Creep Resistance Of Alloy Research Articles

Design considerations for a high temperature creep resistant nickel base alloy

Life of all high temperature materials is decided by their creep properties. The understanding of factors that control their high temperature properties is important in designing creep resistant alloys. Since dislocation movement is primarily responsible for creep, all microstructural parameters that increase resistance to dislocation motion like, low stacking fault energy, association of jogs and vacancies with dislocation and distribution of second phase particles influence the creep resistance of alloys. In this paper, detailed investigations carried out on the evolution of two phase microstructural parameters in nickel base binary and some commercial high temperature alloys are presented. The role of some of the observed microstructures in enhancing creep resistance will be discussed.

Creep Behaviour of AE42 Magnesium Alloy and its Composites Using Impression Creep Technique

The creep behaviour of a creep-resistant AE42 magnesium alloy has been examined in the temperature range of 150 to 240°C at the stress levels ranging from 40 to 120 MPa using impression creep technique. A normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at all the temperatures and stresses employed. The stress exponent varies from 5.1 to 5.7 and the apparent activation energy varies from 130 to 140 kJ/mol, which suggests the high temperature climb of dislocation controlled by lattice self-diffusion being the dominant creep mechanism in the stress and temperature range employed. The creep behaviour of the AE42 alloy has also been compared with its composites reinforced with Saffil short fibres and SiC particles in four combinations. All the composites exhibited a lower creep rate than the monolithic AE42 alloy tested at the same temperature and stress levels and the decrease in creep rate was greater in the longitudinal direction than in the transverse direction, as expected. All the hybrid composites, i.e., the composites reinforced with a combination of Saffil short fibres and SiC particles, exhibited creep rates comparable to the composite reinforced with 20% Saffil short fibres alone at all the temperature and stress levels employed, which is beneficial from the commercial point of view.

The Effect of Cerium on Microstructures and Mechanical Properties of Mg-4Al-2Sn-1Ca Alloy

The development of new creep resistant magnesium alloys has become a major research focus. This study examined the microstructure and creep properties of Mg-4.0 mass%Al-2.0 mass% Sn-1.0 mass%Ca alloys containing Ce. The results showed that Ce could improve dramatically the tensile strength and ductility of the alloy at room temperature and increase the creep resistance at elevated temperatures significantly. With a trace amount of Ce, the morphology changed from a coarse CaMgSn phase to a refined shape and the microstructure of the alloy was remarkably refined. [doi:10.2320/matertrans.M2010042]

Interrupted creep behaviour of Mg alloys developed for powertrain applications

A conventional magnesium alloy, AZ91D, and two creep resistant magnesium alloys, developed for powertrain applications, MRI 153M and MRI 230D, are prepared by high pressure die casting. These alloys are tested for their creep behaviour in the continuous manner, as is the current practice, and in the interrupted manner, which represents the real life situation more closely. It is observed that the interrupted creep tests give rise to a primary creep appearing at the beginning of each cycle resulting in a higher average strain rate than that encountered in the continuous creep tests. Further, the shorter the cycle time, higher is the average strain rate in the interrupted creep tests. A higher average strain rate will give rise to a higher strain over the same period. This is attributed to the recovery taking place during the cooling and heating between two cycles. The effect of additional precipitation during interrupted creep tests depends on the nature of the precipitates. The additional precipitation of β phase during the cooling and heating between two cycles increases the steady state strain rate in the AZ91D and MRI 153M alloys, whereas the additional precipitation of C36 phase during the cooling and heating between two cycles decreases the steady state strain rate in the MRI 230D alloy.

Alloys-By-Design: Application to nickel-based single crystal superalloys

Design rules are proposed by which the compositions of nickel-based single crystal superalloys can be chosen systematically, using models for the most important characteristics: creep resistance, microstructural stability, castability, density and cost. Application of the rules allows the very large compositional space to be reduced to just a few ideal compositions, which are likely to be close to the optimal ones. The procedures have the potential to remove much of the traditional reliance placed upon empiricism and trial-and-error-based testing. It appears that trade-offs must be accepted, however; for example, the most creep-resistant alloys are more dense, more costly and more inherently susceptible to casting-related defects such as freckles during processing. Compositions suitable for both jet propulsion and land-based applications are proposed, for future experimental testing.

First-principles study of typical precipitates in creep resistant magnesium alloys

First-principles calculations were performed to study the mechanical properties of typical precipitates of creep resistant magnesium alloys (Mg2Si, Mg2Ge, Mg2Sn and Mg3Zn3Y2). Formation enthalpies and cohesive energies, elastic constants C11, C12, C44, bulk modulus B0, Young's modulus E, shear modulus G, the ratios of shear modulus to bulk modulus G/B0, negative Cauchy pressure parameter (C12-C44) and Poisson's ratio ν of four precipitates were discussed. The results show that Mg2Si and Mg2Ge are stiffer than Mg3Zn3Y2. The charge density was further investigated to clarify the electronic causes.

Isothermal sections of the Mg-rich corner in the Mg–Sn–Y ternary system at 300 and 400 °C

The phase equilibria of the Mg-rich corner in the Mg–Sn–Y ternary system at 300 and 400 °C have been determined through equilibrated alloy method by means of X-ray diffraction (XRD), scanning electron microscopy with assistance of energy dispersive spectroscopy of X-ray (SEM–EDS), and thus the isothermal sections of this ternary system have been constructed. The equilibrium phase relations have no change with increase from 300 to 400 °C, and there are both four intermetallic compounds in equilibrium with the α-Mg solid solution at the two temperatures, i.e. Mg 2Sn, MgSnY, Sn 3Y 5 and Mg 24+ x Y 5. It is interesting to find that either Y or Sn can be soluble in the α-Mg solid solution but they are not simultaneously soluble. The solubility of Mg in the compound Sn 3Y 5 increases with the temperature from 300 to 400 °C. According to the phase relations and high-melting-point characteristics of these intermetallics in this ternary system, such as Sn 3Y 5 phase, etc., it is expected that the Mg–Sn–Y-based alloy could be a promising creep resistant alloy by appropriate alloy design and processing.

Microstructure Evolution in the Rolled Mg<SUB>97</SUB>Zn<SUB>1</SUB>Y<SUB>2</SUB> Alloy with Long-Period Stacking Phase upon Annealing

Mg97Zn1Y2 alloy (at%) has been studying nearly ten years as an elevated temperature creep resistant Mg-based alloy with long-period stacking (LPS) phase. The deformation processing had been performed upon the alloy so that a highly non-equilibrium state was looked forward to provide particularly advantageous material property profiles for it. Thus the texture evolutions in Mg97Zn1Y2 alloy processed by rolling and annealing the as-cast alloy at different temperatures, are investigated thoroughly. The microstructures of the alloy had been examined using general optical microscopy (OM), laser optical microscopy (LOM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM); the textures were analyzed by X-ray diffraction (XRD). The result shows that the second recrystallization texture had developed primarily when a preliminary annealing treatment of 500 � C for 2 hours was performed to the rolled alloy and that part of the LPS phase had evolved into another phase with hexagonal superstructure which is an approximant to a quasicrystal and even the quasicrystal.

Morphological evolutions of cast and melt-spun Mg 97Zn 1Y 2 alloys during deformation and heat-treating

Mg 97Zn 1Y 2 alloy has been studied as an elevated temperature creep resistant Mg-based alloy for nearly ten years. While, the strength of the cast Mg 97Zn 1Y 2 alloy with long-period stacking(LPS) structure is lower than that of the commercial AZ91 alloy at room temperature. The microstructure evolutions in Mg 97Zn 1Y 2 (molar fraction, %) alloys with LPS phase, processed by rolling and annealing the as-cast alloy and rapidly solidifying/melt-spinning and age treating at different temperatures respectively, were investigated by differential thermal analysis(DTA), X-ray diffraction(XRD), and laser optical microscopy(LOM), scanning electron microscopy (SEM), and transmission electron microscopy(TEM). The evolutionary direction of microstructure prescribed by thermodynamics in the Mg 97Zn 1Y 2 alloy is reflected from experimental data of the as-cast alloy; and the actual evolution paths selected by kinetics are depicted in detail in the as-spun alloy and rolled alloy. The strong influences of thermodynamic non-equilibrium mechanism, which entails the factual complexity of microstructures typically during rapid solidification and deformation processing for strengthening the creep resistant magnesium alloy, are presented.

Interfacial Dislocation Networks and Creep in Directional Coarsened Ru-Containing Nickel-Base Single-Crystal Superalloys

Mechanisms of creep deformation in nickel-base superalloy single crystals in the directional coarsening regime have been studied in alloys with large variations in γ-γ′ lattice misfit and phase composition, achieved by Ru additions and variable levels of Cr and Co. Interfacial dislocation spacings established by long-term annealing experiments under no externally applied stress indicate that the experimental alloys have high-temperature lattice misfits ranging from near-zero to as large as −0.65 pct. Variation in misfit influences the stress-induced directional coarsening (rafting) behavior during creep deformation at 950 °C and 290 MPa. In postcreep deformed material, the density of excess dislocations (defined as the dislocations beyond those necessary to relieve the lattice misfit) at the γ-γ′ interfaces varied with alloy composition, with the most creep-resistant alloy containing the highest excess interfacial dislocation density. In the directional coarsening creep regime, continued deformation requires shearing of the γ′ rafts and is strongly influenced by the resistance of the precipitates to shearing as well as the interfacial dislocation structure. A preliminary model for creep in the rafting regime is developed.

Magnesium Powder Metallurgy: Process and Materials Opportunities

The major efforts in magnesium alloy development for automotive applications have concentrated on creep resistant alloys produced by permanent mould and high-pressure die casting routes. While large components, such as crankcases, will never be produced by powder metallurgy, there are smaller components in and around the powertrain which could be fabricated from powder precursors. This article will explore the potential of some of the more recent powder compaction developments, and discuss the alloy development strategies that emerge for magnesium-based components as a consequence of these process developments. In particular, the viability of direct powder extrusion of semi-finished product, using conventional extrusion or equal channel angular processing, combined with T6 heat treatments, will be considered.

Effect of laser surface treatment on corrosion and wear resistance of ACM720 Mg alloy

A creep resistant Mg alloy ACM720 was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. This treatment was found to be beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarization resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarization resistance values of laser surface treated alloy were twice as high as that for the untreated alloy. The improved corrosion resistance was attributed to the absence of the second phase Al 2Ca at the grain boundary, microstructural refinement and extended solid solubility, particularly of Al, in α-Mg matrix owing to rapid solidification. The laser treatment also increased surface hardness two times and reduced the wear rate considerably due to grain refinement and solid solution strengthening.

Modelling precipitate distribution in reduced-activation steels

The microstructure and carbide distribution in two commercial creep resistant, low-activation, martensitic alloys (F82H and JLF-1) have been modelled using a thermo-kinetic calculation package. The microstructures after manufacturing and after long unstrained thermal aging treatments (13 500 h) at various temperatures (250, 400 and 550 °C), considered to be representative of service temperatures, have been considered. In all cases, the calculated carbide size distributions match in order of magnitude experimental measurements, although the shape of the distribution differs. The obtained results are more accurate for alloy F82H than for alloy JLF-1. Nevertheless, this work shows that it is possible to obtain realistic estimates of the microstructure evolution of creep resistant alloys in long thermal ageing treatments at service-like temperatures.

Effect of Microstructural Inhomogeneity on Creep Response of Mg-Sn Alloys

The development of new creep resistant magnesium alloys has become a major issue in recent years. The alloys investigated in the present work are based on the binary system Mg-Sn. Sn as major alloying element was chosen due to its high solid solubility over a wide temperature range and due to the possible formation of Mg2Sn intermetallic precipitates with a high melting temperature of about 770°C. These characteristics suggest that a fairly large volume fraction of thermally stable Mg2Sn particles can be formed during solidification. This makes it possible that the Mg-Sn alloys can be developed as creep resistant magnesium alloys. In fact, previous investigations indicate that the Mg-Sn alloys have a comparable or even better creep property than AE42 alloy. The present work investigates the microstructure of Mg-Sn alloys with and without creep deformation using SEM and TEM technique. The effects of microstructural inhomogeneity on the creep response are presented. Based on the microstructural analysis, the mechanism responsible for improving the creep resistance will be discussed. It is shown that the grain boundary sliding is a dominant creep mechanism for the Mg-Sn binary alloy.

Thermal analysis and solidification pathways of Mg–Al–Ca system alloys

Mg–Al–Ca alloys are creep resistant magnesium alloys with high application potentials. The solidification pathways and microstructure formation in this alloy system are still under discussion. In this paper, the solidification behavior of AZ91 and AM50 with Ca addition (AZC91 x and AMC50 x alloys) was investigated by a computer-aided cooling curve analysis (CA-CCA) system. Microstructure and phase identification were carried out by SEM and EDX analysis. The results show that the Ca-containing phase formation mainly depends on Ca content and Ca/Al ratio. With increasing the Ca/Al ratio these phases transform from Al 2Ca to (Mg, Al) 2Ca and Mg 2Ca. Moreover, Ca addition decreases the liquidus temperature of Mg–Al alloys, but influences the solidus temperature in a more complex way. Increasing the Ca content also decreases the solid fraction at which dendrite coherency occurs. The relationship between solidification interval, dendrite coherency point, formation of Ca-containing phases and hot tearing is also discussed.

Development of a Magnesium Recycling Alloy Based on AM50

Magnesium applications for structural components in the automotive industry are constantly rising. This is based on the recent development of new alloys, new fabrication processes, and the ambition of car manufacturers to reduce the vehicles weight and CO2 emissions according to the EU and US policy [1, 2]. A rising quantity of magnesium per vehicle leads to a rising quantity of scrap which needs to be recycled according to the European Directive on end-of life vehicles [3]. So far post consumer scrap has not been used for structural parts. But since the metal is still expensive compared to aluminium or steel, and remelting saves more than 90 % of the energy for primary production, magnesium recycling will significantly contribute to cost savings. In comparison to steel or aluminium a recycling cycle for magnesium has not yet been established. Concerning post consumer scrap it is likely that many vehicles will end up in the shredder fraction or at least will be mixed up instead of being dismantled and separated according to their alloy. Thus it is reasonable to define secondary alloys which allow the use of post consumer scrap for structural applications. Creep resistant alloys have the potential of a broad application concerning the weight of the components and therefore a secondary alloy would be reasonable. The aim of this work is to examine a row of AM50-based alloys, modified with additions of Sr, Ca, and Si due to the importance of these elements to increase creep resistance and their usage in modern magnesium alloys. The corrosion properties as well as the mechanical properties and microstructures are investigated in the as-cast and annealed condition. Salt spray tests (using 5 % NaCl) and electrochemical corrosion methods are applied to investigate the corrosion properties which are then compared to the unmodified AM50. Tensile and compression tests at temperatures ranging from 20 °C to 200 °C are applied to investigate the mechanical properties.

Effect of Laser Surface Treatment on Microstructure and Properties of MRI 230D Mg Alloy

A creep resistant Mg alloy MRI 230D was subjected to laser surface treatment using Nd:YAG laser equipped with a fiber optics beam delivery system in argon atmosphere. The laser surface treatment produced a fine dendritic microstructure and this treatment was beneficial for the corrosion and wear resistance of the alloy. Long-term linear polarisation resistance and Electrochemical Impedance Spectroscopy measurements confirmed that the polarisation resistance values of laser treated material were twice as high as that for the untreated material. This improved behaviour was due to the finer and more homogenous microstructure of the laser treated surface. The laser treatment also increased surface hardness two times and reduced the wear rate by 25% due to grain refinement and solid solution strengthening.

Intermetallics in Magnesium Alloys

AbstractIntermetallic phases can be found in almost every magnesium alloy. These intermetallic compounds play a very important role in optimizing the microstructure and mechanical properties. The present paper reviews the effects of intermetallics in magnesium alloys mainly based on their stabilities: dissolvable intermetallics at low temperatures and thermal stable intermetallics at elevated temperatures. The effects of intermetallics are discussed in the age hardenable and creep resistant magnesium alloys, separately. Finally, the further investigations are remarked on the intermetallics, including their precipitation processes, crystal structures and crystallographic orientation relations with magnesium matrix. The aim is to supply useful information in developing new wrought and creep‐resistant magnesium alloys which will be used in the powertrain at elevated temperatures.

Hot Cracking Susceptibility of Ternary Mg-Al-Ca Alloys

Hot cracking of ternary Mg-Al-Ca alloys in permanent mold casting was studied. The alloys are the base of some potential creep-resistant Mg alloys. The Mg-xAl-yCa alloys included Mg-4Al-1Ca, Mg-4Al-2Ca, Mg-4Al-3Ca, Mg-4Al-4Ca, Mg-5Al-3Ca, and Mg-6Al-3Ca. Constrained-rod casting was conducted with a steel mold. Rods were cast with their ends enlarged to act as anchors, which kept the rods from free contraction and thus induced tension in the rods to cause cracking during solidification. The susceptibility to hot cracking was evaluated based on both the widths and locations of cracks in the resultant castings. Both binary Mg-4Al and commercial alloy AZ91E, which is known to have a low susceptibility to hot cracking, were also included for comparison. It was found that within the composition range studied, the crack susceptibility decreased significantly with increasing Ca content (y) but did not change much with the Al content (x).

Formation of hot cracks in the weld zone in welding creep-resisting nickel alloys

Formation of hot cracks in the weld zone in welding creep-resisting nickel alloys