Creep feed grinding induced gradient microstructures in the superficial layer of turbine blade root of single crystal nickel-based superalloy

The service performance of the turbine blade root of an aero-engine depends on the microstructures in its superficial layer.This work investigated the surface deformation structures of turbine blade root of single crystal nickel-based superalloy produced under different creep feed grinding conditions.Gradient microstructures in the superficial layer were clarified and composed of a severely deformed layer(DFL)with nano-sized grains(48–67 nm)at the topmost surface,a DFL with submicron-sized grains(66–158 nm)and micron-sized laminated structures at the subsurface,and a dislocation accumulated layer extending to the bulk material.The formation of such gradient microstructures was found to be related to the graded variations in the plastic strain and strain rate induced in the creep feed grinding process,which were as high as 6.67 and 8.17×10^(7)s^(−1),respectively.In the current study,the evolution of surface gradient microstructures was essentially a transition process from a coarse single crystal to nano-sized grains and,simultaneously,from one orientation of a single crystal to random orientations of polycrystals,during which the dislocation slips dominated the creep feed grinding induced microstructure deformation of single crystal nickel-based superalloy.

Gradient microstructure and superior strength-ductility synergy of AZ61 magnesium alloy bars processed by radial forging with different deformation temperatures

Gradient microstructure modification is a cost-efficient strategy for high strength without compromising ductility,which is urgently needed in the fundamental science of engineering materials.In this study,heterogeneous structures of AZ61 alloy bars with anisotropic gradients(with different grain size distributions from the surface to the center)were observed to exhibit strong strength-ductility synergies under different deformation tem peratures.The results reveal that the grain refinement process under mediumlow temperature deformation conditions(≤350℃)consists of four transition stages along the radial direction,i.e.,twin activations and deformation band formations,dislocation cells and pile-ups,ultrafine sub-grains,and randomly orientated quasi-micron grains.Different deformation temperatures have a great influence on twin activations and deformation band formations,and the high temperature can easily provoke the initiation of non-basal slip.The deformation bands were determined as a primary nucleation site due to their highly unstable dislocation hindrance ability.Analysis in combination with the Radial forging(RF)deformation process,the differences of dynamic precipitates can be attributed to microstructural difference and solubility limit of Al at different tem peratures.By summarizing the tensile test results,the sample forged at 350℃exhibited the best strength-ductility synergy,exhibiting the highest elongation(EL)of 23.2%with a 251 MPa yield strength(YS)and 394 MPa ultimate tensile strength(UTS)in center region,and combined with the highest strength value of 256 MPa YS and 420 MPa UTS in the center region,while the EL was slightly degraded to 19.8%.

Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel

This work investigated the gradient microstructure evolution and tensile property of LPBF fabricated 15-5 precipitation hardening stainless steel in post-process direct ageing(DA)and solution treating&ageing(STA).The varied microstructures for austenite and small-sized oxide inclusions at different sample heights in the as-built(AB)condition was generally preserved after DA treatment.However,austenite was almost disappeared,and oxide particle grew significantly after the STA treatment.As a result,the tensile property differences in sample top and bottom for AB and DA conditions did not occur in the STA samples.For the influence of post-process heat treatment,the STA condition had the highest yield strength due to the highest volume fraction of nano-sized Cu precipitates.However,the DA specimen had the highest ultimate tensile strength and elongation owing to the considerable amount of austenite phase and associated transformation induced plasticity effect.

Gradient microstructure,recrystallization and mechanical properties of copper processed by high pressure surface rolling

The microstructure,hardness and tensile properties have been studied in copper processed by high pressure surface rolling(HPSR)both in the as-deformed condition and after subsequent annealing at 150℃.It is found that a gradient structure with significant differences in the scale of microstructural features is formed by HPSR.The deformed microstructure varies from nano-and ultrafine-scale structures with a large fraction of high angle boundaries near the surface to lightly deformed grains at depths of 1-3 mm below the surface.Tensile tests of 1-mm-thick specimens demonstrate that the asdeformed material has a high strength and a low uniform elongation.Annealing at 150℃results in partial recrystallization,which creates new through-thickness gradients.Except for the topmost layer and several bands in the adjacent layer,recrystallization is more pronounced close to the surface.The fraction recrystallized is at least 80%at depths of 60-300μm after annealing for 960 min.The fraction recrystallized in the subsurface decreases with increasing depth,and the deformed layer at depths greater than 500μm re-mains largely non-recrystallized after annealing.This partially recrystallized condition demonstrates an improved combination of strength and ductility.

Microstructure and Mechanical Properties of ZrO_2-Ni Functionally Gradient Material

The fabrication. microstructure and mechanical properties of ZrO2-Ni functionally gradient materials (FGM ) have been studied. FGM as well as non-FG M of ZrO2-Ni system was developed by powder metallurgical process. X-ray diffractometer (XRD ). electron probe microanalyzer (EPMA), scanning electron microscope (SEM ) and optical microscope were employed to investigate the crystalline phases. chemical composition and microstructure Experimental results demonstrate that the composition and microstructure of ZrO2-Ni FGM have the expected gradient distribution. There are no distinct interfaces in the FGM due to the gradient change of components. that is, the constituents are continuous in microstructure everywhere. Moreover, Vickers hardness and flexural strength were measured for the common composites as a function of composition. It is made clear that the mechanical properties of the FGM vary corresponding to the constitutional changes as well

C^1 natural element method for strain gradient linear elasticity and its application to microstructures

C^1 natural element method （C^1 NEM） is applied to strain gradient linear elasticity, and size effects on mi crostructures are analyzed. The shape functions in C^1 NEM are built upon the natural neighbor interpolation （NNI）, with interpolation realized to nodal function and nodal gradient values, so that the essential boundary conditions （EBCs） can be imposed directly in a Galerkin scheme for partial differential equations （PDEs）. In the present paper, C^1 NEM for strain gradient linear elasticity is constructed, and sev- eral typical examples which have analytical solutions are presented to illustrate the effectiveness of the constructed method. In its application to microstructures, the size effects of bending stiffness and stress concentration factor （SCF） are studied for microspeciem and microgripper, respectively. It is observed that the size effects become rather strong when the width of spring for microgripper, the radius of circular perforation and the long axis of elliptical perforation for microspeciem come close to the material characteristic length scales. For the U-shaped notch, the size effects decline obviously with increasing notch radius, and decline mildly with increasing length of notch.

Effects of normal stress, surface roughness, and initial grain size on the microstructure of copper subjected to platen friction sliding deformation

The effects of applied normal stress, surface roughness, and initial grain size on the microstructure of pure Cu developed during platen friction sliding deformation （PFSD） processing were investigated. In each case, the deformation microstructure was characterized and the hardness of the treated surface layer was measured to evaluate its strength. The results indicated that the thickness of the deformed layer and the hardness at any depth increased with increasing normal stress. A smaller steel platen surface roughness resulted in less microstruc- tural refinement, whereas the microstructural refinement was enhanced by decreasing the surface roughness of the Cu sample. In the case of a very large initial grain size （d 〉 10 mm）, a sharper transition from fine-grain microstructure to undeformed material was obtained in the treated surface layer after PFSD processing.

Tunable 3D Nanofiber Architecture of Polycaprolactone by Divergence Electrospinning for Potential Tissue Engineering Applications

The creation of biomimetic cell environments with micro and nanoscale topographical features resembling native tissues is critical for tissue engineering. To address this challenge, this study focuses on an innovative electrospinning strategy that adopts a symmetrically divergent electric field to induce rapid self-assembly of aligned polycaprolactone(PCL) nanofibers into a centimeter-scale architecture between separately grounded bevels. The 3D microstructures of the nanofiber scaffolds were characterized through a series of sectioning in both vertical and horizontal directions. PCL/collagen(type I)nanofiber scaffolds with different density gradients were incorporated in sodium alginate hydrogels and subjected to elemental analysis. Human fibroblasts were seeded onto the scaffolds and cultured for 7 days. Our studies showed that the inclination angle of the collector had significant effects on nanofiber attributes, including the mean diameter, density gradient, and alignment gradient. The fiber density and alignment at the peripheral area of the 45°-collector decreased by 21% and 55%, respectively, along the z-axis,while those of the 60°-collector decreased by 71% and 60%, respectively. By altering the geometry of the conductive areas on the collecting bevels, polyhedral and cylindrical scaffolds composed of aligned fibers were directly fabricated. By using a four-bevel collector, the nanofibers formed a matrix of microgrids with a density of 11%. The gradient of nitrogen-to-carbon ratio in the scaffold-incorporated hydrogel was consistent with the nanofiber density gradient. The scaffolds provided biophysical stimuli to facilitate cell adhesion, proliferation, and morphogenesis in 3D.

Deformation mechanism and in-situ TEM compression behavior of TB8βtitanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals’surfaces and improve their mechanical properties.However,the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied.Here,ultrasonic surface rolling process(USRP)was used to create a gradient microstructure,consisting of amorphous,equiaxed nano-grained,nano-laminated,ultrafine laminated and ultrafine grained structure on the surface of TB8βtitanium alloy.High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing.In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain,in order to reveal the micromechanics behavior of different grain morphologies.The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers;the ultrafine grained layer and single crystal had an excellent strain hardening rate.The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties.Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy.An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain boundaries caused the grain refinement,where twinning could not be found and no phase transformation occurred.

Effect of rotationally accelerated shot peening on the microstructure and mechanical behavior of a metastableβtitanium alloy

Microstructural evolution and deformation mechanism of a metastableβalloy(Ti-10 V-2 Fe-3 Al)processed by rotationally accelerated shot peening(RASP)were systematically investigated with optical microscopy,X-ray diffraction,electron backscatter diffraction and transmission electron microscopy.Different gradient hierarchical microstructures(gradients inα″martensite andβphase,and hierarchical twins range from the nanoscale to microscale)can be fabricated by RASP via changing the shot peening time.The hardening behavior and tensile mechanical properties of gradient hierarchical microstructure were systematically explored.Novel deformation twinning systems of{112}α″and{130}<310>α″in the kinkedα″martensite were revealed during the tensile deformation.It was found that stress-induced martensitic transformation,twinnedα″martensite and the related dynamic grain refinement contribute to hardness and work hardening ability.Simultaneous improvement of strength and ductility of the metastableα″titanium alloy can be achieved by introducing a gradient hierarchical microstructure.

Gradient Structured Copper by Rotationally Accelerated Shot Peening

A new technology-rotationally accelerated shot peening（RASP）, was developed to prepare gradient structured materials. By using centrifugal acceleration principle and large steel balls, the RASP technology can produce much higher impact energy compared to conventional shot peening. As a proof-of-concept demonstration, the RASP was utilized to refine the surface layer in pure copper（Cu） with an average grain size of 85 nm. The grain size increases largely from surface downwards the bulk, forming an800 ？m thick gradient-structured surface layer and consequently a micro-hardness gradient. The difference between the RASP technology and other established techniques in preparing gradient structured materials is discussed. The RASP technology exhibits a promoting future for large-scale manufacturing of gradient materials.