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Effect of temperature on tensile fracture mechanisms of a Ni-base superalloy

Sajjadi S. A., Zebarjad S. M.

Archives of Materials Science and Engineering

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2007

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Vol. 28, nr 1

34-40

EN

Purpose: The Ni-base superalloy GTD-111 is used in manufacturing of the first stage blades of powerful gas turbines (over 125MW). The alloy posses appropriate microstructure and high temperature properties through precipitation hardening heat treatment. Among the properties, tensile properties of the alloy have strong influence on stability and life of the blades. Design/methodology/approach: Tensile tests over a wide range of temperatures from 25 to 950°C with a constant strain rate of 10 -4s -1 were performed to study the tensile fracture mechanisms of the cast and heat treated superalloy. Scanning electron microscopy was used to provide structural and fractography evidence of the superalloy GTD-111at different temperatures. Findings: The fractography results of the tensile tested specimens were in good agreement with the variation in alloy ductility. Many fractography features such as: transgranular and intergranular fracture with fine dimples, cleavage facets and a combination of them were observed in the specimens tested at different temperatures. Research limitations/implications: Because fatigue is an important fracture mechanism at the service condition of the alloy it is suggested for future research to work on the simultaneous effects of tension and fatigue on the fracture mechanisms although, tensile properties alone are important for the alloy. Originality/value: It was found that different fracture mechanisms operated in different temperature ranges for example, while transgranular dimple fracture was dominant at 650°C, the dominant fracture mechanism at room temperature was intergranular.

2

Study of fracture mechanisms of a Ni-Base superalloy at different temperatures

Sajjadi S. A., Zebarjad S. M.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 18, nr 1-2

227--230

EN

Purpose: The Ni-base superalloy GTD-111 gains its appropriate microstructure and high temperature strength through precipitation hardening mechanism. Because of their service condition, tensile properties of the alloy have strong influence on stability and life of the blades. Design/methodology/approach: Tensile fracture mechanisms of the cast and heat treated superalloy were studied over a wide range of temperatures from 25 to 950°C with a constant strain rate of 10-4s-1. The present paper provides structural and fractography evidence by means of scanning electron microscopy at different temperatures for the superalloy GTD-111. Findings: The variation in alloy ductility was found to correlate well with the fractography results of the tensile tested specimens. Transgranular and intergranular fracture with fine dimples, cleavage facets and a combination of them were shown in the fractographs. Research limitations/implications: Although tensile properties alone are important for the alloy, it is suggested for future research to work on the simultaneous effects of tension and fatigue on the fracture mechanisms. Originality/value: It was cleared that different fracture mechanisms operate in different temperature ranges; while transgranular dimple fracture was dominant at 650°C, the dominant fracture mechanism at room temperature was intergranular.

3

A study on thermal behaviour of HDPE/CaCO3 nanocomposites

Zebarjad S. M., Sajjadi S. A., Tahani M., Lazzeri A.

Journal of Achievements in Materials and Manufacturing Engineering

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2006

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Vol. 17, nr 1-2

173--176

EN

Purpose: In order to improve physical and mechanical properties of high density polyethylene (HDPE) addition of filler, rigid particles and even elastomer to HDPE is very common. One of the most important filler which is added to it is nano size calcium carbonate (CaCO3). To avoid agglomeration of nano size calcium carbonate addition of fatty acids such as stearic acid is very common. Design/methodology/approach: In the current study, nanocomposites with 10vol% nano size calcium carbonate were prepared. To investigate the role of stearic acid on nanocomposite behaviour, nanoparticles were coated at different stearic acid content. Thermal behaviour of high density polyethylene and its nanocomposites reinforced with both uncoated and coated calcium carbonate were investigated. For this purpose differential scanning calorimetry (DSC) test was used. Findings: The results of DSC test showed that addition of 10vol% calcium carbonate to HDPE causes a slightly rise in its melting point but stearic acid content has no significant effect on the melting temperature of HDPE nanocomposites. Research limitations/implications: Agglomeration of nanosized calcium carbonate during sample preparation was the major research limitation. Originality/value: Crystallization temperature of HDPE with addition of 10vol% calcium carbonate increases, while addition of stearic acid causes to decrease it. Both stearic acid content and 10vol% calcium carbonate have no significant effect on crystallinity index of HDPE.