Search

Different grain coarsening behaviors (i.e. abnormal and homogeneous) are prevalently observed in gradient nanograined (GNG) Cu under stress controlled high-cycle and strain controlled low-cycle fatigue tests, respectively. In this paper, to comprehensively understand the intrinsic fatigue mechanism of gradient nanograined structures, both high and low cycle fatigue behaviors of GNG Cu are investigated under strain-controlled fatigue tests with a wide strain amplitude ranges. Cyclic behavior transition from abnormal grain coarsening at small strain amplitude to homogeneous grain coarsening at large strain amplitude is observd in GNG Cu. Microstrucural analysis reveals that the grain coarsening behavior in either abnormal or normal (homogeneous) mode is closely related to the spatial distribution of the cyclic plastic strain in the GNG layer (localized or delocalized) under cyclic loading. Such unique cyclic strain amplitude-dependent fatigue behavior is inherent to the gradient nanostructure, which fundamentally differs from the conventional strain localizing mechanism in metals with homogeneous structures under cyclic loading.

Dense mullite ceramics with excellent mechanical properties were produced using three kinds of mullite powders, namely micro-, submicro-, and nano-powders. The influence of sintering temperature on the microstructural and mechanical properties of the produced mullite ceramics was investigated. As the sintering temperature increased from 1200 to 1500 °C, the relative density of the mullite ceramic produced using micro-powder varied from 65.3 to 99.2%, leading to an increase in the flexural strength and fracture toughness from 87.6 to 245.7 MPa, and 1.4 to 2.5 MPa m1/2, respectively. The densification temperature of mullite ceramics significantly reduced when submicro- and nano-powders were used as raw materials. The relative density and mechanical properties of mullite ceramics fabricated using submicro- or nano-powders increased at first, then decreased following an increase in the sintering temperature. The mullite ceramic produced using submicro-powder became compact after being sintered at 1200 °C, and exhibited a high flexural strength of 254.7 MPa and fracture toughness of 2.9 MPa m1/2, indicating that low-temperature densification of mullite ceramics can be realised by enhancing the sintering activity of the mullite powder.

Fatigue behaviors of two Cu samples with different volume fractions of gradient nanograined (GNG) surface layer, prepared by surface mechanical grinding treatment, are investigated under stress-controlled high-cycle fatigue tests. With increasing the volume fraction of GNG layer from 4.3% to 13%, the fatigue endurance limit (at 107 cycles) of GNG Cu is enhanced from 88 to 98 MPa, much higher than that of coarse grained counterpart. Large volume fraction of GNG layer effectively postpones the extension of abnormal grain coarsening from a deeper subsurface layer to the topmost surface and thereby retards the initiation of surface fatigue crack.

The residual stress significantly influences the cyclic stress response of the hierarchical nanostructured materials. An obvious tension-compression asymmetry with minimum stress in compression larger than maximum stress in tension was observed in gradient nanograined (GNG) Cu under strain-controlled high-cycle fatigue tests, which gradually diminished with increasing cycles or after being annealed at a low temperature. The observed asymmetric response is primarily induced by the presence of the residual compressive stress in the GNG surface layer. The longer fatigue life can be achieved in GNG Cu with a higher residual stress, compared to that of annealed GNG Cu. Obvious tension-compression asymmetry was observed in cyclically deformed gradient nanograined Cu under strain control, caused by the residual compressive stress in the GNG surface layer.

Microstructural evolution of gradient nanograined (GNG) Cu was investigated under strain-controlled cyclic deformation. Mechanically driven abnormal grain coarsening initiated from the ultrafine grained subsurface layer and eventually extended to the nanograined top surface with increasing cycles. Abnormal grain coarsening and the formation of dislocation patterns sustain the cyclic plastic strain and postpone the formation of extrusions/intrusions at the top surface, contributing to the enhancement of overall cyclic properties of GNG Cu.

Cyclic stresses generally lead to fatigue damage and failure with important implications for material and component design, safety, performance and lifetime costs in major structural applications. Here we present unique results for copper to demonstrate that a thin superficial layer of graded surface nanostructure over a coarse-grained core suppresses strain localization and surface roughening, thereby imparting unprecedented resistance to both low-cycle and high-cycle fatigue without compromising ductility. Progressive homogenization of the surface-graded copper is shown to be superior in fatigue properties compared to that of any of its homogeneous counterparts with micro-, submicro- or nano-grained structures. Since the findings here for enhancing resistance to fatigue are broadly applicable to a wide spectrum of engineering metals and alloys, the present results offer unique pathways to mitigate fatigue damage using a broad variety of processing routes, geometric design considerations, and structural parameters in many practical applications. Nanyang Technological University L.L acknowledges the financial support by the National Science Foundation of China (Grants Nos. 51420105001, 51471172 and U1608257) and the Key Research Program of Frontier Science, CAS. S.S. acknowledges support from the Distinguished University Professorship at Nanyang Technological University, Singapore.

Schematic illustrations of the residual stress measurement by X-ray diffraction