2015 EXCELLENCE IN METALLOGRAPHY AWARD

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1 EVOLUTION OF STRAIN-INDUCED PRECIPITATES IN A MOLYBDENUM-BASE Mo-Hf-C ALLOY David Lang,* Jürgen Schatte,** Wolfram Knabl,*** Roland Dallinger,**** Helmut Clemens***** and Sophie Primig****** INTRODUCTION The particle-hardened alloy Mo-Hf-C (MHC) is processed via a powder metallurgy (PM) route. It is known for its high strength at elevated temperatures and its high recrystallization temperatures. Its nominal composition of 0.65 at.% Hf and 0.65 at.% C has been derived from various investigations of arc melted and solution-annealed Mo-Hf-C alloys in the 1960s and 70s. In particular, an alloy with a similar composition to MHC exhibited superior properties after swaging. In contrast to PM processed MHC, all the hafnium and carbon content of the solution-annealed material is in solid solution. The carbon and the hafnium contents of MHC are adjusted in order to produce ~1 vol.% hafnium carbide.1 3 After sintering, the microstructure of MHC consists of a molybdenum matrix, hafnium-oxide particles (5 10 µm diameter), molybdenum carbides decorating the grain boundaries, and large hafnium carbides (1 µm diameter, ~80 nm thick). The residual hafnium content in solid solution is ~ at.% and the typical microporosity is ~4%.4,5 Furthermore, there is almost no carbon in solid solution due to the low solubility of carbon in molybdenum at temperatures below 1,600 C.5,6 Pöhl et al. revealed that the precipitation mechanism of small (nanometre-sized) hafnium carbides in MHC also differs from solution-annealed Mo-Hf-C alloys.5 It was shown that the as-sintered material has to be deformed prior to aging in order to produce a significant amount of small, strain-induced hafnium carbides. Dislocations, at the same time, act as heterogeneous nucleation sites and provide high-diffusivity paths for hafnium and carbon. For the formation of the small plate-like hafnium carbides, carbon is delivered by the dissolution of molybdenum carbide, which is thermodynamically less stable than hafnium carbide at the typical aging temperatures.5 However, there are no data available on the precipitation kinetics of small hafnium carbides. For a full exploitation of the precipitation potential of the MHC alloy, it is 2015 EXCELLENCE IN METALLOGRAPHY AWARD The powder metallurgy processed molybdenumbase alloy Mo-Hf-C (MHC) has excellent properties at elevated temperatures and is therefore used as a structural material for forging dies and rotating X-ray anodes. The nominal composition of this Mo-base alloy is 0.65 at.% Hf and 0.65 at.% C. The micro - structure in the as-sintered condition consists of a molybdenum matrix, hafnium-oxide particles (5 10 µm diameter), molybdenum carbide layers at the grain boundaries, and large, hafnium carbides (1 µm diameter, ~80 nm thick). The residual hafnium content in solid solution is ~ at.%. After deformation and subsequent aging of the as-sintered material, strain-induced plate-like hafnium carbides from 10 nm to 100 nm in diameter are formed. Dislocations act as heterogeneous nucleation sites, while carbon is delivered by the dissolution of molybdenum carbides. The aim of this work was to study the correlation between the compressive strength and the evolution of the size distribution of small, hafnium carbides for MHC specimens aged under different conditions. Scanning and transmission electron microscopy were applied for the micro - structural characterization, and the strength was determined via a deformation dilatometer. *PhD Student, ****Former Student, *****Professor, Head of the Department, Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben, Austria; **Research and Development, ***Head of Research and Development, Plansee SE, A-6600 Reutte, Austria, ******Lecturer, School of Materials, Science & Engineering, UNSW Sydney, Australia; david.lang@unileoben.ac.at 21

2 22 important to understand in detail the microstructural evolution, including the formation of precipitates, and recovery and recrystallization during thermo-mechanical processing. Therefore, the aim of this work was to study the evolution of the size distribution of the small hafnium carbides with scanning (SEM) and transmission electron microscopy (TEM). Furthermore, the particle size was correlated with the compressive strength of the differently aged samples by using deformation dilatometry. EXPERIMENTAL The powder mixture of molybdenum, hafnium hydride, and carbon black for the MHC sample material was cold isostatically pressed and sintered in a hydrogen atmosphere at temperatures above 0.80 T M, where T M is the melting point of molybdenum (2,620 C). The chemical composition of the as-sintered material is shown in Table I. For the determination of the compressive strength after thermo-mechanical processing, cylindrical samples (5 mm diameter 10 mm high) were tested in a deformation dilatometer DIL 805A/D as shown in Figure 1. The samples were heated at 10 K/s to the deformation temperature T Def and held for 5 minutes Subsequently, the first deformation step with a logarithmic deformation degree of φ = 0.35 and a strain rate of φ = 10/s was performed. Then the samples were heated at 100 K/s to an aging temperature of 1,350 C and aged for 1, 30, and 300 minutes. TABLE I. CHEMICAL COMPOSITION OF THE AS-SINTERED MHC SAMPLE MATERIAL (at.%) Element Mo Hf C O Content balance Figure 1. Schematic of the thermo-mechanical process applied in the deformation dilatometer For SEM and TEM investigations of the microstructural evolution the sample material was quenched to room temperature at 100 K/s after aging (blue line). In order to determine the compressive strength of the samples after aging, the samples were cooled to the deformation temperature T Def at 100 K/s and deformed for a second time with the above-mentioned parameters. For each set of parameters, at least two samples were tested. Directly after the second deformation, the samples were quenched to room temperature with gaseous nitrogen at a cooling rate of 100 K/s. Optical light microscopy (OLM) was used to observe the as-sintered microstructure. Prior to that, the material was ground and polished with standard polishing cloths and diamond suspensions down to 1 µm. Additionally, a polishing step with an alumina suspension (OPS) was performed before the microstructure was color etched with the Hasson etchant.4 For SEM microscopy the samples were electrolytically polished with a LectroPol-5 polisher after the polishing step with 1 µm diamond suspension, in order to remove superficial artifacts from the mechanical preparation. The SEM samples were investigated with 10 kv and a working distance of 5 mm with an FEI Versa Dual Beam microscope. For TEM investigations, discs with a diameter of 3 mm were ground and polished to a thickness of ~80 µm and then electrolytically polished with a TenuPol-5. These samples were investigated with a Philips CM12 TEM with an accelerating voltage of 120 kv. A detailed description of the sample preparation can be found elsewhere.4,5 The particle-size distribution of nanometre-sized hafnium carbides was determined using the Gatan Ditigtal Micrograph version software at an appropriate magnification; using 4 to 6 images. RESULTS AND DISCUSSION A characteristic microstructure of MHC after sintering is shown in Figure 2. Figure 2(a) shows an OLM image after color etching. The individual phases are marked with white arrows. Figure 2(b) shows an SEM image in electron-backscatter contrast (BSE) and in Figure 2(c) large, plate-like hafnium carbides can be observed in a TEM image. These hafnium carbides have a Backer-Nutting-type orientation relationship to the matrix that can be observed by the edge-on aligned particles that are perpendicularly oriented to each other.7,8 Typically, no particles smaller than the ones shown in these images can be found after sintering.4 In Figures 3(a) to 3(c), SEM images of the deformed and aged microstructures are shown in BSE contrast. The formation of a distinct sub-grain structure was revealed for every aging time. Such a structure is typi-

3 cal for bcc metals and is caused by extended recovery processes. Furthermore, it is known from published literature that in MHC only particle strengthening and the formation of a distinct sub-grain structure contribute to the increase in hardness and strength. Large hafnium carbides, however, have almost no contribution to the increase in the yield strength.9 Furthermore, no recrystallization occurred during aging of the samples, and similar sub-grain sizes and shape were observable with SEM. In Figures 4(a) to 4(c), TEM images of the microstructure are shown after aging for 1, 30, and 300 minutes. No small particles were observed after aging for 1 minute, but an increasing particle size was Figure 2. Images of the as-sintered MHC microstructure.(a) OLM image, (b) SEM image in BSE (backscattered electron) mode, and (c) TEM image of large, plate-like hafnium carbides. The different phases are marked with arrows Figure 3. SEM images in BSE contrast of the deformed and aged microstructure after aging for (a) 1 minute, (b) 30 minutes, and (c) 300 minutes. A distinct sub-grain structure can be observed for each individual aging condition 23

4 revealed after aging for 30 and 300 minutes. Similar to the large hafnium carbides, the small hafnium carbides also have a plate-like shape and an orientation relationship to the matrix.5,9 This can be observed especially in Figure 4(c), where the matrix is close to the <001> zone axis. The corresponding particle-size distributions for these aging times are shown in Figure 4(d). The particle size increases from 10 +/- 3 nm after aging for 30 minutes to 22 +/- 8 nm after aging for 300 minutes. In comparison with similar material systems with strain-induced precipitates, e.g., high-strength low-alloy steels, where niobium carbides in ferrite exhibit the same orientation relationship as hafnium carbide in molybdenum, the kinetics of the precipitation reaction are comparably slow here.10 A reason for this is that the carbon has to be provided by the dissolution of molybdenum carbide that is located at the original grain boundaries.5 The corresponding stress strain curves for the first deformation step of the as-sintered sample material and the second deformation step after additional aging for 1, 30, and 300 minutes are shown in Figure 5. The Figure 4. TEM images of MHC microstructure after deformation and aging for (a) 1 minute (b) 30 minutes, and (c) 300 minutes. (d) Corresponding particle-size distribution after aging for 30 and 300 minutes 24

5 CONCLUSIONS In order to study the evolution of small hafnium carbides in PM processed Mo-Hf-C alloy (MHC) and to establish a correlation with the obtained compressive strength, transmission and scanning electron microscopy investigations and compression tests were performed. The investigations revealed an ideal particle size of ~10 nm for precipitation hardening after an aging time of 30 minutes. Due to progressing recovery processes and increasing particle size with increasing aging time, the hardening effect drops significantly. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support provided by the Christian Doppler Forschungsgesellschaft. Figure 5. Corresponding stress strain curves for the first deformation step of the as-sintered samples and the second deformation step after additional aging for 1, 30, and 300 minutes at 1,350 C (see Figure 1) curves of all three first deformations are identical, but an increase in hardness can be observed for the second deformation after aging. The highest hardness was observed for an aging time of 30 minutes followed by 1 minute and 300 minutes at an aging temperature of 1,350 C. According to this result, it is suggested that the optimal particle-hardening size for MHC is below or around 10 nm. Additionally, recovery processes, which could lead to additional softening of the material, are regarded as negligible for such short aging periods. After 1 minute aging time no small hafnium carbides were found, which means that only dislocation hardening occurred. This is different after an aging time of 300 minutes. The stress-strain curve of this condition is clearly below the one after an aging time of 1 minute. Possible explanations for this behavior are an increasing size of small hafnium carbides, and progressing recovery processes that reduce the dislocation density. REFERENCES 1. P. Raffo, Thermomechanical Processing of Molybdenum- Hafnium-Carbon Alloys, 1969, Technical Report, Lewis Research Center NASA, Hampton, VA, USA. 2. W. Klopp, P. Raffo and W. Witzke, Strengthening of Molybdenum and Tungsten Alloys with HfC, JOM, 1971, vol. 23, no. 6, pp N. Ryan and J. Martin, The Formation and Stability of Group IVa Carbides and Nitrides in Molybdenum, J Less Common Met, 1969, vol. 17, no. 4, pp C. Pöhl, J. Schatte and H. Leitner, Metallographic Characterization of the Molybdenum Based Alloy MHC by a Color Etching Technique, Mater. Charact., 2013, vol. 77, pp C. Pöhl, D. Lang, J. Schatte and H. Leitner, Strain-Induced Decomposition and Precipitation of Carbides in a Molybdenum- Hafnium-Carbon Alloy, J. Alloy Comp., 2013, vol. 579, pp J.-O. Andersson, Thermodynamic Properties of Mo-C, Calphad, 1988, vol. 12, no. 1, pp N. Ryan, W. Soffa and R. Crawford, Orientation and Habit Plane Relationships for Carbide and Nitride Precipitates in Molybdenum, Metallography, 1968, vol. 1, no. 2, pp Z. Yang and M. Enomoto, Discrete Lattice Plane Analysis of Backer-Nutting Related B1 Compound/Ferrite Interfacial Energy, Mater. Sci. Eng. A., 2002, vol. 332, no. 1, pp C. Pöhl, D. Lang, J. Schatte and H. Leitner, Strengthening Mechanisms of the Molybdenum-Based Alloy MHC, Proc. 18th Int. Plansee Seminar, compiled by L. Sigl, H. Kestler and J. Wagner, Reutte, Austria, 2013, pp. RM75/1 RM75/ S. Hong, K. Kang and C. Park, Strain-Induced Precipitation of NbC in Nb and Nb-Ti Microalloyed HSLA Steels, Scripta Mater, 2001, vol. 46, no. 2. pp ijpm 25