Kinetics of nanoscale precipitation in Ni–Fe–Al alloys: A magnetic monitoring approach

https://doi.org/10.1016/j.jallcom.2011.03.167Get rights and content

Abstract

In this study, time–temperature dependence and kinetic aspects of nanoscale precipitation responsible for strengthening of Ni50FexAl50−x alloys with x = 20, 25 and 30 were investigated by temperature-scan and isothermal magnetic measurements in a vibrating sample magnetometer. Temperature-scan magnetization curves contained a magnetization rise for all studied alloys at temperatures above the Curie transition of the primary phase. These transient rises at relatively higher temperatures were associated with the formation of secondary ferromagnetic precipitates, identified as nano-sized body centered cubic α crystallites by microstructural observations under the transmission electron microscope. The isothermal kinetics of ferromagnetic α-phase precipitation was analyzed with the Johnson–Mehl–Avrami model. The Avrami exponent was determined to be close to unity and independent of the extent of precipitation, annealing temperature and alloy composition. It was concluded that α-phase precipitation was governed by a diffusion controlled growth process with decreasing growth rate, which closely resembles continuous precipitation kinetics. The activation energies, ranging between 75 and 83 kJ/mol, were utilized to construct magnetically assessed isothermal transformation diagrams of precipitation. Conforming to the kinetic analysis, annealing at an intermediate temperature ensures precipitation of fine second phase particles and brings about a significant hardening effect, whereas a higher annealing temperature yields coarser precipitates and a smaller extent of precipitation hardening.

Highlights

► Kinetic aspects of nanoscale precipitation in Ni–Fe–Al alloys. ► Avrami exponent lies near n = 1 independent of the extent of precipitation, utilized isothermal temperature and alloy composition. ► Precipitation is governed by a diffusion controlled growth process with decreasing growth rate. ► Magnetically assessed isothermal transformation diagrams provided a general picture for the nano-sized BCC α-phase precipitation.

Introduction

Nickel–aluminum (NiAl) is one of the most promising intermetallic compounds that have been extensively studied for high temperature structural applications [1], [2], [3]. In addition to its high melting point and low density, the ordered NiAl alloy exhibits excellent corrosion and oxidation resistance. On the other hand, the improvement of room temperature ductility and high temperature strength requires further attention for utilization in demanding applications. In this respect, the incorporation of Fe into the NiAl system has been shown to remarkably modify the structural properties [4], [5], [6] and promote precipitation of a fine body centered cubic (BCC) α-phase, which strengthens the alloy [7], [8], [9]. Although the α-precipitates are softer than the BCC β-matrix, they induce changes in the slip system and strengthen the alloys via precipitation hardening through a dislocation pinning effect [7], [10].

From a general point of view, while very fine precipitates provide the largest strengthening effect, a balance between strength and ductility can be attained with relatively coarser precipitates. However, for long-term applications, evolution of α precipitates at high temperatures should be clarified in order to determine an upper limit where the microstructure coarsens resulting in reduced strength. A convenient annealing heat treatment is also essential to form desirable precipitates. Therefore, in addition to its temperature dependence, kinetic aspects and mechanism of nanoscale α-phase precipitation in Ni–Fe–Al alloys have attracted considerable interest [11], [12]. However, determination of time and temperature dependent changes in these nanoscale precipitates using high-resolution electron microscopy is quite laborious and requires extensive image analysis [11].

In a few studies, magnetic monitoring approach was implemented to replace conventional thermal analysis as a tool to successfully reveal crystallization kinetics in amorphous-matrix nanocrystalline alloys. For instance, Luborsky [13] attempted to model the time and temperature dependent changes in magnetic parameters such as coercivity and anisotropy with the Johnson–Mehl–Avrami equation. Later, this approach was used to understand crystallization in Ni–P precursor films from the amorphous state [14]. Recently, using similar magnetic monitoring approaches, the disproportionation reaction observed in cast rare-earth transition metal alloys [15], [16] and crystallization kinetics of Fe-based soft magnetic amorphous alloys were examined [17]. To the best of our knowledge, kinetics of such precipitation reactions occurring at the nanoscale has not been examined via magnetic measurements for intermetallic alloys. Therefore, this study will be the first to discuss the kinetic aspects of α-phase precipitation in Ni50FexAl50−x (x = 20, 25, and 30) alloys by utilizing a non-conventional characterization technique which exploits the ferromagnetic character of the precipitating phase [18]. In the current work, precipitation was monitored by magnetization measurements encompassing both temperature-scan and isothermal experiments with a vibrating sample magnetometer. Transmission electron microscopy (TEM) and micro-hardness measurements were conducted to support the kinetic analysis.

Section snippets

Experimental procedure

The Ni50FexAl50−x alloys with x = 20, 25 and 30 were prepared by arc-melting of high purity 99.9% Ni, Fe and Al elements under argon atmosphere. Ingots were re-melted for four times to ensure compositional homogeneity.

The kinetics of precipitation was studied using a vibrating sample magnetometer (VSM, ADE Magnetics EV9). Temperature-scan measurements were done under 500 Oe applied field from room temperature to 973 K at a scan rate of 5 K/min whereas isothermal magnetization experiments were

Results and discussion

The study of kinetics in solid state processes such as phase transformations, crystallization or crystal-growth and precipitation reactions often utilizes well-known Johnson–Mehl–Avrami (JMA) equation. Provided that the kinetic process of precipitation obeys the JMA relation, time exponent (n) and activation energy (Ea) for the precipitation reaction can be found.

In addition, measured values of time dependent magnetization under an applied magnetic field (e.g. 500 Oe is applied in this study)

Conclusions

In this study, time–temperature dependence and kinetic aspects of α-phase precipitation in Ni50FexAl50−x alloys for x = 20, 25, 30 were investigated by continuous heating and isothermal experiments in vibrating sample magnetometer. The α-phase precipitation process was treated by the JMA kinetic model where the time exponent for the process assumed values around n = 1, independent of the extent of precipitation, utilized isothermal temperature and alloy composition. The activation energies for the

Acknowledgement

One of the authors (N.D.) acknowledges OYP Program at Middle East Technical University for financially supporting the graduate study.

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