Effect of Sintering Temperature on the Microstructure

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Accepted Manuscript Effect of Sintering Temperature on the Microstructure and Mechanical Properties of Fe-30%Ni Alloys Produced by Spark Plasma Sintering Mxolisi Brendon Shongwe, Saliou Diouf, Mondiu Olayinka Durowoju, Peter Apata Olubambi PII:

S0925-8388(15)30622-8

DOI:

10.1016/j.jallcom.2015.07.223

Reference:

JALCOM 34919

To appear in:

Journal of Alloys and Compounds

Received Date: 12 May 2015 Revised Date:

22 July 2015

Accepted Date: 24 July 2015

Please cite this article as: M.B. Shongwe S. Diouf, M.O. Durowoju, P.A. Olubambi, Effect of Sintering Temperature on the Microstructure and Mechanical Properties of Fe-30%Ni Alloys Produced by Spark Plasma Sintering, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.223. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Effect of Sintering Temperature on the Microstructure and Mechanical Properties of Fe30%Ni Alloys Produced by Spark Plasma Sintering

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Mxolisi Brendon Shongwe*†, Saliou Diouf*, Mondiu Olayinka Durowoju*, Peter Apata Olubambi* *

Institute for NanoEngineering Research, Tshwane University of Technology, Pretoria, South Africa

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Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria,

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South Africa

Fe–30%Ni alloys were produced by sintering in the hybrid hot press spark plasma sintering system using iron and nickel as raw materials. The results indicate that the relative density, microhardness and fracture morphology depend on the sintering temperature which also affects the microstructure. The densification and grain size of the alloys increased with increasing

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sintering temperature, facilitating necking of grains. In the case of the sintering temperature at 1230°C, a relative density of 98.7% and a maximum grain size of around 200µm were obtained,

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and the maximum microhardness of 284Hv1.0, and the microhardness indentations revealed pincushioning indicating better sintering. Microhardness indentations at 1100°C and below were

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characterized by barreling, indicating poor densification and/or microhardness. The fracture type changing from intergranular fracture to transgranular fracture is an indication of improved consolidation of the Fe-30%Ni alloy with increasing sintering temperature.

KEYWORDS: †

Densification, Sintering, Microhardness, Microstructure, Density, Fracture.

Corresponding author. Tel: +27 12 382 5488, E-mail address: [email protected],

[email protected] (M.B. Shongwe, PhD).

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1. Introduction Fe–Ni alloys exhibit good magnetic performances of high permeability and low coercive force,

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which are produced for a wide range of applications in applications including industrial engineering, electronics, fuel injection system components and electrical industry[1–4]. Conventional soft magnetic alloys have been fabricated by the methods of casting and machining

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in the past and currently efforts are being made to improve the quality of the product with various designs. However, mass production of the miniaturization parts with complex shapes is

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greatly limited due to the long production period, low efficiency and high cost. Generally, soft magnetic properties are dependent on density, grain size and the amount of impurities. Not fully dense materials could have their density dependent properties decreased, and also the generated microstructure can be affected, showing pores, that could act as domain-wall barriers reducing

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the soft magnetic performance[5, 6].

The fabrication method of Fe-Ni alloys has been observed to affect the alloys mechanical properties which are attributed to poor densification and microstructural defects[6, 7]. Metal

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injection molding (MIM) which is a near-net shaping technique has been proven to be an effective processing method for applications where complex shape with high dimensional

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accuracy and high density are required[8-11]. The use of arc-discharge technique to process Fe-Ni nanocapsules from mechanically alloyed Fe-Ni powders was also reported[12,

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. Recently,

processing of Fe-Ni by laser fabrication such as selective laser melting/selective laser sintering (SLM/SLS) has been reported and has attracted interest[14, 15]. However, so far, work on Fe-Ni alloys fabricated by spark plasma sintering system has been rarely reported.

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Spark plasma sintering (SPS) is a fast growing and relatively new pressure assisted sintering technology characterized by a fast heating, a short isothermal holding and a relatively low sintering temperature[16,17] (see Fig. 1). These unique features of SPS result in a low ‘‘heat

limited chemical interaction with different constituents[22,

23]

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input’’, which is a favorable prerequisite to sinter powders with a controlled grain size[18-22] and . In SPS a minimum pressure is

required to establish the electrical contact between punches and powders and densification is

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progressively promoted by rearrangement of the powder particles, localized deformation at the contact points and bulk deformation of the particles[24]. This method of sintering offers many

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advantages over conventional sintering methods such as hot pressing, isostatic pressing or atmospheric furnaces owing to the possibility of sintering materials to near full densification with little grain growth. The SPS method has demonstrated its high efficiency in consolidating ceramic and metal nanomaterials, composites, solid materials, functionally graded materials,

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composite materials based on carbon nanotubes and nanofibers, electronic materials, thermo electrics and bio- materials[24].

As there were limited reports in the literature on spark plasma sintering of Fe-Ni alloys, it was

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decided in this study to investigate the spark plasma sintering of Fe-30%Ni alloys produced at

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950°C, 1100°C, 1200°C and 1230°C using HHPD-25 from FCT Germany. The microstructural features, fractography and mechanical properties were analyzed; moreover the reasons behind the microstructural developments were explained.

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Fig. 1. Schematic of the Spark Plasma Sintering apparatus[25].

2. Experimental

Fe and Ni as-received mixed powders of 99.5% purity with a particle size of less than 70µm

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were used as starting materials, and the characteristics are summarized in Table 1. The morphologies of the blended powders are shown in Fig. 2. The Fe-30%Ni has mainly regular spherical particles with negligible agglomeration and the is typical of atomized powders. The

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powders were poured into a graphite die with a diameter of 30 mm and then sintered using the

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spark plasma sintering system (model HHPD-25 from FCT Germany) at different temperatures in vacuum. The sintering temperature was measured by an optical pyrometer which was implanted in the SPS apparatus at 3 mm from the top of the sample surface. The pressure was maintained at 30MPa throughout the whole process. The sintering temperature was varied from 950 to 1230°C with a dwell time of five minutes. Two heating rates were used, from room temperature to 900°C the heating rate was 50K/min, thereafter 40K/min up to the sintering

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temperature was used. Discs of 30mm diameter and approximately 5mm in height were produced. After removing the sintered specimens from the graphite die, the density of the samples was

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measured by Archimedes method. Then the samples were cut and ground for metallographic examination according to standard procedures. The phases present in the sintered specimen were characterized by X-ray diffraction (XRD) using a PANalytical Empyrean model with Cu Kα

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radiation and analyzed using Highscore plus software. Qualitative, quantitative phase and morphological analysis of the sintered bodies’ microstructure and fracture surfaces were

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observed in a scanning electron microscope (SEM) (JEOL JSM-7600F SEM) incorporated with an EDX detector (Oxford X-Max) with INCA X-Stream2 pulse analyzer software, and Back Scattered Electron (BSE) detectors. The INCA analyzer software was set to 70 seconds acquisition time and at a process time of 2s. The samples were etched samples with a 6% nitric

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solution for 15s and the microstructure was analyzed using a Nikon Eclipse LV500 Optical Microscope and also SEM. The Vickers microhardness (Hv) at room temperature were measured by a microhardness tester (Future-tech) at a load (P) 100gf (1.0N) and a dwell time of 10s and

Materials

particle

Fe Ni

Particle

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Shape

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size (µm)

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the test result for each sample was the arithmetic mean of ten successive indentations.