The creep behavior of a binary Mg-15 wt.% Gd alloy was investigated over the temperature range from 523 K to 743 K, i.e., in both the single-phase region (the hexagonal close-packed solid solution of Gd in Mg) and the two-phase region (the solid solution plus Mg5Gd precipitates). The alloy was prepared by the squeeze casting technique. In the higher temperature range, at 723 and 743 K, the specimens were solution treated by in situ annealing prior to testing. At the temperature of 673 K and below, the alloy was tested in the cast state. In the higher temperature range, the behavior was interpreted in terms of the viscous glide, where the dislocation motion was constrained by the presence of solute atmospheres. The dislocation motion was controlled by the rate of the cross slip from the basal to the prismatic planes. At the temperatures of 623 K and 673 K, the creep behavior was rationalized by introducing the threshold stress concept. At the temperatures of 523 K and 573 K, the stresses required to achieve experimentally measurable creep rates were such that dislocations broke away from the atmospheres of foreign atoms. Comparison with a series of magnesium alloys prepared by squeeze casting and creep-tested by the same technique showed that gadolinium can be a favorable creep-resistance enhancing element.

Compressive creep tests were performed on a CoCrFeNiMn equiatomic alloy with the dispersion of (i) aluminum nitride or (ii) boron nitride at temperatures of 973 K and 1073 K. The results are compared with previously published creep rates of the unreinforced matrix alloy and the alloy when strengthened by yttrium + titanium oxides. The comparison reveals that the creep rate is essentially unchanged by the presence of aluminum nitride particles, whereas it is reduced by the presence of oxide particles. Boron nitride particles do not influence the creep rate at low stresses but reduce it substantially at high stresses.

Creep of an alloy based on the intermetallic compound Fe2AlCo was studied by compressive creep tests in the temperature range from 873 to 1073 K. The stress exponent n and the activation energy of creep Q were determined using the multivariable regression of the creep-rate data and their description by means of sinh equation (Garofalo equation). The evaluated stress exponents indicate that the dislocation climb controls creep deformation. The estimated apparent activation energies for creep are higher than the activation enthalpy for the diffusion of Fe in Fe3Al. This can be ascribed to the changes in crystal lattice and changing microstructure of the alloy.

Three Fe-Al-based alloys, a binary with 22 at. % of Al, a ternary with 22 at. % of Al and 7 at. % of Ti and a quaternary with 22 at. % of Al, 4 at. % of Ti and 4 at. % of Nb prepared by arc melting to small button type ingots were studied by small punch test and small punch creep test in order to obtain the high temperature tensile and creep properties. Evaluation of the results shows a significantly improved strength at high temperatures and creep resistance of the ternary and quaternary alloys compared to the binary alloy. The observation of the punched discs fracture surfaces related to the initial microstructure also helps to better understanding of the deformation and fracture behavior of these alloys at high temperatures.

Creep tests were conducted in uniaxial compression to evaluate the creep behavior of magnesium-aluminum-strontium alloy at temperatures from 373 to 673 K. Stress dependencies of the creep rate over the whole interval of temperatures and stresses can be well described phenomenologically by the Garofalo sine hyperbolic equation modified by the inclusion of a threshold stress. The threshold stress increases with decreasing temperature. Creep data normalized by a diffusion coefficient and shear modulus clearly reveal the existence of two different regions. Possible mechanisms by which plastic deformation takes place have been identified in both regions. The critical stress at which dislocations break away from the cloud of foreign atoms agrees well with the value determined by data normalization. At low stresses, a value of the stress exponent of n congruent to 3 is consistent with the model of deformation that takes place through dislocation glide controlled by dragging of solute atoms. At high stresses, the multiple regression yields the activation energy which agrees with that for prismatic glide. (C) 2020 Published by Elsevier B.V. on behalf of Chongqing University.

The evolution of microstructure and texture during high temperature uniaxial compressive deformation of two-phase (gamma+beta) CoNiCrAlY bond coat alloy was investigated. Uniaxial compression tests were carried out at various temperatures ranging from 1073 K to 1473 K with true strain rates from 5.0 x 10(-3)s(-1) to 5.0 x 10(-4)s(-1) up to a true strain of -1.0. It was concluded that dynamic recrystallization (DRX) occurred in both gamma and beta phases, even in two-phase alloys. Under all deformation conditions, the main component of the texture in the gamma phase having fcc structure was {101}. As for the conditions of low Zener-Hollomon parameter Z in the gamma phase, the texture was not developed. This is due to the formation of twin boundaries during grain boundary migration which may weaken the texture. In the beta phase with the B2 structure, the main component of the texture was {111}. Even under the low-Z conditions of the beta phase, the texture was not developed. Because the area fraction of the beta phase in CoNiCrAlY alloy is approximately 25%, and because the beta-phase grains were surrounded by gamma-phase grains, migration of the beta-phase grain boundaries should not occur.

Small punch creep tests of unnotched and notched 8 mm discs of new austenitic steel Sanicro 25 were performed at 700 degrees C. Side notches through the disc thickness proposed by Lacalle of different lengths ranging from 3.0 to 4.5 mm were applied for this study. It was found that the notch length influences the time to rupture and the minimum displacement rate, however no apparent signs of crack initiation were observed on the recorded time displacement and time-displacement rate curves at 400 N force. The shortest time to rupture and fastest displacement rate were obtained for notch length of 3.5 mm, while the longest time to rupture and slowest displacement rate were obtained for notch length of 4.25 mm. The fracture of Sanicro 25 discs had signs of a "star" shape, with multiple cracks, which indicates lower ductility. A larger circular cap which is obvious for materials with high ductility was not present in all the small punch tests performed.

Results of creep tests of two Fe-27 at. % Al-based alloys with additions of 2.7 and 4.8 at. % of niobium conducted in the temperature range from 650 °C to 900 °C in the authors’ laboratory are presented. The purpose of the study is to supplement previous work on Fe-Al-Nb alloys to obtain a more complete overview of creep properties from the dilute alloy with 1% of Nb up to the eutectic alloy with 10% of niobium. At higher temperatures and lower stresses, the creep resistance of the 10% niobium alloy is better than that of the lower niobium alloys. On the other hand, the eutectic alloy loses its preference at lower temperatures and higher deformation rates. This phenomenon is similar to that reported by Yildirim et al. for Fe-50 at. % Al-based alloys and is probably associated with an increased stress sensitivity of the eutectic alloy.

Creep tests of a CoNiCrAlY bond coat alloy were conducted in the temperature range from 1073 K to 1473 K. The alloy was prepared from a powder with a nominal chemical composition of 34.2% Co, 28.6% Ni, 21.2% Cr, 15.6% Al, 0.4% Y (atomic percent is given throughout) by consolidating it under a stress of 60 MPa and annealing it at 1473 K. The final composition of the alloy is 36.3% Co, 29.6% Ni, 21.0% Cr, 12.8% Al and 0.3% Y. The stress exponent n decreases with the increasing temperature from 3.9 at 1073 K to approximately 1 at 1473 K. Consequently, the activation energy of creep is stress dependent and ranges from 512 kJ/mol to 224 kJ/mol with increasing stress. Combined creep and constant strain rate data can be described by the Garofalo (sinh) equation. A comparison of the results with available creep data of free-standing bond coats is presented. Creep resistance of the present alloy clearly overwhelms the resistance of bond coat alloys previously summarized by Wood.

The correlation of the small punch creep (SPC) test results with uniaxial creep test results is challenging due to several factors. The stress state is equibiaxial in the SPC test and the equivalent stress is not constant as the punch is advancing into the disc. The classical use of Chakrabarty membrane theory, with a constant F/

A key issue in the small punch creep test (SPC) is to determine the equivalent stress that results in the same time to rupture in a uniaxial creep test (UAC). A new approach is proposed based on formulas between the ratio of force in SPC to stress in UAC and the deflection at the minimum deflection rate. Another formula is defined between the minimum deflection rate and the minimum strain rate. In both cases, they were created from a large experimental data pool of high temperature creep-resistant steels. The predicted Norton law and the rupture time dependence on stress are in good agreement with the experimental UAC results. In all cases, the predictions using the present approach are much better than those based on Chakrabarty membrane stretch model.

Application of the small punch test in technical practice is critically dependent on the existence of reliable methods for conversion of data measured by means of such a biaxial test to equivalent quantities of uniaxial tests. In the paper, various methods for recalculation of force in small punch test to stress in conventional creep test are investigated. The methods include: (i) classical "CEN Workshop Agreement" model, (ii) empirical "force to stress conversion" method, (iii) modified Chakrabarty method and (iv) "constant deflection rate" method. The methods are examined using the data obtained by testing Fe-Al based alloy with additions of chromium and cerium. The tested alloy enables verification of conversion methods for different crystallographic lattices, namely D03 at lower temperatures and B2 at elevated temperatures.

Mechanical properties of prospective Fe-Al- based alloys were investigated using miniature specimens. This enables testing of material from experimental heats that did not allow preparation of standard specimens. Small punch test and mini-tensile test were applied for estimation of the mechanical properties of Fe-22 Al and Fe-22Al-7Ti (at.%) at high temperatures. It is shown that the force applied in the small punch test can be successfully converted into the equivalent applied stress using the empirical formula suggested by the new European standard for small punch testing. The minimum deflection rate can be converted into the minimum creep strain rate as well. The improvement of the latter conversion based on the Monlanan-Grant relationship is proposed. The small punch testing enables evaluation of rupture properties including the creep rupture strength. This can effectively complement previous investigations of Fe-Al-Ti alloys performed predominantly in uniaxial compression. It is shown that a significant increase in creep resistance is achieved by addition of titanium and a two-phase microstructure analogical to that in gamma'-strengthened nickel-based superalloys is observed.

Creep behaviour of the alloy based on intermetallic compound Fe3Al with additions of 2.6 at.% chromium and 0.02 at.% cerium was studied at temperatures from 500 to 800 degrees C by small punch testing with a constant force. The dependences of the minimum deflection rate and the time to rupture on the applied force follow similar dependences obtained in uniaxial creep tests of the same alloy. The results of small punch tests can be explained by the existence of different crystal lattices occurring at different temperatures. It is shown that the force applied in the small punch test can be successfully converted into the equivalent applied stress using the empirical formula suggested by the new European standard for small punch testing. The minimum deflection rate can be converted into the minimum creep strain rate as well. The improvement of the latter conversion based on the Monkman-Grant relationship is proposed.

Tensile and creep properties of new austenitic steel Sanicro 25 at room temperature and operating temperature 700 °C were investigated by testing on miniature specimens. The results were correlated with testing on conventional specimens. Very good agreement of results was obtained, namely in yield and ultimate strength, as well as short-term creep properties. Although the creep rupture time was found to be systematically shorter and creep ductility lower in the miniature test, the minimum creep rates were comparable. The analysis of the fracture surfaces revealed similar ductile fracture morphology for both specimen geometries. One exception was found in a small area near the miniature specimen edge that was cut by electro discharge machining, where an influence of the steel fracture behavior at elevated temperature was identified.

Creep tests of two equiatomic CoCrFeMnNi alloys were conducted in the temperature range from 973 K to 1073 K. The alloys were prepared by milling blends of powders of pure elements in a planetary ball mill and compacting by the spark plasma technique. Two variants of the alloys were prepared: (i) without and (ii) with the dispersion of oxides. Creep resistance was substantially improved by the presence of oxides. Diffusion creep controlled by lattice diffusion was suggested as a possible mechanism at low stresses. The effective diffusion coefficient calculated for Nabarro-Herring creep was comparable to the lattice diffusion coefficient of Ni in the same high-entropy alloy. At high stresses, the creep behavior was characterized by the presence of a threshold stress invoked by oxide particles.

In order to determine creep properties from small punch creep (SPC) tests, several theoretical models and analytic methods are applied, such as the Chakrabarty’s membrane stretch model and reverse finite element method. However, because the problem is too complicated, differences are always found between the theoretical prediction and the uniaxial creep tests. In this paper, a concept of “Uniaxial test correlation” is proposed without any theoretical assumption and analytic calculation. By comparison of the rupture time in SPC with the uniaxial creep rupture data, the equivalent stress

Small punch testing under constant deflection rate, constant force and constant deflection (i.e. force relaxation) were performed on the new austenitic steel Sanicro 25. Constant deflection rate experiments were correlated to uniaxial tensile tests at room temperature and 700°C with the help of several empirical relationships. Small punch creep testing was performed in as received state. Correlation of the small punch results with uniaxial creep test results was done and the force/stress ratio Ψ and kSP parameter were determined. The constant deflection small punch test was correlated with the uniaxial stress relaxation test and good agreement was reached.

The paper summarizes capabilities of the small punch tests on miniature disc specimens from metallic materials. Results obtained by small punch tests can be correlated with mechanical properties determined by conventional or other miniature tests. Three basic types of small punch test setup: i) static/fracture, ii) creep and iii) stress relaxation are described and the basic quantities that can be obtained are shown. Relations for an assessment of conventional mechanical properties from quantities obtained by small punch test are presented.