Thermal Stability and Superplastic Characteristics in Si₃N₄/Al-Mg-Si Composites

Aug 25, 2026

Differential scanning calorimeter (DSC) measurements and tensile tests at 773–848 K were conducted for four superplastic Si₃N₄/Al-Mg-Si composites. The DSC results showed that each composite’s melting point was lower than that of its matrix alloy, attributed to local melting from solute segregation near reinforcement–matrix interfaces. The optimum superplastic temperature for maximum elongation lay close to its melting point, indicating that superplasticity is achieved within the solid–liquid interfacial region by relaxing stress concentrations and restricting internal cavitation.

LanguageEnglish

CategoryResearch & Report

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Pages7

Reading Time18 min

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I. Introduction

Thermal Stability and Superplastic Characteristics in Si₃N₄/Al-Mg-Si Composites

Differential scanning calorimeter and tensile testing

Materials and alloy composition

*National Industrial Research Institute of Nagoya,I Hirate-cho,Kita-ku,Nagoya 462,Japan # I.Introduction Ceramics whisker or particulate reinforced aluminumalloy composites,i.e.metal-matrix composites(MMC)exhibit a unique combination of high specific room tem-perature strength and modulus,and therefore the compo-sites show promise as structural materials.However,thecomposites have lower tensile ductility than the matrixalloys.This gives rise to high costs in the final formingof the composites.It is,therefore,desirable to improvetensile ductility of the composites for many structuralapplications.The original work by Nieh et al.revealedthat a 20 vol.%SiCw/2124 aluminum composite(the sub-script w represents whisker)behaved in a superplastic-like manner at a high strain rate of 3.3×10-¹s-1(1.Afterthis demonstration,many superplastic MMCs have beenproduced thermomechanically by using powder metal-lurgy.These studies indicate a possibility of superplasticforming of the MMCs(1)-(12).In particular,it is noted thatsome MMCs(1)(4)(7-(12)exhibit superplasticity at high strainrates of more than 10-²s-¹. Recent experimental works of superplastic MMCswere reviewed relating to dependencies of the ability ofsuperplasticity on the chemical composition and grainsize of matrix and morphology,chemistry and volumefraction of the reinforcements,as well as the nature ofthe reinforcement-matrix interfaces(13)(14).It is very im-portant to note that the superplastic elongation stronglydepended on the refinement of the matrix grain structureand the criteria for fracture strength in the interfaces be-tween the matrix and reinforcements.The MMCs con-tain a large amount of ceramics whiskers or particulates,which may include a high possibility that the reinforce- # Thermal Stability and Superplastic Characteristicsin Si₃N₄/Al-Mg-Si Composites Mamoru Mabuchi*and Kenji Higashi** **Department of Mechanical Systems Engineering,College of Engineering,University of Osaka Prefecture,1-1 Gakuen-cho,Sakai,Osaka 593,Japan Differential scanning calorimeter(DSC)investigations and tension tests at 773～848 K have been carried out in fourdifferent superplastic Si₃N₄/Al-Mg-Si composites.The DSC investigations revealed that the melting point of each com-posite was lower than the melting point of its matrix alloy.This is probably attributed to local melting due to segregationof solute atoms near the matrix/reinforcement interfaces.The optimum superplastic temperature for each composite,where a maximum elongation was obtained,was close to its melting point.Therefore this suggested that superplasticelongations are obtained in metal matrix composites within the solid-liquid region because the presence of a liquid phaseat the reinforcement/matrix interfaces serves to relieve the stress concentrations due to sliding and thereby restricts thedevelopment of internal cavitation. (Received December 17,1993) Keywords:differential scanning calorimeter,melting point,interfaces,optimum temperature,maximumelongation,liquid phase ment-matrix interfaces offer a nucleation site for cavitiesduring superplastic flow.Thus,the morphology of thereinforcements is important because it imposes geometricconstraints on the plastic flow of the matrix and affectsthe stress distribution at the reinforcement-matrix inter-face region.Extensive cavitation around the reinforce-ments could cause the premature fracture of MMCs.Thisperennial problem,and the role of cavitation in super-plastic MMCs,are undoubtedly topics that will be consi-dered to achieve enhanced superplasticity(15. It was pointed out very recently that a liquid phasemight play an important role in superplastic deformationof the MMCs and high ductilities were achieved at effec-tive testing temperatures above the solidus temperatureof the matrix alloy,based on a new rheological model inwhich a semi-solid material behaves like a non-Newtoni-an fluid(¹3).But the detailed experimental results from thetemperature dependence of d

FAQs

What did DSC investigations reveal about Si₃N₄/Al-Mg-Si composites?

The melting point of each composite was lower than the melting point of its matrix alloy, likely due to local melting caused by solute segregation near reinforcement–matrix interfaces.

How is the optimum superplastic temperature related to melting behavior?

The temperature that produced maximum elongation was close to the composite’s melting point, supporting superplastic deformation within the solid–liquid region at the interfaces.

Why does a liquid phase at reinforcement–matrix interfaces improve superplastic elongation?

The presence of liquid at the interfaces relieves stress concentrations arising from sliding and helps limit the development of internal cavitation, reducing premature fracture.

Aug 25, 2026

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