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1. Tanzania Industrial Research and Development Organization (TIRDO), P.O Box 23235 Dsm, Tanzania
2. Technical University Hamburg-Harburg, 21071 Hamburg, Germany
Received: April 29, 2011 / Accepted: May 12, 2011 / Published: February 25, 2012.
Abstract: The purpose of this study was to investigate the influence of microstructure on the macrocrack propagation resistance of the titanium alloy Beta 21S. In this work, single-step aging treatments were used to create different microstructures by altering variables such as aging temperature and time. The influence of the ?-precipitate size and distribution in the fatigue crack macrocrack propagation resistance of the resulting microstructures was investigated. The main microstructural feature of ??titanium alloys is the preferential precipitation of the hardening ?-phase at ? grain boundaries forming a continuous ?-layer. The resulting PFZ (precipitate-free zone) adjacent to this continuous ?-layer does not contain any precipitates, and is therefore soft with respect to the surrounding age-hardened matrix. Preferential plastic deformation in the soft zones can have a negative effect on some properties. Two microstructures were evaluated and the results obtained showed that, at low stress intensity (delta K) levels, the cracks propagated transcrystalline and the propagation rate of macrocracks depends on the size and distribution of ?-plates in the ??matrix, while with increasing plastic zone size(high delta K levels) the macrocracks propagated intercrystalline and the propagation rate is influenced by the PFZ path in the microstructure.
Key words: Precipitate-free zones, macrocrack propagation, ?-precipitates, plastic zone size, preferential precipitation.
1. Introduction??
The alloy Beta 21S belongs to the group of ? titanium alloys. These alloys are hardened by ??phase precipitation in the ? matrix. The distribution of the?-platelets in the ? matrix depends temperature and time, and prior working history. At low aging temperatures, the transformation presumably occurs via ? to ???+ ? to ???+ ? and homogeneous distribution is favoured [1]. The ? particles appear to provide preferential nucleation sites for ? precipitation. At higher aging temperatures, the direct heterogeneous nucleation (i.e., at ? grain boundaries and at already
difference between the age-hardened matrix and the soft zones.
This work investigated the influence of two microstructures (created by altering aging temperature and time) on the fatigue macrocrack propagation resistance.
2. Materials and Experimental Procedure
The Beta 21S (Ti-15.10Mo-2.87Al-2.80Nb-0.20Si-0.311Fe-0.132O-0.020C-0.009N wt.%) material used in this investigation was supplied as sheet material of 1.5 mm thickness from the alloy manufacturer (TIMET USA) was delivered in?a?? annealed condition [5]. The microstructure of the as-received sheet material consisted of relatively equiaxed ? grains with average grain size of about 40 μm. In the as-received microstructure the?? phase precipitated along the?? grain boundaries but there were no??-precipitates within the ? grains.
Heat treatment of specimen blanks for fatigue macrocrack propagation tests were performed in hot tube furnaces manufactured by the companies Heraeus and Centronic.
The heat (aging) treatments were done (under argon) in the furnaces which had been pre-heated to the
be explained by considering the details of the crack front geometries for the two aging treatments (Figs. 3-6). At low delta K levels, since the plastic zone size is small [6], the crack front propagates through the ? matrix containing the ?-plates. For the “fine?-platelets” microstructure (8 h 500 °C aging treatment) the crack front profile was smooth (Fig. 3), while for the “coarse ?-plate” microstructure (24 h 725 °C) the crack front profile was rough (Fig. 5). The rough crack front profile caused retardation in the crack propagation rate. The deviation in crack path for the 24 h 725 °C microstructure resulted from crack deflection at coarse ?-plates.
The coarse ?-plate 24 h 725 °C microstructure caused the crack front to move locally out of the main propagation plane and forced the crack to advance by propagating in a very unfavourably direction. On the other hand, the very fine ?-platelets in the 8 h 500 °C microstructure were simply too small to effectively deflect the crack path. Since the local crack deviation from the average propagation plane is proportional to the size of the ?-plate, the effect of ?-plates as crack propagation obstacles is large for the “coarse ?-”microstructure (24 h 725 °C condition), and small for“fine ?” microstructure (8 h 500 °C condition), and this difference in ?-plate size explains the ranking of the da/dN-delta K curves exhibited by these two microstructures in the low delta K regime.
With increasing delta K levels, the size of the plastic zone at the crack tip increases, and as it can be seen in Figs. 4 and 6 the crack tried to follow the soft zones along the ? grain boundaries (but only within short segments when the soft zones were favourably oriented and within the plastic zone size). The crack propagation along the soft zones at ? grain boundaries was more pronounced in the 8 h 500 °C microstructure(Fig. 4) as compared to the 24 h 725 °C microstructure
4. Conclusions
The fatigue crack propagation rate of macrocracks of the alloy depends on the size and distribution of?-plates in the ? matrix (presence of soft zones), and particularly through the effect of these parameters on the crack front profile. At low delta K levels (small plastic zone size), the crack front propagates through the ? matrix containing the ?-plates and presence of coarse ? plates causes stronger retardation of crack propagation rate than fine ? plates due to larger deviation in crack path resulting from stronger crack deflection at the coarse ?-plates(local crack deviation is proportional to the size of the ?-plates). At high delta K levels (large plastic zone size), crack propagation tends to follow the soft zones along the ??grain boundaries causing the crack to move out of the main propagation plane into a rougher crack path. The net effect on the macrocrack
References
[1] T.W. Duerig, J.C Williams, Overview: Microstructure and Properties of Beta Titanium Alloys, Beta titanium alloys in the 1980’s, TMS, Warrendale, USA, 1984, pp. 19-61.
[2] M.A. Imam, C.R. Feng, Study of transformation kinetics in TIMETAL-21S, Titanium ’95: Science and Technology, The University Press, Cambridge, UK, 1996, pp. 2361-2367.
[3] I.J. Polmear, Light Alloys: Metallurgy of Light Metals, Arnold, London, 1995.
[4] G. Luetjering, J.C. Williams, Titanium, Springer Verlag, Berlin Heidelberg, 2003.
[5] J.C. Fanning, Timetal 21S Property Data, Beta Titanium Alloys in the 1990’s, TMS, Warrendale, USA, 1993, pp. 397-410.
[6] S. Suresh, Fatigue of Materials, Cambridge University Press, 1996.
2. Technical University Hamburg-Harburg, 21071 Hamburg, Germany
Received: April 29, 2011 / Accepted: May 12, 2011 / Published: February 25, 2012.
Abstract: The purpose of this study was to investigate the influence of microstructure on the macrocrack propagation resistance of the titanium alloy Beta 21S. In this work, single-step aging treatments were used to create different microstructures by altering variables such as aging temperature and time. The influence of the ?-precipitate size and distribution in the fatigue crack macrocrack propagation resistance of the resulting microstructures was investigated. The main microstructural feature of ??titanium alloys is the preferential precipitation of the hardening ?-phase at ? grain boundaries forming a continuous ?-layer. The resulting PFZ (precipitate-free zone) adjacent to this continuous ?-layer does not contain any precipitates, and is therefore soft with respect to the surrounding age-hardened matrix. Preferential plastic deformation in the soft zones can have a negative effect on some properties. Two microstructures were evaluated and the results obtained showed that, at low stress intensity (delta K) levels, the cracks propagated transcrystalline and the propagation rate of macrocracks depends on the size and distribution of ?-plates in the ??matrix, while with increasing plastic zone size(high delta K levels) the macrocracks propagated intercrystalline and the propagation rate is influenced by the PFZ path in the microstructure.
Key words: Precipitate-free zones, macrocrack propagation, ?-precipitates, plastic zone size, preferential precipitation.
1. Introduction??
The alloy Beta 21S belongs to the group of ? titanium alloys. These alloys are hardened by ??phase precipitation in the ? matrix. The distribution of the?-platelets in the ? matrix depends temperature and time, and prior working history. At low aging temperatures, the transformation presumably occurs via ? to ???+ ? to ???+ ? and homogeneous distribution is favoured [1]. The ? particles appear to provide preferential nucleation sites for ? precipitation. At higher aging temperatures, the direct heterogeneous nucleation (i.e., at ? grain boundaries and at already
difference between the age-hardened matrix and the soft zones.
This work investigated the influence of two microstructures (created by altering aging temperature and time) on the fatigue macrocrack propagation resistance.
2. Materials and Experimental Procedure
The Beta 21S (Ti-15.10Mo-2.87Al-2.80Nb-0.20Si-0.311Fe-0.132O-0.020C-0.009N wt.%) material used in this investigation was supplied as sheet material of 1.5 mm thickness from the alloy manufacturer (TIMET USA) was delivered in?a?? annealed condition [5]. The microstructure of the as-received sheet material consisted of relatively equiaxed ? grains with average grain size of about 40 μm. In the as-received microstructure the?? phase precipitated along the?? grain boundaries but there were no??-precipitates within the ? grains.
Heat treatment of specimen blanks for fatigue macrocrack propagation tests were performed in hot tube furnaces manufactured by the companies Heraeus and Centronic.
The heat (aging) treatments were done (under argon) in the furnaces which had been pre-heated to the
be explained by considering the details of the crack front geometries for the two aging treatments (Figs. 3-6). At low delta K levels, since the plastic zone size is small [6], the crack front propagates through the ? matrix containing the ?-plates. For the “fine?-platelets” microstructure (8 h 500 °C aging treatment) the crack front profile was smooth (Fig. 3), while for the “coarse ?-plate” microstructure (24 h 725 °C) the crack front profile was rough (Fig. 5). The rough crack front profile caused retardation in the crack propagation rate. The deviation in crack path for the 24 h 725 °C microstructure resulted from crack deflection at coarse ?-plates.
The coarse ?-plate 24 h 725 °C microstructure caused the crack front to move locally out of the main propagation plane and forced the crack to advance by propagating in a very unfavourably direction. On the other hand, the very fine ?-platelets in the 8 h 500 °C microstructure were simply too small to effectively deflect the crack path. Since the local crack deviation from the average propagation plane is proportional to the size of the ?-plate, the effect of ?-plates as crack propagation obstacles is large for the “coarse ?-”microstructure (24 h 725 °C condition), and small for“fine ?” microstructure (8 h 500 °C condition), and this difference in ?-plate size explains the ranking of the da/dN-delta K curves exhibited by these two microstructures in the low delta K regime.
With increasing delta K levels, the size of the plastic zone at the crack tip increases, and as it can be seen in Figs. 4 and 6 the crack tried to follow the soft zones along the ? grain boundaries (but only within short segments when the soft zones were favourably oriented and within the plastic zone size). The crack propagation along the soft zones at ? grain boundaries was more pronounced in the 8 h 500 °C microstructure(Fig. 4) as compared to the 24 h 725 °C microstructure
4. Conclusions
The fatigue crack propagation rate of macrocracks of the alloy depends on the size and distribution of?-plates in the ? matrix (presence of soft zones), and particularly through the effect of these parameters on the crack front profile. At low delta K levels (small plastic zone size), the crack front propagates through the ? matrix containing the ?-plates and presence of coarse ? plates causes stronger retardation of crack propagation rate than fine ? plates due to larger deviation in crack path resulting from stronger crack deflection at the coarse ?-plates(local crack deviation is proportional to the size of the ?-plates). At high delta K levels (large plastic zone size), crack propagation tends to follow the soft zones along the ??grain boundaries causing the crack to move out of the main propagation plane into a rougher crack path. The net effect on the macrocrack
References
[1] T.W. Duerig, J.C Williams, Overview: Microstructure and Properties of Beta Titanium Alloys, Beta titanium alloys in the 1980’s, TMS, Warrendale, USA, 1984, pp. 19-61.
[2] M.A. Imam, C.R. Feng, Study of transformation kinetics in TIMETAL-21S, Titanium ’95: Science and Technology, The University Press, Cambridge, UK, 1996, pp. 2361-2367.
[3] I.J. Polmear, Light Alloys: Metallurgy of Light Metals, Arnold, London, 1995.
[4] G. Luetjering, J.C. Williams, Titanium, Springer Verlag, Berlin Heidelberg, 2003.
[5] J.C. Fanning, Timetal 21S Property Data, Beta Titanium Alloys in the 1990’s, TMS, Warrendale, USA, 1993, pp. 397-410.
[6] S. Suresh, Fatigue of Materials, Cambridge University Press, 1996.