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

MARIA TERESA MALACHEVSKY; ALBERTO BARUJ; BERTOLINO, GRACIELA M.; IGNACIO PAPUCCIO

Congreso; MRS Fall 2020; 2020

MRS

Shape memory alloys are able to stand large deformations and to recover their shapeafterwards. 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 forapplications such as damping or actuation. Cu-based shape memory alloys are commercially attractive due totheir relatively low cost compared to Ni-Ti alloys. Multiple research efforts have focused on expanding theirpotential applications by adding features that can improve the material. Porous shape memory alloys weredeveloped in an effort to combine the shape recovery and energy dissipation properties with reduced densityand weight. However, most polycrystalline Cu-based shape memory alloys are prone to intergranular fractureboth in compact and porous samples. Reducing the material grain size could decrease the stressconcentration at grain boundaries and help to avoid fracture. This can be achieved by alloying with otherelements or by selecting an appropriate manufacturing method. As a possible fabrication route, we usedpowder 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, startingfrom 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 at700 rpm for 3 hours. The resulting powder was analyzed by means of X-Ray Diffraction (XRD). The XRDpattern showed no presence of Al or Ni while the Cu peaks were shifted to lower angles indicating the solidsolution of both Al and Ni in the structure. The Cu peaks were also significantly widened due to the strainresulting from the high-energy milling. Some peaks of γ-CuAl were detected, as well as small CW peaksresulting from the milling media contamination. The other precursor was prepared by hand mixing theelementary powders in an agate mortar. Both precursors were mixed with 1 wt.% mixture of PVA and PVB as abinder and, after drying, 30 vol.% ammonium bicarbonate was added as space holder. Cylindrical sampleswere 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 martensiticphase.The samples prepared with the milled powder were investigated by XRD. After sintering, the phases detectedwere α, γ-CuAl and AlNi. The heat treated samples consisted of the shape memory phase 18R accompaniedby α and AlNi. The presence of AlNi in the sintered sample suggests that the alloy resulted short of bothelements 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 consistedof 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 thepresence of the brittle γ-CuAl phase reduced the powder compressibility. The lower density prevented theirmechanical characterization. On the other hand, samples prepared with the hand mixed precursor were wellsintered allowing their complete characterization. Compression uniaxial tests were performed and the shapememory behavior was corroborated. Samples recovered 100% from a 7% strain and 98.56% from 11.5% strainwhen heated to 140 °C. The obtained grain size is about 30 μm.