Fatigue of Ni-Ai-Mo aligned eutectics at elevated temperatures

The elevated-temperature mechanical behavior of two aligned eutectics (Ni-8.1 wt pct Al-26.4 wt pct Mo and Ni-6.3 wt pct Al-31.2 wt pct Mo) has been investigated utilizing monotonic and cyclic testing in vacuum. Tensile yield strength and fatigue resistance increased from 25 to 725 °C, but then were reduced at 825 °C. The fatigue lives of specimens tested at 725 °C decreased sharply with decreasing frequency. A shift from surface to internal crack initiation was observed upon increasing the test temperature from 725 to 825 °C. Stage II crack propagation was observed at both temperatures, in contrast to stage I cracking at 25 °C. The test results are compared to those for other nickel and cobalt-base aligned eutectics to show that the frequency effect on fatigue life is not limited to the Ni-AI-Mo system. formerly Graduate Assistant in the Department of Materials Engineering, Rensselaer Polytechnic Institute     

Deformation of metastable austenite and resulting properties during the ausform-finishing of 1 pct carburized AlSl 9310 steel gears

The process of ausform-finishing in gears involves the deformation of metastable austenite. A critical step in optimizing the deformation process is to determine the link between material deformation behavior and final material properties, such as hardness and microstructure. To this end, uniaxial compression testing was carried out on 1 pct carburized AISI 9310 steel specimens in the low-temperature ausforming regime (85 °C to 230 °C). The work-hardening response of metastable austenite and its relation to the hardness and microstructure was determined from these experiments. High work-hardening rates (work-hardening exponent n=0.4 to 0.7) were caused by deformation-induced transformation of metastable austenite to either martensite or bainite or both. It is postulated that, at the ausforming temperatures in the neighborhood of 230 °C, bainite formed at the highest achievable strains of 50 pct while oriented martensite (loading induced) was detectable at lower strains of 20 pct. The hardness of the resulting ausformed microstructure increased with degree of straining and with reduction in temperature of ausforming. An X-ray determination of the retained austenite content showed that austenite tends to stabilize even after minimal ausforming. A transmission electron microscopy study on ausformed specimens showed the presence of microtwinning and high-dislocation densities. The effect of processing parameters on fatigue response under rolling contact conditions is discussed given current understanding and available fatigue data.     

Effect of residual magnesium content on thermal fatigue cracking behavior of high-silicon spheroidal graphite cast iron

This study investigates the thermal fatigue cracking behavior of high-silicon spheroidal graphite (SG) cast iron. Irons with different residual magnesium contents ranging from 0.038 to 0.066 wt pct are obtained by controlling the amount of spheroidizer. The repeated heating/cooling test is performed under cyclic heating in various temperatures ranging from 650 °C to 800 °C. Experimental results indicate that the thermal fatigue cracking resistance of high-silicon SG cast iron decreases with increasing residual magnesium content. The shortest period for crack initiation and the largest crack propagation rate of the specimens containing 0.054 and 0.060 wt pct residual magnesium contents are associated with heating temperatures of 700 °C and 750 °C. Heating temperatures outside this range can enhance the resistance to thermal fatigue crack initiation and propagation. When thermal fatigue cracking occurs, the cracks always initiate at the surface of the specimen. The major path of crack propagation is generally along the eutectic cell-wall region among the ferrite grain boundaries, which is the location of MgO inclusions agglomerating together. On the other hand, dynamic recrystallization of ferrite grains occurs when the thermal cycle exceeds a certain number after testing at 800 °C. Besides, dynamic recrystallization of the ferrite matrix suppresses the initiation and propagation of thermal fatigue cracking.     

High Cycle Fatigue of (Fe,Ni, Co)3V type alloys

High cycle fatigue tests in vacuum have been performed on ordered (Fe, Co, Ni)₃V alloys between 25 °C and 850 °C. Heat-to-heat variations in fatigue properties of a Co-16.5 wtpct Fe-25 pct alloy, LRO-1, appeared to be due to differing quantities of grain boundary precipitates. Modification of this alloy with 0.4 pct Ti, to produce an alloy designated LRO-23, reduced the density of grain boundary precipitates and increased ductility, resulting in superior fatigue strength at high temperatures. The fatigue lives of LRO-1 and LRO-23 decreased rapidly above 650 °C, and increased intergranular failure was noted. The fatigue resistance of a cobalt-free alloy, Fe-29 pct Ni-22 pct V-0.4 pct Ti (LRO-37), was examined at 25 °C, 400 °C, and 600 °C; there was little evidence for intergranular fracture at any of these temperatures. Fatigue behavior of the LRO alloys is compared to that of conventional high temperature alloys.     

Phase transformations in Ti-6Al-4V-xH alloys

Microstructures, phases, and phase transformations in Ti-6Al-4V alloy specimens containing 0, 10, 20, and 30 at. pct hydrogen were investigated using optical microscopy (OM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and microhardness testing. Alloying with hydrogen was achieved by holding the specimens in a pure hydrogen atmosphere of different pressures at 780 °C for 24 hours. The phases present in the temperature range of 20 °C to 1000 °C were determined by microstructural characterization of the specimens quenched from different temperatures. Increasing the hydrogen addition from 0 to 30 at. pct lowered the beta-transus temperature of the alloy from 1005 °C to 815 °C, significantly slowed down the kinetics of the beta-to-alpha transformation, and led to formation of an orthorhombic martensite instead of the hexagonal martensite found in quenched specimens containing 0 pct H. A hydride phase was detected in specimens containing 20 and 30 at. pct hydrogen. The time-temperature-transformation (TTT) diagrams for beta-phase decomposition were determined at different hydrogen concentrations. The nose temperature for the beginning of the transformation decreased from 725 °C to 580 °C, and the nose time increased from 12 seconds to 42 minutes when the hydrogen concentration was increased from 0 to 30 at. pct.     

High-cycle fatigue of nickel-based superalloy ME3 at ambient and elevated temperatures: Role of grain-boundary engineering

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of cracks to failure due to high-frequency cyclic loading, remains a principal cause of failures in gas-turbine propulsion systems. In this work, we explore the feasibility of using “grain-boundary engineering” as a means to enhance the microstructural resistance to HCF. Specifically, sequential thermomechanical processing, involving alternate cycles of strain and annealing, was used to increase the fraction of “special” grain boundaries and to break up the interconnected network of “random” boundaries, in a commercial polycrystalline Ni-based superalloy (ME3). The effect of such grain-boundary engineering on the fatigue-crack-propagation behavior of large (∼8 to 20 mm), through-thickness cracks at 25 °C, 700 °C, and 800 °C was examined. Although there was little influence of an increased special boundary fraction at ambient temperatures, the resistance to near-threshold crack growth was definitively improved at elevated temperatures, with fatigue threshold stress intensities some 10 to 20 pct higher than at 25 °C, concomitant with a lower proportion (∼20 pct) of intergranular cracking.     

Effects of Fine Particle Peening Conditions on the Rotational Bending Fatigue Strength of a Vacuum-Carburized Transformation-Induced Plasticity-Aided Martensitic Steel

The effects of fine particle peening conditions on the rotational bending fatigue strength of a vacuum-carburized transformation-induced plasticity-aided martensitic steel with a chemical composition of 0.20 pct C, 1.49 pct Si, 1.50 pct Mn, 0.99 pct Cr, 0.02 pct Mo, and 0.05 pct Nb were investigated for the fabrication of automotive drivetrain parts. The maximum fatigue limit, resulting from high hardness and compressive residual stress in the surface-hardened layer caused by the severe plastic deformation and the strain-induced martensite transformation of the retained austenite during fine particle peening, was obtained by fine particle peening at an arc height of 0.21 mm (N). The high fatigue limit was also a result of the increased martensite fraction and the active plastic relaxation via the strain-induced martensite transformation during fatigue deformation, as well as preferential crack initiation on the surface or at the subsurface.     

Heat Treatment of Thixo-Formed Hypereutectic X210CrW12 Tool Steel

Steel is a particularly challenging material to semisolid process because of the high temperatures involved and the potential for surface oxidation. Hot-rolled X210CrW12 tool steel was applied as a feedstock for thixoforming. The samples were heated up to 1525?K (1250?°C) to obtain 30?pct of the liquid phase. They were pressed in the semisolid state into a die preheated up to 473?K (200?°C) using a device based on a high-pressure die casting machine. As a result, a series of main bucket tooth thixo-casts for a mining combine was obtained. The microstructure of the thixo-cast consisted of austenite globular grains (average grain size 46 ??m) surrounded by a eutectic mixture (ferrite, austenite, and M₇C₃ carbides). The average hardness of primary austenite grains was 470?HV_0.02 and that of eutectic 551?HV_0.02. The X-ray analysis confirmed the presence of 11.8?pct ??-Fe, 82.4?pct ??-Fe, and 5.8?pct M₇C₃ carbides in the thixo-cast samples. Thermal and dilatometric effects were registered in the solid state, and the analysis of curves enabled the determination of characteristic temperatures of heat treatment: 503?K, 598?K, 693?K, 798?K, 828?K, 903?K, and 953?K (230?°C, 325?°C, 420?°C, 525?°C, 555?°C, 630?°C, 680?°C). The thixo-casts were annealed at these temperatures for 2?hours. During annealing in the temperature range 503?K to 693?K (230?°C to 420?°C), the hardness of primary globular grains continuously decreased down to 385HV_0.02. The X-ray diffraction showed a slight shift of peaks responsible for the tension release. Moreover, after the treatment at 693?K (420?°C), an additional peak from precipitated carbides was observed in the X-ray diffraction. Thin plates of perlite (average hardness 820?HV_0.02) with carbide precipitates appeared at the boundaries of globular grains at 798?K (525?°C). They occupied 17?pct of the grain area. Plates of martensite were found in the center of grains, while the retained austenite was observed among them (average hardness of center grains was 512?HV_0.02). A nearly complete decomposition of metastable austenite was achieved after tempering at 828?K (555?°C) due to prevailing lamellar pearlite structure starting at grain boundaries and the martensite located in the center of the grains. The X-ray analysis confirmed the presence of 3.4?pct ??-Fe, 84.6?pct ??-Fe, and 12?pct M₇C₃ carbides. The dilatometric analysis showed that the transformation of metastable austenite into martensite took place during cooling from 828?K (555?°C). The additional annealing at 523?K (250?°C) for 2?hours after heat treatment at 828?K (555?°C) caused the precipitation of carbides from the martensite. After tempering at 903?K (630?°C), the thixo-cast microstructure showed globular grains consisting mainly of thick lamellar perlite of the average hardness 555?HV_0.02.     

Hardness of tempered martensite in carbon and low-alloy steels

This paper presents the results of a systematic study of the effect of carbon, manganese, phosphorus, silicon, nickel, chromium, molybdenum, and vanadium on the hardness of martensite in low to medium carbon steels tempered for one hour at 100°F (56°C) intervals in the range 400 to 1300°F (204 to 704°C). Results show that the as-quenched hardness depends solely on carbon content. On tempering, the effect of carbon on hardness decreases markedly with increasing tempering temperature. Studies of carbon-0.5 manganese steels showed that the incremental increase in hardness from 0.5 pct manganese after a given tempering treatment was independent of carbon content. Based on this result, studies of the effects of the other alloying elements were made using a 0.2 or 0.3 pct carbon, 0.3 to 0.5 pct manganese steel base composition. The hardness of the resulting tempered martensite was assumed to be due to a given alloy addition, and when two or more alloying elements were added, their effects were assumed to be additive. Each of the seven alloying elements increased the hardness of tempered martensite by varying amounts, the increase being greater as more of each element was present. Nickel and phosphorus have substantially the same effect at all tempering temperatures. Manganese has essentially the same hardening effect at any temperature in the range 700 (371°C) to 1300°F; silicon is most effective at 600°F (316°C), chromium at 800°F (427°C), molybdenum at 1000 to 1100°F (538 to 592°C), and vanadium at 1200°F (649°C). Using the data obtained, a procedure is established for calculating the hardness of tempered martensite for carbon and alloy steel compositions in the range studied and for any combination of tempering time and temperature. R. A. GRANGE was formerly with U. S. Steel Corporation (retired)     

Influence of cyclic deformation on surface microstructure and hardness of ion-implanted nickel

The effects of cyclic deformation on near-surface dislocation structure and hardness of ion-implanted nickel were characterized and correlated with the evolution of fatigue damage. Polycrystalline nickel fatigue specimens were implanted at 220 °C with 350 keV and 3 MeV nickel ions to a fluence of 1 × 10¹⁶ ions/cm² or at 220 °C and 500 °C with 350 keV aluminum ions to a fluence of 5 × 10¹⁷ ions/cm². Both the self-implantations and aluminum implantations approximately doubled the hardness of the near-surface region. During cyclic deformation, the near-surface regions of self-implanted specimens cyclically softened and formed clear channels through which subsurface persistent slip bands (PSBs) penetrated the implanted region. The near-surface regions of aluminum-implanted specimens maintained a high hardness during cyclic deformation and effectively suppressed the evolution of fatigue damage and extended fatigue life. The results of the study indicate that the cyclic stability of implantation-induced surface hardening is a key factor in the ability of an ion implantation treatment to suppress the evolution of fatigue damage. Formerly with The University of Michigan-Ann Arbor. Formerly with The University of Michigan-Ann Arbor.     

Influence of Temperature and Holding Time on the Interaction of V,Al, and N in Microalloyed Forging Steels

A medium-carbon vanadium microalloyed steel (38MnSiVS5) with three different aluminum levels (0.006, 0.020, and 0.03 wt pct) was used to examine the interaction of vanadium, aluminum, and nitrogen during the heating and cooling cycle for forging. The thermal cycle was simulated using a Gleeble^® 1500. Hold times varied from 5 to 45 minutes and temperature varied from 1323 K to 1523 K (1050 °C to 1250 °C). Thermal simulation specimens and as-received material were characterized by quantitative metallography, hardness, and chemical analysis of electrolytically extracted precipitates. The hardness was observed to be relatively constant for all aluminum levels after all thermal simulations at and above 1423 K (1150 °C). Hardness, pearlite fraction, and austenite grain size decreased with increasing aluminum content at the two lowest temperatures examined, which were 1323 K and 1373 K (1050 °C and 1100 °C). The amount of vanadium precipitated in the lowest aluminum steel was very consistent, approximately 70 pct, for the thermal simulations. The amount of precipitated vanadium decreased with increasing amount of aluminum nitride for the 0.03 wt pct Al level.     

Mechanical and Thermal Properties of Hot-Pressed ZrB2-SiC Composites

ZrB₂-SiC composites were hot pressed at 2473 K (2200 °C) with graded amounts (5 to 20 wt pct) of SiC and the effect of the SiC addition on mechanical properties like hardness, fracture toughness, scratch and wear resistances, and thermal conductivity were studied. Addition of submicron-sized SiC particles in ZrB₂ matrices enhanced mechanical properties like hardness (15.6 to 19.1 GPa at 1 kgf), fracture toughness (2 to 3.6 MPa(m)^1/2) by second phase dispersion toughening mechanism, and also improved scratch and wear resistances. Thermal conductivity of ZrB₂-SiC (5 wt pct) composite was higher [121 to 93 W/m K from 373 K to 1273 K (100 °C to 1000 °C)] and decreased slowly upto 1273 K (1000 °C) in comparison to monolithic ZrB₂ providing better resistance to thermal fluctuation of the composite and improved service life in UHTC applications. At higher loading of SiC (15 wt pct and above), increased thermal barrier at the grain boundaries probably reduced the thermal conductivity of the composite.     

Tempering behavior of iron-carbon sputter deposits with 0 to 5 wt pct C

Iron-carbon sputter deposits with 0.06, 0.18, 0.66, 2, 3, and 5 wt pct C were tempered at temperatures from 100° to 550°C. The 0.06, 0.18, and 0.66 wt pct C sputter deposits were similar to severely cold-worked martensite, both as-deposited and in tempering response. Specifically, these three deposits were much harder than water quenched steels of the same composition, and the deposit hardnesses decreased less than martensite hardnesses on tempering below 400°C. The hardnesses of the 2 and 3 wt pct C deposits increased 40 to 50 Dph units upon tempering at low temperatures (150° to 250°C) and decreased for higher tempering temperatures. The hardness of the 5 wt pct C deposit remained constant (920 Dph) after tempering at 150°C, but increased to 1170 Dph upon tempering at 250°C when monoclinic Hägg carbide (Fe₅C₂) formed. Cementite (Fe₃C) was the only other carbide detected in the tempered deposits, and it formed only at 475°C and above. The columnar grains of the sputter deposits transformed to equiaxed grains upon tempering above 250°C. This change in grain structure was due to recovery and not recrystallization. Some grain growth occurred in the 0.06, 0.18, 0.66, and 2 wt pct C deposits above 300°C, but the grain size of the 3 and 5 wt pct C deposits remained submicron. The hardnesses of the deposits after tempering at 550°C increased with carbon content, the 5 wt pct C deposit having the highest hardness (960 Dph) and the 0.06 wt pct C deposit the lowest (360 Dph).     

Phase relations and crystallographic data for the Pr-Zn system

Metallographic, thermal, X-ray, and resistivity data were employed in establishing the phase diagram of the Pr-Zn system. Eight compounds, three eutectics, and a eutectoid were observed. The compounds PrZn, PrZn₂, and Pr₂Zn₁₇ melt congruently at 882°, 898°, and 978°C, respectively. The compounds PrZn₃, Pr₃Zn₁₁, PrZn_4.46, Pr₃Zn₂₂, and PrZn₁₁ undergo peritectic decomposition at 833°, 855°, 891°, 956°, and 743°C, respectively. The eutectic temperatures and compositions in wt pct Zn are 576°C and 11.9 pct, 833°C and 39.0 pct, and 830°C and 56.8 pct. The eutectoid reaction occurs at 558°C and 5.2 pct Zn. The lattice parameters of the compounds in the system were determined using X-ray powder diffraction methods. Single crystal X-ray techniques were used to show that PrZn₃ has the YZn₃ (space group Pnma) type structure.     

The effect of aging and cold working on the high-temperature low-cycle fatigue behavior of alloy 800h: part ii: continuous cyclic loading

The individual and combined effects of cold working (5 and 10 pct) and aging (4000 and 8000 h in the temperature range 538 to 760 °C) on the high-temperature low-cycle fatigue behavior of alloy 800H have been investigated. The specimens were tested at the aging temperatures. Both the saturation stress range and the fatigue life were found to be history dependent. A history-independent hardening mechanism, dynamic strain aging, was found to operate over the temperature range ~450 to 650°C and to be maximized at ~55O °C. It is speculated that carbon is responsible for this dynamic strain aging. Finally, at temperatures above 538 °C the Coffin-Manson plots show a history-independent deviation from linearity. Formerly a Staff Scientist at General Atomic Co.     

Creep and fatigue crack propagation in a directionally-solidified carbide eutectic alloy

Creep and fatigue crack growth rates and threshold stress intensity amplitudes have been measured for a directionally solidified carbide-eutectic alloy, C73. Fatigue testing temperatures have been ambient, 750 and 950°C for cracking perpendicular and parallel to the solidification direction. In the former cracking direction comparative propagation rates may be understood in terms of the properties of the matrix, which shows a phase transformation from hexagonal to cubic above ~900°C. A situation where crack growth rates decrease with increasing apparent stress intensity amplitudes (ΔK) has been found to exist for propagation parallel to the solidification direction at low ΔK values and high temperatures only. This phenomenon can be related to the occurrence of crack branching and multiple cracking of the carbide fibers. Considerations of plastic zone sizes and critical defect sizes for crack propagation are consistent with the conditions necessary for such crack deceleration to occur. Transformation of the M₇C₃ fibers, present in the as-cast condition, to M₂₃C₆ at cell boundaries of the solidification structure occurs at a temperature of 950°C. Although M₂₃C₆ carbides are easily cracked and therefore probably reduce propagation rates by causing secondary cracking, their presence is known to be detrimental to creep properties. High cycle fatigue threshold stress intensity amplitudes for C73 in either loading direction at room temperature, 750 and 950°C are ~20 pct lower than for the cast nickel-base alloy, EST 738LC,i.e. critical defect sizes are ~10 pct smaller in C73. Despite the known sensitivity of cracking rates and threshold values to factors such as minor fluctuations in loading amplitude it is believed that these differences are significant.     

Effect of microstructure on the corrosion and deformation behavior of a newly developed 6Mn-5Cr-1.5Cu corrosion-resistant white iron

An experimental study has been made of the effect of heat treatment on the transformation behavior of a 4.8 pct Cr white iron, alloyed with 6 pct Mn and 1.5 pct Cu, by employing optical metallography, X-ray diffractometry, and differential thermal analysis (DTA) techniques, with a view to assess the suitability of the different microstructures in resisting aqueous corrosion. The matrix microstructure in the as-cast condition, comprising pearlite + bainite/martensite, transformed to austenite on heat-treating at all the temperatures between 900 °C and 1050 °C. Increasing the soaking period at each of the heat-treating temperatures led to an increase in the volume fraction and stability of austenite. M₃C was the dominant carbide present in the as-cast condition. On heat-treating, different carbides formed: M₂₃C₆ carbide was present on heat-treating at 900 °C and 950 °C; on heat-treating at 1000 °C, M₇C₃ formed and persisted even on heattreating at 1050 °C. The possible formation of M₅C₂ carbide in the as-cast and heat-treated conditions (900 °C and 950 °C) is also indicated. Dispersed carbides (DC), present in austenite up to 950 °C, mostly comprised M₃C and M₅C₂. On stress relieving of the heat-treated samples, M₇C₃-type DC also formed. The hardness changes were found to be consistent with the micro-structural changes occurring on heat-treating. The as-cast state was characterized by a reasonable resistance to corrosion in 5 pct NaCl solution. On heat-treating, the corrosion resistance improved over that in the as-cast state. After 4 hours soaking, increasing the temperature from 900 °C to 1050 °C led to an improvement in corrosion resistance. However, after 10 hours soaking, corrosion resistance decreased on increasing the temperature from 900 °C to 950 °C and improved thereafter on increasing the heat-treating temperature. Deformation behavior responded to the microstructure on similar lines as the corrosion behavior. Although in an early stage of development, the composition thus developed betters the performance of 22 pct Ni containing Ni-Resist irons as far as strength and freedom from pitting and graphitic corrosion are concerned; however, the corrosion resistance is somewhat lower. In conclusion, the usefulness of the different microstructures in attaining a useful combination of corrosion resistance and deformation behavior has been assessed. The data thus generated provide definite clues to developing new materials with improved performance for resisting aqueous corrosion in marine environments. Formerly Postdoctoral Candidate, University of Roorkee     

The effect of frequency on the cyclic strain and low cycle fatigue behavior of cast Udimet 500 at elevated temperature

An experimental investigation was undertaken on cast Udimet 500 to study principally the effect of frequency on low cycle fatigue at temperatures from 730° to 900°C, but mostly at 815°C (1500°F). Total strain range and stress range vs fatigue life relationships were determined which included the frequency following the form of author's recent work. The stress range was found to obey a power law relationship involving the fatigue life and frequency, such that at a specific stress range, a ten-fold decrease in frequency corresponded to a 3.7-fold decrease in life. While stress results were well-ordered with frequency, the scatter in the cyclic strain data was severe. A random crystallographic orientation of the few grains in the cross-section led to an anisotropy in the material which produced as much as a three-fold variation in diametral strain around the circumference. Results were compared to those for directionally solidified Mar-M200. In load-controlled applications cast U-500 gave better life, while in strain-controlled situations, the directionally solidified material excelled. ForN _f>1000 cycles, the difference in life was related to elastic modulus. Evidence for cyclic strain aging is given, the peak effect occurring at 790°C. This temperature corresponding to that where the well-documented tensile ductility minimum occurs.     

The Fe−Ho binary system

The Fe?Ho phase diagram was determined on the basis of data obtained by X-ray diffraction, metallographic and differential thermal analysis techniques. Since emphasis was centered in the region where intermetallic compounds predominate, neither the iron nor holmium terminal regions were included in this study. Eutectic reactions were found to occur at 16.5 wt pct Fe and 875° C, 61 wt pct Fe and 1284° C, and 79 wt pct Fe and 1338° C. The congruent melting points of the compounds Ho₆Fe₂₃ and Ho₂Fe₁₇ were found to be 1332° and 1343° C, respectively. Two other intermetallic compounds were found, HoFe₂ and HoFe₃, and had peritectic decomposition temperatures of 1288° and 1293° C, respectively.