For isoconcentration series Ca 0. But, a large number of amorphous silicon can be found beneath the indenter for 110 and 111 oriented surface beside the bct5 phase and β-Si phase. Elevated temperature nanomechanical testing is becoming a very popular technique to unravel temperature effects on the deformation mechanisms of a number of material systems, especially in those cases where it is the only available technique for mechanical testing, like in thin-films and coatings. We have performed in-situ micropillar compression to investigate the local strain rate sensitivity of single α phase in dual-phase Ti alloy, Ti—6Al—2Sn—4Zr—2Mo wt%. As a consequence of this mechanistic cross-over, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of defor-mation at approximately the same grain size as the onset of the Hall—Petch breakdown. The transition stress is less than the yield stress of the parent austenite which is less than the yield stress of the resulting martensite over a wide range of strain rates, but it eventually exceeds this yield stress once a critical strain rate is exceeded. Measurements of activation energy are few: Wang et al.
Materials tested include metals and alloys, superplastic materials, weldments, glasses, ceramics and polymers. In the present work, it is attempted to modify a recently proposed stress relaxation model with additional coefficients to accommodate the mechanical behavior at different length scales. Nanoindentation creep of nanocrystalline Ni with the as-deposited grain size of 14 nm was characterized at elevated temperatures. These trends appear to hold independent of the processing routes used to refine the grain size and of the testing method employed. This approach gives the opportunity to simulate the formation and the migration volumes of the point defects, taking into consideration the influence of pressure on structure and consequently on energy. Thermally Activated Processes Depending on Crystal Structure, Microstructure, and Solute Solution Content Since strain-rate sensitive deformation behaviors have manifold characteristics and also shed more light into the individual underlying deformation mechanism, in the following two subsections, interesting materials systems will be discussed in closer detail. The creep results are conflicting: while some results indi-cate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected.
The increase of the lattice parameters of maraging steel alloys indicated that the hardening element, which is tungsten, was dissolved after tensile deformation. It was found that the velocity-stress exponent for dislocation motion obtained in this way was identical with that obtained by conventional stress relaxation techniques using bulk samples. This contact stiffness is less prone to thermal drift influences than the recorded indentation displacement. Calculations demonstrate that the effect of temperature on yield strength, activation volume, and strain rate sensitivity is strongly dependent on the grain size. We present models for the emission of partial or perfect dislocations from stress concentrations at a grain boundary or twin boundary.
Local strain rate sensitivity exponent i. The magnitude of this effect is larger than the increase of volume owing to the stretching of the bond between A and B, leading to a negative activation volume. The activation volumes for the two film thicknesses were 4. The mechanical behavior of metals and alloys is strongly related to grain size. An experimental technique, based on classical uniaxial relaxation experiments, was developed to establish a constant strain during repeated load relaxation transients and then to calculate the stiffness of unloading, and therefore the hardness, across the transients with acceptable results. These thermal drift related issues have already been well known for a long time, although they are still ignored in many works. This study proposes a reverse analysis that can be employed to extract elastoplasticity-dependent creep property of commercial diecast Mg alloys and their intermetallic compounds from instrumented indentation with two sharp indenters.
Together, these observations demonstrate that mechanical activation of dislocation glide was not detected in the present experiments. Grain refinement provides a potential approach to achieve these goals in Mg alloys. With the help of a formula of thermal strain and dislocation mechanism of microcrack evolvement, a microcrack growth rate and a probability density distribution function are found. Abstract: The quasi-static and dynamic compression experiments of ultrafine-grained copper fabricated by equal channel angular pressing method were performed at temperatures ranging from 77 to 573K. The results indicate the involvement of multiple dislocation mechanisms in the indentation test. The authors experimental findings and analysis support that the tip-sample diffusion is the dominant mechanism below a critical indentation depth and the effect of grain boundary media mechanisms will emerge as the indenter penetrates deeper.
A maximum strain-rate-sensitivity exponent m of 0. The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0. However, the temperature sensitivity of ultrafine-grained copper to true strain is comparative weaker than that of coarse grained copper. The latter trends indicated that the strain rate regime 105—104 s1 is pivotal in the mechanical response of the particular nanocrystalline Au films. Indentation test using a cylindrical indenter with a flat end is now known as impression test. The reduced modulus E r from the loading segment serves at the same time as basis for the dynamic correction, as shown in Eqs.
Nanoindentation and uniaxial tension methods were used to test the mechanical properties of this membrane in both the thickness direction and length direction at 26°C and 40% relative humidity. © 2019 Wiley Periodicals, Inc. The test is influenced by the material length scale. He was also a Visiting Research Professor in the Department of Mechanical and Aerospace Engineering at Princeton University, 2003-2005. Every parameter can be chosen to the requirement of the application and material. The strain-rate sensitivity exponent and the activation volume provide a fingerprint of the rate controlling mechanisms during thermally activated deformation. Moreover, the quantitative changes upon, for example, different material modifications, loading rates, or temperatures are indicative of the interaction, competition, and change of different contributing plasticity mechanisms.
The plastic deformation behaviour of single crystals of TiAl with a composition of Ti-56 at. The mechanical behavior of coarse grained, ultrafine grained and nanocrystalline Ti, which was fabricated on single crystalline Si substrate using a d. Within the last decade, many studies on different materials classes, microstructures and environmental influences were reported in literature. Table 1 shows some of the values for activation volumes reported in the literature for various free radical polymerizations. To facilitate the wider application of wrought Mg alloys as structural materials, it is desirable to improve the formability and minimize the strength anisotropy inherent in these materials due to low symmetry of their hexagonal close-packed crystal structure.