Present investigation on microstructure and mechanical properties of pseudo NiTi

Transcription

1 Present investigation on microstructure and mechanical properties of pseudo NiTi C. Hinte, S. Reschka, M. Hermann, G. Gerstein, M.A. Swider, H.J. Maier

2 Objectives of the present talk Pseudo NiTi High Entropy Shape Memory Alloys (HESMAs) Microstructure: - Ni 25 Cu 15 Co 10 Ti Zr Hf in the as-cast & heat-treated state difficulties with orientation determination Mechanical properties: - hardness and compression tests difficulties with cracks and sample size Summary and outlook Seite 2 Subgroup-Meeting Dresden

3 High entropy alloys & high entropy shape memory alloys Material development of Ni 50 Ti 50 (B2-structure) Ti-lattice-sides occupied by Ti, Hf, Zr Ni-lattice-sides occupied by Co, Ni, Cu No major base element pseudo pseudo NiTi : B2-related triclinic P1 phase of HESMA (Firstov et al., MATEC Web of Conferences 33, 2015) B2-related triclinic structure lattice distortion Seite 3 Subgroup-Meeting Dresden

4 Transformation temperatures Ni 25 Cu 25 Ti Zr Hf M S 500 K / 227 C Ni 25 Cu 15 Co 10 Ti Zr Hf M S 430 K / 157 C (Firstov et al., Materials Today: Proceedings 2S, 2015) (Firstov et al., Shap. Mem. Superelasticity 1, 2015) increase of the martensite start temperature compared to NiTi ºC vs. ~30 ºC Seite 4 Subgroup-Meeting Dresden

5 EDX: Ni 25 Cu 15 Co 10 Ti Zr Hf as-cast EDX-mapping as-cast Seite 5 Subgroup-Meeting Dresden

6 EDX: Ni 25 Cu 15 Co 10 Ti Zr Hf heat-treated EDX-mapping heat-treated Seite 6 Subgroup-Meeting Dresden

7 EBSD: Ni 25 Cu 15 Co 10 Ti Zr Hf No pattern in interdendritic region Insufficient pattern-quality within dendrits Orientation only determinable in the Zr/Hf-precipitations EBSD-analysis not possible quality-map and pattern of Ni 25 Cu 15 Co 10 Ti Zr Hf possible reasons solution verified insufficient preparation different preparations yes surface amorphization preparation with Focused Ion Beam (FIB) yes overlaid structures TEM-investigation yes / outlook lattice distortion TEM-investigation yes / outlook Seite 7 Subgroup-Meeting Dresden

8 EBSD: Ni 25 Cu 15 Co 10 Ti Zr Hf Electron diffraction wave diffraction Atoms as new wave centers Crystalline (cubic): Ordered structure Periodic eliptical interference-bands atoms as local point source; cubic Superposition of the interferences Kikuchi-pattern Irregular, distorted: No ordered structure No congruent interference-bands Superposition doesn t increase intensity atoms as local point source; distorted no Kikuchi-pattern Seite 8 Subgroup-Meeting Dresden

9 TEM-investigation: Ni 25 Cu 15 Co 10 Ti Zr Hf Overlaid and parallel structures Low differences in element concentration Clear diffraction-pattern of overlaid structures Single structures not differentiable EDX of Ni 25 Cu 15 Co 10 Ti Zr Hf lamella Seite 9 Subgroup-Meeting Dresden

10 Mechanical testing: Materials & processing Material: Ni 25 Cu 15 Co 10 Ti Zr Hf Ni 25 Cu 25 Ti Zr Hf Different manufacturing: Vacuum induction melting (VIM) Arc melting (AM) Available tests: Hardness tests Compression & compression fatigue test Seite 10 Subgroup-Meeting Dresden

11 Vickers hardness in HV1 Vickers hardness in HV1 Comparison VIM AM: hardness AM-ac VIM-ac AM-ht VIM-ht HEA 56-1 VAM-G HEA 56-2 VIM-G HEA 56-1 VAM-H900 HEA 56-2 VIM-H VIM shows a greater Vickers hardness Heat treatment doesn t signifcantly increase the Vickers hardness of AM produced samples Seite 11 Subgroup-Meeting Dresden

12 compressive stress σd in MPa VIM : loading test HEA C VIM-G VIM-ac 50 C HEA C VIM-G VIM-ac 75 C HEA C VIM-G VIM-ac 100 C HEA C VIM-G VIM-ac 125 C HEA 56-2 VIM-G VIM-ac 150 C 150 C HEA 56-2 VIM-G VIM-ac 175 C 175 C HEA C VIM-G VIM-ac 200 C HEA C VIM-G VIM-ac 250 C compression εd in % 10 Curves imply a superelasticity at higher temperatures Compressive strength: Mpa Sample size: Ø 2 mm x 4 mm Seite 12 Subgroup-Meeting Dresden

13 Mechanical testing materials Ni 25 Cu 25 Ti Zr Hf Ni 25 Cu 15 Co 10 Ti Zr Hf (VIM) 10 mm 10 mm 10 mm 10 mm 5 mm Seite 13 Subgroup-Meeting Dresden

14 Heattreatment of Ti16.667Zr16.667Hf16.667Ni25Cu15Co10 quenched air (vacuum) Seite 14 Subgroup-Meeting Dresden furnace (vacuum)

15 Heattreatment of Ti Zr Hf Ni 25 Cu 25 quenched air (vacuum) furnace (vacuum) Seite 15 Subgroup-Meeting Dresden

16 Summary and outlook Heat-treatment isn t able to remove differences caused by the processing Mechanical properties depend on the manufacturing method the superelastic effect can be suspected based on the compression tests The cooling rate is important in order to receive samples sufficient in size for mechanical testing outlook & further investigation: Loading experiments on Ni 25 Cu 25 Ti Zr Hf & further new materials provided by our research partner at Bochum university Loading experiments and tensile tests on homogenized samples Increase of the loading temperature in order to investigate superelasticity Mechanical & functional fatigue testing influenced by temperature Seite 16 Subgroup-Meeting Dresden

17 Best results after controlled cooling Martensite present Crack free Samples with sufficient size Seite 17 Subgroup-Meeting Dresden