Microstructural morphology effects on fracture toughness and crack growth behaviors in a high strength titanium alloy
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Transgranular fracture
Ductility (Earth science)
Fractography
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The hot-pressing sintering studies were carried out by using mixtures of cobalt coated Al2O3 and TiC micro-size and nano-size powders with different cobalt content at different temperatures.The results show that the sintering effects of nano-size powders is super than micro-size powders.The hardness(HRA),flexure strength and fracture toughness of the composites measured are 92.7,782 MPa and 7.81 MPa·m1/2.Compared the fracture surfaces of three materials prepared by the same technology,the fracture modes of Al2O3 and Al2O3/TiC are intergranular mainly.Thus the fracture surfaces of cobalt coated Al2O3/TiC composites(ATC)is the mixture of intergranular and transgranular.Some phenomenon helpful for improving flexure strength fracture toughness such as intergranular-type structure,the tortuous routes of crack propagation,the crack branching,crack deflecting and bridging are observed on the ATC fracture surface.
Transgranular fracture
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In this article, a new WC‐ Al 2 O 3 composite tool material is successfully fabricated by vacuum hot‐pressed sintering process with high hardness and flexural strength as well as fracture toughness, which solved the problem of mutual restriction between hardness and toughness of existing WC‐Co composite tool materials. By adding Al 2 O 3 , the hardness of WC‐based cemented carbide tool is significantly improved, the flexural strength and fracture toughness are also improved. The fracture mode of WC‐Al 2 O 3 composites is transgranular/intergranular mixed fracture mode due to the addition of Al 2 O 3 , and more fracture energy is consumed during crack propagation, thus improving the fracture toughness of WC‐ Al 2 O 3 composites. When Al 2 O 3 content is 5 wt%, the comprehensive mechanical properties of WC‐based cemented carbide is the best, with a fracture toughness of 12.3 ± 1.0 MPa· m 1/2 , a flexural strength of 1385 ± 65 MPa, and Vickers hardness of 22.8 ± 1.6 GPa. In this article, the sintering temperature and holding time are optimized. The WC‐ Al 2 O 3 composite tool materials with excellent comprehensive mechanical properties are fabricated at low sintering temperature of 1450 °C by using appropriate original powder particle size. The hardness is improved while maintaining high flexural strength and high fracture toughness.
Transgranular fracture
Vickers hardness test
Cemented carbide
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The fracture toughness of four different silicon carbides was measured using single‐edge precracked beam (SEPB) and indentation/strength techniques. Two were development grades with similar microstructures and chemistries, and yet exhibited different fracture modes. The grade that exhibited a predominantly intergranular fracture had an SEPB fracture toughness (6.4 MPa√m) 88% higher than the one that showed primarily a transgranular fracture (3.4 MPa√m). The higher fracture toughness was associated with a modest increase in average strength (25%), although there was a significant increase in the Weibull modulus (11–32). Fracture toughness at short crack lengths was assessed by an indentation method that used fracture strengths, crack lengths at fracture, and a new method of estimating the constant δ that characterizes the residual driving force of the plastic zones based on the stable growth of the indentation cracks from the initial ( c 0 ) to the instability ( c * ) lengths. The results showed a rising crack‐growth‐resistance behavior for the grade exhibiting intergranular fracture, while the grade showing transgranular fracture had a flat crack‐growth resistance. Tests on two commercial grades of silicon carbide showed similar behaviors associated with the respective fracture modes.
Indentation
Transgranular fracture
Weibull modulus
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Tinsile, impact, fracture mechanics and dynamic tear tests were carried out on 7075, 7175 and 7475 alloy extrusions. Fracture toughness of Al-Zn-Mg-Cu alloys is considerably influenced by aging condition. Under-aging results in high fracture toughness and resistances to crack propagation and unstable fracture. The fracture toughness decreases with increasing yield strength. The fact is related to localized plastic deformation due to finely distributed precipitate particles and to the resulting strengthened matrix. Over-aged meterials have lower fracture toughness than the other aged materials. Precipitate particles coagulate at the grain boundary and transgranular fracture at under- and peak-aging conditions transits to intergranular fracture at over-aged condition as the aging proceeds.
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Tensile properties of grain-oriented electrical steels up to 300℃were investigated and the corresponding fractography characteristics were observed.It was found that the segregation of phosphor atoms on grain boundaries was decreased with increasing tensile temperature,which resulted in enhancing cleavage fractography with reducing intergranular fracture.The intergranular fracture disappeared at the temperature over 100℃,while the yield strength of the matrix became obviously lower than its cleavage strength,which led to reducing cleavage fractography with enhancing dimples fracture that prevailed at 190℃.The Cottrell atmosphere would be trailed by the moving dislocations during the tensile deformation between 100-160℃,which induced decreasing elongation and limited yield strength drop with increasing deformation temperature.
Fractography
Transgranular fracture
Elongation
Cleavage (geology)
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Transgranular fracture
Austenitic stainless steel
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Fractography
Embrittlement
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Fractography studies were accomplished on two reaction-sintered Si3N4 materials (Norton NC-350 Si3N4 and KBI (Kawecki-Berylco) Si3N4) which had been tested in four-point bending at room temperature, 1200, 1350, and 1500°C in air. At room temperature NC-350 exhibited uniform planar features representing a transgranular fracture mode, uniformly distributed small pores having a size range of 1–2 μm or less, and a few isolated large pores of ∼10 μm. In contrast, the KBI Si3N4 at room temperature exhibited nonuniform, diffuse, and ill-defined fracture features, small pores having a size range of 2–5 μm, and many large pores of ∼50 μm in size. At high temperatures, the fracture appearance of NC-350 Si3N4 was ill defined and diffuse, indicating a change in fracture mode. The fracture originating flaws in NC-350 were primarily the large pores or pore agglomerates. At all temperatures, in the KBI material, it was not possible to observe the mirror and, therefore, the site of fracture initiation. For NC-350, KIC values were 1.2–1.6 MN/m3/2 at room temperature and 2.1–2.4 MN/m3/2 at high temperatures.
Fractography
Agglomerate
Transgranular fracture
Atmospheric temperature range
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Ti3Al/TiC+ZrO2 composites were prepared by mechanical alloying and hot-press sintering. The effects of different sintering temperatures (1500, 1550℃) and addition modes of Ti3Al to TiC matrix on the mechanical properties and microstructure were studied, and the fracture surfaces were analyzed with SEM. The results show that the composites have excellent properties with cooperating effects of Ti3Al and ZrO2. Intergranular cracking, transgranular cracking and some intragranular grains are all presented on the fracture morphology.
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