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Fatigue behavior of powder metallurgy high-speed steels: fatigue limit prediction using a crack growth threshold-based approach
Materials Science and Engineering A 387–389 (2004) 501–504
Fatigue behavior of powder metallurgy high-speed steels: fatigue limit prediction using a crack growth threshold-based approach Y. Torres, S. Rodr´ıguez, A. Mateo, M. Anglada, L. Llanes∗ Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Avda. Diagonal, 647, 08028 Barcelona, Spain Received 25 August 2003; received in revised form 8 December 2003
Abstract Based on the well-known fact that fatigue lifetime of powder metallurgy high speed steels (PM-HSSs) is given by subcritical crack growth of pre-existing defects, a linear elastic fracture mechanics (LEFM) approach is implemented by assessing a fatigue limit–fatigue crack growth (FCG) threshold correlation under infinite fatigue life conditions. In doing so, critical flaw size under cyclic loading is simply defined in terms of FCG threshold. Additionally, it is assumed that the fundamental LEFM correlation among defect size, strength and threshold conditions evaluated for large cracks applies for natural flaws too. Hence, the fatigue limit value for the PM-HSS studied is predicted using the experimentally measured fracture and FCG characteristics, provided that critical flaws are similar, in terms of nature, geometry and size, under monotonic and cyclic loading. The reliability of the applied LEFM approach is sustained through the excellent agreement observed between estimated and experimentally determined fatigue limit values. © 2004 Elsevier B.V. All rights reserved. Keywords: Fatigue limit; Crack growth threshold; PM-high speed steels
1. Introduction The benefits of powder metallurgy (PM) route as a means of manufacturing high-performance steels, over traditional production methods such as melting, casting and hot extrusion, are mostly manifested in high-speed steels (HSSs) for tooling. As a result of the finer and more uniform microstructure that PM-HSSs exhibit, as compared to their conventionally produced counterparts, they also present enhanced cross-sectional hardness uniformity (wear resistance), fracture toughness and fatigue strength, all of them relevant properties in cutting tool design. Concerning mechanical properties of PM-HSSs, opposite to what it is found with respect to hardness, strength or toughness (e.g. Ref. [1]), existing information about fatigue behavior is relatively scarce. Regarding fatigue characteristics, there are only a few works reported in the literature [2–4], and they mostly concentrate on fatigue life evaluation and the corresponding fractographic characterization. From these studies, it is now well-established that (1) PM-HSSs ∗
Corresponding author. Tel.: +34-934011083; fax: +34-934016706. E-mail address: [email protected] (L. Llanes).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.12.072
show a considerable better fatigue resistance than conventionally produced HSSs; and (2) fatigue failure in PM-HSSs is given by the propagation of cracks initiated at intrinsic processing defects, i.e. internal inclusions, carbides or pores. This latter experimental fact is used here as premise in estimating “infinite” fatigue life conditions on the basis of a threshold for subcritical cyclic crack growth. In doing so, the fracture and fatigue behavior of a PM-HSS is thoroughly documented and analyzed within a linear elastic fracture mechanics (LEFM) framework. The referred approach is then implemented by (1) defining critical flaw size under cyclic loading in terms of the fatigue crack growth (FCG) threshold and (2) assuming similitude on the characterization of FCG behavior through LEFM parameters for both large cracks and small natural flaws.
2. Experimental procedure The material studied is a PM-HSS produced by Böhler under the designation of S390 ISOMATRIX. It has a basic composition of 1.6C–4.75Cr–2Mo–5V–10.5W–8Co and was received after being heat treated (annealed, stress re-
502
Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
Fig. 1. Microstructure of the investigated PM-HSS.
lieved, quenched and tempered) under optimal conditions (hardness values about 64–66 HRC). The corresponding microstructure, as revealed by scanning electron microscopy (SEM) examination, consisted of a tempered martensite matrix containing fine and uniformly distributed alloy carbides (Fig. 1). Mechanical characterization included flexural strength, fracture toughness, fatigue limit and FCG behavior. Testing in all the cases was conducted under four-point bending by means of a fully articulating test jig with inner and outer spans of 20 and 40 mm, respectively. Strength under monotonic and cyclic loads was evaluated on 45 mm × 4 mm × 3 mm prisms cut from the primary product. The experimental fatigue limit distribution at 107 cycles (“infinite fatigue life”) was determined following the stair-case test method, at a load ratio R of 0.1 and employing a resonant testing machine (working frequencies of about 146 Hz). Several failed specimens were taken and subjected to a detailed fracto-
graphic examination under scanning electron microscopy (SEM). During SEM inspection, special attention was paid to discern nature, geometry and size of strength-limiting flaws. Crack growth tests were performed using single edge notched bend (SENB) specimens of 45 mm × 10 mm × 5 mm dimensions with a notch length-to-specimen width ratio, a/W, of 0.3. The samples were first precracked by cyclic compression and the resulting cracks (Fig. 2) were then subjected to far-field cyclic tensile loads in order to relieve residual stresses induced by the precracking procedure [5,6]. Fracture toughness was determined by testing the precracked SENB specimens to failure under constant loading rate values. Five samples were employed for strength and fracture toughness evaluation. Finally, FCG behavior was determined following a direct-measurement method using a high-resolution telescope. In this case, tests were also run under R = 0.1 in a servohydraulic testing machine (at frequencies ranging from 0.5 to 10 Hz).
3. Results and discussion 3.1. Fracture and fatigue characteristics Mean flexural strength (σ r ) and fracture toughness (KIc ) values √ for the material √studied were 2530 √ MPa (+0.9 MPa m) and 12.4 MPa m (+ 0.9 MPa m), respectively. They are within the range of those usually reported in the literature for PM-HSSs (e.g. Refs. [1,7,8]). An extended fractographic analysis indicated subsurface origins: pores, carbides and inclusions with sizes ranging from 30 to 40 m (equivalent diameter of circular-like flaws), as typical fracture initiation sites in the flexural strength tests. The complete up-and-down fatigue testing sequence (using 17 specimens) is shown in Fig. 3. After statistical analysis of the experimental data, the mean value for the fatigue
Fig. 2. Typical example of a sharp crack resulting from cyclic compression.
Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
Fig. 3. Up-and-down fatigue tests used for determining mean fatigue limit.
503
Fig. 5. FCG behavior (for a loading ratio of 0.1) of the PM-HSS investigated.
limit determined (σ f ) with a 95% confidence is 1041 MPa (⊕80 MPa). The corresponding fractographic examination revealed that fatigue failure origins were of same type, shape and size as those already identified as strength-controlling flaws under monotonic loading conditions (Fig. 4). Similar remarks may be pointed out with respect to general fractographic features observed within unstable fracture regions. Fig. 5 shows the dependence of FCG rate on K for the PM-HSS investigated. Values for the FCG threshold, Kth , as well as the Paris–Erdogan exponent √ in the power-law dependence observed, m, are 5.4 MPa m and 7, respectively. The relatively high m value found is in agreement with previous studies on other HSSs (e.g. Refs. [2,3]), and should be related to the moderate-to-low effective ductility associated with the hard microstructure (tempered martensitic matrix with embedded carbides) exhibited by these materials [9–12].
design and material selection in fatigue-limited applications involving PM-HSSs. Although this might be attempted following a damage tolerance methodology, it does not seem to be the most suitable route because the relative large prediction uncertainties associated with marked power-law dependences of FCG rates on K as those shown in Fig. 5. Instead, a more classical approach on the basis of fatigue limit and FCG threshold, i.e. from an infinite fatigue life viewpoint, appears to be more appropriate for PM-HSSs. This approach is here implemented by simply defining critical flaw size under cyclic loading in terms of FCG threshold. Under this consideration, fatigue limit, σ f , would then be given by the stress range resulting in the threshold intensity factor range, Kth , of a small non-propagating crack emanating from a defect of critical size, 2acr , according to relationships of type.
3.2. Fatigue limit–FCG threshold correlation
Kth σf ∝ √ acr
The main goal of this investigation is to assess FCG–fatigue life relationships that could optimize proper
Hence, σ f values could be predicted using the experimental findings reported above provided that the fundamen-
Fig. 4. SEM micrographs showing strength-controlling flaws of similar nature and size under: (a) monotonic and (b) cyclic loading.
(1)
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Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
tal LEFM correlation among defect size, strength and threshold conditions evaluated for large cracks applies for natural flaws too. Although it is well known that in many metallic and advanced ceramic materials FCG of small cracks differs considerably from that of large cracks, such anomalous behavior should not be expected in PM-HSSs because their relatively fine microstructure as compared to typical dimensions of intrinsic natural flaws. Following this consideration, a fatigue limit value of 1102 MPa (given in terms of stress range) is estimated from Kth σf = (2) σr KIc under the assumption that strength-controlling flaws are the same, with respect to type, geometry, size and distribution under monotonic and cyclic loading. This latter hypothesis has been shown to apply for other fine-grained PM tool materials [4,13,14] and it has also been experimentally supported in this investigation through fractographic examination by SEM (see Fig. 4 above). A comparison between predicted and observed fatigue limits allows to describe the relative agreement as very good, with a relative error of about 6%. This finding yields strong support to the FCG threshold–fatigue limit correlation proposed in this study and clearly sustains the approach of predicting fatigue limit values directly from the ratio Kth /KIc (i.e. fatigue sensitivity) for (hard/brittle) fine-grained materials whose fatigue life behavior is controlled by the subcritical propagation of pre-existing flaws. 4. Conclusions The fracture and fatigue characteristics of a fine-grained PM-HSS have been studied. In doing so, special emphasis has been placed on the assessment of a FCG–fatigue life correlation within a LEFM framework. It is stated that fatigue behavior of PM-HSSs may be rationalized, under infinite fatigue life conditions, by considering: (1) subcritical growth of pre-existing defects as the dominant stage of fatigue life behavior; and thus, FCG threshold as the effective toughness under cyclic loading; (2) similitude on the characterization
of FCG behavior through LEFM parameters for both large cracks and small natural flaws, and (3) strength-controlling flaws of same type, geometry, size and distribution under monotonic and cyclic loading. Implementation of this crack growth threshold-based approach is validated by means of the excellent concordance found between estimated and experimentally determined fatigue limit values.
Acknowledgements The authors gratefully acknowledge the financial support received from the Spanish Ministerio de Ciencia y Tecnolog´ıa (Grant no. MAT2000-1014-C02-01). They also thank M. Marsal for her electron microscopy assistance and Tyco electronics- AMP España for its stimulating research interest and collaboration.
References [1] J.R. Davis (Ed.) Powder metallurgy high-speed tool steels, Tool Materials, ASM Specialty Handbook, ASM International, Materials Park, OH, USA, 1995, pp. 21–31. [2] H. Berns, J. Lueg, W. Trojahn, R. Wähling, H. Wisell, Powder Metal. Int. 19 (1987) 22. [3] H. Brandrup-Wognsen, J. Engström, O. Grinder, Powder Metal. Int. 20 (1988) 18. [4] F. Meurling, A. Melander, M. Tidesten, L. Westin, Int. J. Fatigue 23 (2001) 215. [5] M.N. James, A.M. Human, S. Luyckx, J. Mater. Sci. 25 (1990) 4810. [6] Y. Torres, D. Casellas, M. Anglada, Llanes, L. Int. J. Refract. Met. Hard Mater. 19 (2001) 27. [7] S.A. Horton, H.C. Child, Met. Technol. 10 (1983) 245. [8] N. Lippmann, H.-J. Spies, Steel Res. 66 (1995) 272. [9] R.O. Ritchie, J.F. Knott, Acta Metal. 21 (1973) 639. [10] R.O. Ritchie, C.J. Gilbert, J.M. McNaney, Int. J. Sol. Struct. 37 (2000) 311. [11] L. Llanes, Y. Torres, M. Anglada, Acta Mater. 50 (2002) 2381. [12] L. Llanes, Y. Torres, A. Mateo, M. Anglada, in: Proceedings of the Euro PM 2003 Conference, EPMA, Valencia, Spain, 2003, pp. 75– 80. [13] U. Schleinkofer, H.G. Sockel, K. Görting, W. Heinrich, Mater. Sci. Eng. 209 (1996) 313. [14] Y. Torres, M. Anglada, L. lanes, Int. J. Refract. Met. Hard Mater. 19 (2001) 341.
Fatigue behavior of powder metallurgy high-speed steels: fatigue limit prediction using a crack growth threshold-based approach Y. Torres, S. Rodr´ıguez, A. Mateo, M. Anglada, L. Llanes∗ Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Avda. Diagonal, 647, 08028 Barcelona, Spain Received 25 August 2003; received in revised form 8 December 2003
Abstract Based on the well-known fact that fatigue lifetime of powder metallurgy high speed steels (PM-HSSs) is given by subcritical crack growth of pre-existing defects, a linear elastic fracture mechanics (LEFM) approach is implemented by assessing a fatigue limit–fatigue crack growth (FCG) threshold correlation under infinite fatigue life conditions. In doing so, critical flaw size under cyclic loading is simply defined in terms of FCG threshold. Additionally, it is assumed that the fundamental LEFM correlation among defect size, strength and threshold conditions evaluated for large cracks applies for natural flaws too. Hence, the fatigue limit value for the PM-HSS studied is predicted using the experimentally measured fracture and FCG characteristics, provided that critical flaws are similar, in terms of nature, geometry and size, under monotonic and cyclic loading. The reliability of the applied LEFM approach is sustained through the excellent agreement observed between estimated and experimentally determined fatigue limit values. © 2004 Elsevier B.V. All rights reserved. Keywords: Fatigue limit; Crack growth threshold; PM-high speed steels
1. Introduction The benefits of powder metallurgy (PM) route as a means of manufacturing high-performance steels, over traditional production methods such as melting, casting and hot extrusion, are mostly manifested in high-speed steels (HSSs) for tooling. As a result of the finer and more uniform microstructure that PM-HSSs exhibit, as compared to their conventionally produced counterparts, they also present enhanced cross-sectional hardness uniformity (wear resistance), fracture toughness and fatigue strength, all of them relevant properties in cutting tool design. Concerning mechanical properties of PM-HSSs, opposite to what it is found with respect to hardness, strength or toughness (e.g. Ref. [1]), existing information about fatigue behavior is relatively scarce. Regarding fatigue characteristics, there are only a few works reported in the literature [2–4], and they mostly concentrate on fatigue life evaluation and the corresponding fractographic characterization. From these studies, it is now well-established that (1) PM-HSSs ∗
Corresponding author. Tel.: +34-934011083; fax: +34-934016706. E-mail address: [email protected] (L. Llanes).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.12.072
show a considerable better fatigue resistance than conventionally produced HSSs; and (2) fatigue failure in PM-HSSs is given by the propagation of cracks initiated at intrinsic processing defects, i.e. internal inclusions, carbides or pores. This latter experimental fact is used here as premise in estimating “infinite” fatigue life conditions on the basis of a threshold for subcritical cyclic crack growth. In doing so, the fracture and fatigue behavior of a PM-HSS is thoroughly documented and analyzed within a linear elastic fracture mechanics (LEFM) framework. The referred approach is then implemented by (1) defining critical flaw size under cyclic loading in terms of the fatigue crack growth (FCG) threshold and (2) assuming similitude on the characterization of FCG behavior through LEFM parameters for both large cracks and small natural flaws.
2. Experimental procedure The material studied is a PM-HSS produced by Böhler under the designation of S390 ISOMATRIX. It has a basic composition of 1.6C–4.75Cr–2Mo–5V–10.5W–8Co and was received after being heat treated (annealed, stress re-
502
Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
Fig. 1. Microstructure of the investigated PM-HSS.
lieved, quenched and tempered) under optimal conditions (hardness values about 64–66 HRC). The corresponding microstructure, as revealed by scanning electron microscopy (SEM) examination, consisted of a tempered martensite matrix containing fine and uniformly distributed alloy carbides (Fig. 1). Mechanical characterization included flexural strength, fracture toughness, fatigue limit and FCG behavior. Testing in all the cases was conducted under four-point bending by means of a fully articulating test jig with inner and outer spans of 20 and 40 mm, respectively. Strength under monotonic and cyclic loads was evaluated on 45 mm × 4 mm × 3 mm prisms cut from the primary product. The experimental fatigue limit distribution at 107 cycles (“infinite fatigue life”) was determined following the stair-case test method, at a load ratio R of 0.1 and employing a resonant testing machine (working frequencies of about 146 Hz). Several failed specimens were taken and subjected to a detailed fracto-
graphic examination under scanning electron microscopy (SEM). During SEM inspection, special attention was paid to discern nature, geometry and size of strength-limiting flaws. Crack growth tests were performed using single edge notched bend (SENB) specimens of 45 mm × 10 mm × 5 mm dimensions with a notch length-to-specimen width ratio, a/W, of 0.3. The samples were first precracked by cyclic compression and the resulting cracks (Fig. 2) were then subjected to far-field cyclic tensile loads in order to relieve residual stresses induced by the precracking procedure [5,6]. Fracture toughness was determined by testing the precracked SENB specimens to failure under constant loading rate values. Five samples were employed for strength and fracture toughness evaluation. Finally, FCG behavior was determined following a direct-measurement method using a high-resolution telescope. In this case, tests were also run under R = 0.1 in a servohydraulic testing machine (at frequencies ranging from 0.5 to 10 Hz).
3. Results and discussion 3.1. Fracture and fatigue characteristics Mean flexural strength (σ r ) and fracture toughness (KIc ) values √ for the material √studied were 2530 √ MPa (+0.9 MPa m) and 12.4 MPa m (+ 0.9 MPa m), respectively. They are within the range of those usually reported in the literature for PM-HSSs (e.g. Refs. [1,7,8]). An extended fractographic analysis indicated subsurface origins: pores, carbides and inclusions with sizes ranging from 30 to 40 m (equivalent diameter of circular-like flaws), as typical fracture initiation sites in the flexural strength tests. The complete up-and-down fatigue testing sequence (using 17 specimens) is shown in Fig. 3. After statistical analysis of the experimental data, the mean value for the fatigue
Fig. 2. Typical example of a sharp crack resulting from cyclic compression.
Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
Fig. 3. Up-and-down fatigue tests used for determining mean fatigue limit.
503
Fig. 5. FCG behavior (for a loading ratio of 0.1) of the PM-HSS investigated.
limit determined (σ f ) with a 95% confidence is 1041 MPa (⊕80 MPa). The corresponding fractographic examination revealed that fatigue failure origins were of same type, shape and size as those already identified as strength-controlling flaws under monotonic loading conditions (Fig. 4). Similar remarks may be pointed out with respect to general fractographic features observed within unstable fracture regions. Fig. 5 shows the dependence of FCG rate on K for the PM-HSS investigated. Values for the FCG threshold, Kth , as well as the Paris–Erdogan exponent √ in the power-law dependence observed, m, are 5.4 MPa m and 7, respectively. The relatively high m value found is in agreement with previous studies on other HSSs (e.g. Refs. [2,3]), and should be related to the moderate-to-low effective ductility associated with the hard microstructure (tempered martensitic matrix with embedded carbides) exhibited by these materials [9–12].
design and material selection in fatigue-limited applications involving PM-HSSs. Although this might be attempted following a damage tolerance methodology, it does not seem to be the most suitable route because the relative large prediction uncertainties associated with marked power-law dependences of FCG rates on K as those shown in Fig. 5. Instead, a more classical approach on the basis of fatigue limit and FCG threshold, i.e. from an infinite fatigue life viewpoint, appears to be more appropriate for PM-HSSs. This approach is here implemented by simply defining critical flaw size under cyclic loading in terms of FCG threshold. Under this consideration, fatigue limit, σ f , would then be given by the stress range resulting in the threshold intensity factor range, Kth , of a small non-propagating crack emanating from a defect of critical size, 2acr , according to relationships of type.
3.2. Fatigue limit–FCG threshold correlation
Kth σf ∝ √ acr
The main goal of this investigation is to assess FCG–fatigue life relationships that could optimize proper
Hence, σ f values could be predicted using the experimental findings reported above provided that the fundamen-
Fig. 4. SEM micrographs showing strength-controlling flaws of similar nature and size under: (a) monotonic and (b) cyclic loading.
(1)
504
Y. Torres et al. / Materials Science and Engineering A 387–389 (2004) 501–504
tal LEFM correlation among defect size, strength and threshold conditions evaluated for large cracks applies for natural flaws too. Although it is well known that in many metallic and advanced ceramic materials FCG of small cracks differs considerably from that of large cracks, such anomalous behavior should not be expected in PM-HSSs because their relatively fine microstructure as compared to typical dimensions of intrinsic natural flaws. Following this consideration, a fatigue limit value of 1102 MPa (given in terms of stress range) is estimated from Kth σf = (2) σr KIc under the assumption that strength-controlling flaws are the same, with respect to type, geometry, size and distribution under monotonic and cyclic loading. This latter hypothesis has been shown to apply for other fine-grained PM tool materials [4,13,14] and it has also been experimentally supported in this investigation through fractographic examination by SEM (see Fig. 4 above). A comparison between predicted and observed fatigue limits allows to describe the relative agreement as very good, with a relative error of about 6%. This finding yields strong support to the FCG threshold–fatigue limit correlation proposed in this study and clearly sustains the approach of predicting fatigue limit values directly from the ratio Kth /KIc (i.e. fatigue sensitivity) for (hard/brittle) fine-grained materials whose fatigue life behavior is controlled by the subcritical propagation of pre-existing flaws. 4. Conclusions The fracture and fatigue characteristics of a fine-grained PM-HSS have been studied. In doing so, special emphasis has been placed on the assessment of a FCG–fatigue life correlation within a LEFM framework. It is stated that fatigue behavior of PM-HSSs may be rationalized, under infinite fatigue life conditions, by considering: (1) subcritical growth of pre-existing defects as the dominant stage of fatigue life behavior; and thus, FCG threshold as the effective toughness under cyclic loading; (2) similitude on the characterization
of FCG behavior through LEFM parameters for both large cracks and small natural flaws, and (3) strength-controlling flaws of same type, geometry, size and distribution under monotonic and cyclic loading. Implementation of this crack growth threshold-based approach is validated by means of the excellent concordance found between estimated and experimentally determined fatigue limit values.
Acknowledgements The authors gratefully acknowledge the financial support received from the Spanish Ministerio de Ciencia y Tecnolog´ıa (Grant no. MAT2000-1014-C02-01). They also thank M. Marsal for her electron microscopy assistance and Tyco electronics- AMP España for its stimulating research interest and collaboration.
References [1] J.R. Davis (Ed.) Powder metallurgy high-speed tool steels, Tool Materials, ASM Specialty Handbook, ASM International, Materials Park, OH, USA, 1995, pp. 21–31. [2] H. Berns, J. Lueg, W. Trojahn, R. Wähling, H. Wisell, Powder Metal. Int. 19 (1987) 22. [3] H. Brandrup-Wognsen, J. Engström, O. Grinder, Powder Metal. Int. 20 (1988) 18. [4] F. Meurling, A. Melander, M. Tidesten, L. Westin, Int. J. Fatigue 23 (2001) 215. [5] M.N. James, A.M. Human, S. Luyckx, J. Mater. Sci. 25 (1990) 4810. [6] Y. Torres, D. Casellas, M. Anglada, Llanes, L. Int. J. Refract. Met. Hard Mater. 19 (2001) 27. [7] S.A. Horton, H.C. Child, Met. Technol. 10 (1983) 245. [8] N. Lippmann, H.-J. Spies, Steel Res. 66 (1995) 272. [9] R.O. Ritchie, J.F. Knott, Acta Metal. 21 (1973) 639. [10] R.O. Ritchie, C.J. Gilbert, J.M. McNaney, Int. J. Sol. Struct. 37 (2000) 311. [11] L. Llanes, Y. Torres, M. Anglada, Acta Mater. 50 (2002) 2381. [12] L. Llanes, Y. Torres, A. Mateo, M. Anglada, in: Proceedings of the Euro PM 2003 Conference, EPMA, Valencia, Spain, 2003, pp. 75– 80. [13] U. Schleinkofer, H.G. Sockel, K. Görting, W. Heinrich, Mater. Sci. Eng. 209 (1996) 313. [14] Y. Torres, M. Anglada, L. lanes, Int. J. Refract. Met. Hard Mater. 19 (2001) 341.