Multiscale Architectured Materials (MAM II): Tailoring Mechanical Incompatibility for Superior Properties: Gradient Materials
Sponsored by: TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Yuntian Zhu, North Carolina State University; Irene Beyerlein, University of California, Santa Barbara; Yves Brechet, Grenoble Institute of Technology; Huajian Gao, Brown University; Ke Lu, Institute of Metal Research, Chinese Academy of Science; Xiaolei Wu, Institute of Mechanics, Chinese Academy of Science
Monday 8:30 AM
February 27, 2017
Location: San Diego Convention Ctr
Session Chair: Ke Lu, Institute of Metal Research; Xiaolei Wu, Institute of Mechanics
8:30 AM Introductory Comments
8:35 AM Invited
Fatigue Behavior of Gradient Nanograined Cu: Qingsong Pan1; Lei Lu1; Jianzhou Long1; 1Institute of Metal Research, CAS
Majority of engineering metals in service suffer from fatigue failure under cyclic loading; however, trade-off between high-cycle and low-cycle fatigue properties prevails for coarse grained (CG) and nanostructured metals, like their strength and ductility trade-off. Here, we report the fatigue behavior of a gradient nano-grained Cu with spatial grain size gradient from nanograin at the top surface to coarse grain in the core under constant stress-controlled and strain-controlled tension-compression fatigue tests, respectively. A synergy increment of fatigue limit and fatigue life was observed in the gradient nanograined Cu, comparing with that of CG-Cu samples. The underlying cyclic stress/strain response associated with the stability of microstructure from top nanograined surface to coarse grained core, confinement of gradient structure, accumulated plastic strain will be discussed.
Strain Incompatibility and Ductility in a Gradient Nanostructure of IF Steel: Xiaolei Wu1; Yuntian Zhu2; 1Institute of Mechanics, Chinese Academy of Sciences; 2North Carolina State University
The origin behind large uniform tensile ductility in the gradient nanostructure (GNS) is still an open issue. Recently, it was reported that the GNS exhibits the Bauschinger effect during the tensile unloading-reloading cycle, indicative of strain incompatibility due to the microstructural heterogeneity in the GNS. Here we report the effect of back stress hardening on the tensile property in the gradient structure. It is observed that the gradient structures increase the yield strength and maintain the large uniform elongation comparable to that of the coarse-grained counterpart. The strain hardening due to back stresses is both depth and tensile strain dependence, which makes up for deficiencies of forest hardening. Even with the presence of the yield-peak, the hardening rate up-turns in the transient to obtain large uniform elongation. Gradient structures exhibit an anti-banana-type yield strength versus ductility synergy.
9:20 AM Invited
Effect of Gradient on Mechanical Behavior of Ni Based Gradient Materials: Y Lin1; R.Q. Cao1; J Pan1; Yi Li1; 1Institute of Metal Research
Materials with spatial gradients of various constituents can achieve superior mechanical properties and performance. The key to improve the properties is to properly optimize the gradient. In our present work through electro-deposition, we demonstrate the production of gradient structure in Ni based materials with various gradient. The constituent gradient can be grain size gradient and phase gradient including amorphous phase. We have found considerable grain growth during tensile deformation independent of gradient. Their effect on the mechanical properties will be reported and duscussed.
Suppression of Surface Fatigue Cracking in Steels with a Gradient Nanostructured Surface Layer: Z.B. Wang1; K. Zhang1; H.W. Huang1; K. Lu1; 1Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences
By means of surface mechanical rolling treatment (SMRT), a gradient nanostructured (GNS) surface layer is generated on steel samples with different microstructures. In a GNS surface layer, the mean grain size is in nanometer scale at the treated surface and gradually increases with depth, accompanied with a decreasing hardness. In comparison with traditional surface treatments such as shot peening and deep rolling, the GNS layers are much thicker in the SMRT samples. In this talk, correlations between microstructure and fatigue properties are studied on different SMRT steel samples in terms of crack initiation and propagation, residual stress, and cyclic deformation response. The results indicated that under cyclic loading, surface crack initiation is effectively suppressed by the GNS surface layers and most cracks are initiated in the subsurface. Fatigue properties of the SMRT samples are significantly enhanced with a higher fatigue strength and/or a longer lifetime relative to the untreated specimens.
10:05 AM Break
10:25 AM Invited
Superior Combinations of High Strength and Ductility in Compositionally Graded Martensitic Steels: Hatem Zurob1; Hamid Azizi1; Olivier Bouaziz2; David Embury1; 1McMaster University; 2University of Lorraine
Steels with more than 0.3 wt%C offer strength levels in excess of 2 GPa, but suffer from very limited ductility. Controlled decarburization of medium carbon steels can be utilized to produce architectured structures with unique combinations of strength and ductility. The improved ductility is attributed to the presence of low carbon martensite at the surface which serves to arrest surface cracks and prevent the propagation of shear bands which are formed within the high carbon core. These graded materials can be cold-rolled and annealed to produce a bimodal microstructure containing a mixture of nanoscale and microscale ferrite grains. This microstructure combines high yield strength and reasonable work-hardening. The heterogeneity of the graded microstructures can be varied by changing the decarburizing parameters. The application of multiple carburizing/decarburizing steps along with diffusion barriers can be used to introduce an additional degree of heterogeneity along the length and width of the material.
Tensile Behaviors of Gradient Nano-grained Copper at 77K: Xiuyan Li1; Xin Zhou1; Ke Lu1; 1Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS
Gradient nano-grained (GNG) metals were reported to have a much improved tensile ductility at room temperature compared with the homogeneous nano-grained counterparts owing to effective suppression of strain localization. In our study, cryogenic tensile behaviors of GNG Cu samples were investigated by using tensile tests at 77 K, including strength, ductility, and work-hardening rates. An even higher tensile ductility was obtained in the GNG Cu at 77 K than that at ambient temperature. Microstructures of the GNG samples before and after cryogenic tension were characterized by using SEM and TEM. The corresponding plastic deformation mechanism of GNG structures at cryogenic temperature will be analyzed based on the structure and property characterizations.
11:10 AM Invited
Stress and Strain Gradients in a Low Carbon Steel Deformed under Heavy Sliding: Xiaodan Zhang1; Niels Hansen1; Xiaoxu Huang1; 1Technical University of Denmark
A recent study  has shown that a microstructure can be refined to a record low of 5 nm and that dislocation glide is still a controlling mechanism at this length scale. In this study, by heavy rotatory sliding of a low carbon steel a gradient structure has been produced extending to about 2.5 mm below the surface reducing the structural scale to the nanometer dimension and increasing the strength to extreme values by dislocation and boundary strengthening. The microstructure and texture gradient are analyzed and stress and strain gradients produced by plastic deformation are evaluated based on the deformation microstructure using the classic stress-structure relationship. Computational and materials modelling has been advanced from bulk to gradient structures leading to dissemination of constitutive stress-strain equations in gradient structures.  D. A. Hughes and N. Hansen, Exploring the Limit of Dislocation Based Plasticity in Nanostructured Metals, PRL 112, 135504 (2014).
Novel Contributions to Deformation and Properties in Gradient Materials: Shan "Cecelia" Cao1; Christian Roach1; Yuntian Zhu1; Suveen Mathaudhu1; 1University of California Riverside
Metallic alloys with gradient microstructures have recently been shown to exhibit enhanced mechanical properties, namely increased strength with minimal loss in tensile ductility. One of the most common ways of engineering a gradient microstructure is via surface mechanical attrition treatment (SMAT) wherein hard balls are ultrasonically accelerated to impinge the surface and cause severe deformation and microstructural refinement. The improved properties are based on a number of proposed mechanisms, including dislocation cascading and mechanical incompatibility between layers, however less studied factors include the contributions of residual stress, texture, hardness and sample/process geometry. In this lecture, we will present research vignettes on each of these factors, and deliberate on their overall roles in property enhancement. The results will be used to explain seemingly incongruent phenomena observed in many reports, and further unravel and decouple deformation mechanisms in gradient materials.