Influence of al concentration on deformation behavior and fracture mode of fe-30mn-6(SI, AL) alloys

Abstract

Microstructural and fractographic observations were systematically done on Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4 mass %) alloys. Optical and transmission electron microscopic observations and X-ray diffractions revealed that the deformation mode continuously shifts from the stress induced fcc/hcp martensitic transformation to the mechanical twinning of the fcc austenite as the Al content increases. It was also clarified by the scanning electron microscopic observations that the microstructural change depending on the Al content is accompanied by the change in the fracture mode from the quasi-cleavage fracture to the ductile fracture. INTRODUCTION Fe-Mn-Si-based shape memory alloys (SMAs) exhibit the shape memory effect (SME) associated with fcc (γ-austenite) / hcp (e-martensite) martensitic transformation [1]. A recoverable strain obtained in a typical Fe-Mn-Si SMA: e. g. Fe-30Mn-6Si (hereinafter compositions are shown in mass%), was reported about 2% in the solution treated condition [2]. This value can be increased to about 4% by so-called the training treatment [3, 4] and fine dispersion of precipitates such as NbC carbides [5-7], etc. One drawback of the alloy was its poor ductility of about 30%. In contrast to this, it was recently reported that the Fe-30Mn-3Si-3Al TWIP (Twinning Induced Plasticity) steel exhibits the ultra-high ductility as much as about 90% [8], but this alloy shows no significant SME. The composition of the Fe-30Mn-3Si-3Al TWIP steel is such that a part of Si in the Fe30Mn-6Si SMA is replaced by Al. In order to systematically investigate the effect of the Al content on the SME and TWIP effect, the present authors prepared four kinds of FeMn-Si-Al alloys by gradually varying the amount of Al substituting Si: i. e. Fe-30Mn(6-x)Si-xAl (x=0, 1, 2 and 4). The following two conclusions were drawn as a result [9]: i) the alloys with x=0 and 1 exhibited similar SME, but no recognizable SME was observed for the alloys with x>2, ii) the ductility linearly increased with increasing the amount of Al. The above-mentioned changing tendencies may originate from the continuous change in the deformation mode from the stress-induced γ → e martensitic transformation to the mechanical γ twinning. However, there has been no systematic study on the effect of the Al content on the deformation and fracture modes between the SMA and the TWIP. In the present paper, microstructural observations using optical microscopy (OPM) and transmission electron microscopy (TEM), phase identification using X-ray diffraction (XRD) and the fractographic observations using scanning electron microscopy (SEM) were carried out to clarify the effect of the Al content on the deformation mode and the corresponding fracture mode of the alloys. EXPERIMENTAL In this paper, hereafter the Fe-Mn-(6-x)Si-xAl (x=0, 1, 2 and 4) alloys are referred to as Al-0,Al-1,Al-2,Al-3 using mass % of Al. The specimens were prepared by vacuum induction melting. After hot forging and rolling at 1270K, the specimens were subjected to solution treatment at 1270K for 3h followed by water quenching. The OPM observations were performed on the samples, which were mechanically and electrolytically polished to obtain smooth surfaces and then extended by about 3%, using a differential interference microscope. The phase constitutions and internal microstructures in the deformed specimens were investigated with a RINT 2500 X-ray diffractometer and with a JEOL 2000FX II transmission electron microscope, respectively. The specimens for TEM observation were carefully prepared to avoid the formation of stress-induced martensite and reverse transformation on heating, using a chemical polishing solution of hydrogen peroxide and hydrofluoric acid mixed in the ratio of 10: 1. The specimens were finally subjected to electropolishing using acetic acid and perchloric acid mixed in the ratio of 20:1 at room temperature, to obtain the TEM foils. Fracture surfaces were examined on the specimens fractured at room temperature with a Hitachi S-3100 scanning electron microscope. RESULTS AND DISCUSSION DEFORMATION BEHAVIOR Figures 1 (a) to (d) show the OPM images observed on the specimens of Al-0 to Al-3, respectively, deformed by tensile strain to 3%. Some grains seen in the figures involves anneal twins. Anneal twin boundaries on {111}γ planes are indicated by arrows in the OPM photos. In each of parent and twin crystals, there are surface striations. It should be noted in Figs. 1(a) to (d) that the width and interval of the striations inside crystals becomes smaller with increase in the Al content. It has been widely accepted that the striations appeared in the Fe-Mn-Si SMAs are formed by the stress-induced γ → e martensitic transformation, while those in the FeMn-Si-Al TWIP steels are due to the mechanical γ twinning. It is inferred from the variation in the microstructures from Figs. 1(a) to (d) that the deformation mode should continuously change from the stress-induced e martensite to the mechanical γ twins, when the Al content is increased from 0 mass % to 3 mass %. Our previous result [9] showing the linear change in the ductility depending on the Al content also supports this speculation. However, it is difficult to distinguish these two deformation products by OPM observations, because both have plate shapes on the {111}γ habit. Figure 1: Deformation microstructures observed by optical microscopy on the specimens of (a) Al-0, (b) Al-1, (c) Al-2, and (d) Al-3. The observations were performed at the tensile strain of about 3%. The phase identification by means of the XRD was, therefore, performed to investigate semi-quantitatively the dependence of the amount of the e phase on the Al content. It was revealed that the intensity of peaks from the e phase relative to that of peaks from the γ phase gradually decreases with increasing the Al content, though not presented here. However, it is impossible to investigate the amount of the mechanical γ twins by the XRD. In order to confirm the existence of the e phase and the γ twins, the electron diffraction pattern analysis using TEM was employed. Figure 2(a) shows the bright field image taken in the Al-0. The plates observed in Fig. 2(a) were identified as the e phase by the corresponding electron diffraction pattern shown in Fig. 2(b). The incident beam is parallel to [011]γ // [21 1 0]e. The diffraction pattern in Fig. 2(b) clearly shows the well known features of the γ → e transformations: i) the S-N orientational relationship between the γ and e crystals, ii) the streaks along γ directions due to small thickness of the e plates. The streaks run in two directions: i. e. and . The (11 1) and (111 ) traces nominal to the corresponding streaks are seen in Fig. 2(a). Figure 2(c) and (d) show an example of the mechanical γ twins (γTM) observation in the Al-3. The zone axes of the diffraction pattern are [011]γ // [011]γTM. A lamella structure consisted of nano-sized twins and the retained austenite is formed in the specimen, being consistent with the previous results in the literature [10]. After a number of careful observations, a very small amount of the e plates were also found even in the Al-3, although it was undetectable in the XRD profile. 200 111 111 MT 200 MT