Nanoindentation technology has been widely adopted to study creep behavior in small regions. However, nanoindentation creep behavior of metallic glass is still not well understood. In the present work, we investigated nanoindentation size effects on creep deformation in a Zr-based bulk metallic glass at room temperature. The total creep strain and strain rate of steady-state creep were gradually decreased with increasing holding depth under a Berkovich indenter, indicating a length-scale-dependent creep resistance. For a spherical indenter, creep deformations were insignificant in elastic regions and then greatly enhanced by increasing holding strain in plastic regions. Strain rate sensitivities (SRS) decreased with increasing holding depth and holding strain at first, and then stabilized as holding depth was beyond about 500 nm for both indenters. SRS values were 0.4–0.5 in elastic regions, in which atomic diffusion and free volume migration could be the creep mechanism. On the other hand, evolution of the shear transformation zone was suggested as a creep mechanism in plastic regions, and the corresponding SRS values were in the range of 0.05 to 0.3.

The crystal orientation effect on mechanical heterogeneity of LiTaO₃ single crystals is well known, whilst the time-dependent plastic behavior, i.e., creep is still short of understanding. Relying on nanoindentation technology, we systematically studied room-temperature creep flows at various holding depths (100 nm to 1100 nm) in three typical orientations namely the X-112°, Y-36° and Y-42° planes. Creep resistance was much stronger in the X-112° plane than the others. In the meanwhile, creep features were similar in the Y-36° and Y-42° planes. The orientation effect on creep deformation was consistent with that on hardness. The nanoindentation length scale played an important role in creep deformation that creep strains were gradually decreased with the holding depth in all the planes. Based on strain rate sensitivity and yield stress, the activation volumes of dislocation nucleation were computed at various nanoindentation depths. The activation volumes ranged from 5 ų to 23 ų for the Y-36° and Y-42° planes, indicating that a point-like defect could be the source of plastic initiation. In the X-112° plane, the activation volume was between 6 ų and 83 ų. Cooperative migration of several atoms could also be the mechanism of dislocation activation at deep nanoindentation.

Relying on nanoindentation technology, we investigated the elastic-to-plastic transition via first pop-in event and estimated the corresponding shear stress for incipient plasticity, i.e., yielding in the three typical orientations, i.e., X-112°, Y-36°, and Y-42° planes. The occurrence of incipient plasticity exhibited a stochastic distribution in a wide range for the three orientations. Accordingly, the obtained values of yield stress were uniform and scattered in the range from about 4 to 7 GPa for LiTaO₃ single crystal. The orientation effect on yield stress at the nano-scale was revealed to be insignificant in LiTaO₃ single crystal. The yield stresses were 5.44 ± 0.41, 5.74 ± 0.59, and 5.34 ± 0.525 GPa for the X-112°, Y-36°, and Y-42° planes, respectively. The activation volumes of dislocation nucleation were computed based on the cumulative distribution of yield stress, which were 12 ų, 8 ų, and 9 ų for the X-112°, Y-36°, and Y-42° planes. The results indicated that point-like defects could be the source of plastic initiation on the surface of LiTaO₃ single crystal.

In modern fossil and nuclear power plants, the components are subjected to creep, fatigue, and creep-fatigue (CF) due to frequent start-up and shut-down operations at high temperatures. The CF interaction on the in-service P92 steel welded joint was investigated by strain-controlled CF tests with different dwell times of 30, 120, 300, 600 and 900 s at 650 °C. Based on the observations of the fracture surface by scanning electron microscope (SEM), the characteristic microstructure of fatigue-induced damage was found for the CF specimens with short dwell times (30 and 120 s). The hardness, elastic modulus and creep deformation near the fracture edges of four typical CF specimens with 30, 120, 600 and 900 s dwell times were measured by nanoindentation. Compared to specimens with post-weld heat treatment (PWHT), lower hardness and creep strength were found for all CF specimens. In addition, significant reductions in hardness, elastic modulus, and creep strength were measured near the fracture edges for the CF specimens with short dwell times compared to the PWHT specimens. Compared to PWHT specimens (0.007), the increased strain rate sensitivities (SRS) of 0.010 to 0.17 were estimated from secondary creep. The increased values of SRS indicate that the room temperature creeps behavior is strongly affected by the decrease in dislocation density after the CF tests.

We systematically studied nanoindentation size effect on creep deformation in a La-based metallic glassy film, including holding depth effect and indenter size effect. Creep displacement was mainly dependent on both holding strain and deformation volume beneath indenter. Under elastic holding, creep strain was merely holding strain–dependent. While for plastic holding, creep strain was greatly enhanced by adopting smaller indenter and/or decreasing holding depth at the same holding strain. A strong nanoindentation size effect on creep resistance was validated. Strain rate sensitivities (SRS) were calculated, which were obviously higher at elastic regions than at plastic holdings. The relationship between SRS value and creep mechanism in metallic glass was discussed.

In fossil fuel and nuclear power plants, welded joints continuously experience creep-fatigue loading, which can result in premature cracking during the in-service term. To study the creep-fatigue interactive (CFI) behavior, the CFI test of P92 steel was performed with different strain rates at 823 K. Results indicate that the short cycle life is measured with the increasing strain rate. Relying on the scanning electron microscope, the fracture mechanism of P92 steel gradually changes from fatigue-dominating to creep-fatigue interactive damage with the increasing strain rate. The hardness (H), elastic modulus (E) and creep deformation were then measured by nanoindentation, and the strain rate sensitivity (m) was estimated. The relation between the degenerated mechanical properties and microstructural evaluations, i.e., enhanced grain size and nucleation of creep voids, was established, and the damage mechanism was discussed.