O. T. Inal

Kinetics of Layer Growth and Multiphase Diffusion in Ion-Nitrided Titanium

Pure titanium specimens were ion nitrided in a nitrogen plasma in the temperature range of 800 ‡C to 1080 ‡C at various nitrogen partial pressures. During ion nitriding, titanium nitrides TiN and Tiin2N and nitrogen solid solution layers (α and Β) were formed consistent with the equilibrium phase diagram. The kinetics of growth of these layers were studied as a function temperature and ion-nitriding parameters. An analytical diffusion model for multiphase diffusion was used to calculate the diffusion coefficients of nitrogen in the phases of the Ti-TiN system from layer growth experiments. Using the layer growth data, the temperature dependence of nitrogen diffusion in TiN (δ), Ti2N (ε), and α titanium was found to obey the following relations: Dinδ=4.4±1.62 × 10su-5 exp-36,500±1400/RT Dinε=2.7±1.05 ×10su-3 exp-35,760±2500/RT Dinα=0.96±0.08 × exp-51,280-505/RTConcentration profiles of nitrogen were measured in specimens ion nitrided at 900 ‡C. The profiles were in good agreement with the predicted ones at high nitrogen concentrations. How-ever, at low nitrogen concentrations, a deviation was observed, assumed to be due to a con-centration dependence of nitrogen diffusion in a titanium.

Formation and growth of iron nitrides during ion-nitriding

In order to clarify the formation and growth kinetics of iron nitrides, Fe2N (ζ), Fe2–3N (ɛ) and Fe4N (γ′), on the surface of iron during ion-nitriding, and their contribution to the mechanism of ion-nitriding coarse-grained specimens (2 to 6 mm) were ion-nitrided in pure nitrogen and nitrogen-hydrogen (20%–80%) plasma at temperatures between 500 and 600°C. Reflection electron diffraction (RED) showed immediate formation of the nitrides. Growth of these phases in latter stages of ion-nitriding was studied using optical microscopy and X-ray diffraction. The mechanism of the nitride layer formation is discussed and compared with the existing gas nitriding data. In the case region, Fe4N (γ′) and Fe16N2 (α″) precipitation was observed to occur under all experimental conditions studied. Case depth is seen to be parabolic with time and nitriding rate increases slightly when nitrogen-hydrogen plasma is used. Discussions are given to explain the difference in the nitriding efficiency under two different plasma compositions.

Ion nitriding behavior of several low alloy steels

Abstract The ion nitriding behavior of several low alloy steels, U.S. Steel T1 type A, AISI 4140, AISI 6150 and Nitralloy 135M, has been examined under varying process conditions using microhardness-depth correlations, friction and wear measurements, optical microscopy and transmission electron microscopy. The process variables included time (2–48 h) and temperature (400-300 °C). All four steels exhibited a diffusion layer case growth rate which was dependent on the square root of the exposure time. The highest nitrided hardnesses and lowest case depths were observed in the Nitralloy 135M steel while the lowest hardnesses and highest case depths were seen in the T1 steel. In general the behaviors of the T1, AISI 4140 and AISI 6150 steels were similar. The white layer thickness was independent of the material and increased with increasing treatment time. Both the case depth and the white layer thickness were found to increase strongly with temperature while the hardness decreased. Observations made by optical microscopy showed that only slight differences arise in the bulk structure during nitriding. The results of the electron microscope investigation show that nitriding produces platelet or disc precipitates approximately 5–20 nm across on {001} matrix planes. These platelets show a striated morphology which is thought to be the result of strain within the matrix. Electron microscopy has also shown tempering to continue in the core region during nitriding. The wear resistance of the steels was observed to increase by factors ranging from 3.7 to 8.5 after nitriding. The coefficient of friction was seen to increase in the nitrided samples, apparently because of the constraints of the test.

Direct observations of vacancies and vacancy-type defects in molybdenum following uniaxial shock-wave compression

Abstract Molybdenum sheet and small diameter wire samples were simultaneously shock-loaded to pressures of 150 and 250 kbar at a constant shock-pulse duration of 2 μsec. Thin foils and emission end-forms prepared from annealed as well as the shock-loaded samples were simultaneously examined by transmission electron and field-ion microscopy respectively. Dislocation loop densities were observed to increase from an annealed value of 5 × 10 9 −4 × 10 14 cm −3 and 7 × 10 14 cm −3 at 150 and 250 kbar, respectively, and the corresponding average loop diameters were measured to be 57, 104 and 152 A, respectively. A full determination of the relevant diffraction conditions and Burgers vectors of the loops in the electron microscope showed that 75% of the loops at 150 kbar and 80% of the loops at 250 kbar were vacancy type. By comparison, direct observations of vacancies and small vacancy aggregates in the field-ion microscope by systematic field-evaporation of atom layers showed a vacancy concentration of 1.2% in the annealed wires as compared with 2.2 and 6.0% vacancies in the 150 and 250 kbar shock-loaded wire samples respectively. Since the corresponding increase in dislocation density was determined to be 1.0 × 10 10 −1.2 × 10 10 cm −2 between 150 and 250 kbar for a concomitant increase in average residual microhardness from 514 to 770 kg/mm 2 in the same pressure range, it is shown that shock induced vacancies contribute significantly to residual shock hardening.

Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 Μm melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 Μm. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450‡C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.

Explosive welding of a near-equiatomic nickel-titanium alloy to low-carbon steel

Abstract Equiatomic and near-equiatomic NiTi alloys are very resistant to cavitation erosion compared with the alloys commonly used to construct pumps and hydroturbines. Thin layers (0.4-1.0 mm) of a near-equiatomic NiTi alloy were explosively welded to low-carbon steel substrates to fabricate high-strength, bimetallic tandems in which the NiTi provided resistance to cavitation damage and the low-carbon steel provided structural strength. Tensile lap-shear tests on the welded material revealed bond strength of up to 387 MPa. As-welded NiTi/steel tandems were less resistant to cavitation erosion than annealed, unwelded samples; however, a post-weld heat treatment at 500 °C recovered most of the lost resistance.

Structure and properties of ion-nitrided stainless steels

The structure and properties of ion-nitrided layers on several stainless steels, 410 martensitic stainless steel, 430 ferritic stainless steel and 321 austenitic stainless steel, has been studied under varying process conditions with microhardness-depth correlations, optical microscopy and transmission electron microscopy. The process variables studied include time (2 to 10 h) and temperature (400 to 600° C). The highest case depth values and hardness levels were observed in martensitic stainless steels. The lowest case depths were observed in austenitic stainless steel. In general, the behaviour of matensitic and ferritic stainless steels were similar. All three steels showed increasing case depths and decreasing surface hardnesses with increasing ion-nitriding temperatures and times. Nitriding depth was found to be parabolic with ion nitriding time in all three steels at all ion-nitriding temperatures investigated, the nitriding reaction being faster in martensitic stainless steel than the others. Electron microscopy showed that almost no structural difference arises in the core of ferritic and austenitic stainless steels whereas recrystallization of the martensitic structure was observed in the core of martensitic steel following ion nitriding. Electron microscopy results also showed that ion nitriding produces platelets or disc-shaped precipitates on {001} matrix planes, coherent with the matrix. These platelets showed a striated morphology which is thought to be the result of the elastic strain in the matrix.

High-temperature crystallization behaviour of amorphous Fe80B20

Samples of 0.003 in. round Fe80B20 amorphous wires were annealed in vacuo for 1 sec to 8 h periods at 780° C and the crystallinity induced in these wires from this heat treatment was studied through X-ray diffraction and field-ion microscopy. X-ray diffraction studies indicate that complete crystallinity is produced following 1 sec anneal at 780° C. However, the initial product is a primitive-tetragonal Fe3B phase unlike the body-centred tetragonal Fe3B observed in low-temperature isothermal transformation studies with this alloy. The Fe3B phase is seen to persist in the diffraction patterns for annealing durations of up to 15 min. Upon annealing for periods of up to 1 h, an intermediate three-phase structure consisting of α-Fe, Fe3B, and Fe2B is seen to result with a gradual decrease in the Fe3B phase corresponding to longer annealing durations. Anneals of more than 1 h at 780° C are seen to result in the disappearance of the Fe3B phase producing a two-phase microstructure consisting of α-Fe(b c c) and Fe2B (b c t). Field-ion-microscopy with a pure neon imaging gas at 78 K likewise indicates that existence of a three-stage phase structural change during the isothermal anneals, and the atomic arrangement of the various species are quite readily discernible because of the different symmetries contained in the three distinct phases.

On the transition from a waveless to a wavy interface in explosive welding

Abstract An analysis and critique are presented of the hydrodynamic theory for wave formation in explosive welding, as proposed by Cowan et al. Based on the recent advances in the understanding of this domain, the above-mentioned study is seen to support a different mechanism, dynamic plasticity, to explain wave generation at the interface of explosively welded couples. The transition from a waveless to a wavy interface was studied by means of impact welding for three different systems (CuCu, FeFe and AlAl). It was found that this transition is dependent on the collision angle and can be expressed by means of an elastic-plastic constant, based on the static properties of the materials. The criterion developed here can be extended to determine the waveless to wavy transition velocity for any system. The necessity for the development of a mathematical model that includes shock wave physics of the phenomena to explain the pulsating pressure instability and wave generation is also emphasized.

On whisker toughening in ceramic materials

Abstract Toughening achieved by incorporation of whiskers to ceramic matrices was calculated based on an approach that combines individual contributions from effective toughening mechanisms. The relative importance of each mechanism, among others, was found to substantially vary depending on the selection of whisker and composite parameters. Analysis of the effect of various parameters showed that whisker strength, pullout length, debond length, and volume fraction of whiskers are the most important parameters in whisker toughening. Very good agreement was found between predictions made by the combinatory approach and experimental toughness values of Al2O3/SiCw composites.