This paper presents a comprehensive review of the current state of knowledge of second harmonic generation (SHG) measurements, a subset of nonlinear ultrasonic nondestructive evaluation techniques. These SHG techniques exploit the material nonlinearity of metals in order to measure the acoustic nonlinearity parameter, $$\beta $$
 . In these measurements, a second harmonic wave is generated from a propagating monochromatic elastic wave, due to the anharmonicity of the crystal lattice, as well as the presence of microstructural features such as dislocations and precipitates. This article provides a summary of models that relate the different microstructural contributions to $$\beta $$
 , and provides details of the different SHG measurement and analysis techniques available, focusing on longitudinal and Rayleigh wave methods. The main focus of this paper is a critical review of the literature that utilizes these SHG methods for the nondestructive evaluation of plasticity, fatigue, thermal aging, creep, and radiation damage in metals.

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Review of Second Harmonic Generation Measurement Techniques for Material State Determination in Metals

Boron modified titanium alloys

A Review on High-Strength Titanium Alloys: Microstructure, Strengthening, and Properties

As a crucial branch for titanium industry, high-strength titanium alloys (HS-TAs, with UTS ≥ 1100 MPa) are indispensable structural materials for advanced engineering applications such as aerospace and marine fields. Along with the expansion of HS-TAs’ market, achieving satisfying synergies of high strength, high ductility (elongation ≥ 6%) and high toughness (KIC ≥ 50 MPa⋅m1/2) has been identified as the uppermost technical bottleneck for their research and development. To overcome the challenge, two primary strategies have been initiated by the titanium community, developing novel alloys and innovating processing technologies. For the former, a dozen of newly-developed alloys were reported to exhibit excellent strength-ductility-toughness combinations, including Ti-5553, BT22, TC21 and Ti-1300, for which the ideal mechanical performances were based on specific microstructures realized by low impurity rate (e.g. oxygen content ≤ 0.15 wt%), complicated processing and complex heat treatment. For the latter, several innovatory forging and heat treatment technologies were originated for the mature alloys to optimize their balanced property by extraordinary microstructural characteristics. In this review, we provide a comprehensive overview over the research status, processing and heat treatment technologies, phase transformation, processing-microstructure-property correlation and strengthening-toughening mechanism of HS-TAs for aerospace engineering applications manufactured via melting-forging process. Finally, the prospects and recommendations for further investigation and development are proposed based on this review. 

High-strength titanium alloys for aerospace engineering applications: A review on melting-forging process

Liquid phase sintering, heat treatment and swaging studies on three tungsten heavy alloys, 93W–4.9Ni–2.1Fe (wt%), 93W–4.2Ni–1.2Fe–1.6Co (wt%) and 93W–4.9Ni–1.9Fe–0.2Re (wt%) were carried out in detail with respect to microstructure, tensile and impact properties. All the alloys were sintered and swaged to 40% deformation. The results indicate that Re addition reduces the grain size of the alloy compared to W–Ni–Fe and W-Ni-Fe-Co alloys. W–Ni–Fe–Re alloy shows superior tensile properties in heat treated condition as compared to W–Ni–Fe and W–Ni–Fe–Co alloys. SEM study of fractured specimens clearly indicates that the failure in case of W–Ni–Fe–Re was due to transgranular cleavage of tungsten grains and W–W de-cohesion. W–Ni–Fe and W–Ni–Fe–Co alloys also failed by mixed mode failure. However, in these cases, ductile dimples corresponding the failure of the matrix phase was rarely seen. Thermo-mechanical processing resulted in significant changes in mechanical properties. While W–Ni–Fe–Re alloy showed the highest tensile strength (1380 MPa), W–Ni–Fe–Co exhibited the highest elongation (12%) to failure. A detailed analysis involving microstructure, mechanical properties and failure behavior was undertaken in order to understand the property trends.

Effect of alloying addition and microstructural parameters on mechanical properties of 93% tungsten heavy alloys

The effect of swaging deformation and low temperature aging on mechanical properties of tungsten heavy alloy (90W–6Ni–2Fe–2Co) was studied. The alloy was prepared by conventional hydrogen sintering followed by vacuum heat treatment and swaging with varying reductions in area (ranging from 10 to 75%). The swaged samples were aged in nitrogen atmosphere at temperatures ranging from 300 to 1100 °C followed by microstructure and mechanical property evaluation. Both microstructure and mechanical properties were sensitively dependent on the amount of deformation imparted during swaging. Subsequently, aging also influenced the properties depending upon the aging temperature. Detailed fractographic study revealed that the fracture mode of the alloy changed from intergranular to transgranular mode depending upon thermo-mechanical processing. Aging treatment after swaging affected the fracture morphology of alloy considerably. Samples aged at 500 °C showed predominant transgranular failure of tungsten grains, whereas the one heat treated at 700 °C showed increasing evidence ductile tear. Based on the results, an approach for optimizing mechanical properties that involves heat treatment and swaging operation was proposed.

Swaging and heat treatment studies on sintered 90W–6Ni–2Fe–2Co tungsten heavy alloy

The microstructural influence on fracture behavior of tensile and Charpy impact toughness properties were investigated for 90W 6Ni 2Fe 2Co and 90W 6Ni 2Fe 1.5Mo 0.5Co tungsten heavy alloys, which are in use of kinetic energy penetrators. The alloys were sintered, heat treated and swaged with 40% reduction in area. Both microstructure and mechanical properties were sensitively dependent on the alloy composition. The W Ni Fe Co alloy exhibited increased tensile strength, ductility and impact toughness compared to the W Ni Mo alloy. Fractographic study revealed that the mode of tensile failure changes from flat to sheared tungsten grains for low to high strength tungsten heavy alloys, whereas the fracture behavior changes from flaws/cracks to transgranular cleavage of tungsten grains for low to high Charpy impact toughness alloys. The results indicate that a strong relationship between microstructure, fracture behavior, tensile and impact properties as a function of alloy composition.

Tensile and impact behavior of swaged tungsten heavy alloys processed by liquid phase sintering

Detailed structure-property correlation has been carried out in high strength tungsten heavy alloys. Alloys of compositions 90W-6Ni-2Fe-2Co, 89W-6Ni-2Fe-3Co, 89.5W-6Ni-2Fe-2Co-0.5Mo, 89.75W-6Ni-2Fe-2Co-0.25Mo, 90W-6Ni-1.5Fe-2.5Co and 90W-6Ni-1Fe-3Co have been prepared by liquid phase sintering followed by large deformation during thermo mechanical processing and studied for microstructure and mechanical properties. Despite differences in composition, higher volume fraction of matrix and lower W-W contiguity in the microstructure result in superior tensile strength and impact toughness. Increasing W content in the matrix enhances mechanical properties by imparting solid solution strengthening, increasing the matrix volume fraction and reducing W-W contiguity. The alloy 90W-6Ni-1Fe-3Co shows superior balance of properties with ultimate tensile strength of 1600 MPa and average impact toughness of 121 J/cm2.

On structure property correlation in high strength tungsten heavy alloys

The present study investigates the interrelationship between microstructure, tensile and impact properties of microwave sintered tungsten heavy alloys. Alloys of two different compositions were microwave sintered and conventionally liquid phase sintered using similar heating rate, followed by microstructure and mechanical property evaluation. The tensile properties of microwave sintered alloys were significantly superior to those of liquid phase sintered alloys in both the compositions. Excellent impact properties were obtained in microwave sintered alloys, without any subsequent heat treatment or processing. In order to understand the reasons for the improvement, a detailed comparative study of microstructural features in both microwave and conventionally sintered alloys was carried out and it was observed that both tensile and impact properties of these alloys were largely influenced by tungsten–tungsten contiguity; the weakest link in the microstructure. Microwave sintered alloys were found to exhibit substantially lower contiguity as compared to the conventionally sintered alloys and consequently their properties were found to be superior as compared to the conventionally sintered alloys.

Tensile and impact properties of microwave sintered tungsten heavy alloys

Five different compositions of tungsten heavy alloy with Ni, Fe, Co and Mo were prepared through powder metallurgy route using liquid phase sintering Three alloys were based on quaternary W-Ni-Fe-Co (with varying Ni to Fe ratio and Co content) and the other two quinary systems with Mo addition The sintered alloy blanks (cylindrical cross section) were subjected to a thermo-mechanical treatment, which comprised three vacuum heat treatments (at 1100 °C for 15 h followed by oil quenching) before and in between multiple swaging operations (at 500 °C) with a total deformation of 53% This was followed by a detailed evaluation of different microstructural parameters in all compositions Mechanical properties such as tensile strength, % elongation and impact were evaluated at the final stage (26 mm dia and 600 mm length) Increase in Co content resulted in better strength and elongation with marginal loss in impact values The alloy containing 025 wt% Mo showed the best combination of tensile and impact properties Further increase in Mo concentration to 05 wt% resulted in marginal increase in strength value but rapid deterioration in elongation and impact values Scanning electron microscopy, electron probe micro analysis, quantitative metallography and fractography were carried out to establish an inter-relationship between microstructure, properties and fracture behaviour of the alloys investigated

T.K. Nandy

Papers

Swaging and heat treatment studies on sintered 90W–6Ni–2Fe–2Co tungsten heavy alloy

Tensile and impact behavior of swaged tungsten heavy alloys processed by liquid phase sintering

On structure property correlation in high strength tungsten heavy alloys

Tensile and impact properties of microwave sintered tungsten heavy alloys

Effect of composition, heat treatment and deformation on mechanical properties of tungsten heavy alloys