The effect of trace element additions on the grain boundary composition of Ir + 0.3 pct W alloys

The grain boundary fracture surfaces of several Ir + 0.3 pct W alloys have been examined using Auger electron spectroscopy. Dopant additions (between 10 and 80 wt ppm) of Al, Fe, Th, Ni and Rh are shown to result in thorium enriched grain boundaries. Inert ion sputtering experiments suggest that the thorium enriched region at the grain boundaries is probably only a few atom layers thick. The other dopants (Al, Fe, Ni and Rh) do not appear to influence the segregation of thorium, and their function (if any) in improving the high temperature impact properties of this alloy is unclear at this time.     

Boron Segregation to Grain Boundaries and Improved Ductility in Pt + 30 Wt Pct Rh + 8WtPctW

Boron additions of up to 75 wt ppm have been observed to improve the room temperature strength and ductility of a Pt + 30 wt pct Rh + 8 wt pct W alloy. Alloys without boron fail intergranularly, and those with 75 wt ppm boron added fail in a mixed intergranular-transgranular mode. Auger electron spectroscopy on intergranular fracture surfaces indicated that boron segregates strongly to grain boundaries in the boron doped alloys. Transmission electron microscopy of alloys with and without boron indicated that both were free of internal precipitates. The observed improvements in strength and ductility appear to be related to boron enrichment within a few atomic distances of the grain boundary.     

Weld Metal Grain Structure and Mechanical Properties of a Th-Doped Ir-0.3 Pct W Alloy (DOP-26)

Weld metal grain structure and mechanical properties of the Ir-0.3 pct W alloy (DOP-26) doped with 60 ppm Th and 50 ppm Al have been investigated by use of a gas tungsten arc (GTA) welding process. The fusion zone grain structure is strongly influenced by heat input and puddle shape and therefore by the bead width. With increasing bead width from 2.5 to 3.7 mm, the grains in the fusion zone show a sharp change in growth direction near the centerline region and develop a fine columnar structure with grains growing parallel to the welding direction. Mechanical properties of the welds and base metal were characterized by tensile and impact tests from 650 to 1150 °C. The ductility and fracture behavior of DOP-26 welds are sensitive to weld bead width, postweld heat treatment, and weld-test orientation. The ductility of the welded specimens increases with increasing test temperature and decreasing weld bead width. The transverse weld specimen with a wide-bead width (3.7 mm) has the lowest impact ductility, and the longitudinal weld with a narrow-bead width (2.5 mm) has the highest elongation at all the test temperatures. The impact ductility of the transverse weld specimen with the narrow-bead width falls between the limits. All the results are discussed in terms of the fusion zone grain structure and fracture path of the welds.     

Influence of cerium additions on high-temperature-impact ductility and fracture behavior of iridium alloys

High-temperature tensile impact testing was carried out on Ir + 0.3 wt pct W alloys doped with cerium and thorium individually, and with cerium and thorium together. Impact ductility was evaluated as a function of grain size and test temperature. Cerium by itself was not as effective as thorium in improving the grain boundary cohesion, even though it segregated more strongly than thorium to the grain boundaries. This lower grain boundary cohesion was responsible for lower impact ductility and higher brittle-to-ductile transition temperature of cerium-doped alloys compared to those of the thorium- or thorium plus cerium-doped alloys. Reduction in thorium content by a factor of 5 (from 50 to 10 appm) in the bulk did not result in any significant reduction in hightemperature impact ductility or an increase in the brittle-to-ductile transition temperature as long as sufficient cerium was added to provide grain refinement. Grain boundary strengths of thorium- and thorium plus cerium-doped alloys were almost identical.     

Grain growth in lr- 0.3 Pct W alloys

The fuel cladding material for radioisotope heat sources operating above 1300°C is currently an Ir-0.3 pct W alloy. The ductility of this material under high-temperature impact accident conditions is reduced by large grain sizes. Knowledge of grain size as a function of time and temperature is required to determine the effects of high-temperature exposure during fabrication, assembly, ground testing, and operation of the heat source. The average grain diameter of three different heats of Ir-0.3 pct W is reported as a function of time at 1583 K (1310°C), 1688 K (1415°C), and 1808 K (1535°C) up to 2000 h. Growth rates were measured both perpendicular and parallel to the sheet surface in longitudinal cross sections of sheet samples. In samples with Th, Al, Fe, Ni, and Rh dopant additions, the grain structure was elongated and grain growth was retarded. Grain size was measured by the line in-tercept method and the data were described by the grain growth equation,d ² -d ₀ ² =kt. An activation energy for grain growth was determined for each heat, in both the perpendicular and parallel directions, and was found to be consistent with the expected activation energy for self-diffusion in the alloy. Abnormal grain growth was not observed within the time and temperature ranges of this experiment. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

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.     

Mechanical properties of Ir-Nb alloys containing Ni and Al

Two alloys made by adding 5 or 10 at. pct, respectively, of Ni-18.9 at. pct Al to an Ir-15 at. pct Nb alloy were investigated. The microstructure and compressive strength at temperatures between room temperature and 1800 °C were investigated to evaluate the potential of these alloys for ultra-high-temperature use. Their microstructural evolution indicated that the two alloys formed fcc and L1₂-Ir₃Nb two-phase structures. The fcc and L1₂ two-phase structures were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The 0.2 pct flow stresses were above 1000 MPa at temperatures up to 1200 °C, about 150 MPa at 1500 °C, and over 100 MPa at 1800 °C. The strength of the quaternary Ir-base alloys at 1200 °C was even higher than that of Ir-base binary and ternary alloys. And the strength of quaternary Ir-Nb-Ni-Al was equivalent to that of the Ir-15 at. pct Nb binary alloy at 1800 °C. The compressive ductility of quaternary (around 20 pct) was improved drastically compared with that of the Ir-base binary alloy (lower than 10 pct) and the ternary Ir-base alloys (about 11 pct). An excellent balance of high-temperature strength and ductility was obtained in the alloy with 10 at. pct Ni-18.9 at. pct Al. The effect of Ni and Al on the strength of the Ir-Nb binary alloy is discussed.     

An approach to developing an alternative hot work die steel

This paper evaluates an approach to developing a steel which combines resistance to softening on tempering with an economical use of alloy elements. To obtain the desired tempering behavior advantage has been taken of the ability of non-carbide forming elements to enhance the secondary hardening response and the tendency of vanadium additions to reduce the coarsening rate of Mo₂C. Five alloys were investigated; these were a base secondary hardening steel, that base steel modified by the addition of 2 wt pct silicon and by the combined addition of 1 wt pct silicon and 1 wt pct aluminum. To these two modified alloys were made additions of 0.4 wt pct vanadium. It was found that both types of additions without vanadium enhance the secondary hardening response to the same significant degree. Both of these alloys soften rapidly as the tempering temperature is increased above 600 °C. However, with the addition of vanadium, hardnesses over R_gC 50 are obtained after tempering at 650 °C. While silicon additions appear indispensable to this resistance to softening on tempering, silicon also favors the retention of primary carbides after austenitizing and, if present in sufficient amounts, can cause brittle intergranular fracture after tempering at high temperatures.     

Preparation and properties of fine grain β- CuAlNi strain- memory alloys

A method has been developed to produce grain sizes as small as 5 μm in alloys of β-CuAlNi. The alloys were of eutectoid composition and a procedure was developed for determining the composition of a eutectoid alloy having any required value for transition temperature (M _s ). The thermo-mechanical treatment involved two sequential stages of warm rolling followed by recrystallization. The alloys produced were single phase β-type with no second phase being present. Characteristic two-stage stress-strain curves were obtained for most of the specimens. It was generally found that the tensile strength and strain to failure increased with decreasing grain size according to a Hall-Petch type relationship down to a grain size of 5 μm. A fracture strength of 1200 MPa and a fracture strain of 10 pct were obtained in the best alloy. It was found that the major recovery mode, whether pseudoelastic or strain-memory, did not have any significant effect on the total recovery obtained. Recovery properties were not affected significantly by decreasing grain size, and 86 pct recovery could still be obtained at a grain size of around 10 μm. Grain refinement improved the fatigue life considerably, possibly due to the high ultimate fracture stress and ductile fracture mode. A fatigue life of 275,000 cycles could be obtained for an applied stress of 330 MPa and a steady state strain of 0.7 pct. At fine-grain sizes most of the fractures were due to transgranular-type brittle fracture and micro void-type ductile fracture, depending on the alloy composition. It was suggested that the difference between the alloys was due to differences in oxygen segregation at the grain boundaries.     

Design of quaternary Ir-Nb-Ni-Al refractory superalloys

We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1₂ two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir₃Nb-L1₂, and Ni₃Al-L1₂—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at. pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir₃Nb, and Ni₃Al—existed in alloys A and C and four-phase equilibria—fcc, Ir₃Nb, Ni₃Al, and IrAl-B2—existed in alloy B. The fcc/L1₂ coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys, such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature use is also discussed.     

High-strength Cu-Ni-Sn alloys by thermomechanical processing

The influence of prior cold work on the aging characteristics and mechanical properties response for copper-rich alloys in the Cu-Ni-Sn system has been investigated. It has been established^15,16 that there exists a spinodal mode of decomposition below a critical temperatureT _R, 200 to 300°C below the equilibrium phase boundary in this system. Significant age hardening response is observed in this region; however, fracture ductility is severely impaired due to a grain boundary precipitate network which develops after relatively short aging times. Cold work prior to low temperature aging is found to have relatively little influence on the incubation time for this embrittling network. It does, however, profoundly enhance the kinetics of the continuous (spinodal) transformation. It is observed that for broad variations in composition, critical combinations of prior cold work, aging time and temperature yield material with unique combinations of. yield stress and fracture ductility (for example, a Cu-9 wt pct Ni-6 wt pct Sn alloy may be processed to exhibit an 0.01 pct offset yield of 174,000 psi in conjunction with a 55 pct R.A. on fracture; significantly higher 0.01 pct offset yield values may be achieved at some reduction in fracture ductility for other NiJSn ratios). It is concluded that the resultant ductileJbrittle properties response is a consequence of a critical compctitive balance between amplitude development in the modulated structure and nucleation of the grain boundary network. The minimum level of prior cold work required to effect this balance in the Cu-9 wt pct Ni-6 wt pct Sn alloy is 75 pct R.A. The present levels of yield stressJfracture ductility values reported, to the best of our knowledge, are unsurpassed by those of any other copper-base alloy system (at a significant cost reduction to the Cu-Be alloys) and suggest the potential yet to be realized in other systems exhibiting this mode of decomposition.     

Microstructure effects on tensile properties of tungsten-Nickel-Iron composites

Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studiedvia slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties. Formerly Graduate Research Assistant at Rensselaer Polytechnic Institute.     

Tensile properties and creep behavior of a new Ti-Al-Nb intermetallic alloy with an O+<Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript> microstructure

A new Ti-Al-Nb alloy with a composition of Ti-27.5Al-13Nb (at. pct) was proposed. The density of this alloy was 4.7 g/cm³, which is about 13 pct lower than that for O+B2 alloys. After hot processing, the alloy was heat treated under two conditions: directly aged at 850 °C (DA treatment), or cooled from above the β-transus temperature with a cooling rate of 3 °C/min and then aged at 850 °C (BCA treatment). Under the present heat-treatment conditions, the phase constitution was primarily O+α ₂. A very fine Widmanstätten microstructure was obtained after the DA treatment, while a microstructure with coarse O plates was obtained after the BCA treatment. The tensile properties were investigated at 20 °C to 950 °C, and the creep behavior was investigated at 650 °C to 750 °C/90 to 380 MPa. The elongation to fracture at room temperature for the DA-treated tensile specimen was as high as 2.6 pct, despite the high Al content in this alloy. In comparison with the O+B2 ternary alloys, this alloy showed higher specific proof stress at temperatures over 800 °C and higher creep strength. The stress exponent and the apparent activation energy for creep were calculated. The fracture mechanism was discussed.     

Microstructures and mechanical properties of dispersion-strengthened high-temperature Al-8.5Fe-1.2V-1.7Si alloys produced by atomized melt deposition process

Dispersion-strengthened high-temperature Al-8.5 pct Fe-pct Si-pct V alloys were produced by atomized melt deposition (AMD) process. The effects of process parameters on the evolution of microstructures were determined using optical metallography and scanning and transmission electron microscopy. The extent of undercooling and the rate of droplet solidification were correlated with process parameters, such as melt superheat, metal/gas flow rates, and melt stream diameter. The size distribution and morphology of silicide dispersoids were used to estimate the degree of undercooling and the cooling rate as functions of process parameters. The tensile properties at 25 °C to 425 °C and fracture toughness at 25 °C of these alloys produced with wide variations in dispersoids size and grain size were determined. Under optimum conditions, the alloy has ultimate tensile strength of 281 MPa and 9.5 pct ductility in the as-deposited condition. Upon hot-isostatic pressing and extrusion, the ultimate tensile strength increased to 313 MPa and ductility increased to 18 pct.     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

Tensile flow and fracture behavior of

The tensile properties, fracture modes, and deformation mechanisms of two DO₃ alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600 °C. The first alloy was produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated formation of antiphase-boundary (APB) bands, while in the Fe-31Al alloy, plasticity occurred by the motion of superlattice dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage fracture modes. The variation of tensile properties and fracture modes with temperature is presented. Formerly with the Materials Laboratory of the Air Force Wright Formerly with the Materials Laboratory of the Air Force Wright Formerly with the Materials Laboratory of the Air Force Wright     

The effects of impurities and heat treatment on the internal friction of tungsten at high temperatures

An improved internal friction technique was used to study the relaxation processes in pure tungsten and the following alloys: commercially doped tungsten (218W), W-1 pct ThO₂, 218W-3 pct Re, and 218W-20 pct Re. Internal friction experiments were performed on worked and recrystallized specimens in the temperature range of 300° to 3000°K. The effects of impurities and alloying additions on the damping and recrystallization behavior of tungsten are demonstrated. It is further shown that the internal friction curves can conveniently be used to determine the temperature of primary and secondary recrystallization. The important effect of impurities on the microstructure and high temperature strength is also revealed by the temperature dependence of shear modulus.