Elsevier

Materials Science and Engineering: A

Volume 552, 30 August 2012, Pages 310-315
Materials Science and Engineering: A

Effect of annealing on microstructure, grain growth, and hardness of nanocrystalline Fe–Ni alloys prepared by mechanical alloying

https://doi.org/10.1016/j.msea.2012.05.045Get rights and content

Abstract

Fe–xNi alloys from x = 0 to x = 15 with an as-milled grain size and hardness in the range of 8–11 nm and 8.5–9.5 GPa, respectively, were synthesized by ball milling. Microstructural changes, hardness, and grain growth due to annealing were characterized using X-ray diffractometry, microhardness, focused ion beam channeling contrast imaging, and optical microscopy. It was found that the composition range of single bcc phase was extended by ball milling. Subsequent annealing of MA samples resulted in reduction of hardness and extensive grain growth. It indicates that nickel has no significant effect on thermal stabilization of iron. Retained austenite was observed for Fe–8Ni and Fe–10Ni alloys annealed in the two-phase region and effect of as-milled structure on retained austenite formation was discussed.

Highlights

► Iron–nickel powders were hardened to 9.5 GPa by the mechanism of grain refinement strengthening using ball-milling. ► We annealed the prepared powders and observed reduced hardness and extensive grain growth above 500 °C. ► The Vickers hardness as a function of the grain size was found to exhibit a Hall–Petch slope. ► Retained austenite was observed for Fe–8Ni and Fe–10Ni alloys annealed in the two-phase region. ► As-milled microstructure plays an important role on the formation of austenite in the two-phase region.

Introduction

This study of iron–nickel alloys has the primary motivation of investigating the effect of annealing and nickel content on microstructure, grain growth, and mechanical properties of nanocrystalline iron–nickel alloys prepared by ball milling. The iron–nickel system has attracted considerable attention for the understanding of steels and other ferrous alloys. Therefore, several studies were performed to investigate phase stability, microstructural, and mechanical properties of iron–nickel alloys [1], [2], [3], [4], [5], [6], [7]. Studies of MA iron–nickel alloys showed that the single-phase bcc and fcc concentration ranges may be extended significantly as compared with those produced by conventional techniques [8], [9]. Another important feature of iron–nickel system is that the annealing of cold-worked alloys in two-phase (α + γ) region may lead to retained austenite formation [10]. The presence of retained austenite has a beneficial influence on the cryogenic toughness of ferritic steels [11]. However, limited research has been done, so far, to determine the effect of nickel content and annealing on grain growth and phase transformation of nanocrystalline iron–nickel alloys prepared by ball milling. Darling et al. [12] reported grain growth behavior of ball-milled Fe–Ni but it was limited to 1 at.% Ni and intermediate annealing temperatures.

Ball milling, that consists of repeated welding, fracturing, and rewelding of powder particles [13], produces powder with nanocrystalline grain size [14] and consolidation of the powders into bulk shapes is necessary for potential applications [15]. The problem is that a combination of pressure and temperature is necessary for complete inter-particle bonding [16]. However, grain growth limits the processing of nanocrystalline powders metals since there is a thermodynamic driving force for reduction of the total grain boundary area [17].

In the present study, we present an attempt to extend single-phase α solid solution and determine the effect of annealing temperature on grain growth and hardness of iron–nickel alloys obtained by mechanical alloying. We compare the obtained results with the published data for pure iron and Fe–Ni alloys.

Section snippets

Experimental

Fe–Ni alloys with Ni content from 0 to 15 at.% (0, 2, 4, 6, 8, 10, and 15) were prepared by mechanical alloying. As starting materials, appropriate masses of elemental Fe (99.9%) and Ni (99.9%) powders were mixed with 440C stainless steel balls and sealed in a hardened steel vial under an argon atmosphere (O2 < 2 ppm) prior to milling. The weight of the mixed powders was 5.1 g and the ball-to-powder weight ratio was 10:1. Ball milling was performed with SPEX 8000 model mixer-mill for 20 h. The

Results

Fig. 1 shows X-ray diffraction patterns from powders of Fe–xNi with 0  x  15 for ball-milling for 20 h. In what follows, all compositions are given in atomic percent. In our investigation, mechanical alloying (MA) of elemental iron and nickel powders leads to the extension of single phase solid solutions up to 15 at.% Ni. Equilibrium solubility of nickel in the iron lattice was reported to be around 3.5 wt% Ni [22], [23] by conventional technique, and extended up to 30 at.% Ni [8] by ball milling.

Discussion

In the present work, we changed the nickel content in iron from 0 to 15 at.% and prepared nanocrystalline alloys by ball milling. The as-milled powder morphology developed from relatively fine powder for pure iron into large spherical particles with increasing nickel content after 20 h milling. This behavior can be associated with the enhanced cold welding and decreased fracture events during milling as a result of a transition from more brittle to more ductile behavior with increasing nickel in

Conclusions

Fe–Ni alloys with Ni content from 0 to 15 at.% were prepared by mechanical alloying. The composition range of the bcc single phase region was greatly extended with respect to the equilibrium state. Using mechanical milling, iron–nickel powders were hardened to 9.5 GPa by the mechanism of grain refinement strengthening. Subsequent annealing lead to reduced hardness and extensive grain growth, suggesting that the nanostructure developed by ball milling is not retained during annealing. Retained

Acknowledgment

The research reported in this paper was supported by NSF-DMR under grant number 1005677.

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