Study on the Role of Stearic Acid and Ethylene-bis-stearamide on the Mechanical Alloying of a Biomedical Titanium Based Alloy ALIREZA NOURI, PETER D. HODGSON, and CUI’E WEN The present study examines the influence of different contents and types of process control agent (PCA), i.e., stearic acid (SA) and ethylene-bis-stearamide (EBS), on the microstructural evo- lution and characteristics of Ti-16Sn-4Nb (wt pct) alloy powders and bulk samples. The char- acterization of the powders and bulk samples was carried out by using chemical analysis, optical microscopy, scanning electron microscopy (SEM) combined with energy-dispersive spectrom- etry (EDS), and X-ray diffractometry. Results indicated that the powder recovered from the ball milling containers increased with increasing amounts of SA and EBS. Furthermore, adding more SA or EBS to the powder mixture resulted in a considerably smaller particle size, with a flaky-shaped morphology for the given ball milling time. Also, a slightly higher effectiveness was found for EBS when compared to SA. Meanwhile, the addition of both SA and EBS led to a delay in the alloy formation during mechanical alloying (MA) and caused contamination of the material with mainly carbon (C) and oxygen (O). An optimum amount of 1 wt pct PCA led to a good balance between cold welding and fracturing, and thus favored the formation of the titanium alloy. The microstructural observation of the bulk alloy showed a homogeneous dis- tribution of fine Nb-rich b-phase colonies within the a-Ti matrix with the addition of PCA less than 1 wt pct. DOI: 10.1007/s11661-010-0207-5 Ó The Minerals, Metals & Materials Society and ASM International 2010 I. INTRODUCTION MECHANICAL ALLOYING (MA) is a powder processing technique, in which initial elemental metal powders are continuously impacted by high energy balls in a container. MA is capable of synthesizing a variety of equilibrium and nonequilibrium materials including supersaturated solid solutions, amorphous metals and alloys, as well as nanostructural and quasi- crystalline phases. [1,2] It is also used to produce several alloys and compounds that are difficult or impossible to be obtained by the conventional ingot melting techniques. [3] Interest in MA has been constantly growing for synthesizing advanced materials, including Ti based alloys. [4,5] Titanium and some of its alloys are among the key advanced materials due to the excellent combination of good corrosion resistance, outstanding biocompatibility, and high strength-to-weight ratio; thus, providing a promising candidate for their appli- cations in automotive, aerospace, biomedical implant materials, etc. [6,7] MA proceeds by the continual cold welding and fracturing of the constituent powder particles when repeatedly subjected to high-energy collision of the milling media. The cold welding and fracturing process deform the particles, creating atomically clean surfaces with minimized diffusion distance. [8] However, the balance between welding and fracture may not be achieved in most cases by MA itself, and the tendency for powders to weld together into larger particles predominates and suppresses the process of alloying. It thus becomes imperative to strike a balance between welding and fracture in a MA process so as to develop the desired fine microstructure and intimate alloying. Organic surfactants, generally known as process control agent (PCA), are often used during MA to obtain the critical balance between cold welding and fracture and enhance the process efficiency. [2,8,9] During the MA process, the PCAs are absorbed onto the surface of the particles, inhibiting excessive cold welding by lowering the surface tension of the solid materials. Among the most commonly used PCAs in the MA process are stearic acid (SA) [10,11] and ethylene-bis-stearamide (EBS). [12–14] Although a number of investigations have studied the effect of PCA on the microstructure, morphology, and thermal properties of powders after MA, [10,13,15–21] little attention has so far been given to the understanding of the microstructure of bulk sintered alloys and to the establishment of a link between the microstructures and the different contents of the PCA. The present study aims to investigate the effect of different SA and EBS additions on the microstructural evolution of powder and bulk sintered biocompatible Ti-16Sn-4Nb (wt pct, ALIREZA NOURI, Postdoctoral Researcher, PETER D. HODGSON, Professor and Director, and CUI’E WEN, Associate Professor, are with the Institute for Technology Research & Innovation, Deakin University, Waurn Ponds, Victoria 3217, Australia. Contact e-mail: cuie.wen@deakin.edu.au Manuscript submitted June 1, 2009. Article published online April 17, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 41A, JUNE 2010—1409