Large Dimension and Low-Cost Fe-SMA Rods

The effects on sample size on abnormal grain growth in inexpensive FeMnAlNi shape memory alloy (SMAs) wires and rod were investigated. It was shown that repeated heat treatments between single phase (bcc) and two phase (bcc + fcc) region resulted in abnormal grains with bamboo structure both in small sized wires and large diameter rods. Microstructural features were investigated using electron backscatter diffraction (EBSD) and it was found that 0.5 mm wires possess strong [011] texture whereas, large diameter rods have random texture after grain growth heat treatments.


Introduction
Shape memory materials with superelastic properties can recover large deformations triggered by changes in stress. Since the loading and unloading paths do not coincide, a hysteresis loop occurs, which signifies energy dissipation. Due to these unique properties, superelastic SMA materials have been considered for a wide range of civil engineering applications, such as bracing systems [1,2], connectors [3], and concreate reinforcement [4][5][6]. As a result, the damage sustained by the structure from adverse events has been decreased and crack propagation has been prevented. Current candidate for such applications is a very well-known NiTi-based alloy called 'Nitinol'. However, NiTi alloys are expensive, are difficult to process and weld at large scales, have a limited range of operating temperatures and limited fatigue resistance [7][8][9]. Also, since NiTi SMAs have mainly been used in medical applications, there is an overall lack of supply in SMAs in large sizes and volumes. Recently discovered, inexpensive Fe-SMAs [10][11][12][13][14] have shown superelasticity and energy dissipation similar to NiTi but with good workability and a cost/weldability on par with advanced high strength steels. Additionally, Fe-SMAs possess an interesting meta-magnetic response where significant changes in magnetic properties occur during superelasticity.
The properties of the Fe-SMAs are sensitive to part size in that the grain size of the material, which can be grown to several inches, should exceed the smallest dimension of the part. In order to obtain large grain sizes through abnormal grain growth in FeMnAlNi SMAs, samples have been heat treated repeatedly between the single phase and two phase region in the phase diagram. Using this technique, several attempts have been made to grow abnormal grains in wires and sheets [13,14]. However, the maximum grain size in large diameter rods, and thus maximum possible dimension of the part is currently unknown. Therefore, in the present work, the grain growth in FeMnAlNi alloys with variable wires and rod diameters was studied.

Methods
Ingots with a nominal composition of Fe-34Mn-15Al-7.5Ni (at.%) were prepared by vacuum induction melting. Cast ingots were hot extruded to 2 mm and 4.6 mm diameter rods. Wire production was performed as follows; cast rods were first hot extruded at 1000 ºC followed by annealing at 1200 ºC for 60 min. Afterwards, rods were heat treated in the two phase region (900 ºC for 30 min) in an inert argon atmosphere prior to wire drawing. Finally, 0.5 mm wires were drawn with a 75% cold work. Cyclic heat treatments were performed in Fe-Mn-Al-Ni alloy rods and wires to promote abnormal grain growth (AGG). Fe-Mn-Al-Ni alloy specimens were encapsulated in quartz tubes and filled with high purity argon. Afterwards, cyclic heat treatments were performed as shown in Figure 1. In cyclic heat treatments, specimens were heated up to 1200 ºC for 30 min and then cooled down to the two phase (bcc matrix + fcc precipitate) region as shown in Figure 1. This heat treatment cycle was repeated for both wires and rod specimens for 5 times. Finally, specimens were solution heat treated at 1300 ºC for 30 min and quenched in water.
Microstructural features of the specimens were investigated using optical microscopy (OM) and crystallographic orientation of the specimens were measured using electron backscatter diffraction (EBSD) technique. OM and EBSD specimens were prepared by mechanical grinding and final polishing using 0.04 µm colloidal silica. Composition of the specimens were measured using wavelength dispersive spectroscopy (WDS).  Fig. 1. Thermal cycling process for grain growth in Fe-34Mn-15Al-7.5Ni (at. %) alloy. Cycling between single phase and two-phase region promotes grain growth.

Results and discussion
Final grain sizes in the 0.5 mm diameter Fe-Mn-Al-Ni alloy subjected to AGG heat treatment are shown in Figure 2. A small number of grain boundaries and triple junctions are observed in the microstructure. Grain boundaries are perpendicular to the wire axis indicating that the wire has bamboo structured grains. In FeMnAlNi alloys, the grain size to diameter ratio (d/D) is an important parameter and has to be larger than 1 in order to obtain recoverable superelasticity [13]. Our results show that the d/D ratio of the 0.5 mm wires is ~8 and therefore satisfies the conditions for superelasticity. Orientation map analysis of the austenite matrix in Figure 2b indicates that the 0.5 mm diameter wire has a strong [011] texture in the drawing direction. According to the theoretical calculations on the orientation effect on superelasticity [17] FeMnAlNi alloys with [011] direction are expected to transform 10%. This has also been shown experimentally in our previous work where [011] textured wires transformed 6.7% when aged at 200ºC for 24h [18].
The 2mm diameter sample wire that was subjected to abnormal grain growth followed by solution heat treatment at 1300ºC is shown in Figure 3. Grain boundaries are visible as indicated with arrows as shown in Figure 3a. Figure 3b presents the EBSD orientation map in the drawing direction (IPFX) and the phase map where yellow and blue colors indicates the FCC martensite and BCC austenite respectively. Similar to the 0.5 mm wire, the microstructure contains very few grain boundaries and triple junctions. Grain boundaries are perpendicular to the wire axis and span across the wire diameter. The d/D ratio for this case is measured around 9. Unlike the 0.5 mm wire, the microstructure of the 2 mm diameter wires contains quench in martensite after solution heat treatment as shown in the phase map. In addition, the orientation of the grains in the drawing direction (IPFX) start to vary from the [011] direction and grain orientations close to the [111] direction are observed. It is known that the orientation of the crystals effects the superelastic response [17] therefore, superelastic properties of the 2 mm diameter wires are expected to differ from highly textured 0.5 mm wires. Abnormal grains were obtained in the 4.6 mm diameter rod as shown in Figure 4. Grain boundaries are indicated with a white dotted line in optical microscopy images in Figure 4a and b. Similar to the wires, large bamboo structured grains are frequently observed in the large diameter rods. On the other hand, small sized grains are found in the microstructure, however their number density is low. EBSD orientation mapping in the drawing direction (Figure 4c) indicates that the microstructure contains large amount of quench in martensite plates after solution heat treatment. Compositional differences caused by different thermomechanical history between wires and rod can be one possible reason for this increase in quench in martensite volume fraction. Grains with different crystallographic orientations are observed in large sized rods after abnormal grain growth heat treatments as shown in EBSD maps in Figure 4. Unlike small diameter wires, strong texture was not present, and texture was random. This is expected because the wire drawing process usually results in strong fibre texture in alloys [19].
The authors would like to acknowledge the financial support from the Transportation Consortium of South-Central States through a grant from the U.S. Department of Transportation/University Transportation Center Program (Grant No. 69A3551747106).

Conclusions
The major findings can be summarized as follows: • Repeated cyclic heat treatments between single phase (bcc austenite) and two phase (bcc austenite + fcc precipitate) result in coarse bamboo grains in both small diameter wires and large diameter rods in FeMnAlNi alloys. After heat treatments, triple junctions are reduced, and grain boundaries are perpendicular to wire axis. • After solution heat treatments from 1300ºC, quench in martensite volume fraction increases as the specimen size increases. This is attributed the compositional variations due to different thermomechanical history of the specimens. • Cyclic abnormal grain growth heat treatments results in strong [011] texture in the drawing direction in 0.5 mm wires, whereas, texture become random as the sample size increases