The 1999 bend-test study of Kobayashi, Hiraoka, Nagae and Takada on TZM alloy (Mo–0.5 mass% Ti–0.08 mass% Zr with trace C) internally nitrided with N2 gas found that nitriding raises yield strength (by about 500 MPa and 750 MPa at controlled surface hardness of 250Hv and 300Hv, Vickers hardness scale) while leaving the critical stress, i.e. the grain-boundary fracture strength of about 2000–2050 MPa, and the transgranular fracture strength of about 2060–2070 MPa essentially unchanged; the reduced low-temperature ductility was attributed to a dispersion of very fine (about 5 nm) disk-shaped nitride particles inside the grains rather than to grain-boundary embrittlement. Two load-bearing elements of this account rest on inferences rather than direct observation: transmission electron microscopy was performed only on the 300Hv specimen, so the particle dispersion in the 250Hv material is inferred; and the absence of grain-boundary nitrogen segregation was inferred from the small intergranular fracture fraction and the unchanged critical stress, because no boundary-sensitive compositional measurement was made. Neither verification measurement has been reported for internally nitrided TZM in the subsequent literature. The problem therefore remains open: (a) direct nanoscale characterization of the nitride dispersion — presence, size distribution, and spatial distribution relative to grain boundaries — in the 250Hv single-step nitrided condition, and (b) direct measurement of nitrogen at grain boundaries of the nitrided alloy relative to the un-nitrided recrystallized material (oxygen being of additional interest in view of the source's cited mechanism of embrittlement by nitrogen alone or by combined nitrogen-and-oxygen segregation), which together would confirm or refute the proposed mechanism for the ductility loss.

Background

TZM (titanium–zirconium–molybdenum) alloy, of nominal composition Mo–0.5 mass% Ti, 0.08 mass% Zr with a trace of carbon, is the most widely used commercial molybdenum alloy, serving in heat-treatment furnace components and light-alloy casting dies. Like other body-centred-cubic refractory metals, molybdenum alloys are brittle below a ductile-to-brittle transition temperature (DBTT), so low-temperature toughness is a primary design limitation.

Internal nitriding is a heat treatment in which nitrogen diffuses into an alloy and precipitates sub-micron nitride particles in its interior; because Ti and Zr are strong nitride formers, dilute Mo–Ti alloys and TZM are natural hosts. Nagae and co-workers had shown (e.g. Nagae et al., Int. J. Refractory Metals and Hard Materials 16 (1998) 127–132) that nitriding dilute Mo–Ti alloys in N2 gas at 1373 K forms an internal nitrided zone without a hard surface nitride layer. The motivation for nitriding a molybdenum alloy is dispersion strengthening; the risk is grain-boundary embrittlement, because molybdenum is notoriously susceptible to intergranular (along-boundary) fracture driven by interstitial segregation: Auger electron spectroscopy, a surface-analytical technique reading the elemental composition of the outermost atomic layers of a freshly fractured surface, showed that nitrogen (together with sulfur and oxygen) segregates to molybdenum grain boundaries (Tuominen and Clough 1979; Kumar and Eyre 1980), and such segregation severely embrittles boundaries (Hiraoka, Edwards and Eyre 1993), whereas coarse nitride precipitates sitting on boundaries can strengthen them (Kurishita et al. 1990). Modern atom probe tomography — a microscopy technique that reconstructs the three-dimensional positions and elemental identities of individual atoms in a needle-shaped specimen — confirms that nitrogen, oxygen and phosphorus still segregate to random high-angle boundaries (boundaries of large crystallographic misorientation between adjoining crystals) in present-day molybdenum materials (Leitner et al. 2017). Whether a nitriding treatment embrittles or safely strengthens a molybdenum alloy therefore depends on where the nitrogen goes — into dispersed intragranular precipitates or onto boundary planes.

The fracture methodology of the Okayama school (Hiraoka and co-workers) characterizes low-temperature fracture by three-point bending over a range of temperatures. The critical stress σc\sigma_c is the temperature-insensitive fracture stress associated with grain-boundary-initiated fracture, read from the intersection of the temperature-dependent yield-strength and maximum-strength curves; the critical temperature TcT_c is the temperature below which the specimen fractures before general yielding (a DBTT-type measure, decreasing as ductility increases); PIF is the area fraction of the fracture surface that is intergranular, measured from scanning electron microscopy fractographs. An empirical relation PIF=k(b−σc)\mathrm{PIF} = k(b - \sigma_c) with k=0.2k = 0.2 for bend tests connects these quantities to the transgranular (grain-interior) fracture strength b≈σtgb \approx \sigma_{tg}.

In this setting, Kobayashi, Hiraoka, Nagae and Takada (1999) bend-tested vacuum-recrystallized TZM (1773 K for 3.6 ks, equiaxed grains of about 14 μm) and two internally nitrided conditions prepared from it: nitriding at 1373 K under 10 atm N2 for 57.6 ks, then grinding about 250 μm or about 200 μm off the nitrided surface to give surface hardness of about 250Hv ('250Hv material') and about 300Hv ('300Hv material') on the Vickers scale. Nitriding raised yield strength by about 500 MPa (250Hv) and 750 MPa (300Hv), left the critical stress (about 2000–2050 MPa, versus 1950 MPa recrystallized) and the transgranular fracture strength (about 2060–2070 MPa) essentially unchanged, kept fracture predominantly transgranular (PIF 1–8% versus 5% recrystallized), and shifted the critical temperature upward with hardness — a net loss of low-temperature ductility. Transmission electron microscopy of the 300Hv material showed disk-shaped particles about 5 nm in diameter dispersed in the molybdenum matrix and almost absent at and near grain boundaries. The authors concluded that the ductility loss is the consequence of intragranular nitride particles raising the yield strength while boundary and transgranular fracture strengths stay unchanged — explicitly inferring (not observing) that the 250Hv material contains the same dispersion, and inferring the absence of grain-boundary nitrogen segregation from the mechanical and fractographic evidence together with earlier bicrystal results, since no boundary-sensitive compositional measurement was performed. Directly verifying these two inferences is the problem addressed here.

Problem Statement

The problem is to experimentally establish the two unverified elements of the mechanism proposed by Kobayashi, Hiraoka, Nagae and Takada (1999) for the low-temperature behaviour of N2-gas internally nitrided TZM alloy. The material state is that of the source study: commercial TZM sheet (TZM: titanium–zirconium–molybdenum alloy, about 0.5 mass% Ti, 0.08 mass% Zr, trace C), vacuum-recrystallized at 1773 K for 3.6 ks (equiaxed grains of about 14 μm), internally nitrided at 1373 K under 10 atm N2 for 57.6 ks, with about 250 μm ground from the nitrided surface so that the tension-side surface hardness is about 250Hv (the '250Hv material').

(1) Directly characterize the second-phase particles in the 250Hv material: their presence, crystal structure and chemistry (TiN, (Ti,Zr)N solid solution, or molybdenum nitrides), their size distribution, and their spatial distribution relative to grain boundaries — thereby confirming, quantifying, or refuting the source paper's inference that the same intragranular dispersion of very fine disk-shaped particles (about 5 nm in the 300Hv material), essentially absent at and near grain boundaries, exists at the lower hardness level.

(2) Directly measure, by a boundary-sensitive compositional technique, the nitrogen content at grain boundaries of the internally nitrided TZM relative to the un-nitrided recrystallized reference — thereby testing, rather than inferring, the claim that grain-boundary segregation of nitrogen does not occur and does not contribute to the reduced low-temperature ductility (the critical temperature — the temperature below which the specimen fractures before general yielding in the bend test — being raised) of the nitrided alloy. Oxygen at grain boundaries is of additional interest, because the source's own discussion invokes the mechanism (Hiraoka et al.) of severe grain-boundary embrittlement by nitrogen alone or by combined nitrogen-and-oxygen segregation; oxygen data would strengthen the test but are not required for testing the nitrogen-segregation claim itself.

Together, the two measurements settle whether the low-temperature toughness of internally nitrided TZM is governed by intragranular nitride dispersion with unchanged boundary strength, as proposed in 1999, or whether a grain-boundary segregation contribution must be added to the mechanism.

References

  1. Makoto Kobayashi, Yutaka Hiraoka, Masahiro Nagae, Jun Takada, 'Low-Temperature Toughness of TZM Alloy which Internally Nitrided by N2 Gas', Journal of the Japan Society of Powder and Powder Metallurgy 46(7) (1999) 705–709. DOI: 10.2497/jjspm.46.705, https://doi.org/10.2497/jjspm.46.705
  2. Masahiro Nagae, Tetsuo Yoshio, Jun Takada, Yutaka Hiraoka, 'Improvement in Recrystallization Temperature and Mechanical Properties of a Commercial TZM Alloy through Microstructure Control by Multi-Step Internal Nitriding', Materials Transactions 46(10) (2005) 2129–2134. DOI: 10.2320/matertrans.46.2129, https://doi.org/10.2320/matertrans.46.2129
  3. Masahiro Nagae, Yoshito Takemoto, Tetsuo Yoshio, Jun Takada, Yutaka Hiraoka, 'Preparation of structurally controlled dilute molybdenum–titanium alloys through a novel multi-step internal nitriding technique and their mechanical properties', Materials Science and Engineering A 406(1–2) (2005) 50–56. DOI: 10.1016/j.msea.2005.06.039, https://doi.org/10.1016/j.msea.2005.06.039
  4. Teruki Hidaka, Masahiro Nagae, Tetsuo Yoshio, Jun Takada, Yutaka Hiraoka, 'Influence of Multi-Step Nitriding Condition upon Mechanical Properties of Dilute Mo-Ti Alloys', Materials Science Forum 426-432 (2003) 4461–4464. DOI: 10.4028/www.scientific.net/msf.426-432.4461, https://doi.org/10.4028/www.scientific.net/msf.426-432.4461
  5. Masahiro Nagae, Naoko Ise, Jun Takada, Yutaka Hiraoka, 'Multi-Step Internal Nitriding of Molybdenum-Zirconium Alloys', Journal of the Japan Institute of Metals and Materials 80(11) (2016) 702–706. DOI: 10.2320/jinstmet.j2016034, https://doi.org/10.2320/jinstmet.j2016034
  6. Masahiro Nagae, Shigetoshi Okada, Makoto Nakanishi, Jun Takada, Yutaka Hiraoka, Yoshito Takemoto, Moritaka Hida, Hideyuki Kuwahara, Myoun Ki Yoo, 'Nitriding of dilute Mo-Ti alloys at a low temperature of 1373 K', International Journal of Refractory Metals and Hard Materials 16 (1998) 127–132. DOI: 10.1016/s0263-4368(98)00005-5, https://doi.org/10.1016/s0263-4368(98)00005-5
  7. Y. Hiraoka, B.C. Edwards, B.L. Eyre, 'Effects of Nitrogen on Grain Boundary Fracture in Molybdenum', Materials Science Forum 126-128 (1993) 153–156. DOI: 10.4028/www.scientific.net/msf.126-128.153, https://doi.org/10.4028/www.scientific.net/msf.126-128.153
  8. Steven M. Tuominen, Stephen P. Clough, 'Grain boundary segregation of sulfur and nitrogen in sintered molybdenum', Metallurgical Transactions A 10 (1979) 127–129. DOI: 10.1007/bf02686418, https://doi.org/10.1007/bf02686418
  9. A. Kumar, Brian Leonard Eyre, 'Grain boundary segregation and intergranular fracture in molybdenum', Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 370 (1980) 431–458. DOI: 10.1098/rspa.1980.0043, https://doi.org/10.1098/rspa.1980.0043
  10. Hiroaki Kurishita, Osamu Tokunaga, Hideo Yoshinaga, 'Effect of Nitrogen on the Intergranular Brittleness in Molybdenum', Materials Transactions, JIM 31 (1990) 190–194. DOI: 10.2320/matertrans1989.31.190, https://doi.org/10.2320/matertrans1989.31.190
  11. K. Leitner (née Babinsky), P.J. Felfer, D. Holec, J. Cairney, W. Knabl, A. Lorich, H. Clemens, S. Primig, 'On grain boundary segregation in molybdenum materials', Materials & Design 135 (2017) 204–212. DOI: 10.1016/j.matdes.2017.09.019, https://doi.org/10.1016/j.matdes.2017.09.019
  12. Zhenxue Zhang, Xiaoying Li, Hanshan Dong, 'Response of a molybdenum alloy to plasma nitriding', International Journal of Refractory Metals and Hard Materials 72 (2018) 388–395. DOI: 10.1016/j.ijrmhm.2018.01.014, https://doi.org/10.1016/j.ijrmhm.2018.01.014
  13. M. Yu. Belomyttsev, 'A Study of Processes of Phase Formation and Hardening in Molybdenum Alloys After Internal Nitriding', Metal Science and Heat Treatment 60 (2018) 243–252. DOI: 10.1007/s11041-018-0268-4, https://doi.org/10.1007/s11041-018-0268-4