Improved Creep Strength Research Articles

Alloy modification of γ-base titanium aluminide for improved oxidation resistance, creep strength and fracture toughness

An attempt to improve the properties of as-cast γ-base titanium aluminides, such as fracture toughness, creep strength and oxidation resistance, from low to high temperatures was carried out through microstructural control and alloy modification. Addition of a small amount of silicon from 0.5 to 1% to the TiAl alloy greatly improved creep strength over binary alloys. However, it was revealed that an excess of silicon was harmful not only to creep strength but also to fracture toughness. Oxidation resistance was dramatically improved by niobium addition. As for fracture toughness improvement, optimization of aluminum content and resultant microstructural control were also important. The TiAlNbSi quarternary alloy with microstructural features of full α 2/ γ lamellae with serated lamellar grain boundaries offered comparable oxidation resistance and excellent specific creep strength around 1200 K over the nickel-base superalloy and had a high fracture toughness value of 32.5 MPa m 1 2 at room temperature.

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M6C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M23C6 carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M23C6 carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.

Creep Deformation Studies in Directionally Solidified MoSi2-Mo5Si3 Eutectics

The high temperature deformation behavior of directionally solidified (DS) MoSi2-Mo5Si3 eutectics was studied and compared to powder processed MoSi2-Mo5Si3 composites having the same volume fraction of Mo5Si3. Decremental step strain rate tests were performed in the temperature range of 1100-1300°C and at strain rates between 10−4 to 10−6/s. A considerable increase in the flow stress was observed for the directionally solidified material. At 1200°C and a strain rate of 10−6/s the flow stress of the DS eutectic was 255 MPa as compared to 20 MPa for the powder processed composite. The high temperature strength of the DS eutectic was unaffected by changes in the scale of the lamellar micro-structure and these results were modeled using a constitutive relation for power law creep. A stress exponent of 4.5 and an activation energy of 300 kJ/mol was determined for the DS eutectic. Evidence of dislocation glide and climb was observed in the MoSi2 lamellae whereas the dislocation density was small in the Mo5Si3 phase. The improved creep strength of the eutectic is believed to be a result of both the fibrous morphology and a stronger interface structure as compared to the powder processed composite.

低炭素窒素添加改良３１６ステンレス鋼のクリープ特性

Effects of carbon, nitrogen and tungsten on the tensile and creep properties of a composition-controlled modified 316 stainless steel for high-temperature use, have been investigated. This steel has a lower carbon and higher nitrogen content than conventional 316 stainless steel (SUS316). The results obtained are as follows.(1) Modified 316 exhibits high creep ductility, which changes little with increasing rupture time. It also has a generally higher creep strength than SUS316, and this advantage increases with longer rupture time.(2) The improved creep strength in the modified 316 is attributable mainly to a lower creep rate in the tertiary creep range, and this reduced creep rate is thought to be explicable in terms of stable solution hardening by nitrogen and suppressed grain boundary embrittlement resulting from the low carbon content.(3) Addition of tungsten increases the creep strength of the modified 316 with little change in creep ductility. A high tungsten content, however, is observed to enhance Fe2(Mo, W) formation. It is thought that the resultant reduction in Mo and W contents in the grain matrix lowers the creep strength as the rupture life increases.

Age hardening and creep ductility in Cr–Ni–Ti, Fe alloys

Abstract In alloys such as 22Cr–33Ni–Fe (wt-%), titanium can improve creep strength, but may also give excessive hardening and low ductility in some regimes. Experimental casts having various Ti, Mn, Mo, and Si contents were solution treated at 1250°C and age hardening at 550–650°C was studied. Two commercial 800 type alloys were included for comparison. Slow strain rate tensile tests were carried out at 550°C to relate flow stress to creep ductility and age hardening. Age hardening was related to Ni3Ti (γ′) particle size and equilibrium volume fraction, and to titanium solute depletion. The critical effect of free titanium ([Ti] – 3·4[N]) on the factors given above was apparent. Manganese suppressed an early stage of hardening at 600°C, apparently by hindering the clustering on cooling that nucleated a fine γ′ precipitate. Aluminium enhanced the effect of titanium, but this could not be attributed solely to substitution in γ′. Intermediate treatments at 850–900°C improved the inherent creep ductility at ...

Effect of antimony on the creep fracture of stainless steel

Stainless steel samples doped with various contents of Sb were crept under the same conditions. With respect to the base material, a low content of Sb (0.1 wt pct or 0.2 wt pct) is found to improve creep strength and creep ductility of the alloys whereas the addition of 1 wt pct Sb decreases the mechanical properties. The microstructure of the samples was examined quantitatively by scanning electron microscopy. The improvement of mechanical properties is attributed to a reduction of the nucleation rate and of the growth rate of cavities. In order to predict the strain to fracture, a criterion on the fraction of grain boundary area cavitated is proposed.

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni5Zr and ZrO2 inclusions, a Zr4C2S2 carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.

Molybdenum solubility differences in MC phases formed in titanium and niobium modified austenitic stainless steels determined using AEM

Molybdenum is added to improve elevated temperature strength and corrosion resistance for type 316 compared to type 304 stainless steel. Strong carbide forming elements, like titanium and niobium, are also added to these steels to improve creep strength and reduce stress corrosion cracking, as well as to improve resistance to irradiation induced swelling and helium embrittlement. This work shows that fairly pure TiC and NbC form in Ti- and Nb- stabilized versions of type 304 stainless steel (types 321 and 347, respectively); however, the Ti-rich MC dissolves Mo considerably whereas the NbC remains compositionally quite pure when these phases form in Ti- and Nb- modified type 316 stainless steels, respectively.

Improvement of Creep strength in a nickel-base single-crystal superalloy by heat treatment

The effect of two heat treatments on the creep behaviour of CMSX-2, a recently developed nickel-base single-crystal turbine blade alloy, was investigated in the temperature range 760–1050 °C. It is shown that the heat treatments had no effect on the γ′ volume fraction, the composition of precipitates and the coherency strains. However, the heat treatment resulting in cuboidal aligned precipitates, 0.45 μm in size, leads to a two-fold improvement in creep lives over the corresponding values obtained with the other heat treatment which produces smaller and more odd-shaped particles. Transmission electron microscopy on specimens crept to various strains at 760 °C showed that, in contrast with the odd-shaped particles, the cuboidal precipitates promoted a very homogeneous deformation which led to the lower creep rates. The precipitate shearing mechanism in the late stages of primary creep is shown to be by {111}〈112〉 slip regardless of the heat treatment. In secondary creep the shearing occurs by (α/2)〈110〉 dislocation pairs but the difference in creep behaviour is thought to be associated with the higher stability of dislocation networks associated with cuboidal precipitates. At 1050 °C the more perfect rafted structure produced by the cuboidal precipitates is responsible for lower creep rates. The γ′ rafts at high stresses were sheared by (α/2)〈110〉 dislocation pairs.

A reevaluation of the mechanical properties of molybdenum- and tungsten-based alloys containing hafnium and carbon

It is the purpose of this paper to reexamine data on the behavior of molybdenum- and tungsten-based alloys which are strengthened by hafnium carbide. This reanalysis suggests that claims of apparently unusual high-temperature behavior in the above systems, specifically (1) the “solute-weakening” effect of hafnium and (2) the fact that increasing the amounts of HfC can have a weakening effect, may not be valid. In the present description, it is postulated that there are significant changes in the amounts of carbide available for precipitation in alloys where the additions of carbide former to carbon are varied about the ratio needed for complete or stoichiometric combination. By recalculating the amounts of hafnium carbide available for precipitation in the molybdenum- and tungsten-based alloys it is possible to demonstrate that excess hafnium does not have a “solute-weakening” effect as had been claimed. Instead, a trend of improved creep strength and high temperature tensile strength with an increase in the amount of HfC available for precipitation is observed.

Tensile creep study of copper with alumina dispersions prepared by high rate physical vapour deposition

Mott has suggested that the ideal creep-resistant material will be one with a fine grain size in which the grain boundaries are filled with some substance, say a refractory oxide, to inhibit the motion of grain boundaries. Such a system, alumina-dispersed copper, was prepared by high rate physical vapour deposition. The process parameters and their effect on structure and texture have been studied. The room temperature mechanical properties have also been reported. This paper deals with a high temperature mechanical property, i.e. tensile creep. Tests were made on a constant-stress vacuum creep rig with a Ferrometic feedthrough to ensure a zero leakage rotary seal. A vacuum of 1.33 X 10 −3 Pa was maintained. The test temperatures were 500°C (0.57 T m) and 700°C (0.72 T m). The stresses applied were 2.07, 3.45, 4.14 and 6.89 X 10 7 Nm −2. Tests were made on as-deposited films and on cold-rolled condensates. Minimum creep rate curves showed the effect of the alumina content in raising the creep resistance of copper. Cold rolling also reduced the minimum creep rate markedly. Varying the temperature and stress affected the shape of these curves. Stress-rupture plots were used to summarize the data. Grain refinement together with a fine stable dispersion seem to give improved creep strength. The critical barrier of the Orowan stress was noted. The stress exponent for a low alumina deposit (0.21 vol.%) was about 8, and the apparent activation energy for creep was about 202 kJ mol −1 (0.13 vol.%). There was an inverse relationship between the rupture life and the minimum creep rate, their product being constant (about 0.2).

Effect of creep deformation on precipitation hardening

The effect of high-temperature deformation on the kinetics of precipitation for several commercial aluminium alloys has been investigated. It has been found that mobile dislocations introduced during high-temperature deformation are efficient sinks for vacancies and retard the ageing process in spite of the additional vacancies produced through deformation. The plentiful supply of dislocations accelerates the loss of coherency of intermediate precipitates. Cold deformation after the quench, but before ageing, refines the precipitate size, which retards loss of coherency and results in improved creep strength.

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.