Improved Creep Properties Research Articles

Effect of thermal ageing on tensile and creep properties of JLF-1 and CLAM steels

The thermal ageing treatments of JLF-1 and CLAM steels were performed at 823 K up to 2000 h to simulate in-service condition and at 973 K for 100 h to provide heavy overageing. Hardness, tensile and creep rupture tests were conducted in the aged and un-aged materials. The results showed increase in hardness at room temperature and improvement in creep properties after ageing at 823 K/2000 h for the both steels, implying the strengthening. However, softening and degradation of creep properties occurred after ageing at 973 K/100 h. Lower normalization and tempering temperature of CLAM steel were suggested to be responsible for higher hardness and tensile strength, lower minimum creep rate and longer rupture time, and also for higher susceptibility to the thermal ageing of relatively short term than those of JLF-1.

Creep deformation mechanism of magnesium-based alloys

Two heat-resistant magnesium alloys AJC421 and Mg-2Nd were prepared. Both as-cast Mg-2Nd and AJC421 alloys exhibited good creep resistance in comparison with commonly used magnesium alloys. The improvement in creep properties through Nd addition to pure magnesium is attributed to both solid solution and precipitation hardening. The stress exponents of 4.5–5.5 and activation energies of 70.0–96.0 kJ/mol obtained from the as-cast Mg-2Nd alloy at low temperatures and low stresses indicate the five power law can be used for predicting the creep mechanism. The additions of alkaline earth elements Sr and Ca into Mg–Al alloys suppress the discontinuous precipitation of Mg17Al12 and form thermal-stable intermediate phases at grain boundaries, leading to effective restriction to grain boundary sliding and migration. However, the mechanism responsible for creep deformation of Mg–Al based alloys with Ca and Sr additions is not consistent with the results of microstructure observations performed on the alloys before and after creep tests.

Effect of combined addition of Si and Sb on the microstructure and creep properties of AZ91 magnesium alloy

Addition of silicon (0.5 wt.%) together with small amount of antimony (0.2 wt.%) to AZ91 alloy has been carried out to study its effect on the microstructure and creep properties at 150 and 200 °C. The results show that the combined addition (Si and Sb) introduces fine polygonal shaped Mg 2Si precipitates at the grain boundaries and in the grains. Besides, the addition suppresses the precipitation of discontinuous Mg 17Al 12 phase during ageing treatment. These lead to an improvement in creep behavior of AZ91 at 150 and 200 °C temperatures. Moreover, extended steady state creep, reduced minimum creep rate and total creep strain are also observed with the combined addition. Presence of fine and stable Mg 2Si precipitates and suppression of discontinuous precipitation are the possible reasons for the improvement in creep properties.

Ruthenium-containing bond coats for thermal barrier coating systems

Bond coats for zirconia-based thermal barrier coating systems applied to nickel-based superalloys are typically composed of the B2 NiAl phase. Since RuAl has the same B2 crystal structure but a melting point 400°C higher than NiAl, ruthenium-modified aluminide bond coats could provide improved system temperature capability. Creep experiments on ternary Al−Ni−Ru alloys demonstrate greatly improved creep properties with increasing ruthenium content. Processing paths for ruthenium-modified NiAl-based bond coatings have been established within the bounds of commercially available coating systems. The oxidation resistance of ruthenium-modified bond coats during thermal cycling has been examined, and potential thermal barrier coating system implications are discussed.

Development of creep resistant die cast Mg–Sn–Al–Si alloy

A study has been made on the tensile properties and high temperature properties of die cast Mg–Sn–Al–Si (TAS831) alloy. The microstructure of TAS831 alloy is characterized by the presence of thermally stable Mg 2Sn particles within matrix and along grain boundaries. It also contains a small volume fraction of thermally stable Mg 2Si particles. It has been shown that TAS831 alloy has better combinations of tensile properties at room and elevated temperatures than die cast AZ91 alloy. Creep properties of TAS831 alloy are also superior to those of AZ91 alloy. Analyses of creep behavior and load-relaxation behavior at elevated temperatures in the context of internal variable theory indicate that the presence of thermally stable dispersoids in TAS831 alloy increases the resistance to dislocation movement, thereby improving creep properties over those of AZ91 alloy.

Formation of carbides and their effects on stress rupture of a Ni-base single crystal superalloy

Creep tests of a nickel–base single crystal superalloy with minor C addition and non-carbon were carried out at different temperatures and stresses. Correlations between microstructural change and testing temperature and stress were enabled through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the rafting microstucture and carbides precipitation. The results showed that minor carbon addition prolonged the second stage of creep strain curves and improved creep properties. Some carbide was precipitated during creep tests in modified alloy. M 23C 6 carbide precipitated at lower temperature (871–982 °C), while (M 6C) 2 carbide precipitated at higher temperature (>1000 °C), all of which was considered to be beneficial to creep properties. A small amount of MC carbide formed during solidification and its decomposition product (M 6C) 1 were detrimental to mechanical properties, which together with micropores provided the site of initiation of cracks and led to the final fracture.

Effect of lamellar structure on microhardness and yield stress of directionally solidified intermetallic Ti-46Al-2W-0.5Si alloy

In recent years ordered TiAl based intermetallic alloys have been extensively studied as potential hightemperature materials in the gas turbine industry [1]. Of the numerous microstructures that can be developed in TiAl-based alloys, the fully lamellar or nearly lamellar microstructures consisting of TiAl (γ -phase and Ti3Al (α2-phase) are very promising for structural applications. Mechanical properties of fully lamellar polycrystalline TiAl alloys depend on the grain size, lamellar thickness, interlamellar spacing and volume fraction of coexisting α2and γ -lamellae [2]. In the case of directionally solidified (DS) TiAl alloys, mechanical properties such as yield stress, tensile elongation, fracture toughness and creep behavior were found to depend strongly on lamellar orientation to loading axis and interlamellar spacing [3–5]. A good combination of room temperature strength, ductility, toughness and creep strength can be achieved when the lamellar orientation is aligned parallel to the tensile direction. The aim of the present work is to study the effect of lamellar structure on room-temperature yield stress and microhardness of directionally solidified Ti46Al-2W-0.5Si (at.%) alloy. The alloy was developed by Nazmy and Staubli [6] for investment cast turbine blades with improved creep properties. Unlike previous studies on mechanical properties of DS TiAl based alloys, this study is focused on fully lamellar alloy reinforced with a constant volume fraction of hard Al2O3 particles. The alloy with the chemical composition Ti-46Al2W-0.5Si (at.%) was provided by Alstom Ltd in the form of cast cylindrical ingots with a diameter of 21 mm and length of 210 mm. Directional solidification was performed in Al2O3 molds (mean grain size of 9.3 μm and purity of 99.5%) of 11/15 mm diameter (inside/outside diameter) under argon atmosphere in a modified Bridgman-type apparatus [7]. All ingots were prepared at a constant growth rate of 1.18 × 10−4 ms−1 using experimental procedure described elsewhere [7, 8]. After directional solidification the ingots were heated to solution annealing temperature of 1633 K at constant heating rate of 0.28 K s−1, stabilized at this temperature for 1 h and then cooled to 973 K at constant cooling rates varying in the range from 0.078 to 0.61 K s−1. Further cooling to room temperature was accomplished by gas fan cooling. All annealing experiments were performed in dynamic argon atmosphere. After heat treatment Vickers microhardness measurements with applied load of 0.64 N were performed on longitudinal sections of specimens. Rectangular compression specimens with dimensions of 4 × 4 × 8 mm were cut from the ingots by electric discharge machining. Statistical evaluation of all compression specimens (8–10 specimens tested for each regime) revealed that the mean angle between lamellar orientation and the compression axis varied in the range 8 ◦ to 15 ◦ and 56 ◦ to 67 ◦ for the specimens with the compression axis perpendicular and parallel to the growth direction, respectively. Roomtemperature compression tests were performed at an initial strain rate of 1.39 × 10−3 s−1 in air using a screw driven universal testing machine Instron. The compression offset yield stress was measured at 0.2% plastic strain. Microstructural analysis was performed by optical microscopy (OM) and scanning electron microscopy (SEM). Quantitative metallographic analysis was performed on digitalized micrographs using a computerized image analyser. The microstructure of DS ingots consisted of columnar grains (3–5) aligned parallel to the growth direction. The microstructure within the columnar grains was dendritic, as shown in Fig. 1. The as-grown ingots contained regular well-aligned α2 (Ti3Al) and γ (TiAl) lamellae. A small volume fraction (about 1 vol%) of elongated B2-particles (ordered β-phase) was formed at the columnar grain boundaries and in the interdendritic region. Fine Ti5Si3-precipitates were observed at the lamellar interfaces and along the grain boundaries [9]. In addition, the as-grown ingots contained constant volume fraction of Al2O3 particles (1.5 vol%), which were formed in-situ due to a reaction between the alumina mold and the melt [8]. The ingots contained tree types of Al2O3 particles (dark color phase in Fig. 1): nearly spherical, needle-like and clusters of particles. The size of particles characterized by a mean value of minor axis calculated from statistical log-normal distribution function was 6 μm. Heat treatment of DS ingots consisting of solution annealing and subsequent cooling at constant rates ranging from 0.078 to 0.61 K s−1 resulted in formation of regular lamellar structure. Fig. 2 shows an example of typical regular lamellar microstructure formed in all heat-treated ingots. In addition, as in the case of the as-grown ingots, the heat-treated ingots contained elongated B2-particles, fine Ti5Si3-precipitates and constant volume fraction of Al2O3 particles (1.5 vol%).

Development of New Die-castable Mg-Zn-Al-Ca-RE Alloys for High Temperature Applications

New die-castable Mg–Zn–Al–Ca–RE alloys are developed and evaluated in order to determine their suitability for high temperature applications. The crystallization of an Al–Ca compound along the grain boundaries and Al–RE compounds both within the grains and along the grain boundaries helps to improve creep properties of the alloys. The creep resistance of diecast specimens of some of the investigated alloys is comparable to that of ADC12 aluminum alloy that is currently used for diecasting of automobile powertrain parts.

Creep Properties affected by Morphology of MX in High-Cr Ferritic Steels

MX-type carbonitrides such as (Nb, V)(C, N) are well known to be effective to improve creep properties of the heat resisting steels at elevated temperatures. The control of precipitation behavior of MX is essential to obtain the stable microstructure that is suitable for heat resisting steels in the Ultra Super Critical plants. In this study, strong relationship between creep properties and precipitation behavior of MX is investigated by using the high-Cr steels with different C/N-balance. MX is categorized to three types from their characteristic morphology, type I, II and III. It is found that the C/N-balance of steel is one of factors to determine which MX is dominant in the tempered steels. Presence of type III-MX that is formed by secondary precipitation of VN adhering to Nb(C, N) seems to be unfavorable to long-term creep resistance by considering relatively decreasing in distribution density of fine VN, type II-MX. Precipitation sites such as Nb(C, N), type I-MX and excess solute nitrogen promote growth of type III-MX at the tempering stage. We conclude that the control of precipitation kinetics of MX is quite important to achieve good creep properties at high temperatures.

Effects of Surface Condition on Room Temperature Tensile Properties of Cast TiAl-Fe-V-B Alloy.

Ti-48.0Al-1.3Fe-1.1V-0.30B(mol%) has been developed to improve creep properties.1) In order for this alloy to be widely employed in structural materials, it is important to understand effects of the surface finishing on mechanical properties. The finishing, such as the turning, the grinding, the beltgrinding (defined as the grinding using the endless buff-belt) of this alloy was carried out under the production scale in this study. The effects of machined surface condition on room temperature tensile properties of this alloy were investigated with metallographic characterization. The room temperature ductility was influenced by surface condition and was within the range from about 0.5 % to about 1.3 %. As a result, the improvement of ductility was attributable to removal of grain distortion at subsurface. Moreover, compared to the results of various machined specimens, the effects of chemicalmilled surface on room temperature tensile properties were also investigated.

Dopant Distributions in Rare‐Earth‐Doped Alumina

The distribution of yttrium and lanthanum dopants has been mapped in yttrium‐ and lanthanum‐doped polycrystalline aluminas using imaging secondary‐ion mass spectrometry (imaging‐SIMS). Both dopants segregate to grain boundaries and pore surfaces. On average, yttrium occupies 7.1%–9.0% of the available grain‐boundary cation sites, whereas lanthanum occupies only 2.0%–5.2%. In 1000‐ppm‐yttrium‐doped alumina, an abundance of yttrium aluminum garnet precipitates also is observed. Implications of these observations to the creep behavior of alumina are discussed. The similarity in the segregation behavior of yttrium and lanthanum highlights the potential of lanthanum‐doped alumina for improved creep properties.

The short‐ and long‐term mechanical properties of filled and unfilled thermotropic liquid crystalline polymer injection moldings

AbstractThe basic mechanical properties of a series of injection moldings of the thermotropic liquid crystalline polymer Vectra have been investigated to determine the effect of fillers on the short‐ and long‐term mechanical properties of these materials. Mineral‐filler addition results in a slight decrease in strength and an increase in stiffness in the flow direction, but has relatively little effect on the anisotropy. Glass fiber addition, on the other hand, may substantially lower the anisotropy depending on the degree of fiber orientation transverse to the flow direction. The glass fiber‐filled grade investigated also showed improved creep properties at high temperature compared with both the unfilled and mineral‐filled grades. © 1993 John Wiley & Sons, Inc.

Influence of carbon and nitrogen on the creep properties of type 316 stainless steel at 873 K

This paper is concerned with the creep properties of a nitrogen-alloyed type 316L (316LN) stainless steel (SS) at 873 K in the stress range 215–335 MPa. A comparison has been made with the creep properties of type 316 SS having a similar grain size and chemical composition with respect to the major alloying elements. Type 316LN SS showed a higher rupture life, a lower steady state creep rate and a higher rupture ductility at all stress levels. The improvement in creep properties has been attributed to the combined influence of precipitation strengthening by fine intragranular and intergranular carbides and strengthening arising from the nitrogen in solid solution. Correlations between metallographic observations and the creep properties as well as the role of interstitial elements in these properties are discussed.

Mechanical properties of welded joints of the reduced-activation ferritic steel: 8% Cr-2% W-0.2% V-0.04% Ta-Fe

A reduced-activation ferritic steel, 8Cr-2W-0.2V-0.04Ta-Fe (F-82H) has been developed by JAERI and NKK to improve creep properties and toughness as compared with HT9. The mechanical properties and phase stability of the steel were reported at the previous conferences, ICFRM-2 and 3. This paper is concerned with the mechanical properties of weld metal and welded joints using a newly-developed filler wire of F-82H which contains less C and Ta than the base metal. The design concept of chemical composition of the filler wire was based on as much reduction of activity after irradiation as possible and considerations of the hardenability and toughness of the weld metal. Mechanical properties, such as tensile strength and toughness, of the weld metal and welded joints produced by GTAW after stress-relieving heat treatment were investigated. The results showed that this welding material has almost the same properties as the base metal.

Elevated temperature low-cycle fatigue of the austenitic stainless steel types 316 and 253 Ma: Influence of microstructure and danage mechanisms: Sandstrom, R., Engstrom, J., Nilsson, J.O. and Nordgren, A. High Temp. Technol. Feb. 1989 7, (1), 2–10

This paper is concerned with the creep properties of a nitrogen-alloyed type 316L (316LN) stainless steel (SS) at 873 K in the stress range 215–335 MPa. A comparison has been made with the creep properties of type 316 SS having a similar grain size and chemical composition with respect to the major alloying elements. Type 316LN SS showed a higher rupture life, a lower steady state creep rate and a higher rupture ductility at all stress levels. The improvement in creep properties has been attributed to the combined influence of precipitation strengthening by fine intragranular and intergranular carbides and strengthening arising from the nitrogen in solid solution. Correlations between metallographic observations and the creep properties as well as the role of interstitial elements in these properties are discussed.

An analytical electron microscopy study of the recently reported “Ti 2Al phase” in γ-TiAl alloys

A variety of experimental techniques has been utilized to establish that the phase reported previously to be a new stable Ti 2Al phase (1–3) is instead Ti 2AlN. The space group of Ti 2AlN has been determined to be P6 3/mmc with lattice parameters, a = 0.304 nm and c = 1.369 nm, in agreement with the results of Jeitschko, et al. (13). Also, since this phase is observed only in alloys which do not contain α 2-Ti 3Al, it was suggested that the solubility of N in α 2 must be rather large, while that in the γ(TiAl) phase is low. Finally, in light of the improved creep properties of γ alloys containing Ti 2AlN precipitates (3), it is suggested that an increase in the volume fraction of this phase, by increasing the N and/or C contents, in these alloys might be used to enhance the mechanical properties of this normally brittle compound.

The mechanisms of improved creep strength in a new austenitic stainless steel

An addition of 40 ppm boron, 0.4 pct vanadium and 0.12 pct nitrogen to an austenitic stainless type steel AISI 316L (0.02 C, 18 Cr, 12 Ni, 2.7 Mo) has considerably improved the creep properties. The improved creep properties are due to a combination of the precipitation of fine stable vanadium nitrides on the dislocations and the precipitation of chromium carbides (M23C6) in the grain boundaries. The latter process is thought to be enhanced by the presence of boron and helps to improve the creep ductility. The precipitation of vanadium nitrides on the dislocations retard the creep rate. The nitrides retain their small size even after long creep testing times. A model is proposed to explain this behavior of the precipitated particles and their interactions with the dislocations.

The creep strength of uranium containing small alloying additions

Sagging bar tests have been carried out on Springfields adjusted uranium and uranium containing low alloying additions of molybdenum, vanadium, zirconium, chromium, aluminium and titanium. They were designed as comparative sorting tests for the development of dilute uranium alloys with improved creep strength. The tests have shown no significant difference in the creep strength of adjusted magnesium-reduced uranium which has been subjected to various quenching and annealing treatments. Adjustments of iron and aluminium to different levels also produced no observable difference in creep strength in tests of 2500 h duration. Additions of molybdenum, vanadium, aluminium and titanium enhanced the creep strength whilst additions of zirconium and chromium lowered the creep strength. Additions of titanium and molybdenum to the uranium-chromium alloy resulted in an improvement in creep properties. Values of the creep moduli, deflection rate and maximum strain rate have been derived. The creep strengths have also been compared by a statistical treatment involving the construction of scatter bands. This method is considered to be the most satisfactory for making comparisons.

The influence of heat-treatment on the creep strength of the magnesium-zirconium alloy ZA

The creep strength of the magnesium-zirconium alloy ZA is examined at 400° C and 200 psi after heattreatment at 600° C in different atmospheres. It is found that while improvements in the creep properties are obtained by an homogenisation anneal and grain growth, further significant improvements in creep strength can be obtained by precipitation of the zirconium in the ZA as zirconium hydride. This precipitate can be formed during the pre-anneal at 600° C or during creep testing at 400° C by reaction with hydrogen or moisture in the annealing or testing atmospheres. It is shown that precipitation of zirconium hydride is not by itself sufficient to produce an improvement in creep strength. Grain growth is also a requirement for improved creep properties.

The effect of a tantalum carbide dispersion on the high-temperature properties of chromium

A marked improvement in creep properties is produced in chromium by the incorporation, by alloying methods, of a fine dispersion of tantalum carbide. The dispersion-hardened chromium has a higher recrystallization temperature than pure chromium (1350°C as against 850°C) and shows some ductility at room temperature.