Recenze: Ing. Radovan Bureš, CSc.

Transkript

1 Hutnické listy č./00, roč. LXIII Methods of Preparation of Based Alloys Metody přípravy slitin na bázi Ing. Petr Štěpán, Doc. Dr. Ing. Monika Losertová, Ing. Daniel Petlák, Prof. Ing. Drápala Jaromír, CSc., Vysoká škola báňská Technická univerzita Ostrava, Fakulta metalurgie a materiálového inženýrství Two methods were used for preparation of alloy. The first method consisted of vacuum electron beam floating zone melting of rod wrapped around with a wire. Solution heat treatment at 900 C for hour in flowing Ar gas, water quenching and precipitation hardening at different temperatures (00, 400 and 600 C) for hour were performed. Plasma furnace melting was the second method for preparation of the experimental alloys with nominal composition of - and -5 (at.%) using (55/45 wt.%) master alloy and pieces. Heat treatment at 00 C for hours in flowing Ar gas and water quenching of prepared samples was realised. Metallographic observation, micro-hardness measurement and general micro-analysis of the prepared alloys were performed in asmelted and heat treated conditions. The effect of heat treatment and content on the micro-structure and microhardness was determined. Bylo provedeno studium možnosti přípravy slitin na bázi dvěma metodami, a to elektronovým zonálním a plazmovým tavením. První technologií byla v elektronové peci pro přípravu slitiny -5 (at.%) přetavena tyč o průměru 0 mm a délce 300 mm, na kterou byl navinut tenký pásek. Odebrané vzorky byly homogenizačně žíhány při 900 C po dobu hodiny v ochranné atmosféře Ar a poté byly precipitačně vytvrzeny při různých teplotách (00, 400 a 600 C) po dobu hodiny v průtoku Ar. Druhou metodou, pomocí které byla předslitina (55/45 hm.%) dolegována kusovým titanem a přetavena v plazmové peci, byly připraveny experimentální slitiny o nominálním složení - a -5 (at.%). Slitiny byly pouze homogenizačně žíhány hodin při 00 C v Ar atmosféře a zakaleny do vody. Na slitinách připravených oběma metodami byl proveden metalografický rozbor, měření mikrotvrdosti a fázová mikroanalýza pro litý i žíhaný stav. Slitina připravená elektronovým zonálním tavením s průměrným složením 73,89 at.% a 6, at.% vykazovala dvojfázovou strukturu (α+β). Mikrotvrdost naměřená pro různé stavy tepelného zpracování dosahovala nejvyšší hodnoty (9 HV) po precipitačním vytvrzení při 400 C, což odpovídalo precipitaci fází α a ω. Mikrostruktura slitin - a -5 připravených plazmovým tavením s průměrným obsahem,57 a 7,3 at.% byla dendritická a po homogenizačním žíhání přešla na lamelární (α+β). Hodnoty mikrotvrdosti byly ve stavu po plazmovém tavení srovnatelné s výsledky po tavení metodou EBFZM, po žíhání se však mikrotvrdost výrazně zvýšila. Byl rovněž prokázán vliv rostoucího obsahu na pokles mikrotvrdosti, což souvisí s množstvím a morfologií matrice β a precipitující fáze α.. Introduction For many years, titanium based alloys were applied in different bio-compatible applications. Shape memory alloys on the base of Ni have been widely used for specific implant materials such as orthodontic archwire and orthopaedic implants due to their unique properties. New -based alloys with bio-compatible elements, such as, Zr, Ta, Sn, have been developed in order to solve the problems with toxicity of some additional elements, such as V, Al and Ni. Recently, β-titanium alloys,, Ta and Zr based alloy systems were studied and found to display both lower elastic module and higher tensile strengths that are preferable for bio-compatible metals and alloys. Binary -based alloys with shape memory effect and good bio-compatibility are investigated as Ni free suitable substitution of Ni-based alloys in bio-medical applications. It has been confirmed [,, 3] for wide range of content ( wt.% ) that alloys exhibited at room temperature the shape memory effect and super-elastic behaviour that is related to stress induced martensite. Niobium as well as possibly other alloying elements (such as Mo, W, Fe, Si, B, Ta, Zr, Sn, V) is β stabilizer so that falls into β- alloys, possibly into β rich (α+ β) alloys. Preparation of alloys with high content of, which is a high melting point metal has difficulties with use of conventional methods. Powder metallurgy (PM), mechanical alloying (MA), electron beam floating zone melting (EBFZM), plasma melting or induction skull melting (ISM) are the suitable techniques for preparation of the -based alloys using elemental metals. In the present study, different methods were 7

2 Hutnické listy č./00, roč. LXIII used for preparation of -based alloys. First, plasma melting of and pieces in water cooled copper crucible was realised, but homogeneous dissolution and distribution of in the prepared alloy was not achieved. Mechanical alloying of pre-hydrogenated pieces of and needed long time of milling and even 40 hours of process did not lead to fragmentation of pieces, only one third of the amount was sufficiently fragmented, the rest remained compact and was low alloyed. Hence, because the plasma metallurgy or MA were not successful and ISM technique was not available, the two methods were used to produce the alloys with the required contents of : vacuum EBFZM of rod wrapped around with ribbon, and plasma re-melting of master alloy with pieces.. Experiment Experimental alloys based on with at.% and 5 at.% were prepared by two different techniques. The first method consisted of vacuum electron beam floating zone melting in vacuum of 6.6x0 - Pa, with anode current of 55 ma and accelerating voltage of 6 kv. The EB-gun travelled along the vertical long axis of specimen at a rate of 3 mm.min -. The rod of diameter of 0 mm and length of 300 mm with wrapped around rolled ribbon of thickness of mm and width of 4 mm was re-melted. The sliced specimens were submitted to the solution heat treatment at 900 C for hour in flowing Ar gas and then they were water quenched. The precipitation hardening was realised at different temperatures: 00, 300 and 400 C for hour in flowing Ar gas and then it was followed by water quenching. In the second method, the melting of experimental - 5at.% and -at.% alloys was performed in plasma furnace with four passes (two passes in Ar and last two passes in Ar-5% H ) in order to obtain the homogeneous material. Heat treatment at 00 C for hours was realised in Linn HT800 furnace in flowing Ar gas and it was followed by water quenching from 700 C. The samples for metallographic observation were polished and etched in Kroll s reagent (8 HF:5 HNO 3 :77 H O) for 0-60 s. The micro-structure was observed using metallographic microscope OLYMPUS DP GX5. Microhardness of the plasma melted specimens as well as of the specimens prepared by EBFZM technique was measured by means of the LECO AMH 000 instrument with load of 0.05 kg and indentation step of mm. Scanning electron microscope (SEM) JEOL JSM LV equipped with EDS INCA X - ACT probe was used to determine chemical and phase compositions of micro-structure. X ray analysis was realised to determine the crystalline state. 3. Results and discussion The micro-structure observed in as-melted samples after EBFZM had duplex and fine precipitate character, as it is seen in Fig. and. Based on average results of micro-analysis performed on two specimens (Table ), the and contents show presence of α and β phases. The results obtained from the micro-hardness measurement are analogous to those for -5 at.% published in [4]. The micro-hardness values showed an obvious relation with heat treatment and aging. The specimens in the as-melted state and aged at 600 C showed similar micro-hardness (73 HV and 7 HV, respectively, in Table ) related with α phase precipitation. The highest micro-hardness value of 9 HV for aging at 400 C is due to α and ω phase precipitation as it was proved in [4]. Conversely, aging at 00 C caused the decrease of the micro-hardness to 44 HV. The X-ray analysis proved that rod was prepared as a single crystal. Hence, no grain boundaries were observed in the specimens. Single crystal growing of inter-metallic alloys is difficult, so the preparation of alloy in a single crystal state could be considered as extraordinary achievement that at present cannot be explained using physical or metallurgical theories. The phase analysis of the prepared alloy was realised in an austenitic state, martensite phase was not observed. Generally, the properties of alloys are strongly affected by content and fraction of α, β and ω phases, but the metallographic observation in the present work was limited by microscopy resolution, so the next analyses by TEM of the aged specimens are needed for confirmation of this effect on microstructure. The micro-structure of - and -5 samples after plasma melting was dendritic (Fig. 3, 4, 7 and 8) with very fine laths. After heat treatment at 00 C β phase with coarse or fine laths of α phase were found (Fig. 5, 6, 9 and 0). Table 3 summarises the results of phase-analysis and general micro-analysis of both alloy compositions. Therefore, the - and -5 alloys were prepared with.57 at.% and 7.3 at.% of content, respectively. The results of microhardness measured for both heat treatment conditions of - (7 and 6 HV) and -5 (35 and 409 HV) alloys as seen in Table 4 showed that annealing increased micro-hardness because of modification of dendritic micro-structure to lamellar one. Furthermore, the higher content in 5 alloy decreased micro-hardness of the samples in as-plasma melted as well as annealed conditions due to lamellar character of micro-structure. 8

3 Hutnické listy č./00, roč. LXIII Tab. The average values of general micro-analysis performed on two as-melted EBFZM specimens Tab. Průměrné hodnoty plošné mikroanalýzy dvou vzorků po elektronovém zonálním tavení Specimen wt. % at. % wt. % at. % Average values Tab. The average values of HV micro-hardness measurements of EBFZM specimens after different heat treatment Tab. Průměrné hodnoty mikrotvrdosti HV pro vzorky po elektronovém zonálním tavení a různém tepelném zpracování heat treatment as-melted 00 C 400 C 600 C micro-hardness 73 ± 6 44 ± 8 9 ± 5 7 ± 7 Fig. Micro-structure of -5 after EBFZM. Obr. Mikrostruktura -5 po elektronovém zonálním tavení. Fig. Obr. Micro-structure of -5 with α and β phases after EBFZM (Detail of Fig. ). Mikrostruktura -5 s fázemi α a β po elektronovém zonálním tavení (Detail Obr.). Tab. 3 Results of phase micro-analysis of - and -5 after plasma melting and heat treatment. Tab. 3 Výsledky fázové mikroanalýzy - a -5 po plazmovém tavení a tepelném zpracování. treatment plasma melted (Fig. 4 and 8) heat treated (Fig. 6 and 0 ) alloy - [at.%] -5 [at.%] micro-structure element white dendrite () dark inter-dendritic space () general analysis of alloy dark lath () bright lath () fine grain () large grain ()

4 Hutnické listy č./00, roč. LXIII Fig. 3 Dendritic micro-structure of - sample after plasma melting. Obr. 3 Dendritická mikrostruktura - po plazmovém tavení. Fig. 4 SEM micro-graph of - sample after plasma melting. Detail of dendritic micro-structure in Fig. 3. with analysed spots. Obr. 4 SEM mikrostruktura - po plazmovém tavení. Detail dendritické mikrostruktury na Obr.3 s vyznačenými body mikroanalýzy. Fig. 5 Large grains with lamellar (α+β) micro-structure of after annealing (h at 00 C). Obr.5 Velká zrna - s lamelární mikrostrukturou (α+β) po žíhání (h při 00 C). Fig. 6 SEM micrograph of - sample with lamellar (α+β) micro-structure after annealing. Detail of Fig. 5. with the analysed spots. Obr.6 SEM snímek - s lamelární mikrostrukturou (α+β) po žíhání. Detail Obr.5. s vyznačenými body mikroanalýzy. Fig. 7 Dendritic micro-structure of -5 sample after plasma melting. Obr.7 Dendritická mikrostruktura - po plazmovém tavení. 30 Fig. 8 SEM micro-graph of -5 sample after plasma melting. Detail of dendritic micro-structure in Fig.7 with the analysed spots. Obr.8 SEM snímek - po plazmovém tavení. Detail dendritické mikrostruktury na Obr.7 s vyznačenými body mikroanalýzy.

5 Hutnické listy č./00, roč. LXIII A Fig. 9 Lamellar (α+β) micro-structure of -5 after annealing ( h at 00 C ). Obr. 9 Lamelární mikrostruktura (α+β) v -5 po žíhání (h při 00 C). Fig. 0 SEM micro-graph of -5 sample with fine grains. Detail of A region in Fig. 9 with the analysed spots. Obr. 0 SEM mikrostruktura -5 s jemnými zrny. Detail oblasti A na Obr.9 s vyznačenými body mikroanalýzy. Tab. 4 Comparison of the results of EDS micro-analysis and micro-hardness measurement in as-melted and heat treated (HT) conditions of and -5 alloys prepared using two different methods: plasma melting and electron beam floating zone melting (EBFZM). Tab. 4 Srovnání výsledků mikroanalýzy a mikrotvrdosti v - a -5 slitinách tavených a tepelně zpracovaných připravených dvěma metodami: plazmovým tavením a elektronově zonálním tavením. Alloy Preparation - Plasma melting HT at 00 C -5 Plasma melting HT at 00 C EBFZM HT at 00 C HT at 400 C HT at 600 C Nominal content [at.%] Measured content [at.%] Microhardness HV HT heat treatment; EBFZM electron beam floating zone melting; 4. Conclusions Metallographic observation and chemical microanalysis of the alloys prepared by both methods proved the composition homogeneity. The micro-structure after EBFZM method, as well as after heat treatment was duplex with α and β phases. After solid solution annealing of plasma melted - and -5 alloys with dendritic micro-structure the presence of β phase with coarse or fine laths of α phase in the microstructure was observed. The martensite phase was not found in the prepared alloy. Micro-hardness values increased with heat treatment and decreased with higher content. Explanation of the effect and heat treatment on micro-hardness need further phase analyses that will be the subject of future research work. Acknowledgement The research work was realised within the frame of the project MSM Processes of preparation and properties of highly pure and structurally defined special materials. Literature [] WANG, Y. B., ZHENG, Y. F.: The micro-structure and shape memory effect of 6 at.% alloy. Materials Letters, 6, 008, [] KIM, H. Y. et al.: Effect of thermo-mechanical treatment on mechanical properties and shape memory behavior of (6 8) at.% alloys. Materials Science and Engineering, A , 006, [3] LI, S. J. et al.: Phase transformation during aging and resulting mechanical properties of two Ta Zr alloys. Materials Science and Technology, 005,, 6, [4] MANTANI, Y., TAJIMA, M.: Phase transformation of quenched martensite by aging in - alloys. Mat.Sci. and Engineering. 006, A , Recenze: Ing. Radovan Bureš, CSc. 3