Composites of iron alloys/hydroxyapatite are interesting biodegradable implants since their individual components have several advantages. It is well known that the Fe-Mn-Si alloys that have high Mn content ≥ 20wt.% are characterized by shape-memory effect (SME) owing to the reversible phase-transition between the face-centered cubic austenite and hexagonal close-packed martensite [1–5]. This characteristic makes these alloys are interesting for civil applications like pipe joint, dampers and other technological applications [6–8]. Early 80s, Sato et al. [9, 10] reported on the shape memory effect (SME) of these alloys. Since then, these materials have been widely studied and received significant consideration due to their low-cost, good workability, weldability, and worthy machinability properties [11, 12]. Recently, it has been reported that Fe-Mn-Si alloys are promising candidate as degradable biomaterials owing to their high cell viability, worthy mechanical properties and acceptable degradation rate [13, 14]. Also, provisional biomedical-devices like cardiovascular-stents and bone fixation plates have been considered as prospective uses of Fe-Mn-Si alloys owing to their sound biodegradability, great biocompatibility and mechanical properties [15–19]. However they have low biodegradable rate due to their dense structure. So, the preparation of these alloys by powder metallurgy (PM) can overcome the lower degradation rate since PM has the ability to prepare porous alloys with controlled porosity. Moreover, the incorporation of hydroxyapatite in this alloys to form new composites, might also increase the bioactivity and other properties of these composites.
Generally, the addition of bioceramic nanoparticles to the Fe-Mn-Si alloys leads to extra improving of the biodegradability and biocompatibility as well as can form a composite with developed properties for commercial purposes. Hydroxyapatite (HA) is the most famous biomaterials which can form good composite when added to Fe-Mn-Si alloys. It is thermo-dynamically the most stable phase in physiological conditions and has the capability to chemically-bond directly to the bone owing to its similarities with composition and structure of bone- and tooth-minerals. It doesn't display any cytotoxic effect but it exhibits outstanding biocompatibility with hard-tissues, skin and muscle-tissues . Furthermore, it shows substantial applications in other fields as catalyst, sensor, and optical applications, etc.  Rodriguez-Lorenzo et al.  reported on the synthesis of hydroxyapatite by precipitation method. The material prepared was stable up to 1200°C and when sintered between 900–1200°C showed dense HA bodies. The optimum sintering temperature was 1100oC; it gave higher mechanical properties with balanced ratio between the total pore area and the average grain size. Over 1100oC, the porosity and density might be increased due to the phase transformation and decomposition of HA into another phases. This indicates that the HA sintered quickly when it is only exist. The presence of pores facilitates the formation of interfacial bond and adhesion between HA or metallic implants and living tissues; consequently develops the strength. [23–26]. When the pore size is very large or very small, the cell cannot spread in the scaffold and cannot form networks . On the other hand, the suitable large porosity increases the surface-area which produces high cell-seeding effectiveness, movement and neovascularization . Pang and Bao synthesized nano size HA powder by chemical precipitation using CaCl2 and ammonium hydrogen phosphate .
There are several approaches for preparing alloys and composites as ingot metallurgy (IM) and powder metallurgy (PM) [30–35]. PM technique has some benefits like cost-effectiveness, high production-rate, can produce complex shapes, and can produce laminated bodies by different layers of powders. Mechanical alloying (MA) is the most famous technique in PM for preparing alloys and composites. From this method, controlled grain size, fine microstructure and homogenous chemical-composition can be accomplished in solid state , therefore avoiding the disadvantages of IM method.
The present work focuses on the preparation of Fe-14Mn-6Si/HA composites by PM technique. The study of sinterability, microstructure, hardness and bioactivity are the main objectives of the present study.