Residual Stress Distribution of Si_(3)N_(4)/SiC Gradient Material and the Effect of Residual Stress on Material Properties

The Si_(3)N_(4)/SiC gradient material with a gradient composition structure was prepared by a hot pressing sintering.The sinterability,distribution of residual stress and the effect of residual stress on mechanical properties of Si_(3)N_(4)/SiC gradient materials were studied.The research results show that,at 1750℃,Si_(3)N_(4)/SiC gradient materials with different ratios can achieve co-sintering,and the overall relative density of the sample reaches 98.5%.Interestingly,the flexural strength of Si_(3)N_(4)/SiC gradient material is related to its loading surface.The flexural strength of SiC as the loading surface is about 35%higher than that of Si_(3)N_(4)as the loading surface.The analysis of the residual stress of the material in the gradient structure shows that the gradient stress distribution between the two phases is a vital factor affecting the mechanical properties of the material.With the increase of SiC content in the gradient direction,the fracture toughness of each layer of Si_(3)N_(4)/SiC gradient materials gradually decreases.The surface hardness of the pure SiC side is lower than that reported in other literature.

Microstructure and ablation behavior of Zr-based ultra-high-temperature gradient composites

To obtain high-performance Zr-based ultra-high-temperature composites,Zr-based ultra-high-temperature gradient composites were prepared by changing the laying method of the infiltrant via reactive melt infiltration.The effects of different infiltrant laying methods on the microstructure and ablative properties of Zr-based ultrahigh-temperature gradient composites were investigated.The results showed that the gradient structure of the Zr-based ultrahigh-temperature gradient composites differed when the composition ratio of the infiltrant was changed.When the thicknesses of the Zr/Mo/Si layers were 6/4/12 mm and 8/2/12 mm,the SiMoZrC solid solution content in the samples increased and decreased along the infiltration direction,respectively.The gradient samples were ablated in an oxyacetylene flame at 3000°C for 40 s.The ablation resistance of the sample was the highest when the infiltrant was a powder and the thickness of the Zr/Mo/Si layer was 6/4/12 mm.

Arc additive manufacturing of gradient hot forging die with shaped waterways

Under the working environment of high temperature and strong load impact,hot forging die is prone to failure which reduces the service life of die.Using arc additive manufacturing in the die cavity,a gradient material hot forging die with high precision,superior per-formance,and conformal cooling channels is developed.This improves the toughness of the die cavity and reduces the working temperature,thereby forming an isothermal field,which is an effective method to enhance the lifespan of the hot forging die.Three kinds of gradient flux-cored wires are designed for the surface of 5CrNiMo steel,and the microstructure and mechanical properties between gradient interfaces were studied.Based on the spatial curved structure of shaped waterways in the hot forging die cavity,a study was conducted on the strategy of partitioned forming for the manufacturing of the die with shaped waterways.In order to avoid interference with the arc gun,the hot for-ging die is divided into four regions,namely the transition region,upper,middle,and lower region,based on a combination of cavity depth and internal U-shaped and quadrilateral structures.The results show that the developed flux-cored wires have good moldability with straight sides of deposited metal under different process parameters and flat surface without cracks,pores and other defects.Under the same working conditions,the life of hot forging die formed by the gradient materials is more than multiple times that of the single material hot forging die,and the temperature gradient field of the shaped waterway die is 7℃/cm smaller than that of traditional straight waterway.

DESIGN AND PROPERTIES OF A FUNCTIONALLY GRADIENT CERAMIC TOOL MATERIAL

Based on the analyses of the severity of cutting process as well as the failure mechanisms of ceramic tools, a model for designing functionally gradient ceramic tool materials with symmetrical distribution is presented, by which a Al 2O 3/(W,Ti)C ceramic tool material FG 2 was developed. Multi objective optimization method was employed in designing the compositional distribution of this ceramic tool material. The results of both continuous and intermittent cutting tests are indicative of the much better cutting behavior of the functionally gradient ceramic tool FG 2 than that of the common ceramic tool SG 4.

NVESTIGATION OF PRODUCTOPN OF FUNCTIONALLY GRADIENT MATERIAL BY ELECTROLESS PLATING

Thispaper proposed a new methodof producing Ceramic/ Metalfunctionally gradient mate rialby electroless platingtechnique. The experimentof producing SiC/ Ni PFGM wascar ried out with self made electroless plating facilities. The results show that the thickness of FGMcoating andthegradientdistribution ofcompositioncanbecontrolled byoptimizingelec trolessplating technology and changing the parameters such as plating time, the additionspeed and concentration of SiCparticles. Analysisdemonstratesthatthereisastrongrelation ship amongthe SiCcontent,the microstructureandthe mechanicalproperty ofthe FGM.

Preparation of 2024/3003 gradient materials by semi-continuous casting using double-stream-pouring technique

The 2024/3003 aluminum gradient alloys are prepared by semi continuous casting. The influences of throttle bore diameter of embedded nozzle and temperature of internal melt on composition distribution, macrostructure, hardness are analyzed, and the stability of gradient distribution of composition, macrostructure and hardness along the axial direction of the ingot is also studied. The results show that diffe rent composition profiles can be achieved by adjusting the processing parameters; the volume fraction of inner alloy in the ingot can be increased by enlarging the throttle bore diameter and elevating the temperature of inner melt; quasi steady solidification can be realized within 20 s during cast processing, and consistent quality ingot is obtained by controlling the casting speed and liquid height of inner melt.

Microstructure and mechanical properties of additive manufactured steel-Al structure materials with nickel gradient layers

The microstructure and mechanical properties of steel/Al structure material produced by additive manufacturing （AM） was investigated in this work based on the cold metal transfer welding. The results show that the microstructure gradually changed from the steel side to the aluminum side. The microstructure in the steel layer consisted of vermiform like ferrite and anstenite structure, while in the aluminum layer the microstructure was constituted by c^-A1 grains and typical reticulate distributive Al-Si eutectic structure. Besides, a 7 y.m thickness Ni-Al intermetallic compound layer was emerged at the interface of nickel and aluminum layer. The maximum room-temperature tensile strength of the Steel-Al structure materials was found to be 54 MPa, the rupture morphology showed a brittle fracture characteristic.

Crack tip field in functionally gradient material with exponential variation of elastic constants in two directions

This paper presents an exact solution of the crack tip field in functionally gradient material with exponential variation of elastic constants. The dimensionless Poisson＇s ratios v0 of the engineering materials （iron, glass …… ） are far less than one; therefore, neglecting them, one can simplify the basic equation and the exact solution is easy to obtain. Although the exact solution for the case v0 ≠ 0 is also obtained, it is very complicated and the main result is the same with the case v0 = 0 （it will be dealt with in Appendix VII）. It has been found that the exponential term exp（ax ＋ by） in the constitutive equations becomes exp（ ax /2 ＋ by/2- kr /2 ） in the exact solution.

AN IN SITU SURFACE COMPOSITE AND GRADIENT MATERIAL OF Al-Si ALLOY PRODUCED BY ELECTROMAGNETIC FORCE

Because of the different conductivities between the primary phase (low electric conduc tivity) and the metal melt, electromagnetic force scarcely acts on the primary phase. Thus, an electromagnetic repulsive force applied by the metal melt exerts on the pri mary phase when the movement of the melt in the direction of electromagnetic force is limited. As a result, the repulsive force exerts on the primary phase to push them to move in the direction opposite to that of the electromagnetic force when the metal melt with primary phase solidifies under an electromagnetic force field. Based on this, a new method for production of in situ surface composite and gradient material by electromagnetic force is proposed. An in situ primary Si reinforced surface composite of Al-15wt%Si alloy and gradient material of Al-l9wt%Si alloy were produced by this method. The microhardness of the primary Si is HV1320. The reinforced phase size is in the range from 40μm to 100μm. The wear resistance of Al-Si alloy gradient material can be more greatly increased than that of their matrix material.

ANTI-PLANE SHEAR CRACK IN A FUNCTIONALLY GRADIENT PIEZOELECTRIC MATERIAL

The main objective of this paper is to study the singular natureof the crack-tip stress and electric displacement field in afunctionally gradient piezoelectric medium having materialcoefficients with a discontinuous derivative. The problem isconsidered for the simplest possible loading and geometry, namely,the anti-plane shear stress and electric displacement in -plane oftwo bonded half spaces in which the crack is parallel to theinterface.

Ti/Al_2O_3 Functionally Gradient Material Prepared by the Explosive Compaction/SHS Process

Ti1Al2O3 Functionally Gradient Material (FGM) was prepared by an explosive compaction/SHS process. Ten sheets of the compounding powder were laminated and pressed to get a green body of FGM. It was then compacted explosively By burying the explosive compaction body into a stoichiometric Al/TiO2 mixture and igniting the combustion of the stoichiometric Al/TiO2 mixture, the SHS reaction of the explosive compaction body was initiated by the heat released from the combustion of the stoichiometric Al/TiO2 mixture. In this way, Ti/Al2O3 FGM was synthesized. The adiabatic temperatures of each gradient layer were calculated when the preheating temperatures were 298 K and 1173 K, respectively The microstructure, composition and properties of Ti/Al2O3 FGM and the reaction mechanism of each gradient layer were studied. It was found that Ti/Al2O3 FGM prepared by the explosive compaction/SHS process had a high density and a high microhardness. Its structure, composition and properties showed apparent gradient distribution. The structure of the standard stoichiometric ratio gradient layer of FGM was a network structure. Its reaction mode could be described as follows: Al powder melted first, then the molten Al penetrated into the TiO2 zone and reacted with TiO2, and big pores were left in the original positions of Al powder. The reaction of gradient layers with the addition of Al3O3 as diluents was similar to that of the standard stoichiometric ratio gradient layer, so were their structure and composition. However, the reaction of gradient layers with the addition of Ti as diluents was more complex and the composition deviated slightly from the designed one

AN ASYMPTOTIC METHOD FOR ANALYZING THE STRESS IN A FUNCTIONALLY GRADIENT MATERIAL LAYER ON A SURFACE OF A STRUCTURAL COMPONENT

A simple and effective method for analyzing the stress distribution in a Functionally Gradient Material(FGM) layer on the su;face of a structural component is proposed in this paper. Generally, the FGM layer is very thin compared with the characteristic length of the structural component, and the nonhomogeneity exists only in the thin layer. Based on these features, by choosing a small parameter I which characterizes the stiffness of the layer relative to the component, and expanding the stresses and displacements on the two sides of the interface according to the parameter lambda, then asymptotically using the continuity conditions of the stresses and displacements on the interface, a decoupling computing process of the coupling control equations of the layer and the structural component is realized. Finally, two examples are given to illustrate the application of the method proposed.

Genetic Algorithm for the Thermal Stresses Optimum Design ofFunctionally Gradient Material Plate

Based on the thermal stress distribution for functionally gradient material (FGM) plates, a Genetic Algorithm (GA) method for the thermal stresses optimum design of FGM plate with computer technologies is given. The minimum thermal stresses combination distribution for FGM is obtained.

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

Synthesis,characterization and biological activity in vitro of FeCrAl(f)/HA asymmetrical biological functionally gradient materials

FeCrAl(f)/HA biological functionally gradient materials(FGMs) were successfully fabricated by the hot pressing technique.Scanning electron microscope(SEM),energy dispersive spectrometer(EDS) and bending strength test machine were utilized to characterize the microstructure,component,mechanical properties and the formation of the Ca-deficient apatite on the surface of these materials.The results indicate that an asymmetrical FeCrAl(f)/HA FGM,consolidating powders prepared by mixing HA with 3%–15%(volume fraction) is successfully prepared.Both of the matrix and FeCrAl fiber are integrated very tightly and bite into each other very deeply.And counter diffusion takes place to some extent in two phase interfaces.The elemental compositions of the FeCrAl(f)/HA FGM change progressively.Ca and P contents increase gradually with immersion time increasing,and thereafter approach equilibrium.The bone-like apatite layer forms on the materials surface,which possesses benign bioactivity,and the favorable biocompatibility can provide potential firm fixation between FeCrAl(f)/HA asymmetrical FGM implants and human bone.

ELASTOPLASTIC OPTIMUM DESIGN OF Ni/TiC FUNCTIONAL GRADIENT MATERIAL

The thermal residual stresses developed in a disk-shaped, Ni/TiC functionally gradient material (FGM) during its fabrication are investigated by an elastic-plastic finite element numerical ap-proach. Constitutive relations for the graded Ni-TiC composite inter-layers between pure metal Ni and ceramic TiC are estimated by using an effectiv-medium approach, and effective plastic strain and stress distributions are computed for simulated cooling from an assumed fab-rication temperature. Analyses are performed for a fixed specimen ge-ometry and the graded region is treated as a series of perfectly bonded equal-thick layers. The results are compared with those obtained by on-ly considering elastic material response to assess the effect of plasticity on the optimum fabrication design of the Ni/TiC FGM. It is demon-strated that the consideration of plasticity is of critical importance for optimization of the metal/ceramic gradient materials.

Hardness Measurement of (TiB_2-TiAl)/TiAl Symmetrically Function Gradient Materials

( TiB2-TiAl)/TiAl symmetrically function gradient materials ( FGM) were prepared by spark plasma sintering ( SPS). Owing to the difference, of the thermal expansion coefficients between TiB2 and TiAl, a compressive surface stress was introduced to the FGM fry the thermal expansion mismatch. The hardness values of the uniform materials and the FGM were tested, respectively. For the FGM with a compressive surface stress, hardness is obviously superior to that of the uniform material. When the FGM was subjected to heat treatment, the hardness decreased due to a partial relaxation of the compressive surface stress.

THERMAL STRESSES ANALYSIS OF CERAMIC/METAL FUNCTIONALLY GRADIENT MATERIAL CYLINDER

In this paper, the composite cylinder system is made of three layers: metal, functionally gradient material (FGM) and ceramics is studied. The formulas of the steady_state temperature distribution and the associate thermal stress distribution in the cylinder are obtained. For ZrO 2/Ti_6Al_4V system, the distribution of steady_state temperature and thermal stress are calculated and discussed.

ANTI-PLANE FRACTURE ANALYSIS OF FUNCTIONALLY GRADIENT MATERIAL INFINITE STRIP WITH FINITE WIDTH

The special case of a crack under mode Ⅲ conditions was treated, lying parallel to the edges of an infinite strip with finite width and with the shear modulus varying exponentially perpendicular to the edges. By using Fourier transforms the problem was formulated in terms of a singular integral equation. It was numerically solved by representing the unknown dislocation density by a truncated series of Chebyshev polynomials leading to a linear system of equations. The stress intensity factor （SIF） results were discussed with respect to the influences of different geometric parameters and the strength of the non-homogeneity. It was indicated that the SIF increases with the increase of the crack length and decreases with the increase of the rigidity of the material in the vicinity of crack. The SIF of narrow strip is very sensitive to the change of the non-homogeneity parameter and its variation is complicated. With the increase of the non-homogeneity parameter, the stress intensity factor may increase, decrease or keep constant, which is mainly determined by the strip width and the relative crack location. If the crack is located at the midline of the strip or if the strip is wide, the stress intensity factor is not sensitive to the material non-homogeneity parameter.

Titanium diboride-metals gradient materials prepared by field activated diffusion bonding process

Functionally gradient samples are prepared by getting metal Ni or Cu bonded with Ni-matrix composites reinforced by TiB2 particles by field activated diffusion bonding process. The intermetallic compound of Ni3Al has been applied as a mediate layer in order to reduce residual stress. The microstracture, phase composition of the interfaces between the metal and Ni3Al are determined and the mechanical properties of the gradient materials are characterized. Elemental concentration profiles across the interfaces between layers showed significant diffusion dissolution and formation of firm bonds. Measured micro-hardness values of the sample increased monotonically from the metal substrate to the surface layer of composites. The values for the surface composite layer ranged from about 2 000 HK to 3 300 HK. The results of this investigation demonstrate the feasibility of field activated diffusion bonding process for rapid preparation of FGMs.