Grain size effect on the structural parameters of the stress induced epsilonhcp: martensite in iron-based shape memory alloy

Fabiana Cristina Nascimento,Paulo Roberto Mei,Jorge Otubo,Lisandro Pavie Cardoso

doi:10.1590/s1516-14392008000100012

Abstract

The aim of this work was to study the effect of austenitic grain size (GS) reduction on the structural parameters of the ehcp - martensite in stainless shape memory alloy (SMA). Rietveld refinement data showed an expansion in c-axis and a reduction in a and b-axis with thermo-mechanical cycles for all samples analyzed. Samples with 75 < GS (µm) < 129 were analyzed. It was also observed an increase of the unit cell volume in this phase with GS reduction. The smallest grain size sample (GS = 75 µm) presented a c/a ratio of 1.649, and approximately 90% of total shape memory recovery.

Highlights

Iron-based shape memory alloys have been extensively studied in the last years due to their good shape recovery properties compared to other materials[1,2,3,4,5,6,7,8,9,10,11,12]
The samples heated at 1323 K presented grain sizes between 75 and 129 μm
The average austenitic grain size was measured using similar images as the ones obtained in Figure 1a, where the grain boundaries revealed by the etching can be seen clearly, as explained in previous works[19,23]

Summary

Introduction

Iron-based shape memory alloys have been extensively studied in the last years due to their good shape recovery properties compared to other materials[1,2,3,4,5,6,7,8,9,10,11,12]. The shape memory effect (SME) is the result of a γ(fcc)↔ε(hcp) martensitic transformation[1,2,3]. The forward and backward movements of Shockley partial dislocations on alternate {111} austenite planes promote the reverse ε(hcp)→γ(fcc) transformation resulting on the shape recovery of these materials[4,7,9,10,11,12,13]. According to the literature[4,7], the thermo-mechanical treatment, chemical composition, and grain size (GS) reduction are some factors that have a strong influence on the SME, mechanical properties and structural parameters of austenitic and martensitic phases. The influence of GS on the shape recovery is a very important parameter to be studied, since some authors believe that the changes in austenitic grain size do not affect the shape recovery performance in stainless shape memory alloys[14]

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Conclusion