Motivation of research

1. Approach Increasing Milling Time Bulk MA MA HIP Ni Ti Elemental Powder Lamellar Structure Amorphous Powder Crystalline Phases Bulk HIP Amorphous Phase HIP = Hot Isostatic Pressing. Motivation of research Amorphous metals offer superior tensile strengths and corrosion resistance when compared with their crystalline counterparts. The dimensions and shape complexity of existing cast bulk amorphous alloys have limitations. Taking advantage of solid state amorphization of multilayers in certain binary alloy systems such as Ni and Ti at low temperatures, mechanical alloying (MA) is used create a lamella structure followed by hot isostatic pressing (HIP) to simultaneously synthesize and consolidate equi-molar bulk amorphous Ni-Ti alloy. Findings Cryogenic MA in an attritor mill produced Ni-rich and Ti-rich lamella structure with 10 - 60 nm spacing, and early stage of amorphization. HIP of milled powder at 370 oC for 2 h produced 93 % dense amorphous structure with 5% amount of crystalline Ti and intermetallics in the amorphous matrix. Nano indentation of the non-porous regions of the compact reflects the presence of the above phases, with the crystalline Ti being the least hard and the amorphous being the hardest. TEM Analysis TEM (bright field) image showing Ni-Ti milled for 10 hours at cryogenic temperature and then annealed for 20 hours at 320°C. XRD Analysis XRD Analysis XRD analysis of Ni-Ti milled for 10 hours at cryogenic temperature then HIPed for 2 hours at 370°C. This pattern shows that a large amorphous halo has formed with low intensity Ni and Ti peaks still present. Selected area diffraction analysis of the above figure. A halo from this analysis as well as diffracted spots indicates that this region is a mixture of amorphous and crystalline phases.

2. EBSD Analysis SEM micrograph of HIPed Ni-Ti powder compact with 93% density. EBSD analysis of a cross-section of the HIPed sample shows almost no crystal grain structure within the Ti-Ni compact compared to the copper used in the canning operation. Broader Impact Three graduate students have participated in this research. They have each received the masters degree, two of whom have stayed on for the Ph.D. degree. Four students extended this work to the studying of surface amorphization of Ti-based metallic implants as their senior project; they have graduated but one has stayed on to continue this work as a graduate student. Education All students working on this project have received hours of hands-on training on TEM, SEM, Nano-indentation and XRD at a new material characterization facility.