Nearly Lamellar Research Articles

A novel Ta-contained TiAl alloy with excellent high temperature performance designed for powder hot isostatic pressing

TiAl alloys offer strong potential for replacing conventional nickel-base alloys in structural applications at high temperatures, which requires good high temperature performance. This study analyzes the creep performance and thermal exposure characteristics for a powder hot isostatic pressing (P-HIP) Ta-contained TiAl alloy. The microstructure characteristics, creep performance, and thermal exposure properties of the nearly lamellar (NL) alloy with a size of about 145 μm were investigated. The alloy remains 0.72 % fracture strain at room temperature after exposure at 750 ℃ for 1000 h. The fitting results of creep curves show that the creep stress index n is 15.82 at 750 ℃, indicating the power law creep mechanism. By reducing the interfacial energy, Ta can facilitate the formation of metastable structures and achieve refinement. The enrichment of Ta element at the lamellar edge, both observed after thermal exposure and creep, inhibits the growth of lamellae and hinders the expansion of cavities due to its low diffusion rate, improving the thermal stabilities and creep performance. The P-HIP Ta-contained TiAl alloy shows exciting high temperature performance.

Thermal stability of a newly developed Mn containing β-solidifying γ-TiAl intermetallic compound at 750 °C

A newly low-cost β-solidifying γ-TiAl alloy Ti-44Al-3Mn-0.4Mo-0.4W-0.1B-0.1C (in at. %, named as TMMW) was invented. The rolled bars with a diameter of 12 mm were successfully fabricated by conventional hot rolling process, directly carrying out from the ingot without the pre-forging or hot-packing. The heat-treated TMMW rolled bar, which had a fine-grained nearly lamellar (NL) microstructure, was then air-exposed at 750 °C for up to 3000 h. The changes of the microstructure during this exposure were characterized using electron probe X-ray micro-analyzer (EPMA), transmission electron microscope (TEM), electron backscattered diffraction (EBSD) and high-energy X-ray diffraction (HEXRD). The tensile properties of the samples before and after thermal exposure were also tested. The fine-grained NL microstructure is found to be thermodynamically stable during the exposure. Meanwhile, the combined addition of Mo and W shows a positive effect on the stabilization of βo phase at colony boundaries, with only slight precipitation of the nanometer βo phases with no other brittle phases, such as Laves, in the metastable α2 lamellae. It is found that the thickness of the α2 lamellae decreases after 500 h exposure, while remains basically unchanged from 500 h to 3000 h, which is contributed by the effective pinning effect of the precipitated βo phase in α2 lamellae. As a result, the exposure-induced embrittlement under room temperature and elevated temperature does not take place. Interestingly, the ductility tested at 800 °C improves significantly after exposure. As for the tensile strength, it is only reduced by less than 10 % after 500 h exposure, while it remains essentially unchanged when increasing the exposure time from 500 h to 3000 h. Finally, the mechanisms of the precipitation of βo phases in the α2 lamellae and the tensile properties evolution during the exposure were carefully discussed.

Hot deformation behavior of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting

TiAl-based alloys are known for their exceptional high-temperature properties but suffer from low hot workability. The emergence of additive manufacturing (AM) technology significantly decreases the workability requirement, while the microstructure heterogeneities in AMed TiAl are troublesome. This paper explores the hot deformation behavior and microstructure evolution of a layered heterogeneous microstructure TiAl alloy prepared by selective electron beam melting. The Arrhenius constitutive model with strain compensation was established, and hot processing maps were constructed. The microstructure showed that at the initial stage of deformation, a large number of dislocation slips occur preferentially in the soft coarse-grained layers. With the increase of strain, dislocation accumulation in heterogeneous interfaces, and dynamic recrystallization (DRX) occurs. Eventually, the coarse-grained region is constantly occupied by fine DRX grains. A high density of deformation twins at high strain rates further promotes the formation of tiny DRX grains. When the hot compression temperature reaches 1200 °C, the initial alternative fine-grained and coarse-grained hetero-structure transforms into duplex (DP) and near-lamellar (NL) microstructure, and microstructure heterogeneity is eliminated. The results enhance understanding of the hot deformation behavior of heterogeneous microstructure TiAl alloys.

A high tensile strength above 900 ℃ in β-solidified TiAl alloy through alloy design and microstructure optimization

As a target for high-temperature applications, β-solidified TiAl alloys have attracted intense attention owing to their low density and superior mechanical properties at elevated temperatures. However, their microstructure instability and the remnant βo phase in β-solidified TiAl alloys result in the deterioration of their mechanical properties. In this work, a novel β-solidified TiAl alloy, Ti-44Al-5.5Nb-0.5W-0.5Cr-0.3Si-0.1C (at%), showing improved high-temperature mechanical properties, was proposed. Appropriate forging and heat treatments were applied to obtain excellent mechanical properties. Hot forging at the (α + β) phase region refined the lamellar colony significantly, and a heat treatment at 1270 °C removed the residual βo phase. Interestingly, undissolved γ laths contributed to the refinement of lamellar spacing. The optimized sample exhibited a nearly lamellar (NL) structure with a colony size of 38.2 µm and lamellar spacing of 29.6 nm, thus presenting a tensile strength of 520 MPa at 950 °C, indicating the present alloy could be used above 900 °C.

Improvement in the high temperature mechanical properties of additively manufactured Ti–48Al–2Cr–2Nb alloy using heat treatment

Gamma Ti–48Al–2Cr–2Nb (Ti4822) alloy, which is manufactured by an electron beam melting (EBM) process, has excellent ductility but poor high-temperature strength, because it has a near gamma structure. The objective of this study was to control the microstructure of Ti4822 by considering various forms of heat treatment to improve the mechanical properties at room and high temperatures. The heat treatment conditions were varied to create nearly lamellar (NL) and fully-lamellar (FL) structures. First, the EBM-built NL-Ti4822 exhibited a lamellar structure with an average layer thickness of about 92 nm and equiaxed γ-phase particles with a size of several tens of micrometers. On the other hand, the EBM-built FL-Ti4822 had an average lamellar thickness of about 347.2 nm, but in this case the equiaxed γ phase was not observed. The results of the room- and high-temperature compression test showed that the mechanical properties of the two heat-treated materials were significantly superior to those of the as-built Ti4822 across all temperature ranges. Moreover, both materials were highly ductile at room and high temperature despite having a lamellar structure, and exhibited intensified yield stress anomaly and dynamic recrystallization as the fraction of lamellar structure increased. Based on the above results obtained by subjecting the materials to different heat treatment methods, the effects of the microstructures of the two materials on the mechanical properties and deformation mechanisms at room and high temperatures were investigated.

Dynamic behavior and microstructural evolution of TiAl alloys tailored via phase and grain size

As a kind of promising aerospace material, TiAl alloys need to withstand extreme conditions such as high-rate impact loads and high temperatures. The mechanism on the failure and fracture of TiAl alloys under extreme conditions is related with the microstructure, including phase and grain size. In the present research, two kinds of TiAl alloys tailored with different microstructures, near lamellar (NL) and near gamma (NG), were fabricated by thermo-mechanical treatment. Microstructural characterization was analyzed by XRD and EBSD. The dynamic behavior of the TiAl alloys under different temperatures ranging from 293 K–873 K was investigated by a split Hopkinson pressure bar. The strain rate sensitivity and temperature sensitivity was analyzed. The microstructural evolution was concerned to understand the failure mechanism of the two kinds of the TiAl alloys. The NG-TiAl had the homogeneous deformation with synergy effect between homogeneous equiaxed grain and lamellar structure, and no failure occurred in NG-TiAl. However, the NL-TiAl showed heterogeneous deformation with both “orange peel effect” and cracks, which was attributed to large equiaxed grain and brittle γ-lamellae with similar orientation. Further, the cracks were easily nucleated and propagated from the interface between γ-lamellae structures, especially in the γ-lamellae structures parallel with the loading direction. Finally, the modified Johnson–Cook constitutive model was proposed to describe the deformation behavior, in which both strain rate hardening and temperature softening terms were expressed as a function of strain and strain rate.

Effect of hafnium addition on microstructure and room temperature mechanical properties of the Ti-48Al-2Cr-2Nb intermetallic alloy

Microstructures, phase transformations and mechanical properties of the Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated. The alloys were fabricated by vacuum arc remelting followed by hot isostatic pressing and homogenization treatment. The results showed that Hf alloying leads to a significant microstructure refinement in terms of both colony size and inter-lamellar spacing. Homogenization at 1400 °C resulted in a fully lamellar (FL) microstructure in 4822 and 2Hf alloys, while nearly lamellar (NL) in 4Hf and 6Hf alloys. Differential thermal analysis (DTA) demonstrated that Hf addition up to 2 at% has a slight contribution to phase transition sequences, but a significant implication to the phase equilibrium of the alloys with further Hf content. Based on the DTA data and annealing at 1450 °C, the solvus temperature of the eutectic phases was estimated to be over the range of 1430–1440 °C. Although the eutectic phases of Al3Hf2 and TiAl2 formed during solidification of the high-Hf alloys did not undergo any phase transition, both size and volume fraction of the eutectic cells increased due to the solvus of these metastable eutectic phases. The orientation relationship {111}Tetragonal//{001}Orthorhombic detected between the eutectic phases and their surrounding matrix confirmed the occurrence of γc (TiAl) → eutectics (Al3Hf2, TiAl2) phase transformation. Small punch tests results showed that the 2Hf and 4Hf alloys exhibit a higher maximum load (Fm) than the base 4822 alloy due to the solid solution effect of Hf and finer inter-lamellar spacing. Nevertheless, the brittle behavior of the eutectic phases dramatically deteriorated mechanical performance such that the 6Hf alloy possessed the lowest Fm and displacement. The major fracture mode changed from trans-lamellar to inter-lamellar as the Hf content increased from 4 to 6 at%.

Effects of Hot Isostatic Pressing and Heat Treatment on the Microstructure and Mechanical Properties of Cast TiAl Alloy

Hot isostatic pressing (HIP) and subsequent heat-treatments (HT) are necessary for titanium aluminide (TiAl) casting components. But there are few studies carefully comparing the microstructure changes from the initial as-cast condition to the final heat-treated condition. In this study, the microstructures of Ti-47Al-2Cr-2Nb (at%) alloy in the as-cast, as-HIPed and as-heat-treated conditions were characterized by optical microscopy and scanning electron microscopy. The mechanical properties after HTs were determined by the tensile tests at 700 °C. The results show that after HIP and HTs, all the microstructures exhibit a nearly lamellar (NL) structure and can be divided into an edge region and a central area. The microstructure after HIP in the edge region is normal, while distorted lamellae and many fine recrystallized grains exist in the central area. The yield strengths after three HTs are nearly the same, but the elongation after the HT at 1310 °C is much more than that after HTs at 1185 °C and 1280 °C. A refinement of colony size induced by distorted lamellae in as-HIPed condition is considered responsible for the great improvement in elongation.

STUDY ON FORMATION OF LAMELLAR MICROSTRUCTURE OF Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y ALLOY BY HIGH TEMPERATURE LASER SCANNING CONFOCAL MICROSCOPE

In order to study the formation mechanism of fine fully lamellar (FL) microstructure of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy, the heat treatments of holding at 1450 ℃ for 35s followed by cooling at the rates of 3-600 ℃/min were conducted by high temperature laser scanning confocal microscope (HTLSCM). Evolution of lamellae were observed in situ and the resultant microstructures were analyzed by SEM and TEM. The results show that the surface of the sample was contaminated by oxygen coming from the alumina crucible and the phenomena observed in situ are not as reliable as those occurred in the interior of the sample. The results observed in situ give some qualitative conclusions on α to lamellar colony transformation: γ lamellae nucleate at α grain boundaries then propagate mainly from one end of grain to the other; at the same cooling rate, the start temperatures of the transformation are different for different α grains; with the increase of the cooling rate, the start temperature of the transformation decreases. The interior microstructures give some quantitative results: the alloy is in α+β two phase region at 1450 ℃; with the increase of cooling rate, the amount of B2 phase increases, the lamellar colony size and lamellar spacing decrease. When the cooling rate is higher than 30 ℃/s, the nearly lamellar (NL) microstructure with volume fraction of B2 phase, lamellar colony size and lamellar spacing of about 2%, about 60 μm and about 100-500 nm, respectively, is obtained.

Continuous-Cooling-Transformation (CCT) Behaviors and Fine-Grained Nearly Lamellar (FGNL) Microstructure Formation in a Cast Ti-48Al-4Nb-2Cr Alloy

The CCT behaviors of a high-Al mid-Nb-containing cast TiAl alloy were experimentally studied in the current work. Samples with the nominal composition of Ti-48.2Al-4Nb-2Cr (at.pct) were preheated to 1420 °C and then cooled at different cooling rates. With preheating time of 8 minutes, the original microstructure was replaced by coarsening α grains. Interrupted tests demonstrated the significant decrease of microsegregation which was also predicted by theoretical calculations. The water-quenched microstructures during continuous cooling were characterized. With the increase of cooling rate from 5 to 700 °C/min, the lamellar spacing was refined from 1.77 to 0.43 μm and the microhardness was heightened from 299 to 426 HV, showing the solute strengthening effect of Nb addition compared with 4722 and Ti-48Al alloys. The CCT diagram of the current alloy was established to guide the microstructure control. Slow cooling contributed to the discontinuous coarsening of γ lamellae, suggesting a nearly lamellar (NL) microstructure. The addition of 4 at. pct Nb increased the temperatures of phase transformations, moved the CCT curves to higher temperature, and decreased the critical cooling rate for the formation of Widmanstatten and feathery structures. Finally, a fine-grained NL (FGNL) microstructure with a potential high performance was obtained.

Microstructural evolution and mechanical properties of a high yttrium containing TiAl based alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed TiAl powder with excessive yttrium addition by spark plasma sintering (SPS) technique. The gas-atomized Ti–43Al–5Nb–2V–1Y (at. %) powders were sintered at various temperatures between 1100 and 1250 °C, applying a uniaxial pressure of 40 MPa. The microstructures of pre-alloyed powders and SPSed TiAl alloy samples were analyzed by scanning and transmission electron microscopies. In addition, the compression tests of SPSed alloys was carried out at room temperature (RT). Attention is focused on the existing form and distribution of excess yttrium in the initial powders and the SPSed samples. The initial powder exhibits a dendrite structure, and yttrium-rich phases (YAl2) are segregated along the interdendritic zone. After sintering, duplex, nearly lamellar (NL) and fully lamellar (FL) microstructures were obtained as a function of the sintering temperature. For duplex and NL microstructures, the YAl2 phases are fine granular precipitates in homogeneous dispersion state. Conversely, stripe-like YAl2 precipitates, which surround the α2/γ lamellar colonies, emerged in the FL microstructures. Excessive sintering temperature leads to grain growth and serious segregation of the YAl2 phases, and it deteriorates the mechanical properties of this alloy eventually.

Friction Weldability of a High Nb Containing TiAl Alloy.

The friction weldability of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy has been investigated by optimizing process parameters and analyzing the microstructures and tensile properties of the joints. The as-cast alloy with a nearly lamellar (NL) microstructure and the as-forged alloys with a duplex (DP) microstructure have been successfully welded. All the joints have a severe deformation zone (SDZ) and a transition zone (TZ) between the parent metal (PM) and SDZ. SDZ, showing a biconcave lens geometry, has a maximum thickness of hundreds of micrometers at the periphery. TZ is hundreds of micrometers thick. All SDZs have a fine-grained DP microstructure with a grain size of a few micrometers. For the joint of the as-cast alloy, the TZ consists of deformed lamellar colonies as the major constituent and partially recrystallized grains as the minor constituent. For the joint of the as-forged alloy, the TZ is similar to both the PM and SDZ, showing a DP microstructure. The grain size, volume fraction of γ grains, and the remnant lamellar colonies all increase with the distance from the SDZ. All joints presented perfect metallurgical bonding. The strengths of the joints are higher than those of the corresponding PMs. This indicates that the studied alloy has good friction weldability.

Dynamic tensile behavior of PM Ti−47Al−2Nb−2Cr−0.2W intermetallics at elevated temperatures

Abstract Split Hopkinson Tension Bar (SHTB) experiments were conducted to explore the dynamic mechanical behavior and deformation mechanism of powder metallurgical (PM) Ti−47Al−2Nb−2Cr−0.2W (at.%) intermetallics with near lamellar (NL) and duplex (DP) microstructures. Results show that, under dynamic loading, the high temperature strength of the PM TiAl intermetallics is higher than that under quasi-static loading, and the ductile to brittle transition temperature (DBTT) increases with the increase of strain rate. Formation of twinning and stacking faults is the main deformation mechanism during dynamic loading. The work hardening rates of the PM TiAl intermetallics are nearly insensitive to strain rate and temperature at high strain rates (800−l600 s −1 ) and high temperatures (650−850 °C). Zerilli−Armstrong model is successfully used to describe the dynamic flowing behavior of the PM TiAl intermetallics. In general, the PM TiAl intermetallics are found to have promising impact properties, suitable for high-temperature and high-impact applications.

Microstructure and properties of friction welding joint of Ti–45Al-8.5Nb-0. 2W-0. 2B-0. 02Y alloy

High Nb containing TiAl alloys have prospect to be used for turbo wheel and exhaust valve for vehicle engine. Friction welding is a suitable process for fabricating these components. For investigating the weldability of these alloys, cylindrical samples of Ti–45Al-8.5Nb-0.2W-0.2B-0.02Y (in at%) alloy with a diameter of 10 mm were welded by continuous drive friction welding process, and the microstructures and the mechanical properties of the joints were analyzed in this paper. The results showed that the weldability of the present alloy was very good. At parameters of rotational speed of 2400 r/min, friction pressure of 320 MPa, burn-off distance of 3 mm and upset pressure of 360 MPa, completely metallurgical bonding interfaces were obtained. The joint had a severely plastic deformation zone (SPDZ) with a biconcave lens geometry. The thicknesses of the center and periphery of SPDZ were about 40 μm and 800 μm, respectively. There existed a transition zone (TZ) between parent metal (PM) and SPDZ. SPDZ exhibited the deformation flow lines and had a randomly-oriented recrystallized duplex (DP) microstructure with the average grain size of 2.5 μm, in contrast, PM presented a nearly lamellar (NL) microstructure with a colony size of 150 μm. TZ showed a mixture microstructure combining deformed lamellar colonies and partially recrystallized grains. At room temperature, 750 °C and 800 °C, welding joints had better tensile properties than PM.

In-situ control of microstructure and mechanical properties during hot rolling of high-Nb TiAl alloy

High-Nb TiAl sheets were fabricated by a hot pack rolling method. During the process, three refined TiAl microstructures can be directly obtained by controlling the rolling temperature: duplex (DP), near lamellar (NL) and fully lamellar (FL). This leads to different mechanical properties. The control of microstructure and mechanical properties was achieved through the hot rolling process, rather than by the conventional subsequent heat treatment. The average colony sizes of NL and FL microstructures are refined to 50 μm and 75 μm, respectively. The β phase can be reduced in NL microstructure and completely eliminated in FL microstructure. High strength of the sheets on both ambient and elevated temperatures is explained by the characteristics of the optimized microstructures. This work provides an alternative approach to tailor microstructure and mechanical properties of TiAl sheets.

Microstructure refinement via martensitic transformation in TiAl alloys

Martensitic transformations are utilized to refine the microstructure of high Nb containing TiAl alloys by rapid quenching from β phase region. Additions of β stabilizing elements, such as Cr and V, are verified to promote the β→α martensitic transformation. The morphologic and crystallographic characteristics of martensites in Ti-42.5Al-6Nb-x(Cr, V) alloys indicate that the martensitic α has a perfect Burgers orientation relationship with the parent β phase, and the lateral size of the martensite decreases with increment of β stabilizing elements. With subsequent heat treatment, a series of refined fully lamellar (FL) and nearly lamellar (NL) microstructures can be obtained, where the FL colony size can reach 30 μm with 2 at.% Cr and 50 μm with 3 at.% V respectively.

Microstructure evolution and mechanical properties of a Ti-45Al-8.5Nb-(W, B, Y) alloy obtained by controlled cooling from a single β region

An accurate temperature-controlled approach was developed to investigate the microstructure evolution of a high-Nb-containing TiAl alloy. The Ti-45Al-8.5Nb-(W, B, Y) alloy was subjected to controlled heating and cooling from the single β phase region (1500 °C) under various processing parameters. Three typical microstructures involving full lamellar (FL), near lamellar (NL) and quenched martensite plates exist in the cooled microstructures. The initial microstructural homogeneity obtained by isothermal holding within the β phase region can effectively reduce the microsegregation in the subsequent cooling process. The fraction of α2 phase and the lamellar spacing significantly decrease with an increased cooling rate. Furthermore, the crystalline orientations between coarsened block γ, retained β/B2 and α2 martensite were identified, and the phase transition mechanism was analysed. Lastly, the microhardness of these treated samples was tested, and the microstructure-dependent mechanical properties were evaluated and discussed.

General features of high temperature deformation kinetics for γ-TiAl-based alloys with DP/NG microstructures: Part I. A survey of mechanical data and development of unified rate-equations

γ-TiAl-based alloys have attracted intense attentions in the past two decades and are regarded as promising candidate materials for high-temperature applications due to their low density, high specific strength, superior creep and oxidation resistance, etc. It is well known that there are four typical microstructures in TiAl-based alloys, namely, full-lamellar (FL), near-lamellar (NL), duplex (DP) and near-gamma (NG). The FL/NL-TiAl alloys possess superior high temperature mechanical performance such as creep resistance, but their deformability is relatively lower. In contrast, the DP/NG-TiAl alloys have much better formability while the creep resistance is reduced. Hence the DP/NG microstructure is preferable for hot-working and thought to the prerequisite for secondary processing, and the present paper is primarily focused on the high temperature deformation behavior of DP/NG-TiAl alloys. Till date, considerable research efforts have been directed towards the deformation kinetics or mechanical properties of various DP/NG-TiAl alloys, and a mass of experimental data has been accumulated. In order to explore their general features of high temperature deformation kinetics, in this paper the tensile/compression mechanical data of various DP/NG-TiAl alloys from the literature published in the past twenty years has been summarized and reviewed. Three deformation mechanisms (dislocation creep, grain boundary sliding and diffusion creep) have been identified, and for each the effects of major factors (i.e., alloy composition, microstructure and grain size) on deformation kinetics have been analyzed and discussed in detail. The results revealed that the high temperature deformation kinetics of DP/NG-TiAl alloys is less sensitive to composition except some dispersion-hardened alloys such as carbon-containing TiAl alloys. Instead, the γ grain size became the major limiting factor. Based on this observation, a series of unified rate-equations has been successfully developed.

Microstructure and tensile properties of containerless near-isothermally forged TiAl alloys

Ti–47Al–2Nb–2Cr–0.4(W, Mo) (mole fraction, %) alloy ingot fabricated using vacuum consumable melting was containerless near-isothermally forged, and the high temperature forgeability, microstructure and tensile properties were investigated. The results show that the TiAl ingot exhibits good heat workability during containerless near-isothermally forging process, and there are not evident cracks on the surface of as-forged TiAl pancake with a total deformation degree of 60%. The microstructure of the TiAl ingot appears to be typical nearly-lamellar(NL), comprising a great amount of lamellar colonies (α 2+γ) and a few equiaxed γ grains. After near-isothermally forging, the as-forged pancake shows primarily fine equiaxed γ grains with an average grain size of 20 μm and some broken lamellar pieces, and some bent lamellas still exist in the hard-deformation zone. Tensile tests at room temperature show that ultimate tensile strength increases from 433 MPa to 573 MPa after forging due to grain refinement effect.

Dynamic behavior and fracture mode of TiAl intermetallics with different microstructures at elevated temperatures

Experimental studies were conducted on the tensile behaviors and fracture modes of TiAl (Ti-46.5Al-2Nb-2Cr) alloys with near gamma (NG) equiaxed and near lamellar (NL) microstructures over a temperature range from room temperature to 840 °C and a strain rate range of 0.001-1 350 s −1. The results indicate that the alloys are both temperature and strain rate dependent and they have a similar dependence. The dynamic strength is higher than the quasi-static strength but almost insensitive to high strain rate range of 320-1 350 s −1. The brittle-to-ductile transition temperature (BDTT) increases with increasing strain rates. NG TiAl yields obviously, while NL TiAl does not. Below BDTT, as the temperature increases, the fracture modes of the two alloys change from planar cleavage fracture to a mixture of transgranular and intergranular fractures, and finally to totally intergranular fracture.