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Solid solution softening of polycrystalline molybdenum–hafnium alloys
Journal of Alloys and Compounds 576 (2013) 250–256
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Solid solution softening of polycrystalline molybdenum–hafnium alloys C. Pöhl a,⇑, J. Schatte b, H. Leitner a,c a
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria Plansee SE, Metallwerk-Plansee-Straße 71, 6600 Reutte, Austria c Christian Doppler Laboratory for Early Stages of Precipitation, Montanuniversität Leoben, Austria b
a r t i c l e
i n f o
Article history: Received 20 December 2012 Received in revised form 7 March 2013 Accepted 20 April 2013 Available online 29 April 2013 Keywords: Molybdenum alloys Powder metallurgy Hafnium Solid solution softening
a b s t r a c t In the present study the solid solution softening of molybdenum–hafnium alloys (0.15–3.0 at.%) was investigated at homologous temperatures ranging from 0.027 to 0.153 (77–443 K). The hardness measurements revealed a significant softening effect of alloys with a hafnium content below 0.63 at.%. In order to determine the distribution of the alloying element and the impurities, a detailed investigation of the microstructure and the chemistry was carried out. With atom probe tomography it could be shown that interstitials were not removed from random solution in the molybdenum matrix during alloying with hafnium. This indicated that solid solution softening in the Mo–Hf system is attributed to an intrinsic mechanism. Furthermore, with atom probe investigations it can be shown that the dissolved hafnium increases the oxygen solubility in molybdenum. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Alloying is a common practice to increase the strength of molybdenum over a wide range of temperature, e.g. for hot metal forming parts, power electronics and high temperature furnace construction [1–5]. In contrast to the hardening effect, a solid solution softening (SSS) has been observed in several molybdenum alloys [6–9]. For example the classic SSS system, molybdenumrhenium, shows a distinct softening below 350 K [10]. The effect of softening through alloying is applied to increase the low temperature ductility of the refractory metals molybdenum and tungsten [11,12]. The SSS effect itself was studied experimentally in body centered cubic (bcc) metal alloys by several authors [6,8,13–15]. The main characteristics of SSS are (a) the effect through alloying is limited to the thermal activated dislocation motion, i.e. below the temperature T 0.15TM (where TM is the melting temperature in K) for maximum thermal activation, and (b) it occurs in bcc single- and poly-crystals but not in face centered cubic metals [6,13]. Furthermore, SSS is a known phenomenon in hexagonal magnesium alloys as well. Akhtar and Teghtsoonian [16] reported a distinct softening of the prismatic slip by Zn and Al solute additions at low temperatures. Blake and Cáceres [17] observed a notable softening for the prism planes in the order of 0.6 at.% Zn in Mg. In general, the amount of SSS depends on temperature and solute concentration and with increasing solute content softening is compensated by solid solution hardening.
⇑ Corresponding author. Tel.: +43 3842 402 4214; fax: +43 3842 402 4202. E-mail address: [email protected] (C. Pöhl). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.138
There is no general model for SSS in the literature and research is still in progress, e.g. the modeling of the softening effect in molybdenum on the basis of ab initio calculations is a recent field of interest [18,19]. Two main theories have been developed in the literature to describe the origin of the alloy softening effect in bcc transition metals [6]. In the first theory, it is assumed that the dislocation motion is impeded by the interstitial impurities carbon, nitrogen and oxygen and thus the strength of the base metal is increased [20]. Through alloying of the base metal the interstitials are removed from random solution of the matrix, i.e. the impurities cannot longer act as solution hardener. Therefore, the yield strength of the alloyed metal is decreased [13,15,21]. This theory of scavenging the base metal lattice with the alloying element leads to a softening by an extrinsic mechanism. In bcc metals the plastic deformation at low temperatures is controlled by kink pair formation and kink migration [22–24]. The second theory is supposed to originate from the enhanced nucleation and motion of kink pairs at screw dislocations by the solute atoms [19]. The mechanism of kink nucleation enhancement is related to a decrease of the Peierls barrier in bcc transition metals and it is an intrinsic feature of the alloys [6,13]. The solid solution of molybdenum–hafnium is the system of interest in the present paper. This alloy system is not as well investigated as other molybdenum solid solution alloys (e.g. Mo–Re). There is a consensus in the literature that Hf act as strong solid solution hardener in molybdenum [7,25]. In contrast, there is a disagreement in the experimental results of the solution softening in the Mo–Hf system. Stephens and Witzke [7] have investigated several binary molybdenum alloys including the Mo–Hf system. In terms of their study just alloys with a hafnium content above
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0.92 at.% were investigated but without detailed investigation of chemistry and microstructure. The authors did not determine an alloy softening in the Mo–Hf alloys. Additionally, it should be mentioned that other Mo alloys, e.g. the Mo–Pt and Mo–Ir system, show a distinct softening below 0.90 at.% solute content. On the other hand, Pink and Arsenault [6,13] have reported that molybdenum exhibited a softening during alloying with hafnium. It was suggested that due to the high chemical attraction between Hf and the interstitials scavenging of the molybdenum matrix was responsible for the softening effect. The aim of the present study is to clarify if hafnium causes a solid solution softening in molybdenum. In the course of this work a detailed experimental investigation of several Mo–Hf alloys is shown. Therefore, alloys were produced by powder metallurgy in the concentration range of 0.15–3.0 at.% hafnium. Hardness tests were carried out in the temperature range of 77–443 K. Due to the fact that hafnium exhibits a high attraction to interstitial elements (O, N, C), special attention was paid to the chemical composition and the distribution of the elements. Therefore, the alloys were investigated with electron microprobe analysis (EMPA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). Furthermore, the distribution of the elements on the atomic scale was measured by atom probe tomography (APT). The APT method provides accurate measurements of low quantities of interstitial elements like carbon, nitrogen and oxygen. The high spatial resolution of the APT technique is able to measure the local composition such as solute clusters in the matrix [26,27]. Therefore, the APT method is appropriate to determine a scavenging of the molybdenum matrix. 2. Experimental In order to study the SSS effect unalloyed molybdenum and Mo–Hf alloys (in the range of 0.15–3.0 at.% Hf) were produced by powder metallurgy. In the course of the alloy production, pure Mo powder (purity 99.9 %, Fisher subsieve size = 4.5 lm), which was reduced from molybdenum oxide, was mixed together with hafnium hydride and a small amount of carbon. The carbon was added to reduce the oxygen in the alloys by the aggregate reaction MoO3 + 3C = Mo + 3CO. After mixing, the powder was uniaxial pressed to form a green compact which was subsequently sintered at 2473 K in hydrogen atmosphere. For a uniform distribution of Hf and a further decrease in oxygen content a homogenization annealing at 2573 K for 35 h was carried out in vacuum. The alloys were forged to remove the porosity from the sintering process. For some of the alloys an annealing in between the forging process was required to reach the minimum total degree of deformation of 60 %. Finally, the material was recrystallized at different temperatures, the alloys A1–A4 at 1873 K and the alloys A5–A6 at 2273 K for 1 h [25]. The unalloyed molybdenum was forged and recrystallized in order to get a dense material with a grain size of approximately 100 lm [28]. After the recrystallization annealing the chemical composition, the homogeneity of hafnium, the microstructure and the mechanical properties were investigated. The grain size of the alloys was determined by the linear intercept method (ASTM E112) using optical light micrographs. SEM investigations were performed with a Zeiss EVO 50. The preparation of the alloys for the metallographic investigations was done with the method described in [29]. For the determination of the mechanical properties a Zwick 3212 hardness testing equipment was used. The specimens for the hardness tests were cut into pieces with the dimension 15 15 5 mm3 from the recrystallized alloys. A minimum of ten Vickers indentations (10 kg load) were made for each testing temperature and alloy. The hardness tests at the temperatures 77 K and 183 K were performed in the cryogenic test unit in Fig. 1a. The reservoir is made of austenitic steel and the thermal isolation of extruded polystyrene foam (XPS). For the testing temperature at 77 K the reservoir was fully filled with liquid nitrogen. The hardness measurements at 183 K were done in liquid ethanol. The temperature of the liquid ethanol was controlled by an addition of frozen ethanol chips. The hardness tests above room temperature, at 443 K, were performed with the heating unit in Fig. 1b. The testing temperatures were measured by a thermocouple which was mounted in a drilled hole of the specimens (see Fig. 1a and b). The hardness at room temperature was determined in three configurations (a) without the cooling/heating unit, (b) with the cryogenic unit and (c) with the heating unit. The interstitial elements hydrogen, oxygen and nitrogen were detected by carrier gas hot extraction (CGHE) with a LECO TCH 600. Combustion analysis (CA) with a LECO CS-230 was used for the determination of the carbon content. The total amount of the alloyed Hf and the accompanying metals tungsten and zirconium
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Fig. 1. Schematic design of the hardness test units. (a) Cryogenic and (b) heating unit.
were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES) with a Thermo iCAP 6500 DUO. The hafnium distribution on the micron scale was investigated by EMPA. In the course of this, line scans with a step size of 10 lm and with a total length of 900 lm were carried out on a JEOL Superprobe JXA 8200. An accelerating voltage of 20 kV was used and pure hafnium served as standard (M.A.C. No.: 8030). Detailed microstructure investigations were performed on a Philips CM12 TEM. The specimens of 3 mm in diameter were ground and polished to a thickness of 80 lm. The final step of the preparation was done by electropolishing in a twin jet apparatus with a solution of 12.5 vol.% H2SO4 in ethanol at 268– 273 K [30]. Atom probe tomography investigations were performed to determine the elemental distribution on the atomic scale. Therefore, sharp needle-shaped specimens of the unalloyed Mo and the Mo–Hf alloys were produced by electropolishing. The electrolyte, 12.5 vol.% H2SO4 in ethanol, was used at 20 °C with a gold ring as counter electrode. The polishing procedure was done with direct voltage between 12 and 15 V. A general description of the specimen preparation for APT can be found in [31]. The atom probe tips were analyzed with a LEAP 3000X HR from Cameca, former Imago Scientific Instruments. The measurements were done in voltage mode at a specimen temperature of 60 K with a pulse fraction of 15% of the standing voltage and a pulse frequency of 200 kHz.
3. Results The aim of the alloy manufacturing was to produce specimens with similar grain size and a defined chemical composition. The total amount of hafnium, the accompanying elements and the interstitials of the investigated materials are listed in Table 1. The elemental content was measured by ICP–OES, CA and CGHE in the recrystallized condition. It can be seen that the oxygen content increases with increasing hafnium content. The content of hydrogen and nitrogen is below the detection limit. The carbon concentration is almost the same in all materials. The tungsten content is constant and the zirconium content increases with increasing hafnium content. The reason is that tungsten and zirconium are accompanying elements of the molybdenum and hafnium raw materials, respectively. In Fig. 2a a typical SEM micrograph of the Mo–Hf alloys is shown. The white appearing particles were identified by EDS measurements as stoichiometric hafnium dioxide (Fig. 2b). With increasing hafnium content the volume fraction of
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Table 1 Chemical composition and grain size of the alloyed and unalloyed molybdenum. EMPA
a b
a
ICP–OES b
CA
CGHE
Grain size
Alloy
Hf (at.%)
Hf (at.%)
Zr (at.%)
W (at.%)
C (lg/g)
O (lg/g)
N (lg/g)
H (lg/g)
d (lm)
Mo A1 A2 A3 A4 A5 A6
– 0.15 0.28 0.63 0.94 1.92 2.92
– 0.1774 0.3386 0.6584 1.0752 2.1652 3.2519
– 0.0026 0.0057 0.0137 0.0241 0.0498 0.0741
0.0078 0.0084 0.0095 0.0094 0.0091 0.0080 0.0099
12 13 8 15 14 25 16
22 35 160 330 560 910 980
<5
<1
99 82 97 95 70 120 200
Content in solid solution. Total content in the material.
Fig. 2. Investigations of the microstructure. (a) SEM micrograph of alloy A6 (backscatter detector). (b) EDS measurement of a particle from (a). The quantification of the spectrum offered the following composition of the particle: 32.4 at.% Hf and 67.6 at.% O. (c) TEM micrograph of alloy A4. Two grains are visible, but no particles were found. (d) TEM micrograph of alloy A6. A triple point is obvious and additionally small particles (red arrows) are present in the right grain, close to the grain boundary [25]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
HfO2 increases. In alloy A6, the alloy with the highest oxygen content, a volume fraction of f = 0.71 vol.% HfO2 was determined by quantitative microstructure analysis. The microstructure of the alloys was additionally investigated by TEM. A representative TEM micrograph, including a grain boundary, of the alloys is depicted in Fig. 2c. In the alloys A1–A5, no submicrometer sized particles could be revealed. In alloy A6 a low amount of small particles, close to the grain boundaries were determined (Fig. 2d). However, there were no submicrometer sized particles found within the grains. Fig. 3 shows the Hf distribution in solid solution measured by EMPA. It can be seen that during the manufacturing process a homogenous hafnium distribution was attained for all alloys. In Table 1 the average Hf contents based on the EMPA measurements are shown. The EMPA measurements consider the Hf concentra-
tions in solid solution. Therefore, these concentrations were used for Figs. 5 and 4. For comparability the EMPA results were converted from weight- into atomic-percent (Table 1). The composition and the distribution of the elements on the atomic scale were obtained by APT experiments. At least 107 atoms were collected for each specimen. The analysis of the datasets was done with the software package IVAS 3.4.3 from Cameca by making a background correction in the mass to charge state ratio (m/n) spectra. In some of the three dimensional reconstructions an oxygen contaminated tip surface, which was caused by the atmosphere, can be observed. Therefore, regions of interest from the unaffected bulk volume were separated and used for analysis. The boxes of equivalent size 30 30 100 nm3 are depicted in Fig. 4, showing the Hf and oxygen distribution. Additionally to
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253
Fig. 3. Hafnium concentration profile of the Mo–Hf alloys.
optical inspection, the l-value was calculated for the dissolved elements of each alloy. The parameter l is independent of the probed volume and it is ranging from 0 to 1, representing the degree of segregation. Where 0 indicates a random distribution and 1 indicates a total segregation of the solute atoms [32]. The results of the APT measurements are summarized in Table 2. Despite the low concentrations of W in the materials the APT results are almost equal to the ICP–OES results. The reason of the lower Zr content in the APT measurements is that some Zr and Mo isotopes are overlapping at the same m/n ratio. The l-value, which is also listed in Table 2, of both accompanying elements indicates a homogeneous distribution. The revealed concentrations of Hf on the atomic scale are in good agreement with the EMPA measurements. Due to the fact that no clustering of Hf was observed in the alloys the l-values are close to zero (Fig. 4). Oxygen could only be detected in the alloys and not in the unalloyed molybdenum. With increasing Hf content the O content increases as well, but no oxygen clusters were revealed in the alloys (Fig 4). Furthermore, no C was determined in solid solution with APT. To sum up, the APT investigations showed a homogeneous distribution of all detected elements in the alloyed molybdenum. The grain sizes of the recrystallized specimens are listed in Table 1. The grain size of Mo and the low concentrated alloys is almost equal. The different grain size of the alloys A4–A6 was
Fig. 5. Hardness vs. hafnium concentration. (a) At 293 K with and without the temperature testing units. (b) For 77 K and 183 ± 3 K. (c) For 443 ± 2 K.
Fig. 4. Three dimensional reconstructions of hafnium and oxygen atoms of the different alloys (box size 30 30 100 nm3). Concentrations and l-values are listed in Table 2.
caused by the different recrystallization temperatures, the number of forging steps and the total degree of deformation. This was required due to the different recrystallization behavior of the higher concentrated alloys. The hardness measurements at different temperatures (77 K, 183 K, 293 K and 443 K) were conducted to reveal the SSS effect of the alloys. Therefore, the testing units in Fig. 1 were designed. Reference measurements at room temperature were performed to ensure that these units have no influence on the hardness measurements. In Fig. 5a, the results of the hardness reference measurements at room temperature with and without the cryogenic and heating units are shown. It can be seen that both temperature units have no significant influence on the hardness measurements. However, it is obvious in Fig. 5a that the low concentrated alloys exhibit an initial softening followed by a rapid increase of the hardness at 1 at.% Hf. The maximum softening can be seen in Fig. 5b, it occurs at temperatures below room temperature, 77 K and
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Table 2 Concentration and distribution of the elements in solid solution determined by the atom probe tomography measurements. Hf
Zr
W
O
Alloy
c (at.%)
l (–)
c (at.%)
l (–)
c (at.%)
l (-)
c (at.%)
l (–)
Mo A1 A2 A3 A4 A5 A6
0 0.1514 0.2870 0.5987 0.9608 1.9140 3.0258
– 0.0096 0.0066 0.0067 0.0135 0.0079 0.0091
0.0000 0.0030 0.0053 0.0112 0.0185 0.0378 0.0558
– 0.0008 0.0000 0.0003 0.0024 0.0005 0.0011
0.0087 0.0081 0.0086 0.0099 0.0082 0.0087 0.0098
0.0016 0.0009 0.0055 0.0024 0.0003 0.0000 0.0011
0 0.0020 0.0032 0.0090 0.0122 0.0311 0.0459
– 0.0002 0.0003 0.0004 0.0003 0.0012 0.0078
183 ± 3 K, and it is limited to the alloys A1–A4 (below 1 at.% Hf). In Fig. 5c the results from the hardness measurements with the heating unit are depicted. At a temperature of T 0.15TM only a hardening of molybdenum is observed. In Fig. 6 the temperature dependence of the hardness is illustrated. The hardness decreases with increasing temperature for the unalloyed Mo and the Mo– Hf alloys. The alloys A5 and A6 show a hardening over the investigated temperature range. The lower concentrated alloys illustrate a softening behavior up to room temperature. It can be seen that with increasing Hf content the temperature dependence of the hardness decreases. The higher errors in the hardness measurements at lower temperatures (see Fig. 5b) are related to the higher sensitivity to small temperature fluctuations (see Fig. 6). The hardness values which are plotted in Figs. 5 and 6 are listed in Table 3. 4. Discussion The metallographic investigations with SEM and TEM revealed second phase particles in the microstructures of the recrystallized alloys. Due to the low volume fraction (f 6 0.71 vol.%) and the size of a few lm (diameter > 5 lm) of the HfO2 particles it was suggested that they have no significant influence on the hardness of the alloys. The smaller particles in alloy A6 were found close to grain boundaries. Since there are no sub-micron sized particles within the alloys the SSS effect cannot be weakened or covered by particle hardening. On the other hand, the formation of the coarse hafnium dioxide particles influences the Hf content in solid solution. The ICP–OES analysis considers the total hafnium content (oxides + solid solution) in the alloys. Therefore, the Hf content in solid solution would be overestimated by this global chemical analysis. However, the EMPA and the APT analysis revealed the true Hf content in solid solution. The results of the CGHE and CA measurements just show the total content of carbon and oxygen in the specimens. Due to the fact that APT analysis revealed no C within the Mo matrix it is assumed that the majority of carbon is located on grain boundaries. The majority of oxygen was used for
the formation of HfO2 particles. The APT measurements (see Table 2) revealed a correlation between the Hf and oxygen content in solid solution. Miller et al. [33,34] conducted atom probe studies on zirconium alloyed molybdenum and they observed an oxygen depletion at the grain boundaries. However, Waugh and Southon [35] have determined an oxygen enrichment on grain boundaries in unalloyed molybdenum by atom probe studies. From these observations Miller et al. [33,34] concluded that an enrichment of oxygen in the intergranular regions occurred with Zr additions. The present results suggest that Hf exhibits a similar effect on oxygen in molybdenum as Zr. APT results of the unalloyed Mo, compared to the alloys, confirmed that hafnium increases the solubility of oxygen in the matrix. Smialek and Mitchell [36] investigated the interstitial hardening in Ta–N, Ta–C, and Ta–O systems. They revealed at 77 K that nitrogen exhibits the greatest hardening effect followed by carbon. The lowest hardening in Ta was caused by oxygen additions. Furthermore, oxygen contents between 20 and 180 ppm caused an almost equal increment of the yield strength. Similar results for a refractory metal system were found by Ulitchny and Gibala [37] for the Nb–O and Nb–N systems at 113 K. It was shown that nitrogen induced a significant hardening effect in Nb, whereas oxygen only causes a moderate increase of the hardness. It is not possible to investigate the hardening/softening effect of oxygen because there is no solubility in pure Mo [38]. Based on the findings of the Ta–O and Nb–O systems [36,37] it was assumed that the dissolved oxygen in the Mo–Hf alloys causes a slight shift to higher hardening values. Nevertheless, the hardness measurements clearly indicate a softening of Mo due to Hf additions up to 0.63 at.%. The lower contents of W and Zr in solid solution do not significantly affect the hardness of the alloys [2]. The hardness of polycrystalline metals with grains at the micron scale can be expressed with the Hall–Petch relationship as 0
1=2
H ¼ H0 þ kHP d
ð1Þ 0 kHP
where H0 is the intrinsic hardness, d is the grain size and is the grain size independent Hall–Petch coefficient [39]. Wesemann et al. 0 [40] determined a Hall–Petch coefficient of kHP ¼ 0:102 HV10 m1/2 for pure molybdenum at room temperature. Orava [41] investigated the effect of different grain sizes on the yield strength of molybdenum. The author revealed an insensitivity of the Hall–Petch
Table 3 Hardness values (HV10) at the testing temperatures (c.u. – cryogenic unit, h.u. – heating unit, w.u. – without unit).
Fig. 6. Hardness vs. testing temperature of the different alloys.
77 K
183 K
293 K
Alloy
c.u.
c.u.
c.u.
h.u.
w.u.
h.u.
443 K
Mo A1 A2 A3 A4 A5 A6
389 ± 2.8 367 ± 4.9 371 ± 2.9 382 ± 5.2 388 ± 4.0 401 ± 5.4 421 ± 5.3
280 ± 1.5 268 ± 1.0 274 ± 1.6 276 ± 1.8 281 ± 2.1 288 ± 2.3 323 ± 1.9
167 ± 1.0 163 ± 1.8 162 ± 1.4 165 ± 1.2 171 ± 1.3 214 ± 1.9 263 ± 1.9
166 ± 1.0 161 ± 1.3 162 ± 0.6 163 ± 2.1 172 ± 1.0 215 ± 2.1 266 ± 2.9
169 ± 0.9 162 ± 1.3 164 ± 1.2 165 ± 1.1 172 ± 0.8 217 ± 1.9 265 ± 1.5
84 ± 0.5 90 ± 0.7 95 ± 0.8 116 ± 0.8 137 ± 0.9 189 ± 1.7 232 ± 1.3
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coefficient at temperatures below 300 K. In the temperature range from 300 K to 450 K the coefficient decreases of approximately 15%. Several authors investigated the effects of substitutional solutes on the Hall–Petch coefficient, mostly in titanium alloys and austenitic steels [42–44]. Chia and co-workers [42] revealed an increase of the Hall–Petch coefficient of 20–30% (between 77 K and 300 K) through alloying titanium with 15.2 at.% Mo. Furthermore, this bcc b-Ti–Mo alloy exhibited a relatively temperature independent Hall–Petch coefficient. Werner [44] investigated austenitic steels with different alloying elements and contents. During highly variation of the Cr, Mn and Mo contents the Hall–Petch coefficient was not significantly changed. It can be seen that the effect of alloying elements on the Hall–Petch coefficient is more ambiguous. However, in the present study the softening effect was investigated 0 in dilute Mo–Hf alloys. Therefore, it was assumed that the kHP of the Mo–Hf alloys was not significantly different to that of pure Mo. Fur0 thermore, a slight variation of kHP has only a small influence on the 0 hardness due to the relatively low level of kHP from Mo [40]. Stephens and Witzke [7] have only investigated alloys above 0.90 at.% hafnium. The results of the hardness tests at temperatures below T < 0.15TM show a distinct softening behavior of the low concentrated alloys A1–A3 with a Hf content up to 0.63 at.%. Thus, the softening effect was not determined by Stephens and Witzke. However, Pink and Arsenault [6,13] mentioned a softening in the Mo–Hf system caused by the scavenging mechanism (extrinsic) but without any detailed results. Therefore, the atom probe tomography investigations were done to reveal a possible scavenging effect of hafnium, due to its affinity to carbon and oxygen. If the hafnium additions remove the interstitial atoms from the random solution a clustering of the relevant elements should occur. From the APT results it can be seen that Hf and the interstitial elements are homogeneously distributed. Furthermore, the technically pure Mo matrix does not even contain oxygen and carbon in solid solution which could be removed by hafnium additions. Therefore, the assumption from Pink and Arsenault [6,13] that scavenging occurs is refuted by the results of the present study. The characteristic of the hardness-concentration curves of Mo– Hf alloys (see Fig. 5), i.e. the low element concentration where SSS occurs and the value of softening, is similar to Mo–Ir and Mo–Pt alloys. This is in contrast to Mo–Re and Mo–Os alloys which exhibit a distinct softening at higher concentrations. A common characteristic of all elements (Pt, Ir, Re, Os and Hf) which cause SSS is the limited or intermediate solubility in Mo. On the other hand, elements (Ta, W) which only cause hardening show a complete solubility in Mo [7,45]. Medvedeva and co-workers [18] investigated the SSS of Hf among other solutes in bcc Mo alloys on the basis of ab initio electronic-structure calculations. They related the experimentally determined hardening effect of Hf, from Stephens and Witzke [7], to the increased generalized stacking fault energy in Mo–Hf alloys. Indeed, their calculations were dealing with medium solute concentrations clearly above 0.63 at.% Hf. This could be an indication that their model was not able to predict the SSS in Mo–Hf system. The low temperature plastic deformation in bcc metals and alloys is influenced by the Peierls potentials and the nucleation and migration of kinks. These intrinsic mechanisms are affected by solute atoms. Due to the fact that no scavenging was observed, the solid solution softening in dilute Mo–Hf alloys is attributed to an intrinsic mechanism.
5. Conclusions This study presents the experimental investigation of the solid solution softening effect of the Mo–Hf system. The aim was to clarify whether alloy softening is present and in consequence to determine an extrinsic behavior.
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Transmission and scanning electron microscopy investigations have shown that hafnium dioxide particles were present in the microstructure. Therefore, a part of the alloyed hafnium was not available for solid solution. Hardness tests at temperatures below T < 0.15TM revealed a solid solution softening effect. The maximum of alloy softening is limited to a Hf concentration of 0.15–0.28 at.%. Stephens and Witzke only investigated alloys above 0.90 at.% Hf [7]. Therefore, they could not determine the softening behavior in the Mo–Hf system. Atom probe tomography measurements have shown that no scavenging of the Mo matrix occurs during alloying with Hf. Thus, the assumption of Arsenault and Pink [6,13] was not correct. The mechanism of solid solution softening in the Mo–Hf system is related to an intrinsic mechanism. Atom probe experiments revealed an increasing oxygen content in solid solution with increasing Hf content. This result suggests that the dissolved Hf increases the oxygen solubility in Mo.
Acknowledgements The authors would like to thank the Department of Applied Geosciences and Geophysics, Montanuniversität Leoben, for the electron microprobe analysis. Plansee SE for the support during the manufacturing of the molybdenum–hafnium alloys. Bruno Krajnc and Julia Lechleitner for the support during the experiments. References [1] W. Chang, in: M. Semchyshen, J.J. Harwood (Eds.), Refractory Metals and Alloys, Interscience Publishers, New York, 1961, pp. 83–117. [2] M. Semchyshen, in: J.J. Harwood (Ed.), The Metal Molybdenum, ASM, Ohio, 1956, pp. 281–329. [3] P.S. Rudman, in: R.I. Jaffee (Ed.), Proceedings of the AIME Conference on Refractory Metals, New York, 1965, pp. 351–368. [4] B. Wilcox, in: I. Machlin (Ed.), Refractory Metal Alloys, Plenum Press, New York, 1969, pp. 1–39. [5] L.L. Seigle, in: N.E. Promisel (Ed.), The Science and Technology of Tungsten, Tantalum, Molybdenum, Niobium and Their Alloys, Pergamon Press, Oxford, 1963, pp. 63–93. [6] E. Pink, R.J. Arsenault, Prog. Mater. Sci. 24 (1980) 1–50. [7] J.R. Stephens, W.R. Witzke, J. Less-Common Met. 29 (1972) 371–388. [8] R. Arsenault, Acta Metall. 17 (1969) 1291–1297. [9] L.A. Gypen, A. Deruyttere, J. Less-Common Met. 56 (1977) 91–101. [10] J. Stephens, W. Witzke, J. Less-Common Met. 23 (1971) 325–342. [11] W.D. Klopp, J. Less-Common Met. (1975) 261–278. [12] D.L. Davidson, F.R. Brotzen, Acta Metall. 18 (1970) 463–470. [13] E. Pink, Z. Metallkde. 64 (1974) 871–881. [14] W.C. Leslie, Metall. Trans. A 3 (1972) 5–25. [15] R. Gibala, T.E. Mitchell, Scripta Metall. 7 (1973) 1143–1148. [16] A. Akhtar, E. Teghtsoonian, Acta Metall. 17 (1969) 1351–1356. [17] A.H. Blake, C.H. Cáceres, Mater. Sci. Eng. A 483–484 (2008) 161–163. [18] N. Medvedeva, Y. Gornostyrev, A. Freeman, Phys. Rev. Lett. 94 (136402) (2005) 1–4. [19] D.R. Trinkle, C. Woodward, Science 310 (2005) 1665–1667. [20] R.L. Fleischer, Acta Metall. 15 (1967) 1513–1519. [21] R.L. Smialek, G.L. Webb, T.E. Mitchell, Scripta Mater. 4 (1970) 33–38. [22] A. Seeger, Z. Metallkde 72 (1981) 369–380. [23] I. Hollang, M. Hommel, A. Seeger, Phys. Status Solidi A (1997) 329–354. [24] R. Arsenault, Acta Metall. 15 (1967) 501–511. [25] C. Pöhl, J. Schatte, H. Leitner, Mater. Sci. Eng. A 559 (2013) 643–650. [26] E.V. Pereloma, I.B. Timokhina, J.J. Jonas, M.K. Miller, Acta Mater. 54 (2006) 4539–4551. [27] M. Thuvander, J. Weidow, J. Angseryd, L.K.L. Falk, F. Liu, M. Sonestedt, K. Stiller, H.-O. Andrén, Ultramicroscopy 111 (2011) 604–608. [28] S. Primig, H. Leitner, H. Clemens, A. Lorich, W. Knabl, R. Stickler, Int. J. Refract. Met. Hard Mater. 28 (2010) 703–708. [29] C. Pöhl, J. Schatte, H. Leitner, Mater. Charact. 77 (2012) 63–69. [30] H. Calderon, G. Kostorz, G. Ullrich, Mater. Sci. Eng. A 160 (1993) 189–199. [31] M.K. Miller, A. Cerezo, M.G. Hetherington, G.D.W. Smith, Atom Probe Field Ion Microscopy, Clarendon Press, Oxford, 1996. [32] M.P. Moody, L.T. Stephenson, A.V. Ceguerra, S.P. Ringer, Microsc. Res. Tech. 71 (2008) 542–550. [33] M.K. Miller, E.A. Kenik, M.S. Mousa, K.F. Russell, A.J. Bryhan, Scripta Mater. 46 (2002) 299–303. [34] M.K. Miller, A. Bryhan, Mater. Sci. Eng. A 327 (2002) 80–83.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Solid solution softening of polycrystalline molybdenum–hafnium alloys C. Pöhl a,⇑, J. Schatte b, H. Leitner a,c a
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria Plansee SE, Metallwerk-Plansee-Straße 71, 6600 Reutte, Austria c Christian Doppler Laboratory for Early Stages of Precipitation, Montanuniversität Leoben, Austria b
a r t i c l e
i n f o
Article history: Received 20 December 2012 Received in revised form 7 March 2013 Accepted 20 April 2013 Available online 29 April 2013 Keywords: Molybdenum alloys Powder metallurgy Hafnium Solid solution softening
a b s t r a c t In the present study the solid solution softening of molybdenum–hafnium alloys (0.15–3.0 at.%) was investigated at homologous temperatures ranging from 0.027 to 0.153 (77–443 K). The hardness measurements revealed a significant softening effect of alloys with a hafnium content below 0.63 at.%. In order to determine the distribution of the alloying element and the impurities, a detailed investigation of the microstructure and the chemistry was carried out. With atom probe tomography it could be shown that interstitials were not removed from random solution in the molybdenum matrix during alloying with hafnium. This indicated that solid solution softening in the Mo–Hf system is attributed to an intrinsic mechanism. Furthermore, with atom probe investigations it can be shown that the dissolved hafnium increases the oxygen solubility in molybdenum. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Alloying is a common practice to increase the strength of molybdenum over a wide range of temperature, e.g. for hot metal forming parts, power electronics and high temperature furnace construction [1–5]. In contrast to the hardening effect, a solid solution softening (SSS) has been observed in several molybdenum alloys [6–9]. For example the classic SSS system, molybdenumrhenium, shows a distinct softening below 350 K [10]. The effect of softening through alloying is applied to increase the low temperature ductility of the refractory metals molybdenum and tungsten [11,12]. The SSS effect itself was studied experimentally in body centered cubic (bcc) metal alloys by several authors [6,8,13–15]. The main characteristics of SSS are (a) the effect through alloying is limited to the thermal activated dislocation motion, i.e. below the temperature T 0.15TM (where TM is the melting temperature in K) for maximum thermal activation, and (b) it occurs in bcc single- and poly-crystals but not in face centered cubic metals [6,13]. Furthermore, SSS is a known phenomenon in hexagonal magnesium alloys as well. Akhtar and Teghtsoonian [16] reported a distinct softening of the prismatic slip by Zn and Al solute additions at low temperatures. Blake and Cáceres [17] observed a notable softening for the prism planes in the order of 0.6 at.% Zn in Mg. In general, the amount of SSS depends on temperature and solute concentration and with increasing solute content softening is compensated by solid solution hardening.
⇑ Corresponding author. Tel.: +43 3842 402 4214; fax: +43 3842 402 4202. E-mail address: [email protected] (C. Pöhl). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.138
There is no general model for SSS in the literature and research is still in progress, e.g. the modeling of the softening effect in molybdenum on the basis of ab initio calculations is a recent field of interest [18,19]. Two main theories have been developed in the literature to describe the origin of the alloy softening effect in bcc transition metals [6]. In the first theory, it is assumed that the dislocation motion is impeded by the interstitial impurities carbon, nitrogen and oxygen and thus the strength of the base metal is increased [20]. Through alloying of the base metal the interstitials are removed from random solution of the matrix, i.e. the impurities cannot longer act as solution hardener. Therefore, the yield strength of the alloyed metal is decreased [13,15,21]. This theory of scavenging the base metal lattice with the alloying element leads to a softening by an extrinsic mechanism. In bcc metals the plastic deformation at low temperatures is controlled by kink pair formation and kink migration [22–24]. The second theory is supposed to originate from the enhanced nucleation and motion of kink pairs at screw dislocations by the solute atoms [19]. The mechanism of kink nucleation enhancement is related to a decrease of the Peierls barrier in bcc transition metals and it is an intrinsic feature of the alloys [6,13]. The solid solution of molybdenum–hafnium is the system of interest in the present paper. This alloy system is not as well investigated as other molybdenum solid solution alloys (e.g. Mo–Re). There is a consensus in the literature that Hf act as strong solid solution hardener in molybdenum [7,25]. In contrast, there is a disagreement in the experimental results of the solution softening in the Mo–Hf system. Stephens and Witzke [7] have investigated several binary molybdenum alloys including the Mo–Hf system. In terms of their study just alloys with a hafnium content above
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0.92 at.% were investigated but without detailed investigation of chemistry and microstructure. The authors did not determine an alloy softening in the Mo–Hf alloys. Additionally, it should be mentioned that other Mo alloys, e.g. the Mo–Pt and Mo–Ir system, show a distinct softening below 0.90 at.% solute content. On the other hand, Pink and Arsenault [6,13] have reported that molybdenum exhibited a softening during alloying with hafnium. It was suggested that due to the high chemical attraction between Hf and the interstitials scavenging of the molybdenum matrix was responsible for the softening effect. The aim of the present study is to clarify if hafnium causes a solid solution softening in molybdenum. In the course of this work a detailed experimental investigation of several Mo–Hf alloys is shown. Therefore, alloys were produced by powder metallurgy in the concentration range of 0.15–3.0 at.% hafnium. Hardness tests were carried out in the temperature range of 77–443 K. Due to the fact that hafnium exhibits a high attraction to interstitial elements (O, N, C), special attention was paid to the chemical composition and the distribution of the elements. Therefore, the alloys were investigated with electron microprobe analysis (EMPA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). Furthermore, the distribution of the elements on the atomic scale was measured by atom probe tomography (APT). The APT method provides accurate measurements of low quantities of interstitial elements like carbon, nitrogen and oxygen. The high spatial resolution of the APT technique is able to measure the local composition such as solute clusters in the matrix [26,27]. Therefore, the APT method is appropriate to determine a scavenging of the molybdenum matrix. 2. Experimental In order to study the SSS effect unalloyed molybdenum and Mo–Hf alloys (in the range of 0.15–3.0 at.% Hf) were produced by powder metallurgy. In the course of the alloy production, pure Mo powder (purity 99.9 %, Fisher subsieve size = 4.5 lm), which was reduced from molybdenum oxide, was mixed together with hafnium hydride and a small amount of carbon. The carbon was added to reduce the oxygen in the alloys by the aggregate reaction MoO3 + 3C = Mo + 3CO. After mixing, the powder was uniaxial pressed to form a green compact which was subsequently sintered at 2473 K in hydrogen atmosphere. For a uniform distribution of Hf and a further decrease in oxygen content a homogenization annealing at 2573 K for 35 h was carried out in vacuum. The alloys were forged to remove the porosity from the sintering process. For some of the alloys an annealing in between the forging process was required to reach the minimum total degree of deformation of 60 %. Finally, the material was recrystallized at different temperatures, the alloys A1–A4 at 1873 K and the alloys A5–A6 at 2273 K for 1 h [25]. The unalloyed molybdenum was forged and recrystallized in order to get a dense material with a grain size of approximately 100 lm [28]. After the recrystallization annealing the chemical composition, the homogeneity of hafnium, the microstructure and the mechanical properties were investigated. The grain size of the alloys was determined by the linear intercept method (ASTM E112) using optical light micrographs. SEM investigations were performed with a Zeiss EVO 50. The preparation of the alloys for the metallographic investigations was done with the method described in [29]. For the determination of the mechanical properties a Zwick 3212 hardness testing equipment was used. The specimens for the hardness tests were cut into pieces with the dimension 15 15 5 mm3 from the recrystallized alloys. A minimum of ten Vickers indentations (10 kg load) were made for each testing temperature and alloy. The hardness tests at the temperatures 77 K and 183 K were performed in the cryogenic test unit in Fig. 1a. The reservoir is made of austenitic steel and the thermal isolation of extruded polystyrene foam (XPS). For the testing temperature at 77 K the reservoir was fully filled with liquid nitrogen. The hardness measurements at 183 K were done in liquid ethanol. The temperature of the liquid ethanol was controlled by an addition of frozen ethanol chips. The hardness tests above room temperature, at 443 K, were performed with the heating unit in Fig. 1b. The testing temperatures were measured by a thermocouple which was mounted in a drilled hole of the specimens (see Fig. 1a and b). The hardness at room temperature was determined in three configurations (a) without the cooling/heating unit, (b) with the cryogenic unit and (c) with the heating unit. The interstitial elements hydrogen, oxygen and nitrogen were detected by carrier gas hot extraction (CGHE) with a LECO TCH 600. Combustion analysis (CA) with a LECO CS-230 was used for the determination of the carbon content. The total amount of the alloyed Hf and the accompanying metals tungsten and zirconium
251
Fig. 1. Schematic design of the hardness test units. (a) Cryogenic and (b) heating unit.
were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES) with a Thermo iCAP 6500 DUO. The hafnium distribution on the micron scale was investigated by EMPA. In the course of this, line scans with a step size of 10 lm and with a total length of 900 lm were carried out on a JEOL Superprobe JXA 8200. An accelerating voltage of 20 kV was used and pure hafnium served as standard (M.A.C. No.: 8030). Detailed microstructure investigations were performed on a Philips CM12 TEM. The specimens of 3 mm in diameter were ground and polished to a thickness of 80 lm. The final step of the preparation was done by electropolishing in a twin jet apparatus with a solution of 12.5 vol.% H2SO4 in ethanol at 268– 273 K [30]. Atom probe tomography investigations were performed to determine the elemental distribution on the atomic scale. Therefore, sharp needle-shaped specimens of the unalloyed Mo and the Mo–Hf alloys were produced by electropolishing. The electrolyte, 12.5 vol.% H2SO4 in ethanol, was used at 20 °C with a gold ring as counter electrode. The polishing procedure was done with direct voltage between 12 and 15 V. A general description of the specimen preparation for APT can be found in [31]. The atom probe tips were analyzed with a LEAP 3000X HR from Cameca, former Imago Scientific Instruments. The measurements were done in voltage mode at a specimen temperature of 60 K with a pulse fraction of 15% of the standing voltage and a pulse frequency of 200 kHz.
3. Results The aim of the alloy manufacturing was to produce specimens with similar grain size and a defined chemical composition. The total amount of hafnium, the accompanying elements and the interstitials of the investigated materials are listed in Table 1. The elemental content was measured by ICP–OES, CA and CGHE in the recrystallized condition. It can be seen that the oxygen content increases with increasing hafnium content. The content of hydrogen and nitrogen is below the detection limit. The carbon concentration is almost the same in all materials. The tungsten content is constant and the zirconium content increases with increasing hafnium content. The reason is that tungsten and zirconium are accompanying elements of the molybdenum and hafnium raw materials, respectively. In Fig. 2a a typical SEM micrograph of the Mo–Hf alloys is shown. The white appearing particles were identified by EDS measurements as stoichiometric hafnium dioxide (Fig. 2b). With increasing hafnium content the volume fraction of
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Table 1 Chemical composition and grain size of the alloyed and unalloyed molybdenum. EMPA
a b
a
ICP–OES b
CA
CGHE
Grain size
Alloy
Hf (at.%)
Hf (at.%)
Zr (at.%)
W (at.%)
C (lg/g)
O (lg/g)
N (lg/g)
H (lg/g)
d (lm)
Mo A1 A2 A3 A4 A5 A6
– 0.15 0.28 0.63 0.94 1.92 2.92
– 0.1774 0.3386 0.6584 1.0752 2.1652 3.2519
– 0.0026 0.0057 0.0137 0.0241 0.0498 0.0741
0.0078 0.0084 0.0095 0.0094 0.0091 0.0080 0.0099
12 13 8 15 14 25 16
22 35 160 330 560 910 980
<5
<1
99 82 97 95 70 120 200
Content in solid solution. Total content in the material.
Fig. 2. Investigations of the microstructure. (a) SEM micrograph of alloy A6 (backscatter detector). (b) EDS measurement of a particle from (a). The quantification of the spectrum offered the following composition of the particle: 32.4 at.% Hf and 67.6 at.% O. (c) TEM micrograph of alloy A4. Two grains are visible, but no particles were found. (d) TEM micrograph of alloy A6. A triple point is obvious and additionally small particles (red arrows) are present in the right grain, close to the grain boundary [25]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
HfO2 increases. In alloy A6, the alloy with the highest oxygen content, a volume fraction of f = 0.71 vol.% HfO2 was determined by quantitative microstructure analysis. The microstructure of the alloys was additionally investigated by TEM. A representative TEM micrograph, including a grain boundary, of the alloys is depicted in Fig. 2c. In the alloys A1–A5, no submicrometer sized particles could be revealed. In alloy A6 a low amount of small particles, close to the grain boundaries were determined (Fig. 2d). However, there were no submicrometer sized particles found within the grains. Fig. 3 shows the Hf distribution in solid solution measured by EMPA. It can be seen that during the manufacturing process a homogenous hafnium distribution was attained for all alloys. In Table 1 the average Hf contents based on the EMPA measurements are shown. The EMPA measurements consider the Hf concentra-
tions in solid solution. Therefore, these concentrations were used for Figs. 5 and 4. For comparability the EMPA results were converted from weight- into atomic-percent (Table 1). The composition and the distribution of the elements on the atomic scale were obtained by APT experiments. At least 107 atoms were collected for each specimen. The analysis of the datasets was done with the software package IVAS 3.4.3 from Cameca by making a background correction in the mass to charge state ratio (m/n) spectra. In some of the three dimensional reconstructions an oxygen contaminated tip surface, which was caused by the atmosphere, can be observed. Therefore, regions of interest from the unaffected bulk volume were separated and used for analysis. The boxes of equivalent size 30 30 100 nm3 are depicted in Fig. 4, showing the Hf and oxygen distribution. Additionally to
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253
Fig. 3. Hafnium concentration profile of the Mo–Hf alloys.
optical inspection, the l-value was calculated for the dissolved elements of each alloy. The parameter l is independent of the probed volume and it is ranging from 0 to 1, representing the degree of segregation. Where 0 indicates a random distribution and 1 indicates a total segregation of the solute atoms [32]. The results of the APT measurements are summarized in Table 2. Despite the low concentrations of W in the materials the APT results are almost equal to the ICP–OES results. The reason of the lower Zr content in the APT measurements is that some Zr and Mo isotopes are overlapping at the same m/n ratio. The l-value, which is also listed in Table 2, of both accompanying elements indicates a homogeneous distribution. The revealed concentrations of Hf on the atomic scale are in good agreement with the EMPA measurements. Due to the fact that no clustering of Hf was observed in the alloys the l-values are close to zero (Fig. 4). Oxygen could only be detected in the alloys and not in the unalloyed molybdenum. With increasing Hf content the O content increases as well, but no oxygen clusters were revealed in the alloys (Fig 4). Furthermore, no C was determined in solid solution with APT. To sum up, the APT investigations showed a homogeneous distribution of all detected elements in the alloyed molybdenum. The grain sizes of the recrystallized specimens are listed in Table 1. The grain size of Mo and the low concentrated alloys is almost equal. The different grain size of the alloys A4–A6 was
Fig. 5. Hardness vs. hafnium concentration. (a) At 293 K with and without the temperature testing units. (b) For 77 K and 183 ± 3 K. (c) For 443 ± 2 K.
Fig. 4. Three dimensional reconstructions of hafnium and oxygen atoms of the different alloys (box size 30 30 100 nm3). Concentrations and l-values are listed in Table 2.
caused by the different recrystallization temperatures, the number of forging steps and the total degree of deformation. This was required due to the different recrystallization behavior of the higher concentrated alloys. The hardness measurements at different temperatures (77 K, 183 K, 293 K and 443 K) were conducted to reveal the SSS effect of the alloys. Therefore, the testing units in Fig. 1 were designed. Reference measurements at room temperature were performed to ensure that these units have no influence on the hardness measurements. In Fig. 5a, the results of the hardness reference measurements at room temperature with and without the cryogenic and heating units are shown. It can be seen that both temperature units have no significant influence on the hardness measurements. However, it is obvious in Fig. 5a that the low concentrated alloys exhibit an initial softening followed by a rapid increase of the hardness at 1 at.% Hf. The maximum softening can be seen in Fig. 5b, it occurs at temperatures below room temperature, 77 K and
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Table 2 Concentration and distribution of the elements in solid solution determined by the atom probe tomography measurements. Hf
Zr
W
O
Alloy
c (at.%)
l (–)
c (at.%)
l (–)
c (at.%)
l (-)
c (at.%)
l (–)
Mo A1 A2 A3 A4 A5 A6
0 0.1514 0.2870 0.5987 0.9608 1.9140 3.0258
– 0.0096 0.0066 0.0067 0.0135 0.0079 0.0091
0.0000 0.0030 0.0053 0.0112 0.0185 0.0378 0.0558
– 0.0008 0.0000 0.0003 0.0024 0.0005 0.0011
0.0087 0.0081 0.0086 0.0099 0.0082 0.0087 0.0098
0.0016 0.0009 0.0055 0.0024 0.0003 0.0000 0.0011
0 0.0020 0.0032 0.0090 0.0122 0.0311 0.0459
– 0.0002 0.0003 0.0004 0.0003 0.0012 0.0078
183 ± 3 K, and it is limited to the alloys A1–A4 (below 1 at.% Hf). In Fig. 5c the results from the hardness measurements with the heating unit are depicted. At a temperature of T 0.15TM only a hardening of molybdenum is observed. In Fig. 6 the temperature dependence of the hardness is illustrated. The hardness decreases with increasing temperature for the unalloyed Mo and the Mo– Hf alloys. The alloys A5 and A6 show a hardening over the investigated temperature range. The lower concentrated alloys illustrate a softening behavior up to room temperature. It can be seen that with increasing Hf content the temperature dependence of the hardness decreases. The higher errors in the hardness measurements at lower temperatures (see Fig. 5b) are related to the higher sensitivity to small temperature fluctuations (see Fig. 6). The hardness values which are plotted in Figs. 5 and 6 are listed in Table 3. 4. Discussion The metallographic investigations with SEM and TEM revealed second phase particles in the microstructures of the recrystallized alloys. Due to the low volume fraction (f 6 0.71 vol.%) and the size of a few lm (diameter > 5 lm) of the HfO2 particles it was suggested that they have no significant influence on the hardness of the alloys. The smaller particles in alloy A6 were found close to grain boundaries. Since there are no sub-micron sized particles within the alloys the SSS effect cannot be weakened or covered by particle hardening. On the other hand, the formation of the coarse hafnium dioxide particles influences the Hf content in solid solution. The ICP–OES analysis considers the total hafnium content (oxides + solid solution) in the alloys. Therefore, the Hf content in solid solution would be overestimated by this global chemical analysis. However, the EMPA and the APT analysis revealed the true Hf content in solid solution. The results of the CGHE and CA measurements just show the total content of carbon and oxygen in the specimens. Due to the fact that APT analysis revealed no C within the Mo matrix it is assumed that the majority of carbon is located on grain boundaries. The majority of oxygen was used for
the formation of HfO2 particles. The APT measurements (see Table 2) revealed a correlation between the Hf and oxygen content in solid solution. Miller et al. [33,34] conducted atom probe studies on zirconium alloyed molybdenum and they observed an oxygen depletion at the grain boundaries. However, Waugh and Southon [35] have determined an oxygen enrichment on grain boundaries in unalloyed molybdenum by atom probe studies. From these observations Miller et al. [33,34] concluded that an enrichment of oxygen in the intergranular regions occurred with Zr additions. The present results suggest that Hf exhibits a similar effect on oxygen in molybdenum as Zr. APT results of the unalloyed Mo, compared to the alloys, confirmed that hafnium increases the solubility of oxygen in the matrix. Smialek and Mitchell [36] investigated the interstitial hardening in Ta–N, Ta–C, and Ta–O systems. They revealed at 77 K that nitrogen exhibits the greatest hardening effect followed by carbon. The lowest hardening in Ta was caused by oxygen additions. Furthermore, oxygen contents between 20 and 180 ppm caused an almost equal increment of the yield strength. Similar results for a refractory metal system were found by Ulitchny and Gibala [37] for the Nb–O and Nb–N systems at 113 K. It was shown that nitrogen induced a significant hardening effect in Nb, whereas oxygen only causes a moderate increase of the hardness. It is not possible to investigate the hardening/softening effect of oxygen because there is no solubility in pure Mo [38]. Based on the findings of the Ta–O and Nb–O systems [36,37] it was assumed that the dissolved oxygen in the Mo–Hf alloys causes a slight shift to higher hardening values. Nevertheless, the hardness measurements clearly indicate a softening of Mo due to Hf additions up to 0.63 at.%. The lower contents of W and Zr in solid solution do not significantly affect the hardness of the alloys [2]. The hardness of polycrystalline metals with grains at the micron scale can be expressed with the Hall–Petch relationship as 0
1=2
H ¼ H0 þ kHP d
ð1Þ 0 kHP
where H0 is the intrinsic hardness, d is the grain size and is the grain size independent Hall–Petch coefficient [39]. Wesemann et al. 0 [40] determined a Hall–Petch coefficient of kHP ¼ 0:102 HV10 m1/2 for pure molybdenum at room temperature. Orava [41] investigated the effect of different grain sizes on the yield strength of molybdenum. The author revealed an insensitivity of the Hall–Petch
Table 3 Hardness values (HV10) at the testing temperatures (c.u. – cryogenic unit, h.u. – heating unit, w.u. – without unit).
Fig. 6. Hardness vs. testing temperature of the different alloys.
77 K
183 K
293 K
Alloy
c.u.
c.u.
c.u.
h.u.
w.u.
h.u.
443 K
Mo A1 A2 A3 A4 A5 A6
389 ± 2.8 367 ± 4.9 371 ± 2.9 382 ± 5.2 388 ± 4.0 401 ± 5.4 421 ± 5.3
280 ± 1.5 268 ± 1.0 274 ± 1.6 276 ± 1.8 281 ± 2.1 288 ± 2.3 323 ± 1.9
167 ± 1.0 163 ± 1.8 162 ± 1.4 165 ± 1.2 171 ± 1.3 214 ± 1.9 263 ± 1.9
166 ± 1.0 161 ± 1.3 162 ± 0.6 163 ± 2.1 172 ± 1.0 215 ± 2.1 266 ± 2.9
169 ± 0.9 162 ± 1.3 164 ± 1.2 165 ± 1.1 172 ± 0.8 217 ± 1.9 265 ± 1.5
84 ± 0.5 90 ± 0.7 95 ± 0.8 116 ± 0.8 137 ± 0.9 189 ± 1.7 232 ± 1.3
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coefficient at temperatures below 300 K. In the temperature range from 300 K to 450 K the coefficient decreases of approximately 15%. Several authors investigated the effects of substitutional solutes on the Hall–Petch coefficient, mostly in titanium alloys and austenitic steels [42–44]. Chia and co-workers [42] revealed an increase of the Hall–Petch coefficient of 20–30% (between 77 K and 300 K) through alloying titanium with 15.2 at.% Mo. Furthermore, this bcc b-Ti–Mo alloy exhibited a relatively temperature independent Hall–Petch coefficient. Werner [44] investigated austenitic steels with different alloying elements and contents. During highly variation of the Cr, Mn and Mo contents the Hall–Petch coefficient was not significantly changed. It can be seen that the effect of alloying elements on the Hall–Petch coefficient is more ambiguous. However, in the present study the softening effect was investigated 0 in dilute Mo–Hf alloys. Therefore, it was assumed that the kHP of the Mo–Hf alloys was not significantly different to that of pure Mo. Fur0 thermore, a slight variation of kHP has only a small influence on the 0 hardness due to the relatively low level of kHP from Mo [40]. Stephens and Witzke [7] have only investigated alloys above 0.90 at.% hafnium. The results of the hardness tests at temperatures below T < 0.15TM show a distinct softening behavior of the low concentrated alloys A1–A3 with a Hf content up to 0.63 at.%. Thus, the softening effect was not determined by Stephens and Witzke. However, Pink and Arsenault [6,13] mentioned a softening in the Mo–Hf system caused by the scavenging mechanism (extrinsic) but without any detailed results. Therefore, the atom probe tomography investigations were done to reveal a possible scavenging effect of hafnium, due to its affinity to carbon and oxygen. If the hafnium additions remove the interstitial atoms from the random solution a clustering of the relevant elements should occur. From the APT results it can be seen that Hf and the interstitial elements are homogeneously distributed. Furthermore, the technically pure Mo matrix does not even contain oxygen and carbon in solid solution which could be removed by hafnium additions. Therefore, the assumption from Pink and Arsenault [6,13] that scavenging occurs is refuted by the results of the present study. The characteristic of the hardness-concentration curves of Mo– Hf alloys (see Fig. 5), i.e. the low element concentration where SSS occurs and the value of softening, is similar to Mo–Ir and Mo–Pt alloys. This is in contrast to Mo–Re and Mo–Os alloys which exhibit a distinct softening at higher concentrations. A common characteristic of all elements (Pt, Ir, Re, Os and Hf) which cause SSS is the limited or intermediate solubility in Mo. On the other hand, elements (Ta, W) which only cause hardening show a complete solubility in Mo [7,45]. Medvedeva and co-workers [18] investigated the SSS of Hf among other solutes in bcc Mo alloys on the basis of ab initio electronic-structure calculations. They related the experimentally determined hardening effect of Hf, from Stephens and Witzke [7], to the increased generalized stacking fault energy in Mo–Hf alloys. Indeed, their calculations were dealing with medium solute concentrations clearly above 0.63 at.% Hf. This could be an indication that their model was not able to predict the SSS in Mo–Hf system. The low temperature plastic deformation in bcc metals and alloys is influenced by the Peierls potentials and the nucleation and migration of kinks. These intrinsic mechanisms are affected by solute atoms. Due to the fact that no scavenging was observed, the solid solution softening in dilute Mo–Hf alloys is attributed to an intrinsic mechanism.
5. Conclusions This study presents the experimental investigation of the solid solution softening effect of the Mo–Hf system. The aim was to clarify whether alloy softening is present and in consequence to determine an extrinsic behavior.
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Transmission and scanning electron microscopy investigations have shown that hafnium dioxide particles were present in the microstructure. Therefore, a part of the alloyed hafnium was not available for solid solution. Hardness tests at temperatures below T < 0.15TM revealed a solid solution softening effect. The maximum of alloy softening is limited to a Hf concentration of 0.15–0.28 at.%. Stephens and Witzke only investigated alloys above 0.90 at.% Hf [7]. Therefore, they could not determine the softening behavior in the Mo–Hf system. Atom probe tomography measurements have shown that no scavenging of the Mo matrix occurs during alloying with Hf. Thus, the assumption of Arsenault and Pink [6,13] was not correct. The mechanism of solid solution softening in the Mo–Hf system is related to an intrinsic mechanism. Atom probe experiments revealed an increasing oxygen content in solid solution with increasing Hf content. This result suggests that the dissolved Hf increases the oxygen solubility in Mo.
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