Erika Coaglia Trindade Ramos

Phase Transformations during the Preparation of Ti6Si2B by High-Energy Ball Milling

Recently, it was identified the existence of a new intermetallic phase in the Ti-Si-B ternary system with atomic composition near Ti6Si2B. In the present work, we report on the phase transformations during the preparation of Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B (at.-%) powders in a planetary Fritsch P-5 ball mill from high-purity elemental powders. To understand the phase transformations, powder Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B samples milled for 90 h were vacuum heated at various temperatures. The starting materials and milled powders were characterized by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopes, and differential scanning calorimetry (DSC). Results indicate that the Ti peaks widen and weaken with the increasing milling time and the silicon was practically dissolved into Ti and TiH2 lattices during milling and formed solid solutions in pre-alloyed Ti-22.2Si-11.1B and (TiH2)-22.2Si-11.1B powders, respectively. The use of titanium hydride instead of titanium as starting material allowed accelerating the mechanical alloying process, i.e., the Ti6Si2B phase was formed during heating at lower temperatures than in case of titanium as starting material. As previously observed, the decomposition reaction of the titanium hydride occurred near 550C. Powder (TiH2)-22.2Si-11.1B sample milled for 90 h presented very fine particle size lower than 20 nm. The ternary Ti6Si2B phase was formed in powder Ti-22.2Si-11.1B samples after heat treatment. Traces of Ti and Ti5Si3 were also detected.

Syntheses of the Ni3Ti, NiTi, and NiTi2 Compounds by Mechanical Alloying

The aim of this work is to prepare the Ni3Ti, NiTi, and NiTi2 compounds by mechanical alloying from elemental Ni-25at.%Ti, Ni-50at.%Ti, and Ni-66.6at.%Ti powder mixtures. The milling process was carried out in a planetary ball mill under argon atmosphere using a rotary speed of 200rpm, stainless steel balls (10 and 19 mm diameters) and vials (225mL), and a ball-to-powder weight ratio of 10:1. Following, the milled powders were heat treated at 900oC for 1h in order to attain the equilibrium microstructures. The milled powders were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and microanalysis via EDS. Similar ball milling behavior of Ni-Ti powders was noted in this work, e.g., a pronounced cold-welding between ductile powders occurred during the initial milling times. The Ni3Ti, NiTi, and NiTi2 compounds were synthesized after milling for 30h. Atomic disordering of the NiTi and NiTi2 compounds was achieved, and amorphous structures were then formed in Ni-50Ti e Ni-66.6Ti powders milled for 60h and 210h, respectively. Homogeneous matrixes constituted by the Ni3Ti, NiTi, and NiTi2 phases were formed in Ni-Ti powders after heat treatments at 900oC for 1h. Iron contamination lower than 2 at-% was measured by EDS analysis in heat-treated Ni-Ti alloys.

Structural Evaluation of Mechanically Alloyed Ti-Nb Powders

This work discusses on the structural evaluation of mechanically alloyed Ti-Nb powders. The Nb amount was varied between 20 and 50 wt-%. The milling process was carried out in a planetary Fritsch P-5 ball mill under Ar atmosphere. The structural evaluation was conducted by scanning electron microscopy, X-ray diffraction, and energy dispersive spectrometry. During ball milling it was noted an excessive agglomeration of ductile Ti-Nb powders on the balls and vial surfaces, and the final amount of remaining powders was then drastically reduced into the vials. This fact was more pronounced with the increased Nb amount in starting powders. Typical lamella structures were formed during ball milling, which were refined for the longest milling times, and fine and homogeneous structures were formed in Ti-Nb (Nb=20-50wt-%) powders. XRD results indicated that the full width at half maximum values of Ti peaks were continuously increased while that the crystallite sizes were reduced for longer milling times due to the severe plastic deformation provided during ball milling of Ti-Nb powders. However, the EDS analysis revealed the presence of Nb-rich regions in Ti-Nb powders after ball milling. The critical ball milling behavior of ductile Ti- Nb powders contributed for reducing the yield powder and increasing the structural heterogeneity.

Effect of Milling Parameters on the TiB and TiB2 Formation in Ti-50at%B and Ti-66at%B Powders

This work discusses on the effect of milling parameters on the TiB and TiB2 formation in Ti-50at%B and Ti-66at%B powders, respectively. Both powder mixtures were processed in a planetary ball Fritsch P-5 ball mill under Ar atmosphere, varying the milling parameters: rotary speed (150 and 200 rpm), size of balls (10 and 19mm diameter) and ball-to-powder weight ratio (2:1 and 10:1). In order to obtain the equilibrium structures the milled powders were heated at 1200oC for 1h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal analysis (DSC). XRD results indicated that extended Ti(B) solid solutions were formed during ball milling of Ti-50at%B and Ti-66at%B powders. After milling for 170h it was noted the TiB and TiB2 formation in Ti-50B and Ti-66B powders processed under higher-energy condition. DSC analysis revealed that the TiB2 formation was completed during heating of mechanically alloyed Ti-66at%B powders only. After heating at 1200oC for 1h, a large amount of TiB and TiB2 was found in Ti-B powders milled under higher energy condition.

Preparation of Ta-12.5Si-25B Powders by Mechanical Alloying

The present work reports on the preparation of the Ta5SiB2 compound by highenergy ball milling and subsequent heat treatment from elemental Ta-12.5at%Si-25at%B powder mixture. The milling process was carried out at room temperature in a planetary ball mill under argon atmosphere. Following the milling process, the powders were heat-treated at 1200oC for 4h under Ar atmosphere in order to obtain the equilibrium microstructure. The milled and heat-treated powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results indicated that the Si peaks disappeared after milling for 1h. It was noted that the broadening and the reduced intensity of Ta peaks occurred continuously up to milling for 10h, suggesting that the Si and B atoms were preferentially dissolved into the Ta lattice during ball milling to form a supersaturated solid solution. A halo was formed in Ta-12.5at%Si-25at%B powders milled for 100h, suggesting that an amorphous phase was achieved. No intermetallic phase was formed in powders milled for 200h. A large amount of Ta5SiB2 was formed after heat treatment at 1200oC for 4h. In addition, peaks of TaB and another unknown phase were also identified.

Syntheses of TiB and TiB2 by High-Energy Ball Milling

The present work reports on the syntheses of TiB and TiB2 by high-energy mechanical milling from high-purity elemental powders: Ti (99.9 wt.-%, spherical, -150 mesh) and B (99.5 wt.%, irregular, -40 mesh). Titanium hydride (99.7 wt.-%, chip) was also used in place of titanium to produce TiB. The high-energy milling was carried out in a planetary ball mill under high-purity argon atmosphere using a ball/powder weight ratio of 2:1, milling speed of 150 rpm, stainless steel vial (225 mL) and high-Cr hardened steel balls (10 mm of diameter). The powders were characterized by means of X-ray diffraction (XRD), scanning electron (SEM) and transmission (TEM) microscopes, and differential scanning calorimetry (DSC). TiB was successfully produced after heating of mechanically alloyed Ti-50at.%B and TiH2-50at.%B powders. The decomposition of the titanium hydride occurred during heating in the temperature range 500 to 600 o C. TiB2 was also formed after heating at 1100 and 1200 o C.

Structural Evaluation of Ti-22.2Si-11.1B Powders Milled with PCA Addition

A large amount of the Ti6Si2B compound can be formed by mechanical alloying and subsequent heat treatment from the elemental Ti-22.2at%Si-11.1at%B powder mixture, but the yield powder after ball milling is reduced due to an excessive agglomeration of ductile particles on the balls and vial surfaces. This work reports on the structural evaluation of Ti-22.2at%Si-11.1at%B powders milled with PCA addition, varying its amount between 1 and 2 wt-%. The milling process was carried out in a planetary ball mill under argon atmosphere, and the milled powders were then heated at 1200oC for 1h under Ar atmosphere in order to obtain equilibrium structures. Samples were characterized by X-ray diffraction, scanning electron microscopy, and thermal analysis. Results revealed that the PCA addition reduced the excessive agglomeration during the ball milling of Ti-22.2at-%Si-11.1at-%B powders. After heating at 1200oC for 1h, the Ti5Si3, Ti3O and/or Ti2C phases were preferentially formed in Ti-22.2at%Si-11.1at%B powders milled with PCA addition, and the Ti6Si2B formation was inhibited.

High-Energy Ball Milling of Ni-Ti and Ni-Ti-Nb Powders

This work reports on the preparation of Ni-50Ti and Ni-40Ti-10Nb and Ni-30Ti- 20Nb (at.%) powders by high-energy ball milling and subsequent heat treatment. The milling process was carried out at room temperature in a planetary ball mill under Ar atmosphere. The as-milled powders were than heat-treated at 900oC for 1h under Ar atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM), and microanalysis via energy dispersive spectroscopy (EDS) were used to characterize the milled and heat-treated powders. A metastable phase was initially formed in Ni-50Ti and Ni-40Ti-10Nb powders milled for 1h. Following the ball milling, the B2-NiTi compound was formed in these powder mixtures. The disordering of the B2-NiTi compound occurred owing the internal lattice strain after milling for 30h. Two phases were identified in Ni-50Ti and Ni-40Ti-10Nb powders milled for 60h: the metastable phase previously reported, and an amorphous phase. In Ni-30Ti-20Nb powders, it was noted the presence of an amorphous halo only. A structural relaxation of the B2-NiTi phase occurred in heat-treated Ni-50Ti, Ni-40Ti-10Nb, and Ni-30Ti-20Nb powders. A small amount of Ni3Ti and NiTi2 was also formed after heat treatment at 900oC for 1h, and an iron contamination lower than 2at.% was found.

Preparation of Nb-40Ti Powders by High-Energy Milling

Porous Ti-Nb alloys are promising candidates for biomedical applications. In the present study, alloy powders containing 60 wt-% Nb were prepared by high-energy milling of Nb, Ti, and/or TiH2 powders. The high-energy milling process was carried out in a planetary ball mill. The starting and as-milled materials were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM). Elemental (Nb, and Ti) and TiH2 powder mixtures with composition Nb-40wt%Ti were mechanically alloyed for 2 to 30 h. The formation of a BCC Nb(Ti) solid solution by high-energy milling using elemental Ti powder to produce Nb-40Ti was observed after milling for 30 h. A HCP-Ti solid solution was formed after milling for 30 h due to the partial decomposition of titanium hydride powder mixture during high-energy milling.

Syntheses of Ta3Si, Ta2Si, Ta5 Si3, and TaSi2 by Mechanical Alloying

Solid state reactions induced by mechanical alloying of high-purity elemental powder mixtures of Ta and Si with atomic compositions of Ta-25%Si, Ta-33.3%Si, Ta-37.5%Si, and Ta-66.6%Si were carried out using a planetary Fritsch P-5 ball mill and stainless steel vial (225 mL). Elemental powder mixtures and hardened steel balls (10 mm diameter) were placed in the vial in an Ar-flushed glove box to avoid contamination. The mass of the powder charge per compound was close to 40 g and the mass ratio of balls to powder was 10:1. The starting and milled materials were characterized by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy, and differential scanning calorimetry (DSC). A supersaturated solid solution of Si in Ta was achieved in powder Ta-25%Si, Ta-33.3%Si and Ta-37.5%Si samples milled for 40 h while Si crystallites were also found in powder Ta-66.6%Si sample. The nanocrystalline Ta3Si, Ta2Si, Ta5Si3, and TaSi2 phases were successfully produced after milling for 40 h and further heat treatment. Introduction The accepted binary Ta-Si phase diagram is based primarily on the work of Schlesinger [1]. The stable solid phases TaSS (ss-solid solution), Ta3Si, Ta2Si, α-Ta5Si3 (low-temperature), β-Ti5Si3 (high-temperature), TaSi2 and Si are indicated. Ta3Si and Ta2Si are essentially stoichiometric and formed from melt by the peritectic reactions L+Ta2Si ⇒ Ta3Si and L+Ta5Si3 ⇒ Ta2Si, and occur at 2340 o C and 2440 o C, respectively. The stoichiometric compounds Ta5Si3 and TaSi2 are formed by congruent transformations at 2550 o C and 2040 o C, respectively. Mechanical alloying is a powder metallurgy processing technique used to produce various oxide dispersion strength alloys, amorphous phases, supersaturated solutions and nanocrystalline materials [2-4]. The solid state reaction induced by the ball milling occurs due to the repetitive cold-welding and fracture of powder particles, which results in the intimate alloying of the starting material on the atomic scale. In a ductile-brittle system, the mechanical alloying process leads to the homogeneous dispersion of brittle phase in the ductile matrix [5,6]. Non-equilibrium processing of materials has been attractive due to the possibility of producing better and improved materials than is possible by conventional methods. Several alloys of the Mo-Si [7-10], Nb-Si [7,11], Ti-Si [7, 12], Zr-Si [2], V-Si [3] and Ni-Si [13] systems have been the subjects of considerable investigation, which can be synthesized by mechanical alloying. In the Ta-Si equilibrium diagram, the non-stoichiometric tantalum disilicide was successfully developed by means of mechanochemical activation under intensive mechanical treatment in an activator-mill of planetary type from elemental powder mixture [14]. The increase of elementary cell parameter takes place during the initial period of milling up to 12 min. The concentration interval from 61.7 to 81.7 at.%Si was investigated. It was observed the increase of Ta lines diminishes, and reflexes of the TaSi2 phase appears during the treatment. However, information concerning the preparation of the Ta3Si, Ta2Si and Ta5Si3 phases by high-energy ball milling was not found. Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 201-206 doi:10.4028/www.scientific.net/JMNM.20-21.201