Indentation Creep Behavior Research Articles

Indentation creep studies of iron aluminide intermetallic alloy

Indentation creep behaviour of iron aluminide intermetallic (Fe–28Al–3Cr) was studied in the temperature range of 843–963 K. The hardness and the plasticity parameter were also calculated from room temperature to 1273 K. The variation of hardness with time showed that the stress exponent ( n) was weakly dependent on temperature. Thermal activation parameter-activation area and energy were also calculated to identify the creep mechanism. The activation energy for high temperature creep was found to be in good agreement with that for self-diffusion of pure iron and in excellent agreement with previous studies. These values of n, activation area and activation energy were found to be consistent with dislocation climb as the rate controlling creep mechanism. Conventional creep curves were also derived from hardness values.

Creep of a TiAl alloy: a comparison of indentation and tensile testing

The present study compares the indentation and uniaxial creep behaviour of a near-γ-TiAl alloy with a duplex microstructure. Indentation creep tests were performed using a cylindrical indenter at temperatures between 750 and 1050 °C and net section stresses between 50 and 1430 MPa. A stress dependent stress exponent ranging from 3 for low net section stresses to 8 for high net section stresses was determined. The apparent activation energy of indentation creep was determined as 343±16 kJ mol −1. The stress as well as the temperature dependence of the indentation and uniaxial creep data are in good agreement. The indentation experiments were also analysed with respect to the deformed volume in front of the indenter and the surface bulge that develops around the indenter. A simple dislocation model is proposed to rationalize volume and shape of the bulge around the indenter. The good agreement of creep parameters obtained by indentation and conventional uniaxial creep testing demonstrates that indentation creep tests are suitable to characterize creep.

Creep of metal-type organic compounds—II. Indentation creep

Temperature and time-dependent measurements of the Vickers microhardness of camphene, camphor, succinonitrile and their composites with alumina particles have been made. The materials have previously been shown to be good analogues of metals as regards their creep behaviour. The indentation creep behaviour of these materials at relatively high homologous temperatures provides new information about the indentation creep process, and is found to be consistent with a back/internal stress description of indentation creep.

Creep Behavior of MoSi2-SiC Composites

AbstractUsing a cylindrical indenter, the indentation creep behavior of hot pressed and HIPed MoSi2-SiC composites containing 0-40% SiC by volume, was characterized at 1000-1200°C, 258-362 MPa. The addition of SiC affects the creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains; by obstructing or altering both dislocation motion and grain boundary sliding; and by increasing the overall yield stress of the material. Comparisons are made between indentation and compressive creep studies. It is shown that under certain conditions, compressive creep and indentation creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the indenter.

Elevated temperature mechanical properties of Be 12Nb

Bend strength, compression strength and fracture toughness of single-phase, vacuum hot pressed Be 12Nb have been assessed at temperatures from ambient to 1200°C. Hot hardness and indentation creep behavior was also determined for both vacuum hot pressed and hot isostatically pressed Be 12Nb over this same temperature range. Bend strength and fracture toughness increased with test temperature for the vacuum hot pressed Be 12Nb. Bend strength increased to a maximum of 220 MPa at 1000°C, while K IC increased from 2.5 MPa m 0.5 at 20°C to 4.5 MPa m 0.5 at 1100°C. Significant plastic deformation in bending was observed only above 1100°C. These results are consistent with a tensile ductile-to-brittle-transition (DBTT) of about 1100°C, with the bend strength at temperatures below 1100°C limited by the low fracture toughness. Tests in uniaxial compression revealed that yielding occured in the vacuum hot pressed material at temperatures above 700°C. High yield strengths (up to 1880 MPa) were measured at intermediate temperatures with a DBTT in compression between 700 and 800°C. Microhardness data indicated a DBTT of about 850°C for both the vacuum hot pressed and hot isostatically pressed Be 12Nb. Vickers microhardness values for the hot isostatically pressed material were significantly greater at low temperatures, but hot isostatic pressing and vacuum hot pressing heats are comparable at temperatures greater than 800°C. Indentation creep results indicated activation energies for dislocation creep of 290 and 220 kJ mol −1 with power law creep stress exponents of 4.3 and 6.6 for the hot isostically pressed and vacuum hot pressed beryllides respectively. This data is discussed using newly acquired information about dislocation slip in this alloy.

Indentation Creep of Na2O · 3SiO2 Glasses with Various Water Contents

Time‐dependent deformation behavior induced by an indentation, called indentation creep, was investigated for Na2O · 3SiO2 glasses with various water contents by measuring the indentation depth as a function of time using a conical indenter. It was found that the indentation creep behavior of the glasses could be best explained as steady‐state inhomogeneous (non‐Newtonian viscous) flow. Water in the glasses appeared to reduce the stress (or yield stress) needed to cause this flow. Crack initiation of the glasses was promoted by water through this inhomogeneous flow.

Biphasic indentation of articular cartilage—I. Theoretical analysis

A mathematical solution has been obtained for the indentation creep and stress-relaxation behavior of articular cartilage where the tissue is modeled as a layer of linear KLM biphasic material of thickness h bonded to an impervious, rigid bony substrate. The circular (radius = a), plane-ended indenter is assumed to be rigid, porous, free-draining, and frictionless. Double Laplace and Hankel transform techniques were used to solve the partial differential equations. The transformed equations and boundary conditions yielded an integral equation of the Fredholm type which was analyzed asymptotically and solved numerically. Our asymptotic analyses showed that the linear KLM biphasic material behaves like an incompressible (ν = 0.5) single-phase elastic solid at t = 0 +; the instantaneous response of the material is governed by the shear modulus ( μ s ) of the solid matrix. The linear KLM biphasic material behaves like a compressible elastic solid with material properties defined by those of the solid matrix, i.e. ( λ s , μ s ) or ( μ s , ν s ) as t → ∞. The transient viscoelastic creep and stress-relaxation behavior, 0 < t < ∞, of this material is controlled by the frictional drag (which is inversely proportional to the permeability k) associated with the flow of the interstitial fluid through the porous-permeable solid matrix. For given values of the Poisson's ratio of the solid matrix ν s and the aspect ratio a h , where a is the radius of the indenter and h is the thickness of the layer, the creep behavior with respect to the dimensionless time H Akt a 2 is completely controlled by the load parameter P 2μ s,a 2 and the stress relaxation behavior is completely controlled by the rate of compression parameter R 0 = kH A V 0h where H A = λ s + 2 μ s and the equilibrium strain u 0 h . This mathematical solution may now be used to describe an indentation experiment on articular cartilage to determine the intrinsic material properties of the tissue, i.e. permeability k, and the elastic coefficients of the solid phase ( λ s , μ s ) or ( μ s , ν s ).

Incubation periods and indentation creep in lead

The experimental variation of the microhardness of lead with dwell time and temperature is reported. At homologous temperatures close to 0.5T m, and using low loads (i.e. 0.1 N), a period of zero indentation creep of the order of 300 sec is observed. This phenomenon, called the incubation period, and the subsequent indentation creep behaviour, are explained in terms of restructuring and recovery of the dislocation network beneath the indenter.

Effects of adsorption on the indentation deformation of non-metallic solids

Abstract Marked changes in indentation microhardness have been observed in a wide range of non-metallic materials due to the presence of adsorbed water and other polar species. Opposite polar faces of crystals with non-centrosymmetric structures, including II–VI, III–V and IV–IV compounds, exhibit large differences in microhardness and anomalous indentation creep behaviour. Such differences are shown to be due to differential effects of water adsorption on the opposite polar faces rather than being an intrinsic polar mechanical property of the crystal. The usefulness of microhardness measurements for absolute determinations of crystallographic polarity of non-centrosymmetric crystals is discussed. Effects of environment on the extent of edge dislocation motion away from the indentation are examined. Possible mechanisms to explain the adsorption-induced changes in microhardness and related mechanical properties of non-metallic solids are critically discussed.