Creep Resistance Of Alloy Research Articles

Effect of friction welding parameters and heat treatment on the quality of welded joints in creep-resisting deformable nickel alloys

Methods of friction welding creep-resisting deformable nickel alloys and heat treatment are investigated. Welding conditions are selected. The results of mechanical tests and the determination of short- and long-term strength and impact bending resistance are presented, and the data obtained in metallographic and fractographic studies of the welded joints are described.

Effect of strontium on the texture and mechanical properties of extruded Mg–1%Mn alloys

Abstract Magnesium–manganese, M1, alloy is preferred for extrusion applications due to its extrudability. It is mainly used as a sacrificial anode or as a creep resistant alloy at elevated temperatures in the nuclear industry. Since Mn does not provide a significant strengthening effect, the alloy is not considered for structural applications. The basal texture which forms after extrusion orients the basal planes parallel to the extrusion direction causing anisotropy in mechanical properties. This basal texture, as well as the low strength of the alloy are the main challenges in its widespread applications. In this study, the effect of Sr addition on the texture and mechanical properties of M1 alloy was studied. M1–Sr alloys showed weakened texture by developing random texture components during extrusion. The texture randomisation is attributed to particle stimulated nucleation (PSN) around Mg–Sr intermetallics during recrystallisation. M1–Sr compositions are found to show improved strength and ductility as well as reduced yield asymmetry.

The Corrosion Resistance of the Mg-Al-Ca-Sr Sand Casting Magnesium Alloys

In this paper, the corrosion resistance of two sand-casting creep resistant magnesium alloys Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr in the salt environment has been investigated. Specimens of each alloy has been immersed in 3.5% NaCl solution at room temperature and successively taken out after 1, 2, 4, 5 and 9 days. After immersion test, the microstructure and the appearances of the corroded structure were examined. The corrosion rates of both investigated alloys increased lineally with increasing the exposure time in both solutions. Mg-9Al-1.5Ca-0.3Sr alloy exhibits the higher corrosion rate during the immersion test than Mg-9Al-2.2Ca-0.8Sr. The corrosion layer of both alloys consists of MgO, MgOH and phases containing Cl, Na, Al and Ca. The increase of Ca content in the Mg-9Al-2.2Ca-0.8Sr alloy improved the corrosion resistance due to the formation of the reticular (Mg,Al)2Ca phase, which acted as an effective barrier against corrosion.

Optimal precipitate shapes in nickel-base γ–γ′ alloys

Precipitate shapes in nickel-base superalloys vary substantially with alloy composition, partitioning of elements to the disordered gamma matrix and the ordered gamma prime precipitates and the degree of coarsening during elevated temperature exposures. These shapes, which vary from spherical to cuboidal to rod-like, have typically not been quantified in Ni-base alloys containing relatively high γ′ volume fractions, in spite of their importance to mechanical properties. Precipitate shapes in a series of new platinum group metal (PGM)-containing Ni-base alloys have been quantified by their two-dimensional moment invariants. Precipitate morphologies were characterized in a total of 17 PGM-containing alloys in the solution treated and aged condition. The average γ′ volume fraction was measured as 0.60, typical of highly creep resistant Ni-base alloys. PGM additions resulted in an unusually large range of precipitate shapes. Precipitate morphologies were quantitatively analyzed using a shape parameter derived from the absolute moment invariant. For the compositions examined, the shape parameter reaches a maximum value at a precipitate–matrix misfit magnitude of 0.4%. A large set of commercial single crystals exhibits the same range in shape parameter values around this misfit magnitude, suggesting that favorable high temperature properties are correlated with an optimum precipitate shape.

Effect of cold rolling on hydrogen sorption properties of die-cast and as-cast magnesium alloys

The effect of cold rolling, as an activation process of materials aimed to hydrogen storage application, was applied to some cast magnesium alloys. The alloys studied in this work are AZ91D and three experimental creep resistant magnesium alloys (MRI153, AXJ530, ZAEX10430) in the as-cast and die-cast states. Experiments showed that AZ91D, MRI153 and ZAEX10430 have faster absorption/desorption kinetic than as-cast pure magnesium. On the other hand, AXJ530 showed the worst hydrogen storage properties. These results were attributed to a possible beneficial effect of aluminum and zinc as alloying elements. Segregation inside primary α-Mg grains may explain the difference of hydrogen storage properties between as-cast and die-cast alloys.

Improving Corrosion Resistance of MRI 230D Mg Alloy by Hybrid Coating of Laser Surface Alloying and Plasma Electrolytic Oxidation

A permanent mould cast creep resistant MRI 230D Mg alloy was laser surface alloyed (LSA) with Al and Al2O3 in order to improve its wear and corrosion resistance. However, this treatment was successful only in improving wear resistance but not corrosion resistance due to the presence of micro−cracks in the coated layer, which has been discussed in an earlier paper. The LSA coated Mg alloy has been further subjected to plasma electrolytic oxidation (PEO) treatment in alkaline silicate electrolyte in order to cover those micro−cracks and improve corrosion resistance, which is discussed in the present manuscript. For comparison, the PEO coating has also been applied on the as−cast MRI 230D Mg alloy. The microstructural characterization of coatings and corroded surfaces was carried out by scanning electron microscope and X−ray diffraction. Electrochemical corrosion tests were conducted in 3.5 wt% NaCl solution having neutral pH to investigate the corrosion behavior. The LSA coatings consisted mainly of β (Mg17Al12) phase, the coatings produced by PEO treatment on MRI 230D Mg alloy consisted mainly of Mg2SiO4 phase, and hybrid coatings of PEO on LSA consisted of Mg2SiO4 and MgAl2O4 phases in the PEO layer. Scanning electron micrographs of the cross−section revealed that the PEO treatment covered the micro−cracks present in the LSA and corrosion tests revealed that it improved the corrosion resistance, though not to the extent of the corrosion resistance of the PEO coated MRI 230D Mg alloy. All the samples exhibited localized form of corrosion.

Mg-Zn-(Y, Gd)鋳物のミクロ構造形成

Commercial magnesium alloys have a great potential for structural applications in automotive due to their significant weight saving. However, they have poor creep resistance at temperature over 125℃, thus making them inadequate for power train applications such as engine pistons, which are operated at temperature up to 300℃. Recently, creep resistant magnesium alloys with rare-earth elements and zinc have been developed and it is suggested that Mg-2Zn-10Y-5Gd-0.5Zr alloy is promising as a candidate material for the engine piston application. However, the detail strengthening mechanism is not clear, hence the detail microstructure of Mg-Zn-(Y, Gd) alloys was investigated in this paper. Gravity casting was performed with Mg-Zn-(Y, Gd)-Zr alloy, followed by T6 treatment. In the as cast condition, both Mg12(Y, Gd)Zn and Mg24(Y, Gd)5 were observed at the α-Mg grain boundaries, while metastable Mg24(Y, Gd)5 was dissolved into α-Mg matrix and surplus (Y, Gd) and Zn were precipitated on the Mg12(Y, Gd)Zn via solution heat treatment at 535℃. After the aging treatment for 24 hours at 225℃, fine β′ precipitates were formed in α-Mg matrix.

Advances in brazing of creep-resisting alloys

The problems of brazing of creep-resisting nickel and intermetallic alloys based on titanium aluminide are investigated. Advantages of using adhesion-active brazing alloys based on Ni–Cr–Zr, Pb–Ni–(X), Ti–Zr–Fe, and Ti–Zr–Mg systems for brazing are described.

Special features of friction welding joints in creep-resisting nickel alloys VKNA-25 and EP975

The technology of friction welding VKNA-25 and EP975 alloys and subsequent heat treatment (prior to and after welding) is discussed. The welding conditions are selected, and the effect of heat treatment on the quality of welded joints is investigated. The welded joints are tested by mechanical tests, metallographic, and fractographic investigations. The efficiency of the welding and heat treatment conditions is determined.

Microstructure and Creep Properties of Casting Mg-8Al-0.3Mn Alloy Containing Ca and Sr

Microstructures and creep properties of AM80 alloy with calcium and strontium additions have been investigated by using OM, X-ray diffraction, SEM and creep tests. The results indicate that the as-cast microstructure of the AM80 alloy consists of the α-Mg matrix, bones-shaped Mg17Al12 and lamellar second precipitation phase at grain boundaries. Calcium and strontium can refine the grain size and the secondary phases. Calcium addition results in the formation of a fishbone Al2Ca eutectic phase in AM80 alloy. With the increase of calcium, reticular Al2Ca phase distribute at the grain boundaries. The creep resistance of the AM80 alloy is significantly improved by a small amount of strontium and calcium addition due to the formation of a grain boundary network consisting of the high melting point Al2Ca phase. Microstructure observations performed on the sample after creep testing reveal that the phase is distorted during creep, reflecting its formation in the as-cast microstructure is unbeneficial to creep properties of the AM80 alloy. The creep resistance of the alloy at elevated temperatures was remarkably increased when calcium was added combined with strontium. The highest creep resistance was obtained from the alloy with xSr and y3Ca addition and its steady state creep rate reached as low as 3.941×10-8s-1, one order of magnitude lower than that of alloy AM80 without strontium and calcium additions.

Influence of HIP processing on microstructure and mechanical properties of superalloy Udimet 720LI

Inert gas (high purity argon) atomised powder of composition conforming to that of the superalloy Udimet 720 of low interstitial grade was hipped at 1200°C/120 MPa/3 h. The hipped alloy has shown near theoretical density and consisted of equiaxed grains with an average diameter of ∼45 μm. While primary γ′-Ni3 (Ti, Al) precipitates with an acicular morphology were found at the grain boundaries, finer secondary γ′ precipitates with near cuboidal morphology were present in the austenite γ matrix. The yield strength (YS) of the as hipped alloy was found to be the same as that of the wrought alloy heat treated for creep applications (termed as creep resistant alloy) at room temperature (RT) as well as at 650°C. However, the ultimate tensile strength (UTS) and ductility were found to be higher than those of the wrought creep resistant alloy. On the other hand, the YS and UTS of the as hipped material were lower than those of the wrought alloy heat treated for high strength applications (termed as high strength alloy), although the ductility of the former was comparable to that of the latter. In order to improve the strength, the hipped alloy was subjected to a heat treatment consisting of solution treatment followed by two-step aging. Extensive precipitation of fine and coarse γ′ precipitates with cuboidal morphology during duplex aging treatment has led to a considerable improvement in YS and UTS of the alloy, although the corresponding ductility dropped moderately at RT and at 650°C. Fractography of the tensile tested specimens has shown ductile transgranular mode of fracture in the as hipped alloy at RT and at 650°C, while the hipped+heat treated alloy exhibited a mixed mode of fracture at those temperature. The stress rupture properties of the as hipped alloy compare well with those of the wrought alloy and have been found to improve significantly after heat treatment. The present investigation reveals that the hipped superalloy Udimet 720LI has substantial potential for use in the development of near net shaped components for aerospace applications.

Stress Corrosion Cracking of Magnesium Alloys

AbstractAn overview is provided of Mg SCC. Intergranular stress corrosion cracking (IGSCC) is typically caused by a continuous second phase along grain boundaries. The second phase causes microgalvanic corrosion of the adjacent Mg matrix. IGSCC is expected for all creep resistant Mg alloys in contact with water whenever creep resistance is produced by a continuous second phase along the grain boundaries. IGSCC can be avoided by appropriate Mg alloy design. Transganular SCC (TGSCC) is caused by an interaction of hydrogen (H) with the microstructure so a detailed study of H‐trap interactions is needed in order to design alloys resistant to TGSCC. This understanding is urgently needed because Mg alloys are being increasingly used in load bearing applications; many common Mg alloys have a threshold stress for SCC of half the yield stress in common environments including high‐purity water.

Nickel and titanium-based high-temperature creep resistant alloy soldering

Nickel and titanium-based high-temperature creep resistant alloy soldering

Dry sliding wear behavior of Saffil fiber-reinforced Mg-10Gd-3Y-0.5Zr magnesium alloy-based composites

Dry sliding wear behavior of the creep-resistant magnesium alloy Mg-10Gd-3Y-0.5Zr and its composites have been investigated in this study. Magnesium matrix composites are prepared by squeezing casting infiltration of Mg alloy into Saffil preforms. Wear tests are conducted using ball-on-flat sliding wear set up under a sliding velocity range of 1-15 cm/s and at an applied load range of 1-8 N for a constant sliding distance of 150 m. According to results, mechanical and wear-resistance properties of magnesium alloy improved by introducing Saffil fibers, and the alumina binder composite has a higher strength and lower wear rate than silica binder composite. The wear rates of the matrix alloy, composites and their counter-face balls increase with increasing applied load. The increment of sliding velocities decreases the wear rate of the matrix alloy under the tested sliding velocities. A critical threshold of sliding velocity for the wear rate of both composites and their counter-faces is about 9 cm/s. Abrasion and plastic deformation are considered to be the dominant mechanism for the matrix alloy in tested conditions, and for both composites under low sliding velocity (<10 cm/s) and at low applied loads (1-5 N). Delamination is the wear mechanism of the silica binder composites at a high applied load (8 N). Adhesion and oxidation are the controlling wear mechanism of matrix alloy and composites under a sliding velocity of 15 cm/s.

Structure and Composition of Nanometer-Sized Nitrides in a Creep-Resistant Cast Austenitic Alloy

The microstructure of a new and improved high-temperature creep-resistant cast austenitic alloy, CF8C-Plus, was characterized after creep-rupture testing at 1023 K (750 °C) and 100 MPa. Microstructures were investigated by detailed scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). Principal component analysis of EDS spectrum images was used to examine the complex precipitate morphology. Thermodynamic modeling was performed to predict equilibrium phases in this alloy as well as the compositions of these phases at relevant temperatures. The improved high-temperature creep strength of CF8C-Plus over its predecessor CF8C is suggested to be due to the modified microstructure and phase stability in the alloy, including the absence of δ-ferrite in the as-cast condition and the development of a stable, slow-growing precipitation hardening nitride phase—the tetragonal Z-phase—which has not been observed before in cast austenitic stainless steels.

The effect of precipitate state on the creep resistance of Mg–Sn alloys

The Mg-Sn system has recently received attention as a potential basis for the development of creep resistant alloys. This is an age hardenable system with the potential to form ~10 vol. % of Mg2Sn. The Mg2Sn precipitate has a high melting temperature (~770 °C) and alloys based on this system might be thought to show some promise for use in applications requiring elevated temperature creep resistance [1]. The age hardening response of the binary alloy is only modest, despite the large volume fraction of precipitates that may be formed [2]. The reason is that the precipitates form with a relatively coarse distribution, in laths parallel to the basal planes of the magnesium matrix. This configuration is relatively inefficient at impeding dislocation motion [3].

Creep resistance in magnesium alloys

<title/>Mg creep resistant alloys have seen increased interest in the 1980s and 90s due to the weight reduction objectives of automotive companies. Development of Mg-Al based alloys with rare earth (RE) and alkaline earth element additions has led to the automotive application of Mg-6Al-2Sr (AJ62) alloy in the BMW engine block and the Mg-4Al-4RE (AE44) alloy in the engine cradle of the Corvette in 2002-2004. Most creep resistant Mg alloy development activities during this period emphasised the creation of stable grain boundary intermetallic phases in the cast microstructure. This elevated the creep performance of automotive Mg alloys to higher temperature and stress combinations (175°C, 70 MPa). Further improvement in creep performance can only arise from an in depth understanding of the creep mechanisms and the related microstructural interactions in Mg alloy systems. This paper gives an in depth review of creep mechanisms in Mg alloys and provides insight into alloy design principles for further development of creep performance in Mg.

Influence of microstructure on creep strength of MRI 230D Mg alloy

The low density of magnesium alloys makes them attractive for lightweight constructions. However, creep remains an important limitation of Mg alloys. To gain a more detailed understanding of the correlation between microstructure and creep properties in Mg alloys, creep tests have been performed on MRI 230D samples featuring various microstructures. For this purpose, the MRI 230D Mg alloy has been thixomolded into a plate with four steps of different height, which gives different microstructures in each step due to different cooling rates. With an increase in cooling rate (e.g., a decrease in step height) the interconnectivity of the eutectic phase increases at virtually constant volume fraction. The creep strength is found to decrease with decreasing interconnectivity of the eutectic phase. This implies that a eutectic phase morphology, which is highly interconnected, benefits the creep properties and should therefore be one goal in further developments for creep resistant Mg alloys.

Effect of Minor Additions of Al and Si on the Mechanical Properties of Cast Mg-3Sn-2Ca Alloys in Low Temperature Range

The Mg-Sn-Ca alloys have shown superior creep properties compared to the creep resistant alloy AE42. In the present study, the effects of small amounts of Al and Si additions on the mechanical properties have been investigated on a Mg-3Sn-2Ca (TX32) alloy. The Al content in the selected alloys was 0.4 wt% and the Si content was varied from 0–0.8 wt% in steps of 0.2 wt.%. The alloys were cast in pre-heated permanent molds. Cylindrical specimens machined from the cast billets were tested in compression in the temperature range 25–250 °C at a strain rate of 0.0001 s–1. The alloy with 0.4 wt% Al shows an increased strength at all test temperatures compared with the TX32 base alloy. This is attributed to a solid solution strengthening of Al in Mg. The alloy with 0.4 wt% Al and 0.2 wt% Si has compressive strength that is closer to that of the TX32 alloy. However, increased additions of Si (from 0.4–0.8 wt%) reduce the strength, more significantly at higher temperatures.

Brazing alloys based on the Ni–Cr–Zr system for brazing creep-resisting nickel alloys

Brazing alloys based on the Ni–Cr–Zr system for brazing creep-resisting nickel alloys