Stage II

2.1 Obtaining samples for hot rolling

Samples were produced at S.C. R & D Consultancy and Services Bucharest, in the form of ingots, made from high purity components, in a FIVES CELES levitation induction furnace using the parameters and technology presented in the Stage I Report. The billets are delivered in their rough state (rotary grinding) to typical sizes of 18 mm diameter and 20-40 mm length, as shown in Fig 1.

Fig.1 Billets received from the R & D Consultancy and Services Bucharest: (a) frontal view; (b) top view

Fig.2 Samples cracking behavior after hot rolling

2.2 Sample thickness reduction

The hot rolling of four prisms samples with a rectangular cross-section was made in two passes, according to the procedure described in Stage I. Despite the high cracking tendency of the samples, fragments shown in Figure 2 did not detach, remaining related to the body of the sample, as seen in the corner marked by an arrow. Another particularity of this cracking tendency is that in each of the four fragments there were three transversal cracks, the hot-rolled samples presented, at the macrographic level, the typical aspect of oligocrystalline samples with a "bamboo" structure characterized by the lack of triple-bound areas between crystalline grains.

2.3 Determining the average grain size

After treatment, due to the abnormal growth of some crystalline grains, some samples have distorted. In the following figures the grain boundaries, visible by optical microscopy, were marked and the crystalline grains (Gi) were numbered. Figure 3 shows the microstructure of a tensile sample of an Fe43.5Mn34Al18Ni4.5 alloy which has undergone an abnormal heat treatment of grains.

Fig.3 Grain boundaries of an Fe43.5Mn34Al18Ni4.5 alloy tensile-tested sample which underwent a cyclic thermal treatment

Fig.4 Details of tensile testing samples that will be tested: (a) constructive dimensions; (b) examples of cut samples

2.4 Obtaining samples for tensile testing and mechanical-dynamic analysis

With the help of the molybdenum spark erosion cutting machine three tensile testing samples were cut and one for mechanical-dynamic analysis. Fig. 4 shows details of tensile testing samples which will be tested.

2.5.1 Effects of thermomechanical treatment on tensile breaking behavior

Fig.5 centralizes the effects of the first three processes, hot rolling (HR), annealing (A) and cold rolling (CR) from the thermo-mechanical treatment and of the chemical composition on static tensile behavior.

Fig.5 Static tensile breaking curves illustrating the effects of sample configuration and processing state (HR-hot rolled, A-annealed and cold-rolled CR) for opm samples of Fe43.5Mn34Al16.5Ni6

Fig.6 The loading/unloading curve of a opm sample, Fe43.5Mn34Al15Ni7.5 in the hot rolled state

2.5.2 Sample behavior during a loading/unloading tensile cycle

Due to ductility favored by cross-section reduction, the opm samples in hot-rolled, annealed or cold-rolled states were subjected to tensile loading/unloading cycles, such as the curves exemplified in Fig 6.

2.5.3 Thermomechanical treatment effects on the tensile behavior over two loading / unload cycles

Fig.7 summarizes the results of the application of two loading/unloading tensile cycles.

Fig.7 Loading/unloading curves of an opm sample, Fe43.5Mn34Al15Ni7.5 in an annealed state

Fig.8 Mechanical tensile cycling of Fe43.5Mn34Al15Ni7.5 opm samples in cold rolled state

2.5.4 Effects of chemical composition on the mechanical cycling behavior and the evolution of superelastic behavior at tensile testing

Next, five cycles of loading and unloading were performed on cold-rolled samples, both Fe43.5Mn34Al15Ni7.5 and those who had 1.5% at Ni substituted with Al. The results appear in Fig.8.

2.6 Conclusion

Following the study, the next conclusions were reached:
  • Substitution of 3 %at Ni with Al, with respect to the classical composition, has resulted in a marked increase in brittleness during hot rolling, so that all the fragments fractured into three segments.
  • Annealing favored softening and shredding of samples, allowing for 45-77% tear elongation, while cold rolling produced cold hardening, allowing maximum tensile stresses of 1-1.2 GPa.
  • The fiber presence in the classic composition sample was accompanied by the highest tensile strength in the cold-rolled Fe43.5Mn34Al15Ni7.5 sample.
  • Despite the application of hot rolling and annealing in an uncontrolled environment (without a protective atmosphere), the traces of contamination (especially by oxidation) could be removed by mechanical grinding, so variations in the chemical composition were negligible.
  • The increase in grain size by "classic" thermomechanical treatment did not allow the average grain size to exceed 0.41 mm.
  • The "classic" thermo-mechanical treatment variant did not allow complete decomposition of the globular form of γ (cfc), which showed a texture after <101> / <112>.
  • For the application of an optimized thermomechanical treatment, thermal processing technology was implemented in a quartz vacuum capsule with a protective atmosphere of argon.
  • The optimized thermomechanical treatment variant led to oligocrystalline structures having the highest average granulations of 3.761 and 2.48 mm respectively, the rest of the samples having a mean granulation over 2 mm.
  • The presence of pseudo-elastic elongation was observed even in the annealed state regardless of the thermomechanical treatment variant and was favored by the increase in the number of mechanical cycles, tensile loading/unloading.
  • The oligocrystalline samples, resulting from the application of the optimized thermomechanical treatment variant, showed levels of loading tension and pseudoelastic elongations of 0.4-0.47%.

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