Nitrogen solubility and aluminum nitride precipitation in liquid iron, Fe-Cr, Fe-Cr-Ni, and Fe-Cr-Ni-Mo alloys

The nitrogen solubility and aluminum nitride formation in liquid Fe-Al, Fe-Cr-Al, Fe-18 pct Cr-8 pct Ni-Al and Fe-18 pct Cr-8 pct Ni-Mo-Al alloys were measured by the Sieverts' method. The temperature range extended from 1823 to 2073 K, and the aluminum contents from 1.01 to 3.85 wt pct Al. Increasing aluminum content increases the nitrogen solubility. The effect of molybdenum additions was determined for 2, 4 and 8 wt pct Mo levels. The first and second order effects of chromium, nickel, molybdenum and aluminum on the activity coefficient of nitrogen in iron were determined. The first and second order effects of chromium, nickel and molybdenum on the activity coefficient of aluminum also were determined. The nitride precipitates were identified as stoichiometric aluminum nitride, AIN, by X-ray diffraction analysis. The lattice spacing was in good agreement with the ASTM standard patterns for AIN in both higher and lower Al content solutions. The solubility product of AIN increases with increasing aluminum concentration and with temperature in liquid iron and the iron alloys studied. However, the magnitudes of the solubility products of AIN in those alloys are different because of the effects of chromium and nickel additions. Additions of molybdenum show little effect on the solubility product of AIN. The standard free energy of formation of AIN in liquid iron is: δG‡ = -245,990 + 107.59 \T J/g-molAIN, based on the standard state of the infinitely dilute solution in liquid iron for aluminum and nitrogen, referred to a hypothetical one wt pct solution, and on the pure compound for A1N.     

Nitrogen solubility and aluminum nitride precipitation in liquid iron,Fe-Cr,Fe-Cr-Ni,and Fe-Cr-Ni-Mo alloys

The nitrogen solubility and aluminum nitride formation in liquid Fe-Al, Fe-Cr-Al, Fe-18 pct Cr-8 pct Ni-Al and Fe-18 pct Cr-8 pct Ni-Mo-Al alloys were measured by the Sieverts' method. The temperature range extended from 1823 to 2073 K, and the aluminum contents from 1.01 to 3.85 wt pct Al. Increasing aluminum content increases the nitrogen solubility. The effect of molybdenum additions was determined for 2, 4 and 8 wt pct Mo levels. The first and second order effects of chromium, nickel, molybdenum and aluminum on the activity coefficient of nitrogen in iron were determined. The first and second order effects of chromium, nickel and molybdenum on the activity coefficient of aluminum also were determined. The nitride precipitates were identified as stoichiometric aluminum nitride, AIN, by X-ray diffraction analysis. The lattice spacing was in good agreement with the ASTM standard patterns for AIN in both higher and lower Al content solutions. The solubility product of AIN increases with increasing aluminum concentration and with temperature in liquid iron and the iron alloys studied. However, the magnitudes of the solubility products of AIN in those alloys are different because of the effects of chromium and nickel additions. Additions of molybdenum show little effect on the solubility product of AIN. The standard free energy of formation of AIN in liquid iron is: δG? = -245,990 + 107.59 \T J/g-molAIN, based on the standard state of the infinitely dilute solution in liquid iron for aluminum and nitrogen, referred to a hypothetical one wt pct solution, and on the pure compound for A1N.     

Nitrogen solution and titanium nitride precipitation in liquid Fe-Cr-Ni alloys

The solubility of nitrogen in liquid Fe-Cr, Fe-Ni, Ni-Cr, and Fe-Cr-Ni alloys up to 20 wt pct Ni and 40 wt pct Cr was measured by the Sieverts’ method. The first and second order interactions in iron between nitrogen and chromium, and nitrogen and nickel were determined. Chromium increases the nitrogen solubility at lower chromium concentrations but the second order interaction term which is of the opposite sign becomes significant at higher chromium levels and compensates partly for the effect of the first order interaction term. Nickel decreases the nitrogen solubility in iron. Titanium nitride formation in liquid Fe-Cr, Fe-Ni, and Fe-Cr-Ni alloys also was investigated. The first and second order interactions between titanium and chromium or nickel were determined at 1600°C. Chromium increases the solubility product of TiN, principally by decreasing the activity of nitrogen in the melt. Nickel decreases the solubility product of TiN by increasing the activities of nitrogen and titanium.     

Nitrogen solubility in liquid Fe-V and Fe-Cr-Ni-V alloys

Nitrogen solubility in liquid Fe, Fe-V, Fe-Cr-V, Fe-Ni-V and Fe-18 pct Cr-8 pet Ni-V alloys has been measured using the Sieverts’ method for vanadium contents up to 15 wt pct and over the temperature range from 1775 to 2040 K. Nitrogen solution obeyed Sieverts’ law for all alloys investigated. Nitride formation was observed in Fe-13 pet V, Fe-15 pet V and Fe-18 pet Cr-8 pet Ni-10 pet V alloys at lower temperatures. The nitrogen solubility increases with increasing vanadium content and for a given composition decreases with increasing temperature. In Fe-V alloys, the nitrogen solubility at 1 atm N₂ pressure is 0.72 wt pet at 1863 K and 15 pct V. The heat and entropy of solution of nitrogen in Fe-V alloys were determined as functions of vanadium content. The first and second order interaction parameters were determined as functions of temperature as: $$e_N^V = \frac{{ - 463.6}}{T} + 0.148 and e_N^{VV} = \frac{{17.72}}{T} - 0.0069$$ The effects of alloying elements on the activity coefficient of nitrogen were measured in Fe-5 pet and 10 pet Cr-V, Fe-5 pet and 10 pet Ni-V and Fe-18 pet Cr-8 pct Ni-V alloys. In Fe-18 pet Cr-8 pet Ni-10 pet V, the nitrogen solubility at 1 atm N₂ pressure is 0.97 wt pet at 1873 K. The second order cross interaction parameters, e _N ^Cr,V and e _N ^Ni,V , were determined at 1873 K as 0.00129 and ? 0.00038 respectively.     

Effect of titanium on the nitrogen solubility in complex liquid Fe−Cr−Ni alloys

The effects of dilute additions of titanium up to 0.20 wt pct on the solubility of nitrogen in two complex Fe−Cr−Ni alloys were examined over the temperature range 1450 to 1600°C. Sieverts' law was obeyed by all titanium-bearing alloys up to some nitrogen pressure below one atmosphere. ‘Breaks’ in each solubility plot were observed that corresponded to the formation of titanium nitride. Titanium additions were observed to lower the nitrogen solubility in each group of alloys. This effect is opposite to that previously observed in pure iron. Calculated values of the solubility product (pct Ti) (pct N) for TiN formation in each alloy increased with rising melt temperature.     

Nitrogen solubility and nitride formation in austenitic Fe-Ti alloys

The equilibrium nitrogen solubility and nitride formation in austenitic Fe and Fe-Ti alloys were measured in the temperature range from 1273 to 1563 K. Specimens 0.5 mm thick were equilibrated with four different nitrogen-argon gas mixtures containing 1 pct hydrogen. The nitrogen solubility in austenitic iron obeys Sieverts' law. The equilibrium nitrogen content was determined to be log (wt pct N)_γ-Fe, P_{N2=1 atm} = (539 ± 17)/T − (2.00 ± 0.01). The precipitated titanium nitride was identified as cubic TiN, and the solubility product was determined to be log(wt pct Ti) (wt pct N) = −14,400/T + 4.94.     

Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys

The thermodynamics of titanium in Fe-Cr alloys and of inclusion formation in Fe-Cr-N-Ti alloys was investigated. A metal-nitride-gas equilibration technique was used to measure the activity of titanium. The equilibrium titanium content of the metal that is in equilibrium with pure solid titanium nitride and nitrogen gas at 1 atm was determined. The activity coefficients of titanium it(f_Ti) relative to 1 wt pct standard state in Fe were calculated for Fe-Cr alloys from the experimental results. The first-order interaction coefficient between titanium and chromium, e _Ti ^Cr , was determined to be 0.024 at 1873 K. The solubility of nitrogen in Fe-Cr alloys was measured and was found to increase with chromium content, which is in agreement with previous work. Thermodynamic calculations were made in order to predict under what conditions titanium nitride will form in 409 stainless steel and was compared with inclusions found in plant samples. The inclusion stability diagrams for 304 stainless steel and Fe-18 pct Cr and Fe-9 pct Cr alloys were computed.     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at 1873 K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logJn, 1873K = 0 to −1.4 as, log(wt pct N)_T = (-247/T-1.22)-(4780/T-1.51) (logf N, 1873K)- (1760/T-0.91) (logfN,1873K)².     

Nitrogen solubility in liquid Fe-Ta,Fe-Cr-Ta,Fe-Ni-Ta,and Fe-Cr-Ni-Ta alloys

The nitrogen solubility in liquid Fe-Ta, Fe-Cr-Ta, Fe-Ni-Ta, and Fe-18 pet Cr-8 pet Ni-Ta alloys was measured using the Sieverts’ method. The experiments covered the temperature range from 1782 to 2031 K, and tantalum contents from 2.0 to 20.0 wt pct Ta. Nitrogen solution obeyed Sieverts’ law and no nitride precipitation was observed in this concentration range. Tantalum increases the nitrogen solubility and the heat of solution of nitrogen is more negative at higher tantalum contents in these alloys. The excess enthalpy and entropy of solution of nitrogen were determined. The first and second order interaction parameters between nitrogen and tantalum were determined as a function of temperature, e _N ^Ta = -101.7/T + 0.018 and e _N ^TaTa = -3.27/T + 0.0022. The effects of alloying elements on the activity coefficient of nitrogen were measured and the second order cross-interaction parameters between nitrogen and Ta with Cr and Ni were determined at 1873 K as e _N ^CrTa = 0.00052 and e _N ^NiTa = 0.00045.     

Solubility of nitrogen in liquid Fe-Cr-Ni alloys containing manganese and molybdenum

The nitrogen solubility in liquid Fe-Cr-Ni alloys containing Mo or Mn was determined by the Sieverts’ method. The first and second order mutual interactions among nitrogen, chromium, nickel, molybdenum, and manganese in iron were determined as a function of temperature. The heat and entropy of solution in these alloys were correlated as functions of the logarithm of the activity coefficient of nitrogen at {dy1873} K independent of the composition of the alloys. An equation was derived to predict the nitrogen solubility in liquid multicomponent iron alloys for the range from logf_N, 1873K = 0 to -1.4 as, log (wt pct N)_T = (-247/T - 1.22) - (4780JT - 1.51) (logf_n, 1873K)-(1760/T -0.91) (logf_N,{dy1873}K )².     

The effect of nitrogen on stacking fault energy of Fe-Ni-Cr-Mn steels

The effect of nitrogen content on stacking fault energy (SFE) has been measured in a series of Fe-21Cr-6Ni-9Mn alloys. Stacking fault energies were determined from node measurements using weak beam imaging techniques in transmission electron microscopy. Nitrogen additions lower the SFE from 53 mJ/m² at 0.21 wt pct to 33 mJ/m² at 0.24 wt pct. Further increases to 0.52 wt pct do not markedly change the SFE. Carbon and silicon had no effect on SFE in the ranges 0.010 to 0.060 wt pct C and 0.17 to 0.25 wt pct Si. The shift in SFE from 0.21 to 0.24 wt pct N is accompanied by a transition to a more planar plastic deformation mode. The sharp transition precludes the use of linear regression analysis for relating SFE to nitrogen content in this class of alloys.     

Thermodynamics of titanium and nitrogen in an Fe-Ni melt

The equilibrium solubility of titanium and nitrogen in Fe-Ni melts was measured in the presence of pure solid TiN under various nitrogen pressures in the temperature range of 1843 to 1923 K. The activity coefficients of titanium and nitrogen relative to a 1 mass pct standard state in liquid iron were calculated from the experimental results for Fe-Ni alloys of nickel contents up to 30 mass pct. Nickel decreases the activity coefficient of titanium, but it increases the activity coefficient of nitrogen in an Fe-Ni-Ti-N melt. Therefore, the effect of nickel on the solubility product of TiN is not significant. The first- and second-order interaction parameters of nickel on titanium (e _Ti ^Ni and r _Ti ^Ni , respectively) were determined to be −0.0115 and 0 at 1873 K, respectively. Similarly, the interaction parameters of nickel on nitrogen (e _N ^Ni and r _N ^Ni , respectively) were determined to be 0.012 and 0, respectively, at 1873 K. The temperature dependence of these interaction parameters was also determined.     

The solubility of nitrogen in liquid pure nickel

A constant-volume Sieverts’ method was used to determine the solubility of nitrogen in liquid nickel. This method has been used for the first time on this type of material. It was found that the solubility of nitrogen in pure nickel at 1600 °C and 1 atm is equal to 0.0020 wt pct (with an experimental error of ± 0.0002 pct). The solubility increases with increasing temperature. The temperature coefficient of nitrogen solubility within the temperature range from 1500 °C to 1750 °C is equal to 1.6 × 10⁻⁶ wt pct/°C. The solution of nitrogen in pure nickel at 1600 °C and pressures up to 1 atm was found to obey Sieverts’ law.     

Thermodynamics of carbon in nickel,iron-nickel and iron-chromium-nickel alloys

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH₄-H₂ gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.     

The apparent solubility of aluminum in cryolite melts

The apparent solubility of aluminum in cryolite melts saturated with A1₂O₃ has been determined by titration with electrolytically generated O₂. The results may be expressed by wt pct Al = − 0.2877 + 0.0268 (NaF/AlF₃ wt ratio) + 2.992 × 10⁻⁴ (temp °C) − 0.00192 (% CaF₂) −0.00174 (% Li₃AlF₆) −0.00288 (% NaCl) with a standard deviation of ±0.017. Ranges covered were ratio 0.8 to 2.3, temperatures 969° to 1054°C, CaF₂ ≤ 14 pct, Li₃AlF₆ ≤ 20 pct, and NaCl ≤ 10 pct. There was no significant effect of adding 0 to 38. pct K₃A1F₆ or 0 to 10 pct MgF₂. It was found that solubility was approximately proportional to activity of aluminum when Al-Cu alloys were used. Possible mechanisms of solution are discussed. Monovalent aluminum is ruled out on the basis of the variation of solubility with NaF/AlF₃ ratio and a_Al. The favored, but not proven, mechanism involves formation of both sodium atoms and a colloidal dispersion of aluminum.     

Hillock-free aluminum thin films for electronic devices

The addition of an alloying element was proposed in order to suppress hillocks which grow on the surface of deposited aluminum conductors after they have been subjected to thermal cycling (200°C to room temperature) or high temperature heat treatment (400°C). The alloying element, tin, which has a small diffusion coefficient, a large binding energy with a lattice vacancy and a large atomic diameter, and manganese, which has a relatively small solid solubility into aluminum, were evaluated since they also have proper values of vapor pressure for ease of evaporation with aluminum. Alloy composition was determined to be just above the solid solubility of the element in aluminum at the deposition temperature of 350°C. It was proved experimentally that Al-0.06 wt pct Sn alloy and Al-0.1 wt pct Mn alloy, which had been selected for the abovementioned reason, had a marked effect in suppressing the growth of hillocks.     

Characterization of stainless steels melted under high nitrogen pressure

Mechanical properties of stainless steels increase with increasing nitrogen concentration. Currently, the maximum nitrogen concentration in commercial stainless steels is 0.8 wt pct. In this study, type 304 and 316 stainless steels were melted and cooled in a hot-isostatic-pressur(HIP) furnace using nitrogen as the pressurizing gas, producing alloys with nitrogen concentrations between 1 and 4 wt pct. These nitrogen levels exceeded the alloys’ solubility limits, resulting in the formation of nitride precipitates with several different microstructures. A new phase diagram for high nitrogen stainless steel alloys is proposed. Several properties of these nitrogen stainless steel alloys with chromium nitrides present were studied: tensile strength was proportional to the interstitial nitrogen concentration; hardness, wear, and elastic modules were proportional to the total nitrogen concentration. Formerly Research Scientist, National Institute of Science and Technology, Boulder, CO, is retired.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

The effects of molybdenum and aluminum on the thermal expansion coefficients of nickel-base alloys

The effects of molybdenum and aluminum on the mean linear thermal expansion coefficients from room temperature to 1050°C were determined for two types of nickel-base alloys. The Solid Solution Alloys were cast and homogenized Ni-Co-Cr-Mo alloys with 0, 312, and 612 nominal wt pct molybdenum concentrations. The Gamma Prime Alloys were wrought and heat-treated Ni-Cr-Mo-Al(Ti) alloys with 0, 2, 5, and 8 nominal wt pct molybdenum in each of four aluminum plus titanium levels (3 pct Al, 412 pct Al, 6 pct Al, or 1 pct Al + 312 pct Ti nominal wt pct). Thermal expansion coefficients were determined on at least two specimens from each alloy. It was found that molybdenum lowers the thermal expansion coefficients of both the cast Ni-Co-Cr solid solutions and the wrought Ni-Cr-Al(Ti) two-phase alloys. Both aluminum and titanium were also observed to decrease expansion coefficients in the two-phase, γ + γ, alloys. Results are discussed in terms of relative melting point effects between solute and solvent elements, and in terms of the volume fraction of the γ phase present.     

Nitrogen solubility in liquid manganese and ferromanganese alloys

The nitrogen solubilities in liquid manganese, manganese-iron, manganese-carbon, and manganese-iron-carbon alloys have been measured by the gas-liquid metal equilibration technique in the temperature range of 1623 to 1823 K. The equilibrium nitrogen content in pure liquid manganese at an atmospheric nitrogen pressure is high, and it does not follow Sievert’s law, i.e., f _N is not unity. The reduced nitrogen partial pressures by dilution with argon enabled us to obtain more reliable information on the thermodynamics of nitrogen in liquid manganese. The nitrogen dissolution follows Sievert’s law at nitrogen contents below 1 wt pct. The standard free-energy change for the dissolution of nitrogen in pure liquid manganese has been determined as −67,222+30.32T J/g atom, with the standard state of nitrogen taken as a 1 wt pct solution. Carbon and iron in manganese-rich melts decrease the nitrogen solubility significantly. The first- and second-order interaction parameters between nitrogen and other elements in manganese alloy melts have been determined. The activity coefficient of nitrogen in a ferromanganese alloy melt can be expressed as