obstructed by precipitates of carbide κ in alloys E and F and by particles of Al₄C₃ in the alloys with either higher content of C or of C and Si. Both carbon and silicon improved the creep resistance, but the effect of silicon was more significant.     

Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

The microstructure and mechanical properties of eight γ-TiAl based alloys with compositions in the range Ti–44Al–8(Nb,Ta,Zr,Hf)–(0–0.2)Si–(0–1)B have been investigated to assess the possibility of improving the properties of γ-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48–2–2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti–44Al binary alloy. In some of the alloys the ω phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900°C. In alloys which contain both beta- and gamma- stabilisers, there is no single α phase field in the transformation sequence and instead there is a (α+β+γ) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of γ-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure.     

Progress in the design of new lead-free solder alloys

Various alloy design approaches have been employed to develop new lead-free solder alloys that can not only substitute for the lead-tin solders, but also offer significantly improved mechanical properties. Three new alloys are described in this article. In Sn-3.5Ag-1Zn (melting point ~217°C), the solidification structure and the eutectic precipitate morphology are6 refined by the addition of zinc. As a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. In Bi-43Sn+2.5Fe, a eutectic alloy (melting point ~137°C), dispersion hardening by magnetically distributed iron particles retards both high-temperature deformation and microstructural coarsening, thus widening the useful service range of Bi-Sn eutectic alloys to much higher homologous temperatures than are typical for the Sn-Pb eutectic alloy. Lastly, Sn-Zn-In based alloys (melting point ~185°C) have been developed for consideration as a drop-in replacement for the neareutectic Sn-Pb alloy(melting point ~183°C).     

Joining of Cu,Ni, and Ti Using Au-Ge-Based High-Temperature Solder Alloys

Au-Ge-based solder alloys are promising alternatives to lead containing solders due to the fact that they offer a combination of interesting properties such as good thermal and electrical conductivity and high corrosion resistance in addition to a relatively low melting temperature (361 °C for eutectic Au-28Ge at.%). By adding a third element to the eutectic Au-28Ge alloy not only the Au content could be reduced but also the melting temperatures could be further decreased. In this study, in addition to the eutectic Au-28Ge (at.%) two ternary alloys were chosen from the Au-Ge-Sb and Au-Ge-Sn system, respectively. The soldering behavior of these alloys in combination with the frequently used metals Cu, Ni, and Ti was investigated. The interface reactions and microstructures of the joints were characterized in detail by SEM and EDX analysis. For the determination of the mechanical properties, shear tests were conducted. Mean shear strength values up to 104 MPa could be achieved.     

High-strength zirconium alloy: Zr-4 wt pct Sn-1.6 wt pct Mo

A heat-treatable, ternary alloy of zirconium is described. This alloy is readily rolled at 800°C and has more than four times the creep strength of pure zirconium at 500°C. The tensile strength of the alloy in the annealed condition is about 90,000 psi, and it may be heat-treated to a strength of more than 140,000 psi. In the annealed condition, the alloy can be cold reduced more than 20 pct. The alloy is harder in the air-quenched condition than as water quenched. This behavior was found to be associated with a reaction similar to age hardening.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.