Hot compression tests and flow curves
To determine suitable forming temperature for the SMA, flow curves in the temperature range from 600 to 1000 °C were measured. These temperatures were chosen to cover the range in which the ductile γ-phase is present for both SMA. Additionally, flow curves for the steel 1.0503 were extrapolated based on data from  since the shell was required to have a similar strength to strength of the core in this temperature range. The SMA flow curves from the hot compression tests and the extrapolated flow curves for the steel 1.0503 are depicted in Fig. 6.
In the temperature range of interest, the flow stresses are similar for both SMA and the steel 1.0503. The differences are smaller than 40 MPa at lower testing temperatures and 25 MPa at higher temperatures.
Samples after forming
The macroscopic appearance of the samples after the forming tests are shown in Fig. 7. The images are obtained from the CoNiGa-steel samples after the tensile, upsetting, rolling and extrusion tests. For the sake of brevity, the NiFeGa samples are not shown as these looked similar after forming. Still, there were differences between the CoNiGa und NiFeGa samples as described in the following. The observed differences in the values of the degrees of deformation leading to damage in the structure of, respectively, CoNiGa and NiFeGa samples are given in the corresponding chapters and described in the following.
Hot tensile tests
Figure 7a and b show X-ray images (Zeiss Xradia 520 Versa) of the CoNiGa tensile samples strained to 10% and 15%. The strain increase from 10% (Fig. 7a) to 15% (Fig. 7b) resulted in the formation of cracks in the CoNiGa core. The cracks occur in those areas where a constriction in the steel shell is visible. These regions are either in the center of the sample or close to the grip section. In the NiFeGa samples, no macro cracks were observed at strains of 10% and 15%, but numerous meso- and microcracks. Further in situ studies under load are under consideration for both alloys in order to analyze the different cracking mechanisms.
Figure 7c depicts the center-cut of an upsetting sample. The tests were interrupted when the maximal capacity of 230 kN of the test system was reached. Height reduction of the whole sample was 41% for the CoNiGa-steel and 52% for the NiFeGa-steel samples. The compression of the cores was 31% (CoNiGa) and 44% (NiFeGa). The lesser deformation of CoNiGa is probably due higher flow stresses of this alloy at temperatures above 700 °C. The free outer surface of the deformed sample had a barrel form typical for upsetting. The angular form at the interface between core and shell is a result of contact friction with the tool due to centering undercuts.
Figure 7d shows the longitudinal section of a hot rolled rod with a CoNiGa core. An elongation ratio l1/l0 of 1.53 was achieved. The final samples had a height of 12 mm and width of 27 mm. Peripheral speed of the rolls was 50 mm s−1. Thus, the mean strain rate was 1 s−1. The surface temperature at the start, middle and end of the sample measured with a HT3305 pyrometer was in the range of 930–970 °C. The mean value of the rolling pressure was 76 MPa. The NiFeGa samples were nearly identical in appearance and shape to the CoNiGa samples shown in Fig. 7d.
After processing, the main part of the extruded rod remains in the extrusion tool until manually removed and it is challenging to measure its temperatures with a pyrometer. The surface color of the rods right after extrusion was bright red to red, but it is difficult to estimate the corresponding temperature due to the lubricant glass layer. However, the temperature was not below 800 °C. In comparison to the other forming methods, the highest degrees of deformation were achieved during extrusion. From the X-ray image of an extruded rod (Fig. 7e), the shape of the formed core could be evaluated non-destructively. In Fig. 7e, a homogenous deformed CoNiGa core in the steel shell is visible. Such homogenous elongation was obtained for all ratio combinations of d0/D0. Despite the different extrusion parameters, the appearance of the NiFeGa samples after extrusion was similar to the CoNiGa samples shown in Fig. 7e.
Characterization of the interface zone between core and SMA shell
Regarding the subsequent separation of the SMA core and the steel shell, the interface between both materials after forming is of interest. Accordingly, backscattered electron images were recorded in a scanning electron microscop (SEM) of the interfaces formed in the CoNiGa-steel and NiFeGa-steel samples. Table 2 shows representative SEM images of the interfaces between the shell and the core.
The analysis of the interfaces formed between the core and the shell materials revealed that distinction in types and degrees of deformation determine a difference in bonding quality. As seen from Fig. 7a, b and c and Table 2a–d, the use of hot tensile testing and upsetting does not promote formation of a metallurgical joint between the investigated materials. The core and shell materials could easily be separated after cutting the final products. Bar rolling with the parameters employed resulted in a strong material bond between the SMA and the steel shell (Table 2e, f). The most intensive metallurgical bond was formed for specific sets of extrusion parameters (Table 2g, h). The interface between the CoNiGa core and the steel shell are shown in Fig. 8 for selected of extrusion conditions. A positive influence of increasing the diameters ratio d0/D0 regarding formation of a strong bonding during extrusion was determined. For a ratio of d0/D0 = 0.22, large voids between the SMA and the steel shell were present (Fig. 8a). By increasing the ratio to 0.32, the voids decrease in size (Fig. 8b) and at some points, a material bond can be assumed. Increasing the ratio to 0.41 causes a full material bond (Fig. 8c).
The results for extruding NiFeGa with a ratio d0/D0 = 0.11 are depicted in Fig. 9. Despite the substantially lower ratio, a firm bond between the NiFeGa and the steel shell was obtained.
Additional line scans were taken to characterize the interface zone. Figures 10 and 11 show the line scans for CoNiGa and NiFeGa cores for d0/D0 = 0.41 (CoNiGa) and d0/D0 = 0.11 (NiFeGa), respectively. The continuous character of the element concentration change in the line scans from the interface zone (Figs. 10b, 11b) indicates a firm material bond.
The porosity at the interface in all cases using a steel shell (Figs. 8, 9, 10, 11) can be a consequence of the Kirkendall effect (condensation and growth mechanism of voids) and its cause is the difference in the diffusion rates of the carbon atoms present in steel and other elements composing the given alloys (Co, Ni, Ga, Fe). This assumption is supported by the absence of porosity at the interface between the two SMA (Fig. 12). Static diffusion tests to clarify the role of carbon atoms and vacancies in pore formation are thus of further interest.
To interpret the SEM images and the EDX line scan data, push-out tests were carried out to characterize the mechanical properties of the bond. The tests were performed for the different ratios of the extruded CoNiGa samples, and the results of the push-out tests are summarized in Table 3. The shear stresses τc are quite high compared to the samples taken from tensile test specimens, where a direct separation of parts of the sample was observed after preparation. In fact, the highest achieved bonding strength is close to the nominal strength of steel employed for the shell. This can be explained by two effects. The first one is a material bonding at interface between core and shell, and adhesion spots with remains of the steel shell were visible on the surface of the pushed-out cores. The second effect is a higher thermal expansion coefficient of steel compared to the SMA core leading to a shrink fit.
Increasing the ratios d0/D0 for a constant extrusion ratio ER led to an increase in the bond strength. The bond strength at d0/D0 = 0.41 is about two times higher compared to tests on samples with d0/D0 of 0.22 and 0.32. This confirms the metallographic data showing that increasing ratios of d0/D0 lead to a firm material bonding with a distinct transition zone.
Hot bar rolling of SMA compounds
As joining of the SMA and the steel shell by hot bar rolling was successful, an investigation of the potential to join two different SMA by bar rolling for the manufacturing of SMA compounds was undertaken. In these experiments two half cylinders made of CoNiGa and NiFeGa were placed in a steel shell that was subsequently hot bar rolled using the process parameters that were employed for the single SMA cores. An SEM image and a line scan of the joining zone between the two SMA after bar rolling are shown in Fig. 12. A bonding between the two materials was successfully achieved and a transition zone of about 25 µm is visible in Fig. 12a. The chemical composition in this cobalt-rich transition phase (Fig. 12b) demonstrates a distinct stoichiometric ratio. In this regard, we can assume the possibility of a change in the stoichiometric ratio of the alloy composition. Unlike to the combination of SMA and steel, no Kirkendall pores occurred in the transition phase indicating a more balanced diffusion process in the absence of carbon in the atomic lattice.
In the optical image of Fig. 13, this phase appears light brown–grey and was identified as the γ-phase [3, 14] with a chemical composition of Ni48.6Fe17.1Mn4.1Ga23.7Co6.5. It has been observed that the formation of this transition γ-phase layer at the interface between core and shell depends on the local deformation conditions.
As can be seen from Fig. 13, the volume fraction of the γ-phase in the core increases with increasing degree of deformation of the material (from left to right in the figure). This means that besides a heat treatment one of the possible mechanisms for regulating the amount and nature of the γ-phase distribution can be the degree of plastic deformation as well.