High Surface Compressive Stresses Research Articles

Experimental and numerical studies of fatigue behavior of Ti6Al4V alloy treated by laser shock peening

Laser shock peening (LSP) is applied as the laser surface treatment technology to improve the fatigue resistance of aircraft parts with complex geometric features. In this paper, the effects of laser energy inputs on residual stresses and fatigue properties for hole-structured specimens of Ti6Al4V alloy treated by LSP are studied. Changes of residual stress distributions, fatigue behaviors and fracture morphologies are analyzed with different laser-induced shock pressures. The results show the fatigue life is prolonged by 137% compared with the untreated specimens due to the introduction of compressive residual stresses. The sectional observations of fatigue fracture morphology show the inwards movement of fatigue crack initiation positions and the complexity of fracture cracks after LSP treatment. The simulation results confirm that the fatigue life with different laser energy inputs depends strongly on the three-dimensional stress distribution around the hole. Among them, high surface compressive stresses and deep stress affected layers play a decisive role in improving fatigue life. The results in this study broaden the knowledge of fatigue behavior and explore engineering potentials of improved fatigue life for complex industrial specimens.

Application of hybrid additive manufacturing technology for performance improvement of martensitic stainless steel

Additive manufacturing (AM) is considered to be an attractive technology that offers tremendous advantages in rapidly producing complex-shaped components close to their final/net shape. However, uncontrolled metallurgical defects (pores, cracks, and inclusions), deleterious tensile residual stresses (RS), as well as scattered and unsatisfactory material performance have hindered its applications. In this study, layer-by-layer ultrasonic micro-forging treatment (UMT) was combined with directed energy deposition (DED) to substantially improve the mechanical performance (e.g., strength-ductility synergy, and fatigue behavior) and corrosion resistance of as-built 15–5PH stainless steel (SS). The surface RS distribution, mechanical properties, corrosion resistance, and microstructural properties of the non-UMT and UMT specimens were analyzed. In particular, focused ion beam (FIB) was employed to prepare the specimen for transmission electron microscopy (TEM) observations of the surface severe plastic deformation (SSPD) zone after UMT. The results indicated that the process with UMT resulted in higher surface compressive stress (~−750 MPa), reduced oxide inclusions, and refined martensite blocks (23.8% finer than that of the non-UMT specimen). The improvement of these characteristics induced by UMT further led to a comprehensive increase in the strength, elongation, fatigue resistance, corrosion resistance, and surface hardness of the DED fabricated parts. This work demonstrated the tremendous potential of integrated DED technology for tailoring the microstructure, controlling the quality, and producing martensitic SS with excellent mechanical performance and corrosion resistance.

Atomistic Understanding of Ion Exchange Strengthening of Boroaluminosilicate Glasses: Insights from Molecular Dynamics Simulations and QSPR Analysis.

Ion exchange (IOX) is an effective and widely used method to enhance mechanical properties of various glass products ranging from the touch screen of consumer electronics to window shields of airplanes and spacecrafts. IOX or chemical strengthening is achieved through the creation of a compressive surface layer on the glass product. Although widely studied experimentally, the fundamental understanding of the IOX strengthening process is still limited. In this work, we have applied large-scale atomistic simulations to understand IOX-induced mechanical property changes and their relation to the glass composition and structural characteristics. Two series of borosilicate glasses are studied to elucidate the composition effect, with boron oxide for silica and alumina for silica substitutions, respectively, on the mechanical properties of different levels of K+ to Na+ ion exchanges by using molecular dynamics (MD) simulations with a set of recently developed effective partial charge potentials. The linear network dilation coefficient (LNDC), a common measure of IOX behaviors, was calculated for each of the glass compositions. Quantitative structural property relationship (QSPR) analysis based on the MD-generated structural features was used to establish the structure-property correlations of mechanical and other properties. The results show strong composition dependence of the LNDC, hence the suitability of IOX strengthening. This behavior is discussed based on glass structure features of the glasses. It was found that glass compositions with a higher amount of mixed glass formers, higher network connectivity, and less complex components tend to show higher calculated LNDC and higher surface compressive stress. MD simulations, in combination with QSPR analysis, can thus provide atomistic insights into how the glass composition and structural characteristics affect IOX behaviors.

Design and fabrication of TiB2–TiC–Al2O3 gradient composite ceramic tool materials reinforced by VC/Cr3C2 additives

In this study, TiB2–TiC–Al2O3 gradient composite ceramic tool materials reinforced by VC/Cr3C2 additives were prepared through vacuum hot-press sintering. The effects of gradient combination, thickness ratio and sintering process (sintering temperature and soaking time) on the mechanical properties and microstructures of the composites were investigated. The results showed that the high surface compressive stress produced by the adequate gradient combination and thickness ratios refined the grains and increased the crack growth path. By comparison, the sintering temperature and soaking time influenced the grain size and porosity. In addition, the effects of VC/Cr3C2 on the grain size were analysed, and it was found that VC/Cr3C2 inhibited TiC dissolution and accelerated Al2O3 re-precipitation, which in turn refined the TiC grains and coarsened the Al2O3 grains. Consequently, crack deflection and intergranular fracture occurred around the TiC grains, whereas trans-granular fracture occurred inside the TiB2 and Al2O3 grains because rapid Al2O3 precipitation facilitated TiB2 grain growth. The optimum mechanical properties were hardness of 23.59 GPa, fracture toughness of 7.45 MPa m1/2, and flexural strength of 764.11 MPa.

Determination of microstructure and mechanical properties of functionally graded WC-TiC-Al 2 O 3 -GNPs micro-nano composite tool materials via two-step sintering

The effect of Al2O3-TiC additions on graded WC-Al2O3-TiC-GNPs composite synthesized employing two-step sintering was investigated. Al2O3-TiC additions with a proper weight ratio can not only facilitate the sintering but also make Al2O3 grains homogeneously distributed in the matrix and prompted the WC (0001) plane preferred orientation. Nano-grained TiC entered into the WC matrix and formed (Ti0.8, W0.2) C solid solution phase intragranular structure, which contributed to the formation of WC grains with irregular shape. High surface compressive stress, strong interface bonding, crack bridging, crack deflection and Al2O3 pullout are the major mechanisms contributing to the enhanced flexural strength and fracture toughness.

Determination of microstructure and mechanical properties of VC/Cr 3 C 2 reinforced functionally graded WC-TiC-Al 2 O 3 micro-nano composite tool materials via two-step sintering

Abstract The effect of VC, Cr3C2 and combination (Cr3C2 and VC with varied weight ratios of Cr3C2/(Cr3C2+VC)) on the microstructure and mechanical properties of functionally graded WC-Al2O3-TiC composite synthesized employing two-step hot-pressing sintering (heated to 1700 °C and then immediately cooled to 1600 °C with a soaking time of 30min) was comprehensively investigated. Combined addition of VC and Cr3C2 with an appropriate ratio to WC-TiC-Al2O3 composite performed a more pronounced effectiveness on inhibiting grain growth and improving the homogeneous dispersion of Al2O3 nano-particulates in WC matrix than single addition of VC or Cr3C2 did. Furthermore, single Cr3C2 addition outperformed VC in grain refinement and uniform distribution of Al2O3. The experimental results show that excellent mechanical properties are achieved for Cr3C2-VC additions with Cr3C2/(Cr3C2+VC) weight ratio 0.6 with a hardness of 25.64 GPa, a flexural strength of 1209.6 MPa, a fracture toughness of 11.49 MPa mm1/2 and a surface residual stress of −591.9 MPa. High surface compressive stress, strong interface bonding, crack branching and bridging, crack deflection, microcracking and Al2O3 pullout are the major mechanisms contributing to the drastically enhanced flexural strength and fracture toughness. The five-symmetric-layer graded ceramics developed in this paper provide an exceptional combination of high toughness and high hardness, which are conducive to the application as metal cutting tools.

Containers for the 21st century: Opportunities for lightweighting

Potential methods for decreasing the weight of glass containers are reviewed. The greatest potential lies in methods for producing high surface compressive stresses. Glazing, controlled crystallisation and ion-exchange techniques appear most promising, but all have serious disadvantages.

Stability of ion exchange-strengthened plate glass to local microdefects

investigations of the past few years show that industrial sodium calcium silicate plate glass acquires strength equal to 500-700 MPa as a result of treatment in potassium salt melts. The possibilities of using it as a structural component operated under conditions of abrasive surface damage have been discussed widely in the literature. Some authors substantiate the lack of prospects for using ion exchange-strengthened plate Nass in articles of structural optics by the shallow depth of the compressed layer, while others, on the contrary, note its good operating properties that are manifested due to the high surface compressive stresses. This contradiction of opinions is explained to a significant degree by the absence of a generally accepted method and criterion for evaluating the abrasives resistance of silicate glass.

Increased durability of optical fiber through the use of compressive cladding

Durable fibers having a thin Ti-doped silica surface layer were fabricated by using the refined fabrication process of compressive cladding. High surface-compressive stresses up to 0.69 GPa (100 kpsi) resulted in significant improvement in fatigue resistance of optical-fiber waveguides.