The effect of mischmetal addition on the structure and mechanical properties of a cast Al-7Si-0.3Mg alloy containing excess iron (up to 0.6 Pct)

The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356) alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 °C) were evaluated. A structure-property correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume percentage of Fe-bearing intermetallic compounds (β and π phases), contributing to the loweryield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg; and improve the mechanical properties of the alloys both at room and elevated temperatures.     

The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al₉FeMg₃Si₅) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al₅FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

Microstructural refinement of an As-cast Al-12.6 wt pct Si alloy by repeated thermomechanical treatment to produce a heavily deformable material

A certain degree of cold working is advantageous in developing a fine microstructure with minute silicon crystals for eutectic and/or hypereutectic Al-Si cast alloys. A novel process, repeated thermomechanical treatment (RTMT), was applied to an Al-12.6 wt pct Si cast alloy. The process involves multiple-pass cold working (less than a 20 pct reduction in section area) and heat treatment at 793 K for 3.6 ks. Cold-work annealing was repeated up to about an 80 pct reduction in section from the beginning. The RTMT material showed a refined microstructure with high ductility. Most silicon crystals were fragmented to only a few micrometers and were spheroidized. The RTMT material showed such marked plasticity that it could be wrought up to a 99 pct reduction in section at room temperature. The Cold-worked RTMT materials exhibited an excellent balance between tensile strength and elongation and a higher strain hardening than the cast material.     

The effect of matrix reinforcement reaction on fracture in Ti-6Ai-4V-base composites

Samples of Ti-6A1-4V containing 10 vol pct of either TiC or SiC have been tested in tension at temperatures up to 760 °C, and the mechanical properties have been compared with those of the unreinforced matrix alloy. It has been found that the yield stress and the tensile strength of the TiC-containing composite are superior to those of the SiC composite at room temperature but that this behavior is reversed at the higher temperatures. The ductility of the TiC composite is about 2 pct at room temperature and increases with increase of temperature. No ductility is found for the SiC composite at room temperature, but some ductility is observed at the higher temperatures. These observations are interpreted in terms of the extent and nature of the reaction zones between the matrix alloy and the reinforcement and in terms of the failure mechanisms observed using scanning (SEM) and transmission electron microscopy (TEM). D.G. KONITZER, formerly with the Aluminum Company of America, Alcoa Center, PA. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

On the development and investigation of quaternary Pt-based superalloys with Ni additions

The objective of this work is to mimic the microstructure and strengthening mechanisms of Ni-based superalloys in a new group of high-temperature alloys based on the system Pt-Al. The elements Cr and Ni were chosen as further alloying components. Having a face-centered cubic (fcc) crystal structure with an Ll₂-ordered and coherently embedded phase, these new alloys should increase creep and corrosion resistance beyond Ni-based superalloys. After arc melting and heat treatment, the alloys were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the aged condition, the alloy composition 13 at. pct Al, 3 at. pct Cr, 7 at. pct Ni, and balance Pt showed the most promising microstructure with cubical precipitates, 30 pct precipitate volume fraction, and a lattice misfit of about −0.1 pct at room temperature. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Influence of calcium on the microstructure and properties of an Al-7Si-0.3Mg-xFe alloy

The effect of Ca addition on the microstructure, physical characteristics (density/porosity), and mechanical properties (tensile and impact strength) has been investigated in an Al-7Si-0.3Mg-xFe (x=0.2, 0.4, and 0.7) alloy. The size of Al-Fe intermetallic platelets (β-Al₅FeSi) increased with increasing Fe content. The addition of Ca modified the eutectic microstructure and also reduced the size of intermetallic Fe-platelets, causing improved elongation and impact strengths. A low level of Ca addition (39 ppm) reduced the proosity of the alloys. The tensile strength was decreased marginally with Ca addition. However, Ca addition improved the ductility of the alloy by 18.3, 16.7, and 44 pct and the impact strength by 44, 48, and 15.8 pct for Fe contents of 0.2, 0.4, and 0.7 pct, respectively.     

Effect of heat treatment on the microstructure,tensile properties,and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10_p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.     

Deformation and fracture behavior of a directionally solidified β/γ′ Ni-30 At. pct Al alloy

The effect of a ductile γ′-Ni₃Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni₃Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately 850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties.     

Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys. The work was carried out when the authors were with Chrysalis Technologies Inc., Richmond, VA. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Deformation behavior of dilute SnBi(0.5 to 6 at. pct) solid solutions

Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct, SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature. The test temperatures were − 20 °C, 23 °C, 90 °C, and 150 °C, and the strain rates ranged from approximately 10⁻⁸ to 10⁻¹ 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures, only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 × 10⁻⁴<σ/E<7.5 × 10⁻⁴. The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation is about equal in all the alloys (∼60 kJ/mol) and the stress exponents are high (10<n<12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6 at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount of dissolved Bi.     

Effects of combined high and low temperature deformation processing of β III titanium

The structure and properties of β III titanium alloy (nominal composition: 11.5 pct Mo, 6 pct Zr, 4.5 pct Sn, bal Ti) were studied as a function of combined high and low temperature ther-momechanical processing. A water quenched extrusion was deformed various amounts by swaging at room temperature prior to the aging treatment. No re-solution heat treatment was employed. The swaging introduced mechanical twinning and a small amount of stress induced orthorhombic martensite. Following a 900°F, 8 hr aging treatment, substantial increases in yield and tensile strength were observed, combined with a severe decrease in tensile ductility in samples with small amounts of swaging. The orientation and morphology of the deformation products have a critical influence on tensile ductility. A decrease in the plane strain fracture toughness accompanied the large increase in tensile strength.     

The effect of processing and microstructure development on the slip and fracture behavior of the 2.1 wt pct Li AF/C-489 and 1.8 wt pct Li AF/C-458 Al-Li-Cu-X alloys

Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458, respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than 10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation while encouraging matrix precipitation of the T₁ (Al₂CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However, no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating a very large processing window. Grain size and δ′ (Al₃Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Influence of silicon and phosphorous on the mechanical properties of both ferrite and dual-phase steels

A study has been made of the influence of up to 2 pct Si and 0.42 pct P upon the strength and ductility of ferrites over a wide grain size range; the grain size was varied fromd ^−1/2 mm^−1/2 = 4 to 14. Although the ductility decreased with increasing strength for all the alloys, the 2 pct Si alloy had the best combination of strength and ductility. The ferrites containing 2 pct Si and 0.2 pct P had greater uniform elongations than conventional HSLA steels at the same tensile strength; it is thought that the ductility of the ferrites is enhanced by the presence of Si while the ductility of conventional HSLA steels is reduced by the presence of carbide precipitates. With the theory for a composite of two ductile phases and the results for the fine-grained alloyed ferrites, the change in uniform elongation as a function of tensile strength was predicted for dual-phase (martensite plus ferrite) steels. Good agreement was found between the prediction and experimental results for dual-phase steels containing up to 0.2 pct P or 2 pct Si; the 2 pct Si alloy had the best combination of strength and ductility of all dual-phase steels so far reported. This study again emphasizes the importance of the high strength, high ductility ferrite in controlling the properties of dual-phase steels.     

Quench Sensitivity of an Al-7 Pct Si-0.6 Pct Mg Alloy: Characterization and Modeling

The quench sensitivity of a cast Al-7 wt pct Si-0.6 wt pct Mg alloy was characterized by tensile tests and scanning electron microscopy. Specimens were cooled from the solution treatment temperature following 58 different cooling paths including interrupted and delayed quenches. Analysis of the microstructure showed that quench precipitates were Mg₂Si (β), which nucleated heterogeneously on Si eutectic particles as well as in the aluminum matrix, presumably on dislocations. The quench sensitivity of the alloy’s yield strength was modeled by multiple C-curves, using an improved methodology for quench factor analysis. The three C-curves used in the model represented loss of solute by (1) diffusion of Si to eutectic particles, (2) precipitation of β on Si eutectic particles, and (3) precipitation of β in the matrix. The model yielded a R ² of 0.994 and a root-mean-square error (RMSE) of 7.4 MPa. The model and the implications of the results are discussed in the article. This article is based on a presentation made in the symposium entitled “Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

Correlation of deformation mechanisms with the tensile and compressive behavior of NiAl and NiAl(Zr) intermetallic alloys

To identify the mechanisms controlling strength and ductility in powder-extruded NiAl and NiAl + 0.05 at. pct Zr, tensile and compressive testing was performed from 300 to 1300 K for several grain sizes. Grain size refinement significantly increased yield stress in both alloys and, in some cases, slightly lowered the ductile-to-brittle transition temperature (DBTT), although no room-temperature tensile ductility was observed even in the finest grain size specimens. The small Zr addition increased the DBTT and changed the low-temperature fracture mode from intergranular in NiAl to a combination of intergranular and transgranular in the Zr-doped alloy. Scanning electron microscopy (SEM) of compression specimens deformed at room temperature revealed the presence of grain-boundary cracks in both alloys. These cracks were due to the incompatibility of strain in the poly crystalline material, owing to the lack of five independent slip systems. The tendency to form grain-boundary cracks, in addition to the low fracture stress of these alloys, contributed to the lack of tensile ductility at low temperatures. The operative slip system, both below and above the DBTT, was {110} 〈001〉 for both alloys. The change from brittle to ductile behavior with increasing temperatures was associated with the onset of diffusional processes.