The influence of the powder metallurgy precursor on the microstructure and mechanical properties of a porous Cu-Al-Ni alloy

M.T. MALACHEVSKY; G. BERTOLINO; A. BARUJ; I. PAPUCCIO

Virtual

Conferencia; MRS 2020 Fall Meeting; 2020

Materials Research Society

Shape memory alloys are able to stand large deformations and to recover their shape afterwards. Depending on the temperature, the shape recovery could take place after heating or by the so-called pseudoelastic behavior. These particular properties make these smart materials good candidates for applications such as damping or actuation. Cu-based shape memory alloys are commercially attractive due to their relatively low cost compared to Ni-Ti alloys. Multiple research efforts have focused on expanding their potential applications by adding features that can improve the material. Porous shape memory alloys were developed in an effort to combine the shape recovery and energy dissipation properties with reduced density and weight. However, most polycrystalline Cu-based shape memory alloys are prone to intergranular fracture both in compact and porous samples. Reducing the material grain size could decrease the stress concentration at grain boundaries and help to avoid fracture. This can be achieved by alloying with other elements or by selecting an appropriate manufacturing method. As a possible fabrication route, we used powder metallurgy starting from a small particle size.Among Cu-based shape memory alloys, Cu-Al-Ni alloys are an alternative with good properties and low cost. In this work, we explored the fabrication of porous Cu-14Al-3Ni by conventional powder metallurgy, starting from mechanically pre-alloyed powder and a mixture of the elemental metal powders. For preparing the pre-alloyed powder, we employed a high-energy Fristsch Pulverissette 7 planetary mill, processing under Argon at 700 rpm for 3 hours. The resulting powder was analyzed by means of X-Ray Diffraction (XRD). The XRD pattern showed no presence of Al or Ni while the Cu peaks were shifted to lower angles indicating the solid solution of both Al and Ni in the structure. The Cu peaks were also significantly widened due to the strain resulting from the high-energy milling. Some peaks of ϒ-Cu9Al4 were detected, as well as small CW peaks resulting from the milling media contamination. The other precursor was prepared by hand mixing the elementary powders in an agate mortar. Both precursors were mixed with 1 wt.% mixture of PVA and PVB as a binder and, after drying, 30 vol.% ammonium bicarbonate was added as space holder. Cylindrical samples were prepared by uniaxial cold pressing at 700 MPa. After sintering at 1000 °C, the samples were solution-treated for 24 h at 900 °C under Ar. After the treatment, they were water-quenched to obtain the martensitic phase. The samples prepared with the milled powder were investigated by XRD. After sintering, the phases detected were α, ϒ-Cu9Al4 and AlNi. The heat treated samples consisted of the shape memory phase 18R accompanied by α and AlNi. The presence of AlNi in the sintered sample suggests that the alloy resulted short of both elements and consequently the solution treatment was performed in the α-β two-phase field. After quenching, α remained as a secondary phase. Meanwhile, the samples prepared with the hand mixed precursor consisted of the 18R and 2H shape memory phases. X-ray microtomography images of the porous samples were acquired with an Xradia Micro XCT-200 microscope. The samples prepared with the pre-alloyed precursor presented internal cracks indicating that the presence of the brittle ϒ-Cu9Al4 phase reduced the powder compressibility. The lower density prevented their mechanical characterization. On the other hand, samples prepared with the hand mixed precursor were well sintered allowing their complete characterization. Compression uniaxial tests were performed and the shape memory behavior was corroborated. Samples recovered 100% from a 7% strain and 98.56% from 11.5% strain when heated to 140 °C. The obtained grain size is about 30 µm.