# Development of Zirconium-Based Alloys with Low Elastic Modulus for Dental Implant Materials
## 1 Introduction  
Today, metallic materials have good biomechanical properties and suitability for sterilization processes in the biomaterials industry because they have excellent mechanical and corrosion properties. Biomaterials are widely used due to the aging of the population, and metallic materials such as gold, stainless steel, Ti, Co-Cr alloy, and Ni-Ti alloy are used [1; 2; 3]. However, the selection of materials for the various components of a biomedical device depends on several factors. First, the metallic implant materials should have good biocompatibility without any side effects. They remain in long-time contact with body fluids and tissues. Therefore, they may cause corrosion or release of alloying elements in the human body, which cause adverse effects [1; 2; 3; 4]. Second, they should have good mechanical strength and fatigue resistance to mimic the mechanical properties of bone substitutes [1; 2; 3]. For the mechanical strength, most metallic materials have excellent strength properties such as yield strength, tensile/compressive strength, shear strength, fatigue limit for the bone substitutes. However, there is a huge discrepancy of elastic modulus between the present implant metallic materials and human bone. Third, the osteointegration is the direct contact between human bone and the implant materials. For the improvement of osteointegration, many researchers have studied this for a long time and reported many applicable surface treatment techniques such as plasma electrolytic oxidation (PEO) and electrophoretic deposition (EPD) [5; 6; 7; 8; 9; 10]. At present, dental materials researchers are also concerned about the contact surface of implants and much research is being done. Recent research shows that surface research is a great help in rehabilitation in the field of dental implants. It has been reported that the application of surface treatments to the alloplastic materials used in rehabilitativesurgery, such as titanium surfaces, has shortened the healing time, especially the time required for osteointegration [11]. These reports can provide better care for clinicians and patients and can be of great help in the field of orthopedic and implant surgery. The Ti and Ti-6 Al-4V alloys are metallic materials that are widely used in dental implants, artificial joints, bone plates, etc. However, many recent studies have shown that both Al and V ions can cause long-term health problems [12; 13]. Vanadium has previously been shown to be toxic to humans [14; 15]. Element V has been found to react severely with animal tissues, and Al may be associated with nervous system disorders and Alzheimer's disease [12; 13]. Moreover, a low elastic modulus is required to transfer the appropriate mechanical stress of the bone to the surrounding bone [2; 3]. Low elastic modulus is important for mimicking the bone substitutes to neglect the stress shielding effects due to the unbalanced stress distribution between bone and implant materials [14; 15]. The elastic modulus of Ti-6 Al-4V alloy is about 110 GPa, which is lower than that of Co-Cr-Mo alloy (230 GPa) [16]. However, Ti-6 Al-4V alloys are still significantly higher than the human bone elastic modulus (10-30 GPa) [16]. The difference in elastic modulus between the metallic biomaterials and the human bone causes the stress shielding effect resulting in bone resorption. To solve this stress shielding problem, many new Ti alloys, especially single \(\beta\)-Ti phase alloys, have been developed. Unfortunately, the elastic modulus of most \(\beta\)-type Ti alloys is still high about 60-80 GPa, which is far from the requirement for hard tissue replacement (HTR) [17; 18]. The Zr and Zr alloys, which are similar to the Ti and Ti alloys, have excellent biocompatibility and are an advantageous non-toxic metal, are also of interest [19; 20]. The Zr is considered a biomechanical advantage because of its low elastic modulus, which minimizes the stress shielding of host bones compared to Ti and Ti alloys [21].