Impression Creep Technique Research Articles

Impression creep properties of a semi-solid processed magnesium–aluminum alloy containing calcium and rare earth elements

The creep properties of a thixoformed magnesium–aluminum alloy containing calcium and rare earth elements were studied under shear modulus-normalized stresses ranging from 0.0225 to 0.035 at temperatures of 150–212 °C using the impression creep technique. Analysis of the creep mechanism based on a power-law equation indicated that pipe diffusion-controlled dislocation climb is the dominant mechanism during creep. The alloy has a better creep resistance than high-pressure die-cast magnesium–aluminum alloy.

Creep behaviour of δ-phase of U–Zr system by impression creep technique

In this study, the impression creep behaviour of δ-phase of U–50 wt.% Zr (U–72.29 at.% Zr) system was studied in the temperature range 525–575 °C at different stresses. The velocity of the punch at different stresses and temperatures were evaluated for the above alloy. The stress exponents and thermal activation parameters of the above alloy were determined. A power law behaviour is displayed with the stress exponents range from 6.5 to 7. The activation enthalpy for the δ-UZr 2 was found to be independent of stress with an average value of 106 kJ/mol.

Determination of activation parameters using impression creep data

Impression creep technique is a modified indentation creep test wherein the conical or ball indenter is replaced by a cylindrical, flat bottomed punch. In this study, the impression creep behavior of a few Al-U-Zr alloys was studied in the temperature range 693–783K at different stresses. Impression creep curves generated for Al-22U-2Zr and Al-46U-3Zr (composition in wt %) show a wavy pattern. The reason for the unusual creep curve is found to be in the fact that microstructure is not stable at the creep testing temperature. The velocity of the punch at different stresses and temperatures were evaluated for each of the alloy covered in this study. The stress exponents and thermal activation parameters of the above alloys were determined.

Microstructure and impression creep of age hardenable AA2219 aluminium alloy modified by Sc, Mg and Zr additions

The present work pertains to impression creep of age hardenable AA2219 Aluminium-copper (6.3%) alloy correlated with microstructure with the addition of scandium, magnesium and zirconium. These additions were systematically varied by preparing alloys of different composition using gas tungsten arc melting. Impression creep technique was used to study the high temperature stability of the alloys. Modified compositions of the alloy resulted in fine equiaxed grains, refined eutectics, large number of high temperature stable and fine precipitates. Among all the compositions, 0.8%Sc+0.45%Mg+0.2 %Zr addition was found to be significant in improving the high temperature stability of AA2219 alloy. From the calculated values of stress exponent (n) and activation energy (Q), mechanisms of dislocation-climb and glide interaction with the precipitate particle are proposed.

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.

Effect of Surface Roughness on Determination of Creep Parameter using Impression Creep Technique

AbstractThe indentation creep test, especially the impression creep, exhibits a magic appealing in the determination of creep properties of small structures in industry for its simplicity, efficiency and non-destruction merits. Most of previous researches of indentation or impression creep neglect the effect of surface roughness of materials, which plays a crucial role in extracting creep properties of materials. The FE results showed that the surface roughness has no effect on the determination of creep exponent when the punching stress is larger than 150MPa. However, under a smaller punching stress the stress exponent is decreased due to the “Tuner” effect of asperities. The conclusions drawn in the present study provide an important guidance on experiment results amendment for impression creep technique.

Microstructure and high temperature stability of age hardenable AA2219 aluminium alloy modified by Sc, Mg and Zr additions

The present work pertains to the improvement of high temperature stability of age hardenable AA2219 aluminium–copper (6.3%) alloy. Addition of scandium, magnesium and zirconium to the base metal AA2219 was adopted to improve this high temperature stability. These additions were systematically varied by preparing alloys of different composition using gas tungsten arc melting. Long time ageing studies and impression creep technique were used to study the high temperature stability of the alloys. These modified compositions of the alloy resulted in fine equiaxed grains, refined eutectics, large number of high temperature stable and finer precipitates. Among all the compositions, 0.8% Sc + 0.45% Mg + 0.2% Zr addition was found to be significant in improving the high temperature stability of AA2219 alloy. This may be attributed to the possible microstructural changes, solute enrichment of the matrix and pinning of the grain boundaries by the finer precipitates.

Impression creep technique—An overview

Impression creep technique is a modified indentation creep test wherein the conical or ball indenter is replaced by a cylindrical, flat bottomed punch. The usefulness of this technique, pioneered by Prof. Li, is illustrated by application to a variety of problems in this laboratory. High temperature creep behavior of a number of metals and alloys, particularly estimation of the thermal activation parameters aiding the identification of the rate controlling mechanisms of creep, has been investigated. The technique has also been exploited to assess the ‘single crystal’ creep behavior vis a vis that of a polycrystalline sample. Utilizing the impression creep test, the creep behavior of individual zones in steel weldments has been examined. The simplicity and the utility of the impression creep test have been further demonstrated by its application to the study of superplastic behavior in alloys. This paper presents a cross section of the results obtained in the above investigations. It is concluded that the impression creep test technique is capable of yielding much of the information that can be obtained from tensile creep testing. Furthermore, it can provide data which are either impossible or extremely difficult to obtain with conventional creep testing.

Microstructural characterization and creep behaviour of as-cast titanium aluminide Ti–48Al–2V

Solidification paths were established in a two-phase (α 2+γ) alloy of nominal composition Ti–48Al–2V (at.%). The alloy buttons were prepared by vacuum arc melting. The phases present in the alloy were characterized by X-ray diffraction, optical microscopy and scanning electron microscopy. Room temperature microstructure of the alloy consisted of colonies of (α 2+γ) lath dendrites and interdendritic γ segregate. The (α 2+γ) lath colony size was determined to be about 76 μm. Based on the dendritic morphologies and the orientation of the (α 2/γ) lath striations observed in the shrinkage cavities, it was established that primary β solidification occurs in the alloy. Creep behavior of the alloy in the as-cast condition was studied by employing the impression creep technique in the temperature range of 1033–1098 K and in the stress range of 187–420 MPa. The alloy exhibited power law creep in this regime. The stress dependence of the steady-state creep rate showed a stress exponent value of 4.0–4.6 and the activation energy for creep was found to be in the range of 320–340 kJ mol −1. This value is slightly higher than that for diffusion of Ti in single phase γ (TiAl). These results suggest the operation of a diffusion-controlled creep mechanism in the alloy. Microstructure of the deformed region under the indenter revealed formation of fine-scale lamellae and randomly oriented grains as a result of creep. The creep parameters obtained in the present study are compared with those in the literature on lamellar (α 2/γ) alloys.

Effect of Microstructure on the Impression Creep of Two-phase Titanium Aluminide

An alloy containing 49 at.% aluminium was heat treated at different temperatures within the α+γ two-phase field to get five different volume fractions of lamellar constituent. Creep studies were carried out on these samples at five different temperatures in the interval between 1023 and 1223 K, by impression creep technique using four levels of stresses at each temperature. For a given temperature and stress, steady state impression velocity decreased with increasing volume fraction of lamellar constituent. Activation energy for steady state creep increased linearly with lamellar content from 185 kJ/mol at 22 vol.% to 362 kJ/mol at 100 vol.% The stress exponent was found to be around 1.2 in all the cases. The results show that a fully lamellar structure has a superior behaviour where creep is an important factor.

Application of the reference stress method for interpreting impression creep test data

Impression creep tests have been performed on a 316-stainless steel at 600°C, for which conventional uniaxial creep test data are available. It is shown that the technique, based on the reference stress approach, for converting impression creep test data to equivalent uniaxial creep data, is accurate. The results show that the impression creep technique can be used for obtaining creep properties for materials which have high creep resistance at high temperature and test pressure conditions. The difficulties and limitations associated with such situations are described and methods of dealing with them are outlined.

Investigation of superplasticity by the impression creep technique

Impression creep tests have been carried out on a Zn-22% Al eutectoid alloy in two heattreatment conditions, as a function of applied stress and at temperatures of ∼25 to 270° C. At small grain sizes (∼ 0.9μm) and in the temperature range ∼ 180 to 270° C, the observed deformation response conforms to Regions I, II and III typical of superplastic behaviour. Furthermore, strain-rate index values determined for Region II fall in the range 0.44 to 0.51, consistent with superplasticity. It is concluded that the impression creep technique offers considerable potential for characterization of superplastic alloy systems on small specimen volumes.