Microstructure and Mechanical Properties of a High-Strength Ti-4Al-2Fe-3Cu Alloy Fabricated by Sintering and Hot Extrusion

A Ti-4Al-2Fe-3Cu (wt pct) alloy containing only low-cost alloying elements was fabricated by vacuum sintering a blend of TiH₂, Al, Fe, and Cu powders at 1200 °C for 1 hour followed by hot extrusion at the same temperature. The as-extruded alloy exhibited a microstructure consisting of mainly α/β lamellar colonies and Ti₂Cu as a minor phase. The average colony size and lamella thickness were 118 and 12 µm, respectively, and Fe and Cu were predominantly distributed in the β lamellae. The as-extruded alloy had a high tensile yield strength (YS) and ultimate tensile strength (UTS) of 1248 and 1270 MPa, respectively, but a limited ductility (elongation to fracture: 2.3 pct). Annealing at 750 °C for 4 hour caused the average colony size and lamella thickness of the alloy to increase to 145 and 17 µm, respectively, and the volume fraction of the β phase decreased with the annealing. These microstructural changes resulted in a slight decrease of the YS and UTS to 1221 and 1253 MPa, but a clear increase of the ductility with the elongation to fracture reaching 4 pct. This work demonstrates that a combination of relatively low-temperature vacuum sintering, hot extrusion, and annealing can be effectively utilized to fabricate a low-cost Ti-4Al-2Fe-3Cu alloy with high strength and appreciable tensile ductility.

     

Importance of slip mode for dispersion-hardened β-titanium alloys

The mechanical properties of Ti-7 Mo-7 Al and Ti-7 Mo-16 Al (in at. pct) were correlated to the microstructure. The mechanical properties of the alloy with low aluminum content, consisting of α+ β phases, were dependent on the size of the α particles. Although the α phase is softer than the β phase, the small α particles, upon plastic deformation of the alloy, functioned as typical hard agents in a dispersion-hardened system and the volume fraction of the particles controlled the macroscopic ductility. A rapid strain-hardening behavior of the small α particles seemed to be responsible for this effect. Large α particles behaved like soft, incoherent particles, the volume fraction having little effect on the inherent ductility of the alloy. The two phase (β+ Ti₃Al) microstructure of the alloy with high aluminum content resulting from high temperature aging to 900°C exhibited a yield stress of 130 ksi and an elongation to fracture of 5 pct. The ductility of this microstructure was controlled by the volume fraction of the Ti₃Al particles inducing homogeneous slip. The favorable ductility properties of the microstructures with low Ti₃Al volume fraction were lost if the slip mode was changed from homogeneous slip to planar slip. Formerly Staff Member, Materials Research Center, Allied Chemical Corp., Morristown, N. J.     

Effects of Friction Stir Processing Parameters and In Situ Passes on Microstructure and Tensile Properties of Al-Si-Mg Casting

Friction stir processing (FSP) was applied to modify the microstructure of an as-cast A356 alloy. The effects of rotation rate, travel speed, in situ FSP pass, FSP direction, and artificial aging on microstructures and tensile properties were investigated. FSP broke up the coarse eutectic Si phase into 2.5 to 3.5 μm particles and distributed them homogeneously, and resulted in the dissolution of the coarse Mg₂Si particles and the elimination of porosity, thereby improving both the strength and the ductility of the casting. Increasing the rotation rate was beneficial to breaking up and dissolving the particles, but it contributed little to eliminating the porosity. The travel speed did not affect the size of the particles apparently, but lower speed was beneficial to eliminating the porosity. 2-pass FSP showed an obvious advantage in the microstructure modification and tensile properties compared with the single-pass. However, a further increase of FSP passes only resulted in slight improvement. The FSP direction of the following pass did not show distinct effect on the microstructure and tensile properties. After post-FSP artificial aging, the strengthening phase (β″-Mg₂Si) precipitated, which increased the strength and decreased the ductility of the FSP samples.     

Tensile and Fracture Properties of Cast and Forged Composite Synthesized by Addition of Al-Si Alloy to Magnesium

Cast Mg-Al-Si composites synthesized by addition of Al-Si alloy containing 10, 15, and 20 wt pct of Si, in molten magnesium, to generate particles of Mg₂Si by reaction between silicon and magnesium during stir casting has opened up the possibility to control the size of these particles. The microstructure of the cast composite consists of relatively dark polyhedral phase of Mg₂Si and bright phase of β-Al₁₂Mg₁₇ along the boundary between dendrites of α-Mg solid solution. After hot forging at 350 °C, the microstructure has changed to relatively smaller sizes of β-Al₁₂Mg₁₇ and Mg₂Si particles apart from larger grains surrounded by smaller grains due to dynamic recovery and recrystallization. Some of the Mg₂Si particles crack during forging. In both the cast and forged composite, the Brinell hardness increases rapidly with increasing volume fraction of Mg₂Si, but the hardness is higher in forged composites by about 100 BHN. Yield strength in cast composites improves over that of the cast alloy, but there is a marginal increase in yield strength with increasing Mg₂Si content. In forged composites, there is significant improvement in yield strength with increasing Mg₂Si particles and also over those observed in their cast counterpart. In cast composites, ultimate tensile strength (UTS) decreases with increasing Mg₂Si content possibly due to increased casting defects such as porosity and segregation, which increases with increasing Mg₂Si content and may counteract the strengthening effect of Mg₂Si content. However, in forged composite, UTS increases with increasing Mg₂Si content until 5.25 vol pct due to elimination of segregation and lowering of porosity, but at higher Mg₂Si content of 7 vol pct, UTS decreases, possibly due to extensive cracking of Mg₂Si particles. On forging, the ductility decreases in forged alloy and composites possibly due to the remaining strain and the forged microstructure. The initiation fracture toughness, J _IC , decreases drastically in cast composites from that of Mg-9 wt pct. alloy designated as MA alloy due to the presence Mg₂Si particles. Thereafter, J _IC does not appear to be very sensitive to the increasing presence of Mg₂Si particles. There is drastic reduction of J _IC on forging of the alloy, which was attributed to the remaining strain and forged microstructure, and it is further lowered in the composites because of cracking of Mg₂Si particles. The ratio of the tearing modulus to the elastic modulus in cast composites shows a lower ratio, which decreases with increasing Mg₂Si content. The ratio decreases comparatively more on forging of cast MA alloy than those observed in forged composites.     

Direct consolidation of γ-TiAl-Mn-Mo from elemental powder mixtures and control of porosity through a basic study of powder reactions

A gamma titanium aluminide was made by elemental powder metallurgy. For consolidation of the alloy from powder blending, either hot extrusion or hot forging was used. A good combination of tensile yield strength and ductility was obtained by hot extrusion that produced a grain size of 50 μm. Consolidation by forging, however, resulted in a porous microstructure. On the basis of an investigation of the cause of the porosity by an Al/Ti diffusion couple experiment and by characterization of the temperature peaks due to an exothermic reaction among elemental powder particles, it was concluded that a transient phase such as TiAl₃ was the culprit. Being the source of Al diffusion, the transient phase leaves behind Kirkendall voids when it forms prior to the major exothermic reaction among elemental powder particles. From this study, two processing techniques to circumvent the porosity were proposed and verified: a fast heating to the consolidation temperature or sufficient soaking above the reaction temperature prior to consolidation. A sound, fully lamellar, β-phase controlled microstructure was obtained by these methods.     

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65å and a small amount of incoherent η (MgZn₂). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35å. Aging the 7050 alloy to produce particles larger than 50å gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δε_p/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters.     

Microstructure and properties of Ti-17 powder compact prepared through powder sintering and isothermal forging

Abstract

Powder sintering and dual isothermal forging were utilised to prepare titanium alloy. Owing to the disturbance effect of residual pores during sintering, the microstructures of two sintered Ti-17 powder compacts prepared with about 80–150 and ?150 mesh powders were composed of large residual pores and low aspect ratio of α platelets and small residual pores and high aspect ratio of α platelets respectively. Residual pores can be closed during the first isothermal forging above the β transus due to the excellent plastic deformation capability of the β phase. The microstructure can be effectively broken during the second isothermal forging below the β transus. Isothermal forging with 10^?2 and 10^?4 s^?1 strain rates can obtain uniform microstructure, whereas isothermal forging with 1 s^?1 strain rate just refined the microstructure non-uniformly. Closure of the residual pore and change of the microstructure’s morphology during dual isothermal forging effectively improved the ductility of the compact even though the oxygen and nitrogen contents exceeded the standard requirements.     

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al₅FeSi, α- Al₁₅(Fe,Mn)₃Si₂, and Al₂Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.     

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Newly Developed Austenitic Heat-Resistant Alloy

The hot deformation characteristics, microstructure evolution, and dynamic recrystallization (DRX) mechanism of the newly developed austenitic heat-resistant steel Fe–18Cr–10Ni–0.3Nb–2.5Cu were systematically investigated by thermal compression tests combined with microstructure characterizations. The activation energy (Q) map, Zener–Hollomon parameter (Z) map, and processing map were plotted according to the stress–strain curves to reveal the inherent connection between the three maps and the hot deformation characteristics of this alloy. The high η region in the processing map does not precisely correspond to the region where DRX developed. Nevertheless, the flow instability map accurately predicts the microstructure. The variation pattern of Z corresponded more closely to the hot deformation microstructure evolution than did the variation pattern of Q. The degree of DRX increases with decreasing Z. The optimal process parameters are 1000 °C/0.01 s⁻¹/0.8 and 1100 °C/10 s⁻¹/0.8 (temperature/strain rate/strain), and they result in complete DRX and a narrow range of Z values. The DRX mechanism at high strain rate is characterized by the combined enhancement of discontinuous DRX (DDRX), continuous DRX (CDRX), and twin-DRX (TDRX). The dominance of the particle-stimulated nucleation (PSN) mechanism at intermediate strain rate results in the formation of incompletely recrystallized microstructures with approximate orientation. Sufficient time at low strain rate promotes the development of DDRX and CDRX.

     

FRACTURE BEHAVIOUR OF A DISPERSION-STRENGTHENED SAP-TYPE ALLOY

Abstract

The fracture surfaces of AT-400 (Al-Al₂O₃, SAP-type) dispersion-strengthened alloy specimens, broken in impact over the temperature range 20 to–170°C, have been examined visually and by electron microscopy. Test temperature, dispersed-particle morphology, grain size, and method of alloy fabrication all affected the mode of fracture.

Conventional fabrication of aluminium SAP-type alloys appears to lead to grain-boundary porosity associated with reduced ductility. Specimens fabricated using a vacuum annealing treatment before final consolidation have more ductility than specimens of commercial-purity aluminium.

The presence of the grain-boundary porosity, coupled with the fine Al₂O₃ dispersion, can produce independently nucleated microcleavage facets in the aluminium matrix. The appearance of this type of fracture is very similar to that observed for dispersion-strengthened non-metallic crystalline materials.     

Microstructure and mechanical properties of PM Ni56Fe19Al25 alloy

Abstract

Scanning electron microscopy and X-ray diffraction analysis were used to study microstructure and mechanical properties of PM Ni₅₆Fe₁₉Al₂₅ alloy. The results indicate that as sintered specimen is (β+γ) dual phase structure, and its density is 6·54 g cm^?3 (the relative density is 94·0%), tensile strength is 771 MPa and the total strain is 4·3%. As quenched specimen presents a large superelasticity with the maximum recovery strain of 4·5%, and its tensile strength is 850 MPa and the total strain is 9·2%. The fracture modes of Ni₅₆Fe₁₉Al₂₅ alloy is transgranular, intergranular and tough mixed type.     

Effect of composition on the properties of strain-transformable β titanium alloys

A number of metastableβ titanium alloys were examined to determine the effects of composition on strain-transformation behavior and precipitation hardening response. Maximum ductility as solution heat treated was observed in alloys slightly richer in alloy content than the minimum alloy content required to retain an all-β microstructure on quenching. Such materials transformed to a martensitic structure upon straining and, as a result of strain transformation, developed room temperature ductility exceeding that found in unalloyed titanium. Uniform elongation of 35 to 45 pct was observed in a number of compositions of this type containing major additions of Mo, V, Cr, or Mn. Auxiliary alloy additions of Sn, Al, or Zr, or ternary alloying with molybdenum were necessary to preventω embrittlement during quenching in alloys containing V, Cr, or Mn. Alloying with Fe, Cu, Co, or Ni resulted in low ductility as solution heat treated, but it is probable that optimum amounts of these additions were not studied in this investigation. Oxygen above about 1200 ppm also had a detrimental effect on ductility. All alloys studied showed precipitation hardening when heat treated in the 800° to 1100°F range. Tensile strengths of 170 to 190 ksi were readily attainable in most alloy systems. Ductility as precipitation hardened appeared to be higher in alloys containing at least 6 pct Mo or V than in other alloys studied.     

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.     

Solidification Microstructure and Mechanical Properties of Cast Magnesium-Aluminum-Tin Alloys

The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg₁₇Al₁₂, and Mg₂Sn phases. The amount of Mg₁₇Al₁₂ and Mg₂Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content, whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial AZ91 alloy.     

The relationship of microstructure to monotonic and cyclic straining of two age hardening aluminum alloys

The effect of microstructure on the monotonic and low cycle fatigue properties of a high purity, large grain, ternary aluminum-zinc, magnesium (Al-Zn-Mg) alloy and a high strength 7050 aluminum alloy was investigated. The best combination of fatigue life, strength, and ductility for the ternary alloy resulted when aged to produce a microstructure containing predominately η′ having a Guinier radius of approximately 65? and a small amount of incoherent η (MgZn₂). Superior fatigue life, strength, and ductility were found when the 7050 alloy was aged to produce the maximum number of partially coherent η′ precipitates having a Guinier radius of approximately 35?. Aging the 7050 alloy to produce particles larger than 50? gave a microstructure that had lower fatigue properties at the low plastic strain amplitudes, δε_p/2 <1.0 pct. The empirical CoffinManson relationship was found to hold for a given deformation process, however changes in deformation character resulted in changes in the Coffin-Manson parameters. This research was supported by the U.S. Air Force Office of Scientific Research (AFSC) under Grant No. AFOSR-74-2615, Dr. Alan H. Rosenstein, Contract Monitor.     

Rapid solidification processing of a Mg-Li-Si-Ag alloy

A Mg-13Li-4Si-lAg (wt pct) alloy with improved ductility and thermal stability was developedvia the rapid solidification (RS) processing technique. Silicon was added to the alloy as the third alloying element in order to form a thermally stable intermetallic dispersoid phase required for improved mechanical properties at ambient and elevated temperatures. The microstructure of the as-spun and heat-treated alloy was characterized using differential scanning calorimetry (DSC), X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Microhardness measurements were conducted on as-spun and heat-treated alloy in order to obtain qualitative prop-erty data and to investigate the extent of the degradation of properties at elevated temperatures. It was found that the melt-spun Mg-Li alloy possessed a microstructure consisting of a fine dispersion of Mg₂Si phase in a fine-grained body-centered cubic (bcc) Mg-Li solid solution, resulting in the desired improvements in thermal stability and mechanical properties. Formerly Graduate Student, Department of Materials Science and Engineering, University of California     

Effect of Ni on Microstructure and Mechanical Properties of CrMnFeCoNi High Entropy Alloy

The effect of Ni content on microstructure and mechanical properties of the CrMnFeCoNi high entropy alloy (HEA) has been studied. The Ni content varied from 0 to 20 at% in the composition (CrMnFeMn)_100?xNi_x, where x?=?0, 2.5, 5, 10, 15, and 20 at%. The alloys were synthesized by vacuum arc melting and the microstructure as well as hardness of the as-cast alloys were studied. Alloys with low Ni content (x?≤?2.5%) consists of a two-phase microstructure of dendritic and inter-dendritic regions with fcc (matrix) and tetragonal (sigma) crystal structure, respectively. When the Ni content is 5 at%, two-phase structure with fcc (matrix) and bcc (secondary phase) is observed, with the addition of Mn-rich inclusions that are present in the entire matrix. Alloys with higher Ni content (x?≥?10, at%) exhibit a single phase of fcc structure. Hardness of the HEAs decreases from 320 to 120 Hv with increase in Ni content, and the high hardness of these alloys with low Ni content is due to the mixture of both fcc and hard tetragonal (sigma) phases.

     

Thermoelastic martensite and shape memory effect in B2 Base Ni-Al-Fe alloy with enhanced ductility

A ductile shape memory alloy of the Ni-AI-Fe system has been developed using principles through the control of microstructure. Addition of Fe to the binary Ni-Al shape memory alloy allows the introduction of a ductile face-centered cubic (fcc) phase in an otherwise extremely brittle β phase alloy, leading to an improvement in its ductility while retaining its ability to exhibit shape memory arising from the martensitic transformation of theβ phase to Ll₀ structure. It is shown that the transformation temperature in the ternary Ni-AI-Fe alloy can be easily controlled by the preannealing in the β+ γ region. Experimental results on the effect of different annealing treatments on the microstructures and the shape memory behavior in this alloy are presented and discussed.     

Effect of thermomechanical treatment on microstructure and mechanical properties of AISI 301LN stainless steel

Abstract

A wide range of cold thickness reduction (10–80%) and subsequent annealing were carried out on AISI 301LN stainless steel. X-rays and Feritscope MP30 were used to identify the strain induced α′-martensite phase and its volume fraction respectively. The microstructure was observed by optical micrograph and scanning electron microscope. The results show that shear bands were present and strain induced α′-martensite nucleated at their intersections. The volume fraction of α′-martensite increased with the increased cold reduction by the continuous growth of embryos, which resulted in the increasing yield and tensile strength. The reversion of α′-martensite to austenite occurred after subsequent annealing. The grain size variation of austenite was related to the annealing regime. A good combination of strength and ductility can be obtained after annealing at 650°C for 30 min. The effect of grain size on yield strength conformed with the Hall–Petch relationship in the entire range of our analysis.